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  Corvetteforum.com
  1968 - 1982: C3 Corvettes Archived Topics
  "COWL" INDUCTION (Page 1)

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Author Topic:   "COWL" INDUCTION
JDG/76-ZZ4
Senior Member

Posts: 279
From: Alamogordo, NM
Registered: Sep 1999

posted 12-31-1999 03:46 PM     Click Here to See the Profile for JDG/76-ZZ4   Click Here to Email JDG/76-ZZ4     Edit/Delete Message   Reply w/Quote
I have been toying with the cowl (hi pressure) induction on my car. I have drilled holes both horizontally and verically to get as much cool air to the air filter (360 deg)as possible. I took some temp meausurements today and am posting this as a general ref point and are "snapshots":

Ambient 66F
60 MPH at cowl entrance 53F
75 MPH 50F
IDLE 101F

With the sensor at the rear of filter:

Ambient 66F
Idle 115F
40 MPH 88F
65 MPH 69F

After looking at the numbers, I can see why there is a need to try and bring cool air into the intake system.

------------------
JD 76/ZZ4
K&N A/Filter, Valentine1, Remote Engine Oil Filter,Tranny/Engine oil Coolers
MSD 6A, SONY Cd Player, BAZOOKA subwoofer, Blaupunkt speakers,
Crossover(H) Pipe, EDEL 795 CFM QJET, ACCEL Coil/Wires,
>>Where the END justifies
the MEANS >>>>


[This message has been edited by JDG/76-ZZ4 (edited 12-31-1999).]

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Vetterodder
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Posts: 1260
From: Stockton, CA
Registered: Feb 99

posted 01-02-2000 01:59 AM     Click Here to See the Profile for Vetterodder   Click Here to Email Vetterodder     Edit/Delete Message   Reply w/Quote
JD, I'm kinda confused (so what else is new? ). Did you drill the holes in the cowl? Where are you refering to as "cowl entrance"? If ambient is 66, how did you get 53 and 50 readings? BTW, I ran a factory plenum fed air cleaner (the plenum fed thing, not the zillion pound hood system used on later years) on a `67 Chevelle years ago. It was good for a consistent 1 mph increase in the 1/4 but et suffered slightly. That, and the fact that if you used the vents or heater you sucked gas fumes into the interior, caused me to take the first $20 offered. As I do about most of the homes and many of the cars I've had, I cringe when I think about what that puppy would be worth now .

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Jack Sweet
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From: Mission Viejo, Calif.
Registered: Mar 99

posted 01-02-2000 02:05 AM     Click Here to See the Profile for Jack Sweet   Click Here to Email Jack Sweet     Edit/Delete Message   Reply w/Quote
Are you drilling out the part of the hood immediately under the "grate" at the rear of the '76 hood?

I've actually thought about doing that myself. Is that all you've done or have you fabbed up an airbox to go with it?

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LymanSS
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From: Washington DC. USA
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posted 01-02-2000 02:16 AM     Click Here to See the Profile for LymanSS   Click Here to Email LymanSS     Edit/Delete Message   Reply w/Quote
Just a thought, but I would imagine that the drop in temperature would indicate that the pressure was dropping too. Perhaps the high speeds lower the pressure in the rearward facing cowl induction hood. This would cause the lower than ambient temps. I have yet to actualy measure any of the effects of my cowl induction L88 style hood. I hope to get some decently cold measurements when I do.

Scott

Scott

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Ken73
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From: Houston, TX
Registered: Aug 2000

posted 01-02-2000 10:59 AM     Click Here to See the Profile for Ken73   Click Here to Email Ken73     Edit/Delete Message   Reply w/Quote
Scott; I'm thinking along the same lines you are. A drop in temperature would indicate a drop in pressure as well. If you look at cowl induction a little closer, it would actually appear (and flow dynamics will tell you) that the flow of air up over the hood and windshield will actually create a low pressure area at the cowl induction.

My suggestion would be to do some more readings with a extrememly-sensitive low-pressure gauge, such as a manometer. Gauges can be bought from http://www.gaugestore.com/ for good prices. They have vacuum and pressure gauges that measure in inches of water. (About $30 for one.) That way you could get very accurate results to post and not have to spend a fortune on test equipment.

Ken

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redvetracr
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Posts: 637
From: WI
Registered: Aug 1999

posted 01-02-2000 01:18 PM     Click Here to See the Profile for redvetracr   Click Here to Email redvetracr     Edit/Delete Message   Reply w/Quote
Look at EVERY Cup car they all use the cowl for carb air intake!

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JDG/76-ZZ4
Senior Member

Posts: 279
From: Alamogordo, NM
Registered: Sep 1999

posted 01-02-2000 01:34 PM     Click Here to See the Profile for JDG/76-ZZ4   Click Here to Email JDG/76-ZZ4     Edit/Delete Message   Reply w/Quote
The holes are right under the grate on the black portion of where the "cowl" would have been...I started out with holes in a horizontal plane and then in a vertical plane. Also under the hood, I did the horizontal holes...This way air comes in and down and in and to the air filter.....

The temp was meausured with a digital thermometer who has an external wired sensor. I put the sensor at the entrance hole(s).

The temp dropped from ambient to 53 deg doing about 70 MPH (about 13 deg).

I then took the sensor and placed it on the filter itself...That is where I got the other temp meausurements....

My rough test showed that at about 60 MPH, I was able to get the same temp at ambient and at the FILTER which is still cooler than w/out the holes drilled.

