Minimum amount of vacuum required for CI flapper to operate - Chevelle Tech
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post #1 of 7 (permalink) Old May 29th, 19, 1:32 PM Thread Starter
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Mike
 
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Minimum amount of vacuum required for CI flapper to operate

Hi All,

I just installed a cowl induction hood on my '71 Chevelle, and am in the process of setting up the outer flapper door. Took me a while to find a diagram of the necessary parts, but I think I've got it now. Since I have cold air intake tubes routed to the front headlight area, the CI flapper will just be for show, and I had anticipated to just put it on and "bolt" it in the open position. Once I found out that the vacuum canister is what holds the flapper open, I realized that I might be able to get it to actually open and close while driving - again, just for s**** and giggles.

My question is: does anyone have an idea of the minimum amount of vacuum necessary to fully close the flapper door? My engine is far from stock (427 c.i. SBC, Dart SHP block, AFR 220 heads, Clay Smith Cams 237/243 @ 0.050 on 108 LSA, Holley sniper fuel injection), and I think I'm only pulling around 4-6" vacuum at idle (can verify this with the sniper EFI).

I'm not gonna be too torn up if it won't close at idle, but would be a nice bonus. I'm also going to just try it out, and can report back what I find.

Anyone else have an engine with low vacuum at idle or cruise, with CI flapper that works?

Thanks in advance for the help/discussion. Pic of engine/hood below.



Mike
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post #2 of 7 (permalink) Old May 29th, 19, 2:57 PM
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Bruce
 
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Re: Minimum amount of vacuum required for CI flapper to operate

Sorry I can't help, but I have a suggestion: Be sure to block off the air intake in the CI Hood, otherwise the thing will act like a chimney and vent engine compartment smells into your passenger compartment.

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post #3 of 7 (permalink) Old May 29th, 19, 3:03 PM Thread Starter
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Mike
 
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Re: Minimum amount of vacuum required for CI flapper to operate

Oh that's a good call - didn't even think about that. Will definitely try something to block that off. Thanks!
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post #4 of 7 (permalink) Old May 29th, 19, 7:23 PM
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Stan
 
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Re: Minimum amount of vacuum required for CI flapper to operate

As you've likely figured out by now, the engine vacuum closes the door. Not running, door is open, start engine and flap closes. The design is that as engine vacuum decreases the flap slowly opens and at a point where "when vacuum drops off sufficiently, and stays there, the flap will remain wide open. Set correctly and with a normal 10-13 vacuum, the flap will open very little until the gas pedal is mashed to the floor. On a big cam engine with lower than stock vacuum the door generally opens well before your foot is through the floor, and you can get it active simply by revving the engine in neutral.
My 454 car has a mere 6-7" vacuum and the flap is very active throughout the rpm range, but on start up the door closes and stays closed at idle.
My 396 car is stock and the flap hardly opens till you get to WOT. Because I like the "cool" factor of an active flap, I put a restrictor in the vacuum line and I can get some action before WOT
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post #5 of 7 (permalink) Old May 29th, 19, 7:37 PM Thread Starter
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Mike
 
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Re: Minimum amount of vacuum required for CI flapper to operate

@stanski

That's exactly the information I was looking for! Thanks!

And yeah, I understand that the default position for the door is to remain open under atmospheric pressure conditions (zero manifold vacuum/WOT, or engine off), and that the spring in the vacuum canister is overcome by the vacuum seen at the diaphragm during idle and part throttle conditions.

I'm really happy to hear that you can get movement in the flapper door from very little vacuum! I also like the cool factor of a moving flap in relation to throttle position/engine load, so this seems very promising.

Do you have the one-way flow restrictor in your vacuum line in your 454 to aid in gently shutting the door, or does yours just snap shut quickly?
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post #6 of 7 (permalink) Old May 29th, 19, 9:03 PM
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Stan
 
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Re: Minimum amount of vacuum required for CI flapper to operate

Not to confuse a restrictor in the line with the vacuum check valve in the line. Yes the purpose of the valve is to prevent the toilet seat from slamming down. The 454 and 396 both have them, but IMHO i don't think they really do anything. The restrictor I referred to is my own idea, just took a straight through vacuum hose fitting, filled it with JB Weld and then drilled through with the smallest bit I could find. As I said, this allows the 396 to be a bit more active prior to WOT.
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post #7 of 7 (permalink) Old May 29th, 19, 10:00 PM Thread Starter
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Mike
 
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Re: Minimum amount of vacuum required for CI flapper to operate

Oh I see what you mean. That does make sense, and I've read that some people do use the check valve, and others don't. Seems to be preference, or depends on characteristics of the engine.

Neat idea with the flow restrictor - I'll have to keep that in mind if I'm not really satisfied with how it's working. Thanks again!
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