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Old 12-02-2009, 09:36 AM   #1
moonsteel
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Default 73/74 cowl induction Q

Couple of Q's

1) I can't think of any reason not to have this open all the time, other than noise, is there any reason GM chose to have it only open at WOT?

2) the two openings at the front, do the snorkels from later C3's fit these so the engine doesn't breathe in the hot air in the engine bay?
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Old 12-02-2009, 09:50 AM   #2
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Noise & emissions to keep intake temp. warm.

I run cowl induction open full time.
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Old 12-02-2009, 10:03 AM   #3
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Removed the flapper and taped up the 2 snorkels with a/c duct tape.

No problems and didn't notice any increased noise.

It's always warm here.
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Old 12-02-2009, 10:09 AM   #4
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Noise was the only reason for the flapper valve but the whole assembly is all 'show' and no 'go' anyway. There's no actual performance difference with it working or not.
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Old 12-02-2009, 10:44 PM   #5
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Most would disagree that there's "no actual performance difference" from a cowl induction system.

Every 11 degree reduction in induction air temperature will equate to a 1% HP increase. Underhood temps get pretty warm, especially in a Vette.

Also, some Chevy engineers named Roe, Piggins, and Shinoda designed the cowl hood induction system, first designed for SCCA and Nascar racing. Would GM put three of their top engineeers to work on an "all show and no go" project?

More here:

http://www.camaro-untoldsecrets.com/...es/rpo_zl2.htm

Also, blocking the snorkels will block the flow of fresh air. When the cowl hood valve (flapper) opens the snorkels allow the cold dense air to vent from the air cleaner. In other words, the flow of air through the snorkels is reversed. This keeps the fresh air in the air cleaner fresh and cool.

Lastly, rain and snow were the reasons Chevy decided on employing the flapper.

FWIW

Last edited by 73, Dark Blue 454; 12-02-2009 at 11:14 PM.
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Old 12-02-2009, 11:15 PM   #6
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Greetings,
Cowl on my 73 is wide open, and so are my snorkels. The air cleaner seal is in place and seals properly. This forces fresh air flowing from the cowl into the top of the open air cleaner to vent out the snorkels. IMHO this helps keep the engine bay cool, especially at higher speeds. I have a 10.1cr 383 and run a 190* stat and never have a temp issue. I believe that having the cowl open improves efficiency and possibly throttle response, and clearly contributes to mitigating heat. It has been demonstrated in many previous threads that having the cowl open 100% with the air cleaner sealed increases airflow from the cowl. Whether you get any measurable increase in RWHP is probably very debatable.

peace
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Old 12-02-2009, 11:21 PM   #7
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Quote:
Originally Posted by 73, Dark Blue 454 View Post
Most would disagree that there's no performance gain from cowl induction.
..............
Also, blocking the snorkels will block the flow of fresh air. When the cowl hood valve (flapper) opens the snorkels allow the cold dense air to vent from the air cleaner. In other words, the flow of air through the snorkels is reversed. This keeps the fresh air in the air cleaner fresh and cool.

Lastly, rain and snow were the reasons Chevy decided on employing the flapper.

FWIW
Most have never taken the time to really understand the failings of the design, so yes most falsely believe that it works and 'disagree'. Cowl induction can and does work, given the correct design, application and enough vehicle speed, but this particular design ('74-74 Corvette) misses the mark on all counts.

Go unplug your cowl flap on your car and do a bunch of quarter mile passes. Then plug it in and do more passes. Tell me if there's any difference (I already know there won't be).

Looks very zoomy, sounds great and really good in advertising but that's about it.

I think there was a poster here who (years ago) put a thermocouple inside the air cleaner housing and measured temps with the flap open and closed at all sorts of speeds. Guess what? No difference.

Your statement about reversing the airflow just baffles me. I guess you're under the impression that there's enough pressure created in the plenum to force cool air inside the duct. Nope. The plenum is also shared by the Astro ventilation floor ducts so any 'pressure' would be bled off by those. Not that there is any- not at legal highway speeds.