The holes were just drilled and no box was fabed...There are no fumes in to the cab since the air goes under the hood. I repainted the area where the holes are black so unless you are specifically looking for the holes, they are hard to see...I had also put a string in one of the holes to get an effect of air going in the cowl and although not scientific, I could see the string go from a layed down position to a completely horizontal.
All I wanted was to somehow get cooler air to the air filter and I have been able so far. There is still room for maybe 2+ inch holes which I will probably do today. Right now they are about 1.5 ".
Hope this helps.....

I am quoting a part of Roe's book on pg 114: Cold air gives more improvement than ram air because about 1% HP increase is gained for each 5 deg decrease in temp (asuming mixture, etc... is correct).
"Duct cold air from the cowl just ahead of the windshield. This is a high pressure zone that will supply cool outside area to the carburetor".
------------------
JD 76/ZZ4
K&N A/Filter, Valentine1, Remote Engine Oil Filter,Tranny/Engine oil Coolers
MSD 6A, SONY Cd Player, BAZOOKA subwoofer, Blaupunkt speakers,
Crossover(H) Pipe, EDEL 795 CFM QJET, ACCEL Coil/Wires,
>>Where the END justifies the MEANS >>>>


[This message has been edited by JDG/76-ZZ4 (edited 01-02-2000).]

[This message has been edited by JDG/76-ZZ4 (edited 01-02-2000).]

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JDG/76-ZZ4
Senior Member

Posts: 279
From: Alamogordo, NM
Registered: Sep 1999

posted 01-02-2000 05:50 PM     Click Here to See the Profile for JDG/76-ZZ4   Click Here to Email JDG/76-ZZ4     Edit/Delete Message   Reply w/Quote
A couple more bits of info...I noticed on my previous drop base filter that only about an inch or less of the filter was directly exposed to any type of direct air flow over the top of the carb...the remainder of the fiter was below the top of the carb. I replaced it with a flat base thus the whole filter (2") are over the carb esp with any type of air flow coming in from the "cowl".
Another thing I noticed is that up until about 30MPH when some strong air flow started coming in the drilled holes, warm/hot air from under the hood would be "vented" out.. The temp readings at the entrance prove that.. I would think this is a good thing at low/idle speeds...I wanted to make the holes bigger for more volume but the weather is not permitting...
I dont have a way of quantifying all this as a power mod but maybe someone can...

------------------
JD 76/ZZ4
K&N A/Filter, Valentine1, Remote Engine Oil Filter,Tranny/Engine oil Coolers
MSD 6A, SONY Cd Player, BAZOOKA subwoofer, Blaupunkt speakers,
Crossover(H) Pipe, EDEL 795 CFM QJET, ACCEL Coil/Wires,
>>Where the END justifies the MEANS >>>>


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JDG/76-ZZ4
Senior Member

Posts: 279
From: Alamogordo, NM
Registered: Sep 1999

posted 01-02-2000 08:03 PM     Click Here to See the Profile for JDG/76-ZZ4   Click Here to Email JDG/76-ZZ4     Edit/Delete Message   Reply w/Quote
Scott...If you decide to take temp meausurements, RShack has the indoor/outdoor thermometers with the long wire and sensor. Gives you flexibility for meausrements...Just a note...

------------------
JD 76/ZZ4
K&N A/Filter, Valentine1, Remote Engine Oil Filter,Tranny/Engine oil Coolers
MSD 6A, SONY Cd Player, BAZOOKA subwoofer, Blaupunkt speakers,
Crossover(H) Pipe, EDEL 795 CFM QJET, ACCEL Coil/Wires,
>>Where the END justifies the MEANS >>>>

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Juliet Page
unregistered
posted 01-02-2000 10:36 PM           Edit/Delete Message   Reply w/Quote
I'm not overly familiar with the cowl induction systems, but I can add some simple aerodynamics / physics info to this thread. (Please suppress those yawns while I put on my Aerodynamics Teacher hat OK?)

Bernoulli's Equation for incompressible flow in a duct (under Mach 0.3, safe assumption for incomressibility here)
P + (1/2)*rho* Veloc*Veloc = constant

P = pressure
rho = density
Veloc = airspeed in the duct

Point is, as velocity increases the pressure drop goes with velocity squared.

Other relationship for air (perfect gas law):

P*V = n*R*T

P = pressure of the gas
V = Volume
n, R = constants
T = temperature

Here you can see that the Temperature drops in direct proporation to the pressure.

From an aerodynamics perspective I would *guess* that the extra holes drilled allowed an increase in velocity resulting in the decrease in pressure and hence a decrease in temperature. Of course the volume as the air traves into the induction system is not constant, so there should already be some sort of effect on temp due to an unmodified system... theorteically one can calculate the X-areas and temp / pressure drops. I'm sure the auto designers have more sophisticated tools for doing this (which account for friction drag, losses, leaks, choked flow etc) than my simple equations. But maybe this helps understand the aero situation anyways. Please feel free to correct me if I screwed this up... back from a week's vacation skiing (& drinking Long Island Iced Teas) and the brain ain't quite in gear yet for work tomorrow.

------------------
~Juliet ...overlooking Mill Creek on the Chesapeake Bay...
Loaded Bridgehampton Blue on Blue '70 350/300Hp TH400 with a White Ragtop

[This message has been edited by Juliet Page (edited 01-02-2000).]

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Robert Holtman
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From: Corning, CA, USA
Registered: Jun 99

posted 01-02-2000 11:42 PM     Click Here to See the Profile for Robert Holtman   Click Here to Email Robert Holtman     Edit/Delete Message   Reply w/Quote
Gee wiz! The only equation I know is More power + less weight = goes faster. I bet Juliet cooks (and looks) better than I do too.


