Join Date: May 2016
Location: Sonoma County, CA
Minimum amount of vacuum required for CI flapper to operate
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.