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One cylinder produces the same vacuum as four. (archive)

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Posted by (e.g. brake booster line) on June 29, 2001 at 16:04:27:

In Reply to: dual 40s have less vacuum assist posted by ///M Chris on June 29, 2001 at 15:42:02:

You do not get "more" vacuum (in terms of the gauge pressure (psi)) out of 4 cylinders than one. The manifold vacuum gauge will read the same whether it is connected to one cylinder of the sidedraft setup, or all four.

However the manifold vacuum for a given throttle position is less for sidedrafts than it is for a smaller-throttle downdraft setup. Also, there will be less vacuum on 45's than on 40's, and less at idle on lumpy-monster-cam setups than stock cam setups.

Another difference is that the sidedraft setup takes more time to "recharge" the canister than the downdraft, because the change in manifold vacuum caused by opening a single manifold runner to the canister is more significant than opening all 4 runners to the canister.


I have a 1600 with a tii brake booster.

When I switched from a 32/36 to dual 40's I notice a BIG difference in brake assist. Because my manifold only gives vacuum off one cylinder. That's 1/4 of the assist it used to have.

I saw a manifold at Rebello in Pacheco, CA where they attached a tube to each sidedraft manifold to get more vaccuum for brake assist.

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