Dumber than a box or rocks but still a 'Vette Guy BBBBob

------------------
BBBBob

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JDG/76-ZZ4
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Posts: 279
From: Alamogordo, NM
Registered: Sep 1999

posted 01-02-2000 11:44 PM     Click Here to See the Profile for JDG/76-ZZ4   Click Here to Email JDG/76-ZZ4     Edit/Delete Message   Reply w/Quote
Juliet..The formulas in Roes' book touches briefly on your theory...I read on one of the restoration books that cowls were part of the Vettes introduced in 73 until I believe 75. I believe they were solenoid controlled but many people didnt like them because of the howling noise (Corvette Black Book)....Hence in 76 te air induction was moved to the front..The mod I did is coarse since I have no "real" control but I believe it helps to some degree....Based on my rough temp numbers and if the formula in Roe's book is correct, I tend to pickup app 8% HP in the 3500RPM area..As to noise, well I cant tell if it is noisier or not...Thnx and glad you enjoyed your vacation....

------------------
JD 76/ZZ4
K&N A/Filter, Valentine1, Remote Engine Oil Filter,Tranny/Engine oil Coolers
MSD 6A, SONY Cd Player, BAZOOKA subwoofer, Blaupunkt speakers,
Crossover(H) Pipe, EDEL 795 CFM QJET, ACCEL Coil/Wires,
>>Where the END justifies the MEANS >>>>

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Jack Sweet
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From: Mission Viejo, Calif.
Registered: Mar 99

posted 01-03-2000 12:32 AM     Click Here to See the Profile for Jack Sweet   Click Here to Email Jack Sweet     Edit/Delete Message   Reply w/Quote
There are a few reasons I had been toying with the idea of drilling holes or cutting slots into the vertical wall in the '76 hood almost directly under the grate at the top rear of the hood.

First, up until 1975 or so, that's where Corvettes got their air via a solenoid-controlled "flapper" door.

Second, I've long been aware of the benefits of full-on cowl induction with an airbox.

Third, when the car was wet from a wash, I noticed droplets of water falling from the grate and from the back edge of the hood while the car was in motion. Instead of dropping straight down into the wiper channel, those droplets seemed to be blown or sucked forward toward the base of the hood.

The benefits of this mod would seem to be limited without fabbing up a sealed airbox and channel from the cowl inlet to the air cleaner. Buuuuut, if informal, seat-of-the-pants type tests seem to indicate that underhood temps are decreased somewhat by the addition of the holes it may not be a bad idea.

Based on your initial post it looks as if the temps at the holes in the cowl decreased, but that the temps at the air filter stayed up slightly.

What were the temps at the exterior of the cowl, the interior of the cowl and the air cleaner before the holes were drilled or cut?

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Vetterodder
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From: Stockton, CA
Registered: Feb 99

posted 01-03-2000 01:34 AM     Click Here to See the Profile for Vetterodder   Click Here to Email Vetterodder     Edit/Delete Message   Reply w/Quote
JD, cooler air is better air and it looks like you've found some.

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JDG/76-ZZ4
Senior Member

Posts: 279
From: Alamogordo, NM
Registered: Sep 1999

posted 01-03-2000 02:13 AM     Click Here to See the Profile for JDG/76-ZZ4   Click Here to Email JDG/76-ZZ4     Edit/Delete Message   Reply w/Quote
Jack..I never took meausurements before the holes were drilled...I did it on the spur of the moment a while back...The temp that would count the most, I would think would be the one on the rear of the filter (pleats)...The "snapshot" at idle I got was about 115F and I am sure climbing with 66 ambient...The best I could do was drop to 69F at about 65 MPH with 88F at 40MPH..So there is an obvious decrease in air temp going into the carb itself....Had I had more time and distance I could have seen the difference at 75MPH(Juliet's theory)..The temp at the filter would not be as low as the outside entrance for obvious reasons...However in the snapshot, a drop of about 45 degrees is substantial even though cooler would be better....How I could do a duct or box, I dont know since I have a 360 K&N open filter..maybe someone with a 73-75 that has a cowl induction would help.....For now, as soon as the weather clears I want to increase the size of the holes without damaging the hood release cable, to allow maximum volume of air flow...

------------------
JD 76/ZZ4
K&N A/Filter, Valentine1, Remote Engine Oil Filter,Tranny/Engine oil Coolers
MSD 6A, SONY Cd Player, BAZOOKA subwoofer, Blaupunkt speakers,
Crossover(H) Pipe, EDEL 795 CFM QJET, ACCEL Coil/Wires,
>>Where the END justifies the MEANS >>>>

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LymanSS
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From: Washington DC. USA
Registered: Apr 99

posted 01-03-2000 06:39 AM     Click Here to See the Profile for LymanSS   Click Here to Email LymanSS     Edit/Delete Message   Reply w/Quote
Juliet,
Good points with the formulas, but there is a piece there that doesn't work well for me. Bernoulli's principle applies to the effects that a moving fluid has on a surface. It determines the drop in the pressure that a gas applies to a surface as it slides over it. However, the P*V=n*R*T law (or Pervert as my chem teacher taught us), is used to determine the attributes of still, contained gasses. What I am saying is that I do not think that the bernoulli effect will lower the pressure of the gas. It will simply lower the pressure that the gas applies to the surfaces over which it is flowing. In order to lower the pressure, the gas has to be put into an area of larger volume. This will make pressure and temperature drop. I believe that this is what is happening. The air is whipping across the car very fast. All of a sudden as it passes the cowl, we expect some of it to reverse direction, and go toward the front of the car, into the cowl, and then the carb. What is there to force the air into the cowl? Nothing but the ambient air pressure. So the air will not enter the cowl untill there is a significant difference between the ambient air pressure and the air pressure in the cowl. So the engine is "sucking" this air in against its will....sort of the opposite of ram air. This causes the low pressure, and the resultant drop in temperature. Sorry if I am sounding disjointed and rambling here.

