Cowl Induction - Flap or not to Flap - Chevelle Tech
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post #1 of 34 (permalink) Old Sep 10th, 06, 8:13 PM Thread Starter
meb
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Cowl Induction - Flap or not to Flap

Got a completly rebuild cowl induction system. Electronics work, Vacuum works, flow valve installed with gold side towards engine and it is set up correctly.

The top flapper is not opening during heavy throttle. The 396/350hp manual trans has a slightly larger than stock cam (.571 -vs- .610). Hardly worth mentioning. I do understand that when the manifold pressure drops, that the flapper is suppose to open. The valve then allows it to shut slowly. However, it does not.

I spent a bit of time pouring over the search engine forr cowl induction problems. Didn't find any answers.

Today; I put a smaller hose (6 mm) on it and it did help. If I punch it, I can get it to raise up about a 1/4 inch as opposed to almost shutting. I'm not sure what the slightly larger hose was, but it did fit.

Any ideas how to lower the amount of vaccum coming from the intake? The hose is attached to the intake at the back on the same outlet as the power brakes. Its all stock.

If you look at my web site, you can see a pic of how it is setup.

appreciation in advance.

70 Chevelle SS 396; Manual Trans; build sheet; matching numbers; bench; fathom blue; white pearl interior; 63k documented miles.

65 Chevelle SS 283; Powerglide; Protect-o-Plate; matching numbers; danube blue; two-tone blue interior; 51k documented miles.

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post #2 of 34 (permalink) Old Sep 10th, 06, 9:01 PM
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Re: Cowl Induction - Flap or not to Flap

Had a problem like that with my ragtop. You need to watch where the hose passes over the cowl seal, it can get pinched and not let the flapper respond to the vacuum signal. If it opens OK with the hood up v/s not opening with the hood down, that's your problem.

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post #3 of 34 (permalink) Old Sep 10th, 06, 9:24 PM Thread Starter
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Re: Cowl Induction - Flap or not to Flap

Thank you and will check. Didn't think of that test.

Any other suggestions? The web link below shows a picture of the setup. The hose is supported with the proper plastic strap on the under side of the hood.

Again, that smaller hose did help; it raises about 1/4 inch -vs- wanting to close. However, Any smaller diameter host will most probably not work since I will not be able to fit it over any of the orafices.

I read a number of others having this same problem...just no resolutions.

Perhaps the vacuum acutator is bad? Its the only part not replaced. It looks brand new. It does open when the car is shut off and it does respond to a vaccum gun. Perhaps it is just not accurate for the application (i.e. broke).

70 Chevelle SS 396; Manual Trans; build sheet; matching numbers; bench; fathom blue; white pearl interior; 63k documented miles.

65 Chevelle SS 283; Powerglide; Protect-o-Plate; matching numbers; danube blue; two-tone blue interior; 51k documented miles.

TC# 3484
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post #4 of 34 (permalink) Old Sep 11th, 06, 12:23 AM
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Sid
 
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Re: Cowl Induction - Flap or not to Flap

Also-if you've put a new cowl seal on, esp. ones made by SoftSeal, they don't compress too well, which also pinches the hose. Good luck!

BTW, once you get the flapper door working, your gas mileage will go WAY down, ask how I know

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post #5 of 34 (permalink) Old Sep 11th, 06, 6:18 AM
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Re: Cowl Induction - Flap or not to Flap

Quote:
Originally Posted by Sid Coleman View Post
BTW, once you get the flapper door working, your gas mileage will go WAY down, ask how I know
Thats why I leave mine disconnected--it stays up all the time.......Now if I only could say the same for me

72 ss 454 "W" code,w/cowl induction (how many left?)
matching # LS5 454+.060" 9.2 :1, 2.19/1.88 valves,Comp HR cam 289/295 234/240 .613/.613 110* (dyno'd @ 557hp @ 5400 & 583tq @ 4100)
matching # M22 w/CF dual friction clutch.
Holley 750 HPs dp(82751) on Weiand Stealth & MSD Pro billet HEI
Hedman Elite headers/Flowmaster exh. (w/super 40s)
TC#3095
Garage find parked from 1978-2004


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post #6 of 34 (permalink) Old Sep 11th, 06, 7:58 AM
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Re: Cowl Induction - Flap or not to Flap

Mine is hooked up...nothing like watching that door open when the secondaries open

..yes,I have experienced the hose getting pinched too


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post #7 of 34 (permalink) Old Sep 11th, 06, 8:57 AM Thread Starter
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Re: Cowl Induction - Flap or not to Flap

Just a thought about a pinched hose...Wouldn't that keep the flapper open? I.E. Not getting enough air and therefore not being able to close.

My problem is the it will not open.

Thanks for the suggestions (and humor) I'm going to take a closer look. Probably should replace the actuator, I replaced everything else LOL. Like I stated earlier, I put a vacuum gun on it and the flapper does go up and down. I'm wondering if the actuator is just not responding the the changes properly based on application? I.E. It works with the gun at a particular pressure, but not at the pressure that my application is providing (maybe this is all obvious).

I'll replace the actuator and I'll post back.

70 Chevelle SS 396; Manual Trans; build sheet; matching numbers; bench; fathom blue; white pearl interior; 63k documented miles.