How would rain and snow make the 180* turn and enter the intake? The inertia would cause them to keep going straight, not turn. Why is it OK at full throttle to have the flap open but not at part throttle if contamination was a problem?

There's theory, then there's reality.
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Old 12-03-2009, 12:19 AM   #8
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MW,..Did you even open the link I posted above? Take three minutes and read the piece.

Do you not believe cooler air for induction is better for HP? If not, then fine.

"How would rain and snow make the 180* turn and enter the intake?" Again, read from the link I posted above. You obviously have a small understanding of the cowl induction system. The 73-75 Corvette system is identical to the 69 Camaro system (ZL-2 option) discussed in the link.

So to say it "this particular design ('74-74 Corvette) misses the mark on all counts." says you don't have any faith in the engineers and engineering behind the system. Perhaps you're unfamiar with Vince Piggins, Doug Roe and Larry Shinoda. And that's fine too.

Here are photos showing how air is sourced to provide a cooler, denser intake charge, using pieces of yarn taped to the back of the hood.

This is at idle:

http://s421.photobucket.com/albums/p...=YarnTest1.jpg

...and this is the Vette in motion:

http://s421.photobucket.com/albums/p...=YarnTest2.jpg

http://s421.photobucket.com/albums/p...=YarnTest3.jpg

Last edited by 73, Dark Blue 454; 12-03-2009 at 12:21 AM.
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Old 12-03-2009, 12:56 AM   #9
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Quote:
Originally Posted by 73, Dark Blue 454 View Post
MW,..Did you even open the link I posted above? Take three minutes and read the piece.
Yes, I've read it before, and I re-read it again. The design used on the Camaro is not identical to the Corvette. The stock Corvette windshield plenum (where the wipers are) is vented- NO pressure is created. The Camaro has no such vented plenum, nor does the Corvette in the pictures. Apples and oranges.

The Corvette air cleaner ***'y has two open snorkels which are always open. Any pressure created at the base of the windshield would be bled out through the snorkels. To quote the article you've hung your hat on:

"Ducting pressurized fresh cold air from this area to the carburetor creates a more dense fuel charge with increased volumetric efficiency resulting in more horsepower"

Since there's no positive pressure available, you might want to argue that the snorkel creates a path for cold air to be drawn in rather than foreced. Again there's two open snorkels in the air cleaner housing that are much bigger in area than the snorkel (go measure it like I did) and offer a much easier path for the air to enter. Result? The vast majority of air entering the air cleaner is from the underhood area.

Proof? No change in temp as proven by the experiment conducted by the member here on the CF.

More proof? Go to strip and test it yourself.

I guess you don't understand basic physics regarding the inertia of matter. Newtons' first law:
"An object at rest tends to stay at rest and an object in motion tends to stay in motion with the same speed and in the same direction unless acted upon by an unbalanced force." Tell me what makes the rain drops and snow flakes defy this law.
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Old 12-03-2009, 09:27 AM   #10
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MW,..did you not open three photos I posted and see for yourself, that show that yes, high pressure is created at the base of the windshield? And that yes, rain would be pulled into the air cleaner if the Vette were at speed?

Camaros (I currently own three 69's) have a cowl cavity like most any other Chevy of the day, including Vettes. The body panel just forward the windshield on a first gen Camaro is called a cowl vent grill. Vette's don't have that, but I've no idea how that's germane to this discussion.

The fresh air is sourced from the high pressure area at the base of the windshield, where the wipers are, on both the Camaro and Vette.

Yes, both systems (Vette and Camaro) are almost identical in theory and function.

I've supported my position with photos and an article. Your turn. Other than your anecdotal evidence, please forward a doc or two that supports your position, that there's no positive pressure at the base of the windshield and/or fresh is not introduced to the air cleaner on 73-75 cowl hood cars and/or fresh air in no way impacts performance.

Lastly, we appreciate your reference to Sir Issac Newton. Sorry he's not around. He could explain the fresh air cowl hood system to you.