In any case, i am certain that it is better than hot engine compartment air. I would like to get some test figures on my car, but i have just begun a project (new brakes, and steering box) so it will be off the road for at least a week i think.

Scott

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JDG/76-ZZ4
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From: Alamogordo, NM
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posted 01-03-2000 08:30 AM     Click Here to See the Profile for JDG/76-ZZ4   Click Here to Email JDG/76-ZZ4     Edit/Delete Message   Reply w/Quote
Scott..I am definitely not a physics type but in the "cowl" effect, isnt the air being forced back by the windshield..A portion is deflected up and a portion is bounced back (cowl) induction...I thought in the ram air function, the air is actually "scooped" in or rammed in by the scoop on the hood...that is why many low rams are actually very inefficient based on their low height over the hood...I have some literature on this I need to dig out.....

------------------
JD 76/ZZ4
K&N A/Filter, Valentine1, Remote Engine Oil Filter,Tranny/Engine oil Coolers
MSD 6A, SONY Cd Player, BAZOOKA subwoofer, Blaupunkt speakers,
Crossover(H) Pipe, EDEL 795 CFM QJET, ACCEL Coil/Wires,
>>Where the END justifies the MEANS >>>>

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vettfixr
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Posts: 1187
From: Sewell, NJ
Registered: Jul 1999

posted 01-03-2000 09:31 AM     Click Here to See the Profile for vettfixr   Click Here to Email vettfixr     Edit/Delete Message   Reply w/Quote
Here's my $.02. It seems like two different topics are being co-mingled here. One is the effect of cooler air on the charge which we all know is good. Cooler air = more oxygen = more horsepower. The second topic is the effect that high pressure (ram-air) has on horsepower. I've often wondered if a setup could be used which would duct air in from the fender wells or shark vents. In the back of most of the Chevy books is an ad for a ram-air cleaner which is enclosed and has spouts which can be connected to ducting. If these spouts could be connected to where the flaps for the control arms or the shark gills are, then you would be able to pull in cool air but without the ram effect. If ram air is required the only area that I know of which would consistently be of high pressure are the front vent area (where the parking lamps are)or the area just in front of the spoiler. It seems, as others have noted, that the cowl induction only supplies cold air and not a ram effect. Just thinking out loud.

------------------
vettfixr
74 4-Speed L-48 T-Top
Clear over Black Lacquer
Some "Grinnin" Done to Engine.
Not Stock but the Way I Like It.
Sewell,NJ
users.snip.net


[This message has been edited by vettfixr (edited 01-03-2000).]

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Juliet Page
unregistered
posted 01-03-2000 01:42 PM           Edit/Delete Message   Reply w/Quote
Hi, very cool thread BTW. I'm enjoying this one.

What is the performance limitation for a typical vette setup? Is it the stock carb CFM being choked at WOT, or is it fuel flow? Exhaust backpressure etc? My newbie status is showing again. Maybe it's somewhat choked flow in the cowl inlet passages leading to the diffuser section, which would certainly be helped by the additional holes.

I think that the basic premise of the cowl induction is that it sucks in air near the windshield near a stagnation point... IE high pressure, low velocity flow, and it just happens to be cooler too. The vehicle aerodynamics have already paid the drag penalty to slow it down, might as well use the "dead air" and reduce the boundary layer thickness at the same time. (Sorta similar to the Laminar Flow Control F-16XL project I worked on). So now this flow goes from low velocity to higher velocity (eventually at the carb) thorough some sort of ductwork with varying area. Then it goes into the more open plenum (the area around the filter) which is clearly much larger and acts as a subsonic diffuser, which greatly decreases pressure and increases density.

Also, correct me if I'm wrong, but I think that the CFM ratings for carbs are determined assuming sonic flow (Mach=1.0) at the throat for WOT (yes, incompressibilty not applicable there). So the change in velocity HAS to affect the pressure, but then that's inside the carb.

One can also state the gas law as P = rho*R*Temp. There's a mixing effect of Temp decrease and density increase even for constant pressure, but yes the pressure is decreasing. You're probably right about the suction "driving" the flow, otherwise why and how would it want to enter the cowl to begin with, but one can also draw a comparison with "starting" the flowfield, and once it's established, what does it take to mantain it?

The other thing I wanted to mention is that fluid can not support a pressure gradient (without motion - Bernoulli, or the compressible flow eqns) and always transmits it, therefore the force on the walls of the duct (pressure) and the pressure of the flow will be the same for a given cross section. (Equal and opposite)

I agree that I probably muddled the issue somewhat(that's my automotive newbie status showing) Sorry about that guys, but there has to be a blend of both effects in order to get that kind of temperature change, and in order to go from stagnation air to moving air. If it was pure momentum induced the T/To=.975 (66 vs. 53 in Rankine, subsonic compressible flow gamma=1.4) would imply flow M=0.35 at the measurement points. That's hauling butt, and probably doesn't exist in the induction system! So yes, it's definately not entirely due to momentum effects, but they do have some effect.

Also there is heat addition to the system due to the engine, which is more pronounced near the rear of the filter (higher temps) than at the cowl entrance (lower temps). We haven't discussed that yet.

JD, you could always get some "before" measurements simulated by taping the holes closed perhaps? The other thing which I don't really picture is where the holes were drilled... Is there a picture online of the cowl induction system? I'm trying to visualize this.

