1974 Cowl Induction System
Melting Slicks
Joined: Mar 2005
Posts: 2,838
Likes: 10
From: Austin TX
1) As stated above, the system isn't designed to "force" air into the induction system. It's to allow cooler ambient air into the air cleaner. Temps taken at the snorkel of air cleaners run from 120-150 degress. Every 11 degress reduction in the temperature of the intake charge, equates to 1% HP increase.
2) Yes, the snorkels are integral to allow air to "out" believe it or not, when the cowl induction system is activated. If the system were closed, the air in the air cleaner would be hotter. Instead a flow of cool fresh air flows in from the cowl. Unused air flows out the snorkel(s) which keeps the air fresher and cooler.
3) Of course it works. Would GM assign two of their top engineers with it's design? (Doug Roe and Vince Piggins). More here:
http://www.camaro-untoldsecrets.com/...es/rpo_zl2.htm
4) Lastly, this CI cold air system was designed and developed for Mark Donhue's Camaros that won damn near every SCCA Trans Am race in the late 60's. The cold air system used on old #9 was functionally the same as the cowl induction system on Chevelles, Camaros, and 73-74 Vettes (obviously no flappers, plungers, and switches on a race car).
2) Yes, the snorkels are integral to allow air to "out" believe it or not, when the cowl induction system is activated. If the system were closed, the air in the air cleaner would be hotter. Instead a flow of cool fresh air flows in from the cowl. Unused air flows out the snorkel(s) which keeps the air fresher and cooler.
3) Of course it works. Would GM assign two of their top engineers with it's design? (Doug Roe and Vince Piggins). More here:
http://www.camaro-untoldsecrets.com/...es/rpo_zl2.htm
4) Lastly, this CI cold air system was designed and developed for Mark Donhue's Camaros that won damn near every SCCA Trans Am race in the late 60's. The cold air system used on old #9 was functionally the same as the cowl induction system on Chevelles, Camaros, and 73-74 Vettes (obviously no flappers, plungers, and switches on a race car).
Last edited by 73, Dark Blue 454; Apr 24, 2008 at 12:15 AM.
Race Director
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2) Yes, the snorkels are integral to allow air to "out" believe it or not, when the cowl induction system is activated. If the system were closed, the air in the air cleaner would be hotter. Instead a flow of cool fresh air flows in from the cowl. Unused air flows out the snorkel(s) which keeps the air fresher and cooler.
Let's say it is a 'suck' design. The air will enter the air filter housing by the path of least resistance. That's the snorkels, not the duct.
If it's a 'blow' design, then there's not enough pressure in the plenum at legal speeds to do any pushing. If you continued to increase speed (and blocked off the astro ventilation ducts) till you built pressure, then the ductwork would be insufficient to accommodate 100% of the airflow needs. Result, you still suck through the snorkels. Net gain, probably zero.
How about all you bench racers putting your contradictory theories to the test and explaining why it is entirely possible to disable the flapper in the closed position and it makes NO difference in performance?
Yes, I know that magazine articles claimed that the duct dropped 1/4 mile times by 1 full second, but we all know that this statement was marketing BS.
Race Director
Joined: Feb 2005
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WHY AN AIR VALVE?
"The air valve was specified fur use on the production RPO ZL2 option for the sake of better overall drivability. Taking into account the diversity of climatic conditions and their effects on the system, it was determined that the system should function only when called on. If the system remains open in extreme cold weather, icing of the carburetor could occur. The valve also affords longer air filter service life"
When I did the "yarn" tuft test on my 69,the yarn was "blown" into the cowl opening before I reached 30 mph due to ambient air from a high pressure area at the base of the windshield...
Pic at 30 mph...
My 78 air intake system is more a "ram" air type design because it gets ambient air via intake duct at top of radiator...I have "improved " that by adding an air scoop under center valance opening which directs more air into rad and duct intake...in fact,at 80 mph,coolant temps dropped 5 degrees which was my main objective in that case.
Pic...
"The air valve was specified fur use on the production RPO ZL2 option for the sake of better overall drivability. Taking into account the diversity of climatic conditions and their effects on the system, it was determined that the system should function only when called on. If the system remains open in extreme cold weather, icing of the carburetor could occur. The valve also affords longer air filter service life"
When I did the "yarn" tuft test on my 69,the yarn was "blown" into the cowl opening before I reached 30 mph due to ambient air from a high pressure area at the base of the windshield...