Absent a supporting doc or photo to bolster your position, I'm done here.

Last edited by 73, Dark Blue 454; 12-07-2009 at 09:31 PM.
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Old 12-03-2009, 10:25 AM   #11
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I give up.

The car in your photos is NOT a '73-'74 Corvette. The hood and cowl area is very different. You've based your assertions that 'it works' based on that and a magazine article that talks only about Camaros and dances back and forth between that and NASCAR racing in the 60s. Not one word of test results on production Camaros and mention about Corvettes.

I've pointed out the DIFFERENCES between the two cars- any of which would defeat the purpose of the system. I've referred to real world testing which indicates that there's no proof that it works.

You didn't understand why I referred to Newtons law of inertia to explain why rain and snow will not enter the duct. Nothing to do with cowl induction.

Edit: I found some of the data I was referring to above. It's from 2003. Please read carefully.

http://forums.corvetteforum.com/c3-t...in-the-c3.html

Especially posts 9, 11 and 61. Post 9 is from the guy who taped the wool on his hood. Even he says 'This tells me that cowl induction is ineffective for ram air at most driving speeds. '

As I said, I give up. Good luck in dreamland.

Last edited by Mike Ward; 12-03-2009 at 10:46 AM.
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Old 12-03-2009, 11:08 AM   #12
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Mike is famous around here for taking the contrarian view on subjects, just IGNORE him! I would say a properly set-up cowl hood will add 5-10HP to your 73 and later Vette (I think system was done by 76 as I recall.) You may want to investigate insulating the aircleaner housing with some heat shielding and blocking the 2 intake snorkles would be a good idea to get some positive pressure happening at speed. Compared to the earlier C3 Vette, the 73 and up intake set-up is FAR SUPERIOR to what the early C3 had unless you have a ZL-1/L-88 Vette.
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Old 12-03-2009, 11:57 AM   #13
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Quote:
Originally Posted by Solid LT1 View Post
Mike is famous around here for taking the contrarian view on subjects, just IGNORE him!
Yes, you've got me right. Sorry you think that someone who offers counterpoints on technical subjects should be ignored. Either of you bench racers got any proof to back up what you 'think' and your theories?

I think I'll listen to knowledgeable, experienced people like Lars before either of you two.

"flyn -
There is no question that the factory "ram air" systems (not only on Vettes, but on cars such as the GTO, Firebird, Camaro, etc) have very little, if any, effect on performance: I've measured and documented similar results showing little, if any effect from the systems, and I've seen similar documented results during my research at GM Engineering. Rather, the systems were marketing tools, and the marketing department knew that the cool sound and "ram air" words sold cars."

Yes I know he used the words 'ram air' instead of cowl induction. Good luck with your fantasies.
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Old 12-03-2009, 12:15 PM   #14
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Marketing

Wonder why the marketing department didn't make the cowl induction available for the 6 cyl Camaros.

I guess that's why they didn't sell a lot of them.
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Old 12-03-2009, 03:00 PM   #15
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I have a '73 with functioning cowl induction. I have no idea about how well it works but I tend to think GM did some actual test analysis before spending the money to introduce it into the vehicle.

As an engineer myself I learned a long time ago to "bring the data" and "One test is worth 1,000 expert opinions".

I'm going to Google this just for fun. I would like to find some empirical data to support the effectiveness of the system.
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Old 12-03-2009, 03:09 PM   #16
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Anyone running EFI that uses the same air filter setup as a carb, a little data logging would easily show any temp differences between closed and open cowl induction.
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Old 12-03-2009, 04:18 PM   #17
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Quote:
Originally Posted by 08vycpe View Post
I have a '73 with functioning cowl induction. I have no idea about how well it works but I tend to think GM did some actual test analysis before spending the money to introduce it into the vehicle.

As an engineer myself I learned a long time ago to "bring the data" and "One test is worth 1,000 expert opinions".