------------------
~Juliet ...overlooking Mill Creek on the Chesapeake Bay...
Loaded Bridgehampton Blue on Blue '70 350/300Hp TH400 with a White Ragtop
[img]http://www.annapolis.net/members/julepage/Juliet1970.jpg[/img]

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Jack Sweet
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posted 01-03-2000 03:11 PM     Click Here to See the Profile for Jack Sweet   Click Here to Email Jack Sweet     Edit/Delete Message   Reply w/Quote
I don't think anybody's talking about trying to create a "ram air" effect. I don't even thing we're talking about creating a true cowl induction setup, either. The question is whether drilling holes or cutting slots in the vertical portion of a 1976 'Vette's hood underneath the grate at the top rear of the unit will play a role in creating lower underhood temps. I think it might. I'm not an engineer but I think the temps underhood could be lowered in one of two ways by adding the holes.

Scenario 1 involves my earlier observation of water droplets from the rear edge of the hood cowl and the grate being either sucked or blown forward when the car is moving. This was probably caused by turbulent air at the rear of the hood. It seems that the air flow would be split--part of it continuing smoothly up and over the windshield and roof of the car, and part of it being deflected forward and down into the windshield wiper well. If that air could get under the hood, it might play a part in reducing the temps under there. This wouldn't create any kind of "ramming" effect, nor would it make for a true "cowl induction" setup but that additional air might make the underhood temp drop.

Scenario 2 goes like this: Corvettes are bottom breathers. They take air in from underneath the chin and it's deflected up, over and through the radiator. Some of that air may be able to escape through the fender vents, but it would have to get down to the bottom rear of the engine compartment and make a dramatic 180-degree turn to do so. I would think that having holes at the rear of the hood under the grate would provide that air a path of lesser resistance for getting out from under there.

I don't have any idea which of these scenarios of more plausible. This is just a shadetree guy talking--I don't have any flow of fluids engineering to back me up other than the rudimentary grasp of Bernoulli's principle I learned in ground school. I would be interested in seeing a before-and-after test, though. It can only be a good thing if the holes make the underhood temps go down.

What's the theory? A 10-degree drop in intake temperature can help make what, 2 or 3 h.p. or so?

BTW, Juliet--I dig a chick who can do math...

[This message has been edited by Jack Sweet (edited 01-03-2000).]

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gkull
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From: reno nevada
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posted 01-03-2000 03:12 PM     Click Here to See the Profile for gkull   Click Here to Email gkull     Edit/Delete Message   Reply w/Quote
I have a large aftermarket cowl induction hood with a 4X20 or so opening . Right now (because I have to) I am having a local race shop fab up an aluminum NASCAR type air box that seals against the bottem of the hood and the top of the carb. Then I have my open element K&N sitting in the open cold ram air space.

I had to do something because of high speed water temp problems. I hung 6 inch black electrical tape off the rear of the hood. At less than 40 mph the fan plus flow though the grill makes heat waves and the tape swing out towards the windshield. From there on up it swings all the way in and at some point causes very little flow through the radiator. I even fabed up ducting that all of the air that hit the Datona front nose had to go in the direction of the radiator and installed a larger flex fan. In testing even with the stock electric on I had water temps over 235 at high speed. So I borrowed my original hood back and never saw 205 degrees at sustained 140+ mph

From the race books I've read hp at 120 mph is 3% higher in a well designed cold(very close to ambient) ram air induction.

[This message has been edited by gkull (edited 01-03-2000).]

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JDG/76-ZZ4
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From: Alamogordo, NM
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posted 01-03-2000 07:25 PM     Click Here to See the Profile for JDG/76-ZZ4   Click Here to Email JDG/76-ZZ4     Edit/Delete Message   Reply w/Quote
Great physics discussion...I guess in simple terms,the following was accomplished with minor mods:
* I was able to drop about 20 to 45deg in air temp hitting the filter at about 65mph.
* At idle/low speed, I was able to exhaust some of the high underhood temps.
* The holes are not distracting since they are right under the grate on the backside plate of the hood...
*It appears the deflection from the windshield causes the air to flow in through the holes as speed is increased.
* I can plug the holes and get a temp reading on the filter to see the actual delta (before/after holes).
*Drilling the holes is messy with all the powder flying around...
* Under ideal conditions, Roe calculated using :
Outside air density=460 +temp under hood/460+temp outside air density*under hood air temp.
He wrote" Cold air gives more improvement than ram air because about 1%HP is gained for each 5F drop in temp; assuming mixture is adj to comp for density change and there is no detonation problems". This I cant quantify...
* He calls the area just ahead of the windshield as a high presure area. You guys smarter than me can sort this as high or low.....
Juliet..true on the temp differences on the filter and cowl entrance...see my first post....

------------------
JD 76/ZZ4
K&N A/Filter, Valentine1, Remote Engine Oil Filter,Tranny/Engine oil Coolers
MSD 6A, SONY Cd Player, BAZOOKA subwoofer, Blaupunkt speakers,
Crossover(H) Pipe, EDEL 795 CFM QJET, ACCEL Coil/Wires,
>>Where the END justifies the MEANS >>>>

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Ken73
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posted 01-03-2000 10:30 PM     Click Here to See the Profile for Ken73   Click Here to Email Ken73     Edit/Delete Message   Reply w/Quote
Another strange thought - any "ram air" effect would be nullified anyway since any excessive pressure would just escape through the normal snorkels. It might appear that maybe the underhood temps are cooling off because the hot air is being siphoned off by the low pressure area at the cowl? (If indeed there IS a low pressure area, which seems to make more sense?) If this is the case though, might be a bit helpful to put a sheet of metal around the back 180 degrees of the filter so the cooler air "rams" into the front of the filter. Just a thought.

