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kenhaw
04-12-2009, 04:46 AM
Why does somebody plug the vacuum port on the distributor and what are the pros and cons?

68Formula
04-12-2009, 08:48 AM
Dedicated drag cars do it because a lot of times they run locked at a fixed timing. Big cams, race gas, and high stall convertors can not only allow this, but require it.

Some believe it can cause slight timing changes at WOT, which if working properly it shouldn't since there is virtually no vacuum at WOT to activate it.

And some just disconnect it because after making changes to the mechanical or intial timing, the car pings at part throttle. This not because of the vacuum advance, it's because the vacuum advance needs to be re-tuned due to other changes made. This can be done with $15 worth of parts.

A street vehicle will waste a lot of fuel without the vacuum advance. Throttle response can suffer. If the car is intended to run it connected to full manifold vacuum, the car will idle poorly. The only time you should have the vacuum port plugged on a street vehicle is temporarily, to set your intial timing.

kenhaw
04-12-2009, 09:00 AM
ok, so can I connect the distributor vacuum fitting to a vacuum source which comes under pressure when my supercharger makes boost? And if this vacuum advance only works under vacuum that will also mean that over a certain rpm, lets say 2000-2500 rpm, it will have no affect on timing?
If my timing is adjusted with the vaccum fitting plugged does that mean that I just connect the hose now and the timing will still be spot on?

Btw the disributor is a MSD E-Curve.

Thx

68Formula
04-12-2009, 12:29 PM
Yes.

Yes.

Sort of. Usually the factory has it plugged to ensure the static timing is properly set (initial). If the setup is designed to work with full manifold vacuum, once you hook it up, it should add more timing. Which is okay if that's how the engine likes to run. Now if it's setup to work on ported manifold vacuum (the fitting draws from above the throttle blades), then you shouldn't see any additional timing. However, if the throttle blades are open too far at idle either because of tuning, or poor vacuum signal, then you could see some additional timing come on when it's hooked up. If that's the case, you need to try more adjustment with your mixture screws. If they aren't responding, try adding in more intial timing and see if that helps. But keep in mind, more initial timing means more total timing, so you'll need to adjust the centrifugal down. Nice thing about the E-curve is that its really easy to experiment with different settings.

Now, the thing is with boost, the added cylinder pressure means the engine tolerates less timing compared to an naturally aspirated engine. Otherwise you can blow a headgasket or worse. Will you be running a boost retard setup?

How much boost are you adding and what is your compression ratio?

kenhaw
04-12-2009, 03:00 PM
Compression ratio is 9,6:1 and boost levels is 7-8 psi.
I am running a Summit Street and Strip Boost Master with boost retard.
How will I know where to take the vacuum signal from, before or after throttleblades? My system is a blowthrough.


Just checked manual, the distributor wants ported vacuum from before the throttleblades.

kenhaw
04-19-2009, 11:42 AM
Just got an answer from Summits techline. They say that I have to leave the vacuum nipple blocked of on my application, does that make sense??
Its a MSD E-Curve dizzy , Summit Street Boost timing master and Procharger F1 setup.

ponchopwr70
04-23-2009, 02:05 PM
I don't use mine ran into problems with pining issues. I have no ill side effects. I have a decent cam 10.8:1 3000 converter and an hp950.

68Formula
04-23-2009, 05:02 PM
Just got an answer from Summits techline. They say that I have to leave the vacuum nipple blocked of on my application, does that make sense??
Its a MSD E-Curve dizzy , Summit Street Boost timing master and Procharger F1 setup.


From what I read the MSD unit should take 25psi of positive pressure, but you should check with them first (not Summit). If they say it's okay, I'd hook it up, and try a little at a time.

MonzaRacer
04-27-2009, 08:56 PM
If your using a boost timing control and boosted engine should not be using vacuum advance at all.
But then most of engines I tune have electronic advance/retard and no mechanical or vacuum advance used ie locked dist.
Set your base timing and tune electronicly from there.
This reason I am getting more people tunred into FI people, lap tops much easier to deal with on some projects.
BUT as I have always found ported vacuum has done nothing for me in the life of tuning cars.
Heck girl at work thinks her "cam" should sound like a top fueler loping or one of them alky guzzling blown race cars.
Man if she would let me setup her dist/timing/carb her car would be pretty tight.
Had fella bring over a aluminum headed 355 to work, sounded like her car, I shot the timing, added 15 degrees to the base, lightened up the advance springs and hooked up manifold vacuum advance and adjusted it to limit to no more than a total of 60 degrees advance and showed him how to remove timing from both centrifical and vacuum and how to keep advancing it. He also have one of my prototype "knock" indicators on dash. 10.2 to 1, .658 lift cam and lots of timing and his fuel mileage went from 8mpg to 23, next comes removing a few jet sizes from BG carb, hope this one turns out better than his last one for race car, that POS must have came from floor holding door open at BG as it was NEVER right and had so much dirt/metal in it I never got it to run.
OH well Gotta love aluminum heads and Evan Cooling NPG-R.