65 Chevelle SS 283; Powerglide; Protect-o-Plate; matching numbers; danube blue; two-tone blue interior; 51k documented miles.

TC# 3484
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post #8 of 34 (permalink) Old Sep 11th, 06, 9:08 AM
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Re: Cowl Induction - Flap or not to Flap

I'd leave the actuator alone.
Try running the the hose thru the firewall where the winshield washer hoses run thru
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post #9 of 34 (permalink) Old Sep 11th, 06, 11:48 AM
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Re: Cowl Induction - Flap or not to Flap

FWIW, mine doesn't open under acceleration or full throttle either, never has and I have everything in place for proper operation. It stays shut all the time. In fact there is so much vacuum applied that the hood door is even pulled down below hood level and is actually bent from all the years it has been pulled shut! Need some vacuum restriction for sure, however I don't want to go with a smaller hose as meb has done trying to alleviate the vaccum - it might collapse due to the large amount of vacuum.

Last edited by Super70; Sep 11th, 06 at 11:49 AM. Reason: corrections
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post #10 of 34 (permalink) Old Sep 11th, 06, 1:07 PM Thread Starter
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Re: Cowl Induction - Flap or not to Flap

The smaller diameter hose has a pretty good thickness to it, so I dont see any problems. I got it at NAPA (6mm hose). I'm going to try to move the vacuum source to another area. I'm wondering if on of the carburetor base plate vacuum sources will work?

I'll try it when I get home.

70 Chevelle SS 396; Manual Trans; build sheet; matching numbers; bench; fathom blue; white pearl interior; 63k documented miles.

65 Chevelle SS 283; Powerglide; Protect-o-Plate; matching numbers; danube blue; two-tone blue interior; 51k documented miles.

TC# 3484
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post #11 of 34 (permalink) Old Sep 11th, 06, 1:16 PM
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Re: Cowl Induction - Flap or not to Flap

My 70 model has a 1/4 inch hose from the vacuum servo, straight to direct manifold vacuum and it works fine. I'm wondering if the check valve could be causing problems. Have you tried a direct line from intake to servo? I would imagine that the check valve's main purpose is to keep a severe back-fire from damaging the servo diaphram. I tried a smaller hose but it seemed to make the flapper less responsive. I think that the larger the hose, the better, to allow for instant pressure changes.
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post #12 of 34 (permalink) Old Sep 11th, 06, 5:33 PM Thread Starter
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Re: Cowl Induction - Flap or not to Flap

Just got home and put some larger diameter hose on, as suggested, and gave it a test. I had some success; the cowl induction is more responsive with the larger diameter hose.

I also looked quite carefully and indeed the hose is being pinched by the cowl seal. I can physically see that it is being pinched. Like someone said, it needs to be able to react to the pressure change and I now believe that the pinching is definitely causing the problem.

I cannot do it now, but I will re-route the hose as suggested on this thread through (umm) something else. I believe someone suggested the water washer tube hole on the firewall. I'll figure it out and report back tomorrow evening with the results.

Thanks again.

70 Chevelle SS 396; Manual Trans; build sheet; matching numbers; bench; fathom blue; white pearl interior; 63k documented miles.

65 Chevelle SS 283; Powerglide; Protect-o-Plate; matching numbers; danube blue; two-tone blue interior; 51k documented miles.

TC# 3484
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post #13 of 34 (permalink) Old Sep 11th, 06, 8:10 PM
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Re: Cowl Induction - Flap or not to Flap

Quote:
Originally Posted by Sid Coleman View Post
BTW, once you get the flapper door working, your gas mileage will go WAY down, ask how I know
The outer vacuum-operated door is just for show. Plenty of air can flow through the cowl screen at the back of the hood bulge - even when the flapper is completely shut.

The INNER electrically-operated door is the one, and the only one, that determines whether or not outside air goes to the air cleaner.

So I've got my outer flapper doors operational, but I've rigged the inner doors to be open all the time.

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post #14 of 34 (permalink) Old Sep 12th, 06, 6:10 AM
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Re: Cowl Induction - Flap or not to Flap

Quote:
Originally Posted by dcarr View Post
The outer vacuum-operated door is just for show. Plenty of air can flow through the cowl screen at the back of the hood bulge - even when the flapper is completely shut.

The INNER electrically-operated door is the one, and the only one, that determines whether or not outside air goes to the air cleaner.

So I've got my outer flapper doors operational, but I've rigged the inner doors to be open all the time.
OK so how did you do that?

72 ss 454 "W" code,w/cowl induction (how many left?)
matching # LS5 454+.060" 9.2 :1, 2.19/1.88 valves,Comp HR cam 289/295 234/240 .613/.613 110* (dyno'd @ 557hp @ 5400 & 583tq @ 4100)
matching # M22 w/CF dual friction clutch.
Holley 750 HPs dp(82751) on Weiand Stealth & MSD Pro billet HEI
Hedman Elite headers/Flowmaster exh. (w/super 40s)
TC#3095
Garage find parked from 1978-2004


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post #15 of 34 (permalink) Old Sep 12th, 06, 8:41 AM
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Re: Cowl Induction - Flap or not to Flap

I removed the inner flapper on mine as well.Its all bagged up and put away.
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