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Stewart Way · 01-05-2008, 07:31 PM

Lee
Don't doubt that you have seen 0 vacuum at WOT but I would bet most S and SS cars see some vacuum because they have to run the factory undersized carb. Reading info from Maxrace Software, and Larry did some testing on 283 Chevy SS combos with the 4GC carb and WOT vacuum was 4-7 "hg. If the engine can pull more air than can flow thru the 4 little barrels then you will have a restrictive carb which results in vacuum in the intake.
The Holley Carburetors & Manifolds book says "If the carburetor is too small, pressure drop at wide-open throttle will be greater than the desired 1 inch Hg (for high performance). My guess is that most S and SS class cars have too small a carb because it was sized for driveability rather than all out race.
So back to the original comment that got this started. If you can energize a solenoid at WOT and introduce a metered amount of air and the correct amount of fuel is injected by the computer, you have a HP increase. This is the same thing the 2 bbl stick shift stockers used to do in the early 70's by spacing up the carb from the intake and jetting up in the carb. It would pop and bang on the line but under load it made more power but it was a trial and error thing. Thats where the rule came from that required all vacuum leaks plugged and all carbs bolted tightly.
If I get to the shop next week I will pull some Hemi dyno runs and see if we recorded vacuum.

01-05-2008, 07:31 PM	#105
Stewart Way Member Join Date: Aug 2003 Location: Atlanta Posts: 476 Likes: 20 Liked 70 Times in 22 Posts	Re: Dommination of FI cars. Lee Don't doubt that you have seen 0 vacuum at WOT but I would bet most S and SS cars see some vacuum because they have to run the factory undersized carb. Reading info from Maxrace Software, and Larry did some testing on 283 Chevy SS combos with the 4GC carb and WOT vacuum was 4-7 "hg. If the engine can pull more air than can flow thru the 4 little barrels then you will have a restrictive carb which results in vacuum in the intake. The Holley Carburetors & Manifolds book says "If the carburetor is too small, pressure drop at wide-open throttle will be greater than the desired 1 inch Hg (for high performance). My guess is that most S and SS class cars have too small a carb because it was sized for driveability rather than all out race. So back to the original comment that got this started. If you can energize a solenoid at WOT and introduce a metered amount of air and the correct amount of fuel is injected by the computer, you have a HP increase. This is the same thing the 2 bbl stick shift stockers used to do in the early 70's by spacing up the carb from the intake and jetting up in the carb. It would pop and bang on the line but under load it made more power but it was a trial and error thing. Thats where the rule came from that required all vacuum leaks plugged and all carbs bolted tightly. If I get to the shop next week I will pull some Hemi dyno runs and see if we recorded vacuum.