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.

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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 >>>>

Scott

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.

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".
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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).]

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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 >>>>

------------------
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 >>>>

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.

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

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

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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 >>>>

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.

------------------
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 >>>>

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.

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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 >>>>

------------------
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

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.

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~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]

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...

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.

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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 >>>>

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 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 ??

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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 >>>>

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

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

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'.

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.)

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

Ken

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

------------------
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 >>>>

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.

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?

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

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