I'm going to purchase a super-low-pressure gauge here soon - a 30" of *water* vacuum gauge. (30 inches of water is about 1 inch of mercury, if that.) I have the cowl induction deal on my '73 but I'm not sure if it's the same as the cowl induction on the '76 that appears to be our test vehicle. (Sorry, JD.)

If anyone is interesting in gauges of different sorts for pressure/vacuum, look at the gaugestore place I mentioned above. The gauge I'm talking about costs $28 or so.

Ken

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Jack Sweet
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posted 01-04-2000 01:20 AM     Click Here to See the Profile for Jack Sweet   Click Here to Email Jack Sweet     Edit/Delete Message   Reply w/Quote
Ken, I believe the '76 hood is unique. By that I mean that the area under the rear of the hood and directly under the "hump" and the decorative "grate" is a vertical wall between the bottom on the rear of the hood and the seal around the engine compartment. It's almost as if GM "forgot" to cut in the passage and plumb in the mechanism from the flapper-actuated cowl induction system used in some previous cars. This "wall" or whatever is about 20" wide by about 1 3/4" tall. You can see it from the drivers' seat or when you pop the hood. It would seem that drilling holes or cutting a couple of slots in this area would either give warm underhood air another escape route, thus causing cooler temperatures under there or it would allow some of the turbulent eddies of air normally swirling around in the wiper well to actually enter the engine compartment. This could also reduce underhood temps. It's certainly not a "ram air" or "cowl induction" system in the strictest sense. It's more like adding vents. It's pretty difficult to punch louvers in fiberglass.

[This message has been edited by Jack Sweet (edited 01-04-2000).]

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JDG/76-ZZ4
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From: Alamogordo, NM
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posted 01-04-2000 07:28 AM     Click Here to See the Profile for JDG/76-ZZ4   Click Here to Email JDG/76-ZZ4     Edit/Delete Message   Reply w/Quote
Jack is right since the option is not a 'true cowl" in the 76 even though that was the same basic setup except it operated in the 73-75 hoods and hence my quotes around the word 'cowl"...Another good source of info on this subj is "Max perf Chevy V3 by David Vizard . He claims much lke Roe in one example of 190F to 100F temp drop underhood temp using a hood "shaker" in a Trans Am was worth 25ft lbs anywhere in the RPM range. He has a comparison chart using cold air/ram air ad a combination of. It also avg about 1% gain per 5 degree drop in temp. So for those that have this style of hoods or want to modify theirs, it is a simple means of gaining some more power.

Ken you are right since you have the actual operational flapper that was discontinued in 76 due to the "howling" noise in the cab. I have yet to hear any more howling than when I had no holes.....Do you get more noise when you activate the flapper ??
Also am curious to see if we lost any "cool air" when the design went to the snorkel over the rad for air to the filter ??

------------------
JD 76/ZZ4
K&N A/Filter, Valentine1, Remote Engine Oil Filter,Tranny/Engine oil Coolers
MSD 6A, SONY Cd Player, BAZOOKA subwoofer, Blaupunkt speakers,
Crossover(H) Pipe, EDEL 795 CFM QJET, ACCEL Coil/Wires,
>>Where the END justifies the MEANS >>>>

[This message has been edited by JDG/76-ZZ4 (edited 01-04-2000).]

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Ken73
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posted 01-04-2000 10:28 AM     Click Here to See the Profile for Ken73   Click Here to Email Ken73     Edit/Delete Message   Reply w/Quote
JD, can you take pics of your setup? I'm not familiar with the '76 air cleaner setup. I know that mine has two bare snorkels with NO hot-air flapper things. It has a rubber seal that fits up to the top of the hood, and at the back of the hood (near the windshield) is a solenoid-operated flapper door that activates by the gas pedal switch. (The same one that downshifts the automatic transmission.)

I can post pics of my setup if anyone's interested, as well.

Ken

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swhiteh
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posted 01-04-2000 10:54 AM     Click Here to See the Profile for swhiteh   Click Here to Email swhiteh     Edit/Delete Message   Reply w/Quote
These are great dicussions. Nobody has commented on this so far though.

We can look at Bernoulli and try to make correlations between pressure and temperature to try to understand the drop in temperature, but we have to ask if the temperature that is being recording is correct. In my professional opinion, the huge temperature drops that are being seen is due to an instrumentation error. The digital thermometer being used to take measurements is not designed for measurements in a free stream. A termometer measures the temp of itself. With the increased air flow there is an increase in convective heat transfer. The greater heat transfer causes the thermocouple to give erroneous readings, lower than actual. I don't know of any commercially available thermometers that are designed to operate in a free stream. It would be interesting to see what the temperature measurements actually are.

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Da Freaky 1
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From: South Georgia, USA
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posted 01-04-2000 11:51 AM     Click Here to See the Profile for Da Freaky 1   Click Here to Email Da Freaky 1     Edit/Delete Message   Reply w/Quote
Ken73, please post a few pics. I've got an unmoded '76 hood on my car and wonder if the old system could be 'grafted' in.

BTW everyone, I'm thinking this weekend I might try and get some unmolested before temp readings since JDG/76-ZZ4 didn't.

Great string, lots of info here.

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Jack Sweet
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posted 01-04-2000 12:39 PM     Click Here to See the Profile for Jack Sweet   Click Here to Email Jack Sweet     Edit/Delete Message   Reply w/Quote
You know, there's a way to take this cowl mod one step farther and perhaps wind up with a real cowl induction setup on a '76.

