z ya Posted March 25, 2002 Share Posted March 25, 2002 Ok, I have the timing set on the Z and when I hook the line up to the proper ported output on the carb the timing increases. I know the vac adavance should not work while at idle. The distrbutor is a Proform HEI that is brand new as is the Holley 3310 750. Whats the deal? Quote Link to comment Share on other sites More sharing options...
Guest needwaymorespeed Posted March 25, 2002 Share Posted March 25, 2002 You want vacume advance at idle and part throttle none at wot. Quote Link to comment Share on other sites More sharing options...
z ya Posted March 25, 2002 Author Share Posted March 25, 2002 You dont want vac adavance at idle just part throttle. If you wanted it at idle you would not disconnect your vac advance when you time it. Quote Link to comment Share on other sites More sharing options...
DAW Posted March 25, 2002 Share Posted March 25, 2002 You must be getting that vacuum signal from somewhere below the throttle plate (manifold pressure), rather than just above it at idle (ported). Either that, or the throttle is cracked way more than it should be for the engine to idle for some reason (valve timing, mixture, etc), and you're into the ported vacuum source prematurely. How wide is the throttle plate actually open at idle? DAW Quote Link to comment Share on other sites More sharing options...
Guest greimann Posted March 25, 2002 Share Posted March 25, 2002 In general, emissions controlled low comperssion motors want ported vacuum and high compression, hi-po street driven motors want full vacuum. The reason is this: A high compression motor does not want much initial advance (4 to 6 degrees) in order to resist pinging under heavy load, however, that small initial timing makes the motor idle like crap-ola (to use a technical term). Full vacuum at idle brings the timing up and the idle quality as well. As soon as you put your foot in it, the vacuum advance goes away and you have purely initial + mechanical advance working for you. Part throttle cruise brings vacuum advance back into play for good economy. An emission controlled, low compression motor uses the ported vacuum to bring in vacuum advance that is more dependent on the engine rpm, rather than load. Best bet is to tune the motor both ways and see what works best for your particular setup. Quote Link to comment Share on other sites More sharing options...
Mike C Posted March 26, 2002 Share Posted March 26, 2002 Nice explanation. Also, it is much easier to accurately dial in mechanical + initial for overall advance than it is with vac advance. The Proform is setup to run with vac advance off of a ported vac connection, however. One drawback to running GM style HEI is that there is no adjustability built into the mechanical advance. You have to either slot the advance plate and/or weld up the pins to change it. One of the REAL benefits to running an aftermarket distributor like MSD. Quote Link to comment Share on other sites More sharing options...
Recommended Posts
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.