Vacuum to advance
Tech Contributor
Joined: Aug 1999
Posts: 14,379
Likes: 6,390
From: At my Bar drinking and wrenching in Lafayette Colorado
The timing requirements of the engine are more related to cam and compression than intake manifold type. The type of carb used does not affect whether an engine wants vacuum advance at idle or not. Engines with very mild cams ("RV" cams or "station wagon cams") will usually produce better idle quality with initial timing in the 12-16 degree range (running ported vacuum to keep the timing down in this range). Bigger cams want a lot more timing advance, and you can usually obtain much better idle quality and off-idle throttle response by using manifold vacuum (to get the timing kicked up at idle). My advice is to simply try it out and give the engine what it wants - whichever source produces the best quality idle (not necessarily fastest idle) is the source you want to use, providing emissions are not a concern. Listen to the car when you tune and try things out - it will tell you what it wants.
Drifting
Joined: Sep 2004
Posts: 1,959
Likes: 1
From: North Bend Ohio
You're not going to get a lot of change in vacuum signal with different intakes and no change with different carburetors. What can make a big difference is camshaft profile. A long duration camshaft will have give reduced manifold vacuum.
BigBlockk
Later.....
BigBlockk
Later.....
Team Owner
Joined: Aug 2006
Posts: 24,125
Likes: 15
From: Columbia Missouri