I'm going to Google this just for fun. I would like to find some empirical data to support the effectiveness of the system.
Great idea. Please post if you find anything. While you're at it, look for info on C2 sidepipes. You'll see an ever better example of marketing winning out over engineering.

Last edited by Mike Ward; 12-03-2009 at 07:35 PM.
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Old 12-03-2009, 05:54 PM   #18
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Interesting thread. My flapper has been removed.

I do not normally drive in the rain but last May I got caught in a real downpour for about two hours at highway speeds. The 1974 ran well. The rain revealed a leak on the convertible top that was corrected shortly thereafter.

I am not an engineer but as a biologist I did not see algae developing on the paper air filter in the following weeks. The rain went up and over the windshield.

As to whether the two front snorkels suck or blow?? The notion that they blow is ludicrous. They suck. Big time.

As to the suggestion of fooling about at a dragstrip with whatever induction on a 1974 L-48 .....? A tuned Honda Civic will beat you up. These are old heavy cars. Just enjoy them for what they were 35 years ago.

Click the image to open in full size.

Click the image to open in full size.

Last edited by paul 74; 12-03-2009 at 07:14 PM.
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Old 12-04-2009, 03:34 PM   #19
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Well, instead of sitting around pontificating, pulling numbers out of the air, assuming, or quoting from aged magazine articles but primarily to maintain my image of being contrary without due cause, I went out today after lunch and did some real world testing. How about that. Real data.

Ambient temp today is 48*F. I'll quote all degrees in F as I know very few of the Luddites here understand degrees Celsius. He's dead (just like Newton), but I digress.

Test vehicle is my '73 small block, with a moderately modified small block, but dead stock outward appearance including air cleaner and functioning cowl induction just like GM designed and installed.

Test location is a four lane highway running roughly east-west on level ground surrounded by open fields. This will become relevant further along. Very little or no other traffic midday on certain stretches.

Test instrumentation was a thermocouple placed inside the air cleaner housing mounted to not touch the housing itself and to take full advantage of the air stream. This was attached to a RF transmitter that sent a signal to a recording receiver located inside the cabin. I located the display panel just below normal line of sight so I could associate the varying temps with different conditions.

Temps recorded:

Idling in the driveway: 93
Driving to/from in town the 'test track' : 90
Steady state speed on the highway going to the test track: 88
Accelerating eastbound to red line in each gear and holding WOT to 100mph: 87
Accelerating westbound to red line in each gear and holding WOT to 100mph: 86

I repeated the test 4 additional times, and repeatedly the westbound test was always 1 degree cooler than eastbound. I moved the t-couple to be directly in line with one of the two snorkels vs. the orignal location at the rear and did the test one more time, no change.

So, results of the 12 runs?

Min change 1 degree F- all eastbound
Max change 2 degrees F- all westbound
Average 1.5 degrees F

Steady state speed on the highway returning from the test track: 87

Why the 1 degrees variation east vs west? (betcha Paul 74 would know this) Our prevailing wind is from the west. At the time it was blowing from the west at approx 15-20 mph. It seems that driving into the wind accounted for the cooler temps. It could be interpreted that this proves that the cowl induction works, albeit a lousy 2 degrees.

BUT!

There was also a 1 degree variation east/west while driving at steady speed (flapper closed), so it seems that if the effect of the wind is factored out, the actual temp difference is a whopping 1.5 degrees.

Using the quoted 11 degrees reduction= 1% gain in HP, or 0.136%. On a '73 LS4, that's a whopping .374 HP increase. On Paul's L48, his neck would snap clean off with the extra .262 HP.

I stopped recording the temps at around 140 degrees. That was with the engine stopped and the car sitting in the garage. I was laughing too hard to see clearly anyway.

Now we know. Let's all race heading to the wind!

Last edited by Mike Ward; 12-04-2009 at 03:36 PM.
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Old 12-04-2009, 03:47 PM   #20
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Your tests were done with the cowl induction as GM designed it?
I wonder what change, if any, would happen if you removed the flap and either plugged the front snouts or snorkeled them into cooler air.
Any thoughts on a difference that might make?
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