I forget the company's name, but you see the small B/W display ad for this product in the back of magazines all the time. It's an enclosed air cleaner with two snorkels that's marketed as a "Ram Air" box. The thing is avaialable in two configurations, the difference being the angle between the snorkels. Basically you put this box on your carb and run ducting from the snorkels to wherever you want to draw air from.

Say somebody were to cut two slots in the undercowl "wall" of a '76 hood--for discussion purposes, let's say 8" x 1". Then you could fabricate a couple of plastic flanges that could be bonded to the inside of the hood. This would form two rectangular channels to the engine compartment.

Put the "Ram Air" airbox on the carb backwards. Run your ducting from the air box snorkels to the hood flanges you just installed and Viola! cowl induction.

There are only three minor problems with this:

First, the flanges in the hood would be rectangular and the snorkels on the "ram air" box are round so you you would have to make the size of the hood slots and flanges a function of the diameter of the ducting you would have to use. (Unless you work for a plastics manufacturer and you could shoot flanges that transition from rectangular to round in about 3" or 4".)

The second is attaching the ducting to the flanges to go driving. You'd probably have to leave the ducting a little longer than necessary then reach under the hood while it was still open 3" or 4" to attach it to your hood flanges.

The third may be a distributor clearance issue, but that could be overcome with careful cipherin' and measurin'.

It's certainly not as sanitary as a factory airbox installation but it would work, I think. And maybe marketable?

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Ken73
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posted 01-04-2000 12:50 PM     Click Here to See the Profile for Ken73   Click Here to Email Ken73     Edit/Delete Message   Reply w/Quote
I'll post some pics tonight; I have to take them first, of course.

For the intake temp readings, I think you may be right. It makes plenty of sense that way. Thought: infrared thermometer? (Non-contact?) Not sure where you'd be able to get one, but it might be worth looking into if you could build one easily.

I still say you ought to try the pressure readings with the super-low pressure gauge. (Inches of water, not mercury.)

Ken

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JDG/76-ZZ4
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posted 01-04-2000 07:39 PM     Click Here to See the Profile for JDG/76-ZZ4   Click Here to Email JDG/76-ZZ4     Edit/Delete Message   Reply w/Quote
Ken..I dont have any way of taking pics and posting them....But if you look at your flapper, that is where the horizontal holes go....Only drilled the area directly under the grill..About a foot wide...If you raise the hood and look at where your hood release cable is screwed on is the vertical area where I drilled the holes...That area where your cable screws on is about 2"wide. I came from the inside, and drilled outward. The cable holder precludes the air from having a straight horizontal path since there is about a 1/2" or so drop (lower) than the holes from the outside. So in essence, I have air coming in, dipping and then going straight in (so to speak). At the same time, I also have the holes on the cable holder (lack of better words) which routes the air in a vertical (down) flow to compensate for the dip between the internal/external holes..
I dont get"direct" air to the filter due to the layout. Which is why I wanted to enlarge the holes so I could have more volume...
As I said in the beginning, this is a very crude idea. And it is very possible I am getting some erroneous readings. My main concern originally was deltas regardless of actual specfic numbers.....
Jack I agree that a "box" could be made but it is definitely a snug area. It could be a "snorkel" type of filter assy or "device" . Or like you said attach to the 'cable holder" a duct to go directly to the back of the filter..It would take some imagination to build but I can see where it could be done... On my hood, I have maybe a 1/2 " under the blanket and the lid top of the air filter...

------------------
JD 76/ZZ4
K&N A/Filter, Valentine1, Remote Engine Oil Filter,Tranny/Engine oil Coolers
MSD 6A, SONY Cd Player, BAZOOKA subwoofer, Blaupunkt speakers,
Crossover(H) Pipe, EDEL 795 CFM QJET, ACCEL Coil/Wires,
>>Where the END justifies the MEANS >>>>

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Ken73
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posted 01-04-2000 07:43 PM     Click Here to See the Profile for Ken73   Click Here to Email Ken73     Edit/Delete Message   Reply w/Quote
Here's some pics of my '73 hood.

This is the stock setup. The air cleaner seals up against the hood. Air comes in(goes out?) from the backside of the grill.

Ken

[This message has been edited by Ken73 (edited 01-04-2000).]

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Juliet Page
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posted 01-05-2000 08:28 PM           Edit/Delete Message   Reply w/Quote
Great pictures! Thanks, worth a million words... I'm definately the visual type. Forgive my ignorance here, but how does the air get from that rear grille into the circular depression (?) on the underside of the hood? Is that the flapper on the lower edge of the grill in the top picture, right above the black "button looking thing"... does the air travel in between the top of the hood and the underside shown in the picture? Is that where the holes were drilled? What's the difference between the '73 and JD's 76 vette?

------------------
~Juliet ...overlooking Mill Creek on the Chesapeake Bay...
Loaded Bridgehampton Blue on Blue '70 350/300Hp TH400 with a White Ragtop

[This message has been edited by Juliet Page (edited 01-05-2000).]

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JDG/76-ZZ4
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posted 01-05-2000 09:57 PM     Click Here to See the Profile for JDG/76-ZZ4   Click Here to Email JDG/76-ZZ4     Edit/Delete Message   Reply w/Quote
Ken..Good shots...I dont see the flapper though....Pic 1 with the grille is where the holes are at (right below it)....Is pic #3 with a little tunnel ducting the air to the filter ??? That is where the air is flowing on mine....Obviously yours has the contained filter where mine is open....Since someone volunteered to take temp meausurements, I knocked out as much of the back wall as possible to let all the air possible....I am still toying with the idea of putting some kid of "tunnel" from where the holes are to the backside of the filter to "directthe flow"...just not sure how to to make it and what to make it out of......

