Vacuum advance

[Straight six]

I have a edelbrock 500cfm also a new hei distributor. I'm getting so much conflicting info about we're to hook up vac advance is it the pass side or drivers side port. Some say pass side some say drivers side. The left side is port vac and the right side is manifold vac....
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[Straight six]

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[Jabes1]

Full manifold vacuum.

[geezer#99]

Depends!! How's that for an answer!
What's your normal idle rpm?
Can you adjust your idle rpm lower right now?
What's your initial timing?
Can you adjust the mixture screws all the way in without the motor trying to quit?

[Jabes1]

The only reason not to put it to full manifold vacuum is for emissions purposes. Here is an article that will be very helpful to you and dispel any misinformation. You can skim over the EFI, stuff if you want, but please read 'Timing and Vacuum Advance 101.'
http://www.chevytalk.org/fusionbb/sh...hp?tid/248441/

[geezer#99]

Good link, but every motor is different. Some like ported, some like manifold. Depends on your mods.

[Jabes1]

Originally Posted by geezer#99

Good link, but every motor is different. Some like ported, some like manifold. Depends on your mods.

Please enlighten me... I'm curious.

[geezer#99]

Originally Posted by Jabes1

Please enlighten me... I'm curious.

1. Stock motor, stock low duration and lift cam, stock timing of 4-6 degrees, idles at 550 rpm would like ported vacuum. THe idle is low enough, the throttle plate is closed enough to allow correct adjustment of the mixture screws. THe transition slot is covered enough with the throttle closed so you're not trying to idle on the power circuit.
A stock motor will run on manifold too as long as the vac advance isn't too much. Stock motor tends to detonate easier at low rpm and can't handle the extra timing.

2. Higher compression, radical cam, good heads, headers would need much more initial timing plus manifold vacuum to speed up the motor so the carb could be set up so the mixture screws and the transition slot is working correctly. Without extra timing from the increased initial plus manifold the idle would need to be increased on the carb which puts the carb into the power circuit and runs too rich at idle.

Most stock motors up to the late sixties had only manifold vacuum for the distributor. But they where set up with low initial timing too.

So you see it depends on your set-up.

[nheller76]

Try it both ways see how your motor responds. I know mine does better with Venturi vacuum witch is above the blades on my demon carb. If u have manifold vacuum it may give u too much advance at high rpm and through your timing off. Play with it see what happens. Do u have it timed correctly? If so it will make it a lot easier to find out the best vacuum for u. This is all stuff that my engine builder told me, I am running a very different carb then u so I may be talking out my behind.
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[Jabes1]

I think I see what you're trying to say. That setup could run more efficiently than it does. If you want a factory tune, sure set it up that way, but with todays gas prices why not set it up to run as efficient as possible and get more from it? His truck isn't exactly stock especially with that Edelbrock Carb. Tom Langdon from Stovebolt does amazing things with these I6's. I don't think you understand exactly what the function of Vac Advance does or its initial purpose. Here's one of Landon's tech tips from his site http://stoveboltengineco.com/2002/04/vacuum-advance/ its pretty basic. There was a really good article not long ago where someone was having a stumble and the truck wasn't accelerating like it should. Landon recommends initial timing at 15-18 degrees advanced with the vac can unplugged and then running the vacuum can at full manifold vacuum. Needless to say it really woke the truck up. I wish I could find the article! You definitely know your I6, but I think we all can learn something new every day.

Stock cams produce more vacuum than performance cams. Lean mixtures need more ignition lead to ignite than rich mixtures. Idle is a low load, high vacuum lean mixture. Cruise is high vacuum, low load, lean mixture as well. I'm sure his I6 will idle real low if he wants it too, but it takes a lot to get that carb to start sipping off of the power circuit. It would totally have to be setup incorrectly. The vac can will raise the idle and smooth it out when hooked up to full manifold vacuum as it should, he can then adjust the idle to where he wants it, hook up a vac gauge next and then adjust the screws the same amount to achieve the highest vacuum at idle.

Higher compression and a big cam have low vacuum and most people with those setups have nice heads that burn the mixture very efficiently which require less advance to burn. I've set up a few Edelbrocks/Carters to run on stock mills to blown mills and I've found that idle is set up in the same manner for both. His truck will run better and more efficient with the vac can hooked up to full vacuum. Ultimately its his truck, his decision. Nice truck geezer#99, looks clean!