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Discussion Starter · #1 ·
I am restoring a '71 Chevelle with a 454. I just got the engine back in the car and I've added several parts to get the cowl induction hood working again. Now, when I start the engine, the vacuum modulator closes the flapper door. Is that right? I thought it was supposed to close when you step on the gas. It's like it gets too much vacuum when it's idling. Please help - this is the first Chevelle I've owned with cowl induction.
 

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When you start the engine the cowl door should close. It will only open back up upon hard acceleration so as to suck air into the carb. Its working right, as long as when you romp on it that flapper door opens to draw in air. Your A OK bud.
 

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Discussion Starter · #3 ·
Thanks for the info. I thought it was the other way around - Flapper door stays open when idling and closes when you step on the gas. So is it right to assume the engine generates less vacuum when you step on the gas than it does while idling? Thanks again...

Chad Oudekerk
 

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If I understand correctly, vacuum pulls the flapper door closed. So when you have zero vacuum...engine is off, or at wide open throttle...door is open allowing cool air in. If engine is idling, or at partial throttle, vacuum is higher and door should be closed.

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Mike Newby

69 Chevelle 355/TH200-4R
97 Grand Prix GTP
64 Chevy C10 Pickup
79 Suzuki GS550
 

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Correct, engine vacuum (at idle) holds the flapper door closed. At wide-open-throttle, vacuum drops, and the spring pushes the door open.

If you really look at the hood geometry and cowl induction system, it is apparent that the outer door is just for show. Even when the outer door is closed there is still a huge ventilation opening through the screen under the hood bulge at the back.

What really controls the air flow is the electrically-operated inner door under the hood. It is wired to the gas pedal - push far enough on the gas pedal and the switch closes, sending 12 volts through the solenoid to pop open the inner door.



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Dan Carr
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Thanks Dan, I didn't know there was an electric switch as well. Is this the same, or similar, as the TH400 kickdown switch?

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Mike Newby

69 Chevelle 355/TH200-4R
97 Grand Prix GTP
64 Chevy C10 Pickup
79 Suzuki GS550
 

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I'll just pop in on this one. The switch for the electric door is similar to the 400 switch. It mounts above the accelerator pedal rod and energizes the solenoid for the electric door under the flapper at WOT. If you are using a new hood or have just painted the underside of your hood, make sure your solenoid has a good ground or it wont work. I found that out the hard way
 

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