------------------
JD 76/ZZ4
K&N A/Filter, Valentine1, Remote Engine Oil Filter,Tranny/Engine oil Coolers
MSD 6A, SONY Cd Player, BAZOOKA subwoofer, Blaupunkt speakers,
Crossover(H) Pipe, EDEL 795 CFM QJET, ACCEL Coil/Wires,
>>Where the END justifies the MEANS >>>>

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Ken73
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posted 01-05-2000 11:12 PM     Click Here to See the Profile for Ken73   Click Here to Email Ken73     Edit/Delete Message   Reply w/Quote
You can't see the flapper. It's under the funky bulge between the circular area and the grill. I thought I'd take it apart and take a picture, but I still don't think the idea would come across as easy as if I drew it, so here's a drawing.

The blue part is the actual filter element itself. The green line is the flapper door in the closed position and the red dots are supposed to represent the flapper in the open position. The purple is where the grill at the end of the hood is.

Hope this gives a little better concept of how it works.

Ken

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Juliet Page
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posted 01-06-2000 02:52 PM           Edit/Delete Message   Reply w/Quote
Ken, great diagram. How does the air get past the black vertical wall (which is drawn below and on the left edge of the purple grille) and into the duct so it can get to the flapper? Is that a blocked entrance which shows up in the top picture or is the bottom or sides or something else open for the air passage? Thanks, Juliet

[This message has been edited by Juliet Page (edited 01-06-2000).]

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Jack Sweet
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posted 01-06-2000 07:02 PM     Click Here to See the Profile for Jack Sweet   Click Here to Email Jack Sweet     Edit/Delete Message   Reply w/Quote
Actually, you wouldn't have to fab up an airbox. The site below is what I was talking about--it's a commercially available product.
http://www.ramairbox.com/

Put this thing on so the snorkels face the rear of the engine compartment. Then run ducting from this to the flanges you installed in the slot you cut at the rear vertical wall under the "cowl" on your hood.

Do-able?

[This message has been edited by Jack Sweet (edited 01-06-2000).]

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Ken73
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posted 01-06-2000 07:54 PM     Click Here to See the Profile for Ken73   Click Here to Email Ken73     Edit/Delete Message   Reply w/Quote
Juliet, there's a small plastic mesh where the vertical black line is. It's about 1.5" tall and maybe 14" (or more) wide? The air comes in (or goes out?) through there.

Hmm. Anyone know how to do a smoke test? (Airstream analysis?)

Ken

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Juliet Page
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posted 01-07-2000 02:29 AM           Edit/Delete Message   Reply w/Quote
Aren't the forward facing snorkels also breathing in air? I would be concerned with choked flow if you put a restricted duct from the snorkels for any kind of a long length. My guess is that the length of the snorkels is probably sized so that when the air is really being sucked in and the capacity of the original cowl induction system is not able to supply enough air that's when the snorkels start drawing in air... Sort of a path of least resistance thing. The cowl induction is at high pressure in the windshield area and will naturally travel to lower pressure (carb area), whereas the flow through the duct (considering boundary layers etc) and a lower ambient pressure in the engine bay (relative to the windshield air) *maybe* they don't contribute *as much* except when the system's really cranking... ?? Maybe? Just grasping for straws here. Other thing to consider is the "reservoir" of air around the filter from which the carb draws what it needs. Does it need to be fed at 3 places, roughly equally distributed around the perimeter? Or if all the inlets for fresh air are placed aft, what would that do to the area at the front of the filter? Maybe nothing... but perhaps something to think about.

I've never done a smoke test in a wind tunnel before.... but I have done some laser doppler measurements & plenty of flow-visualization. But, unless you have some NASA wind tunnels & support crew at your disposal.... Actually though, one could quite easily "tuft" things to see what the air flow is doing. Take some contrasting color thin yarn (like 1 -ply thick of the typical cheap 4-ply acrylic stuff you find at wal-mart). Try to get cotton though. It's not as susceptible to static adhesion as acrylic. Cut it into 4-5 inch lengths and tape the end of each one down with a piece of masking tape in a nice overall grid on the surface in question. Problem would be looking inside with the hood closed to see the airflow paths inside. Maybe some sort of boroscope (the flexible kind) might let you see something... but without affecting what you're seeing will be hard... not to mention finding a place on the front hood to hang on while someone drives 70-mph down the road. (Just kidding!!) I dunno... anyone have a miniature video recorder which can accept digital boroscope output... or telemeter the data? Then we need a light source? Hmmm... Geeze I'd better give it a rest for tonight. ~Juliet

[This message has been edited by Juliet Page (edited 01-07-2000).]

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jbs75
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posted 01-07-2000 09:44 AM     Click Here to See the Profile for jbs75   Click Here to Email jbs75     Edit/Delete Message   Reply w/Quote
Verry interesting thread. I have seen overheating problems at high speed also. I have also completely replaced the radiator core, water pump, foam seals, thermo-clutch, etc. What made the biggest improvement however was putting the spoiler under the nose on. I suspect this has two effects; one is force air up into the radiator(bottom feeder), and two is to create a low pressure zone behind the spoiler(kind of like blowing accross a straw) which further enhances air flow through the radiator. An interesting test would be to measure the pressure versus ambient, and in front of the radiator simultaneously. I got an air plenum as is used on later models to feed the snorkels, and when I get finished with paint and the trailing arms(I see the light at the end of the tunnel, and it's not another train...I hope); I'll find out if it makes a difference. The foam seal on the transmission tunnel also helps cool the engine, in addition to the interior. I suspect this enhances the low pressure zone effect. Jim

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