Pic at 30 mph...
My 78 air intake system is more a "ram" air type design because it gets ambient air via intake duct at top of radiator...I have "improved " that by adding an air scoop under center valance opening which directs more air into rad and duct intake...in fact,at 80 mph,coolant temps dropped 5 degrees which was my main objective in that case.
Pic...
Last edited by rihwoods; Apr 23, 2008 at 12:32 PM.
Melting Slicks
Joined: Jun 2003
Posts: 2,802
Likes: 241
From: Niles, MI
1) As stated above, the system isn't designed to "force" air into the induction system. It's to allow cooler ambient air into the air cleaner. Temps taken at the snorkel of air cleaners run from 120-150 degress. Every 11 degress reduction in the temperature of the intake charge equates to one HP.
2) Yes, the snorkels are integral to allow air to "out" believe it or not, when the cowl induction system is activated. If the system were closed, the air in the air cleaner would be hotter. Instead a flow of cool fresh air flows in from the cowl. Unused air flows out the snorkel(s) which keeps the air fresher and cooler.
3) Of course it works. Would GM assign two of their top engineers with it's design? (Doug Roe and Vince Piggins). More here:
http://www.camaro-untoldsecrets.com/...es/rpo_zl2.htm
4) Lastly, this CI cold air system was designed and developed for Mark Donhue's Camaros that won damn near every SCCA Trans Am race in the late 60's. The cold air system used on old #9 was functionally the same as the cowl induction system on Chevelles, Camaros, and 73-74 Vettes (obviously no flappers, plungers, and switches on a race car).
2) Yes, the snorkels are integral to allow air to "out" believe it or not, when the cowl induction system is activated. If the system were closed, the air in the air cleaner would be hotter. Instead a flow of cool fresh air flows in from the cowl. Unused air flows out the snorkel(s) which keeps the air fresher and cooler.
3) Of course it works. Would GM assign two of their top engineers with it's design? (Doug Roe and Vince Piggins). More here:
http://www.camaro-untoldsecrets.com/...es/rpo_zl2.htm
4) Lastly, this CI cold air system was designed and developed for Mark Donhue's Camaros that won damn near every SCCA Trans Am race in the late 60's. The cold air system used on old #9 was functionally the same as the cowl induction system on Chevelles, Camaros, and 73-74 Vettes (obviously no flappers, plungers, and switches on a race car).
Mike, high pressure at the base of the windshield forces air into the hood duct not the carb, thus it is a cool air system not ram air.
Any evidence to back up that claim? Cold air induction has been used for years right up to the present on just about every car built. Why do you doubt its usefullness?
Race Director
Joined: Jul 2001
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Thanks for the post! I couldn't have said it better myself. I was wondering when truth and logic were going to enter this thread. The bs has been flying.
Mike, high pressure at the base of the windshield forces air into the hood duct not the carb, thus it is a cool air system not ram air.
Any evidence to back up that claim? Cold air induction has been used for years right up to the present on just about every car built. Why do you doubt its usefullness?
Mike, high pressure at the base of the windshield forces air into the hood duct not the carb, thus it is a cool air system not ram air.
Any evidence to back up that claim? Cold air induction has been used for years right up to the present on just about every car built. Why do you doubt its usefullness?
From my point of view, NO hard truth and logic proving anything has been presented, just unrelated stuff on other car types. We're talking about production standard '73-75 Corvettes here.
We all know the basic theory behind cold air induction and ram air induction and their respective potential for increasing performance. No argument there. I've got 30 years worth of test data from aircraft and engine design I've participated in that would knock your socks off in terms of cold air/ram air benefits.
The OP and myself amongst others are talking about the real life practicality and functionality as installed on Corvettes.
Irrespective of whether anyone believes that it is designed to be a suck or blow system, I've pointed out the design flaws that keep it from being effective. As measure of that, I've run my own car with the solenoid working and not working. I saw no difference in performance, just more noise.
Somebody please go do some A/B real life 1/4 mile tests and prove me wrong. Please.
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Race Director
Joined: Apr 1999
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From: NM
We discussed this a couple of years ago here on the forum (but I cannot find that thread). We did the math on the air flow and it was interesting.
The Q-jet that is used on the Vette is a 750 CFM carb. Good thing about the Q-jet is that it is designed to provide only the amount of air that the engine can use (unlike the Holly which can overload the engine). The math that we did showed that the cowl induction could only deliver about half the air that a 750 CFM carb needed. This was due to the very small cross section of the ducting through the hood. So this restriction would depend upon what engine you had. The base L48 could never use the full 750 CFM. Then you had the L82 which would come much closer, but a full bore SBC could easly use the full 750 CFM. So if you had the underhood snorkles blocked off, a base enigne car would never notice it. The L82 might, but a modified engine would most deffently notice the restriction.
I will keep looking to see if I can find that old thread to get the math.
tom...
The Q-jet that is used on the Vette is a 750 CFM carb. Good thing about the Q-jet is that it is designed to provide only the amount of air that the engine can use (unlike the Holly which can overload the engine). The math that we did showed that the cowl induction could only deliver about half the air that a 750 CFM carb needed. This was due to the very small cross section of the ducting through the hood. So this restriction would depend upon what engine you had. The base L48 could never use the full 750 CFM. Then you had the L82 which would come much closer, but a full bore SBC could easly use the full 750 CFM. So if you had the underhood snorkles blocked off, a base enigne car would never notice it. The L82 might, but a modified engine would most deffently notice the restriction.
I will keep looking to see if I can find that old thread to get the math.
tom...
Safety Car
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Melting Slicks
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From: Piedmont Va
St. Jude Donor '11-'12-'13,'19-'20
Melting Slicks
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From: Airdrie Alberta
Chevy did away with the cowl induction to help sooth the passangers ears.
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From: Scottsdale Arizona
Melting Slicks
Joined: May 2007
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From: Bay Area CA
On my 4sp, I had the Ign on the other day, "Vette not running. Stepped on the gas all that way and in about a 1/2 second, heard a swoop and a slap. let off the gas it closed, stepped on it again, happened again. They are easy to test, engage at the very end of the gas pedal travel, no lock out on my stock 4 spd. slap! 
Melting Slicks
Joined: Mar 2005
Posts: 2,838
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From: Austin TX
Forget ram air,..that's not what the system does. Q-Jets can't meter air that's forced into the carb. It needs a smooth flow of air which is why air cleaners and filters are integral to proper carb metering. How many morons do you know who remove their air cleaner completely and can't understand why their car runs like crap at speed.
Again, the cowl induction system simply flows colder air through the air cleaner for more power. Pulling air from the cowl area is a race proven technique. I disagree there won't be enough cold air available. Look at the earlier photo of the yarn at 30 mph.
Also, I'm sure Vince Piggins and Doug Roe and other engineers at GM figured out the needs of the carb and designed the system to meet those needs. Those boys knew what they were doing.
FWIW.
Again, the cowl induction system simply flows colder air through the air cleaner for more power. Pulling air from the cowl area is a race proven technique. I disagree there won't be enough cold air available. Look at the earlier photo of the yarn at 30 mph.
Also, I'm sure Vince Piggins and Doug Roe and other engineers at GM figured out the needs of the carb and designed the system to meet those needs. Those boys knew what they were doing.
FWIW.
Race Director
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From: Scottsdale Arizona
First, when the engine is warming up, I would think the air under the hood would be warmer and make it idle and run smoother.
The real reason is that part of the idea of a flapper is to have the cool action of the door openning when you stomp on the gas. Truth is, it would probably help if the door were open cruising at highway speed.
Safety Car
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Two reasons, I think.
First, when the engine is warming up, I would think the air under the hood would be warmer and make it idle and run smoother.
The real reason is that part of the idea of a flapper is to have the cool action of the door openning when you stomp on the gas. Truth is, it would probably help if the door were open cruising at highway speed.
First, when the engine is warming up, I would think the air under the hood would be warmer and make it idle and run smoother.
The real reason is that part of the idea of a flapper is to have the cool action of the door openning when you stomp on the gas. Truth is, it would probably help if the door were open cruising at highway speed.
Miles
Melting Slicks
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