HEI vacuum advance

I'm looking to put an adjustable vacuum advance on a DUI dist. All I'm seeing are ones for V8's. Anyone out there using an adjustable, if so where did you get it, who makes it?

 

As an afterthought, anyone using a lockout plate for the vacuum advance?

 

The DUI and HEI use the same parts so the adjustable vacuum advance would share the same part number.

 

Ya, I wondered if there was any difference between a V8 vac adv and a V6. Summit's site listed numerous adj. vac adv units, but they specifically stated that they were for V8 GM HEI and didn't list any applications for V6 except for the Pertronix Billet Dist. I'm going to pull my vac adv and take it to Summit and compare it to one of the V8 units. I can't imagine that there is any difference.

 

I did a major amount of research on vacuum advance units and what I found in different applications was the location and or orientation of the vacuum connector, the strength of the internal spring and the distance of travel of the control rod. For the DUI/HEI dist you need the long tongue. It is far simpler to get the adjustable one because you can then adjust the amount of advance in relation to the vacuum applied at the desired throttle position (meaning inches of vacuum @ RPM).

 

Roger that, Walt. Thanks for the heads up guys. I'm off to Summit Racing this morning. I'm going to take my vac adv with me.

 

I've been running my HEI V6 for many years with the vacuum advance line disconnected and plugged. The diaphram leaks on the junkyard HEI unit I have.

With the high price of gas I finally ordered an adjustable unit and I'm waiting for it now. Pretty sure it's an Accel brand. I'm assuming the V8 and V6 are the same.

 

I just bought an Accel p/n 31035 adjustable vac adv. Once I get it installed I'll post up the results. I considered just plugging off the old one, but my buddy read an article that said you loose low end performance and tend to load up the heads with carbon if you don't have the vac adv. working. If you order one from Summit Racing the p/n is ACC-31035. A friend of mine who's been researching the vac adv. found that with the stock vac adv on his HEI he was running close to 50* BTDC at 3000 rpms. Of course his engine ran like crop and had no power at 2800 - 3000 rpms. I also bought one for him. I've been experiencing the same issue with my engine. Great out of the hole up to about 2600 rpms where the power fell off. It was OK for the trail, but not good on the road.

 

Know that the vac advance can be used different ways. Originally it was on a PORTED vacuum source (the signal that is generated by airflow through the carb, NOT manifold vac.) With 8 degrees initial advance, and 12 in the distributor (works out to 24 crank) you had 32 deg total. Then 4 more was added with the vac advance at WOT. Then some were put on a manifold vac which gave lots of advance at part throttle which I think was for emissions/economy. So you need either a tape on the balancer or a dial back timing light to figure total advance and what RPM it comes in at.

 

Here's what I did.
http://www.earlycj5.com/forums/show...um-advance-on-on-HEI&highlight=vacuum+advance

 

I found a pretty good write up on how to properly setup HEI timing. I'm going to use the tape on the crank technique to check the timing as I don't have a dial back timing light. In the article I found he is using "manifold" vacuum. I was under the assumption that "ported" was the proper way to draw vac for the advance. As always the internet is a wealth of conflicting information.

 

Not sure but I think it would work backward on manifold vacuum?

 

Thanks for the heads up jc588. I went back and re-read your thread. Don't suppose you snapped a pic of the aluminum "stop" you made?

 

when I built my motor I sent out the dizzy to a shop told them what gears ,headers ,carb,etc.... and they set it up for me ,was like $60 plus parts , he said ONLY use GM stuff inside , since they have millions of cars on the road.I can look and see what my specs worked out to .......my is run off the ported vac so it moves when throttle is applyied if you had it on manifold it would be advanced all the time ,,,,,execpt at WFO ,where there should be NO vacume,,, but how often does the jeep see that .

I blueprinted the 442s also and now THAT dist. recuved and timed make a HUGE difference, niehter vehicle pings anymore,,,,unless the fuel is really bad.

 

I can get one later this evening or in the morning. It's really simple but effective. I bought a cheap Mr gasket advance the accel or other brands may respond better to adjustments.

 

Vacuum advance should be hooked up to ported vacuum. As throttle opens and rpm increases manifold vacuum decreases which will decrease the vacuum advance. Just the opposite of how you want it.

 

jc588: Pic would be great.

nickmil: That might not be a bad thing if you are trying to back off too much advance at higher rpms. Using manifold vac you'd still get advance at lower rpms, but less total advance above say, 2000 - 3000 rpms? From what I've read, these engines run well at a total timing advance of 35* at 2800 - 3000+ rpms. Set at initial timing of 12* +/- I get around 50* total advance using the ported vacuum. I'm hoping the adjustable vac adv does the trick, but it looks like I may still have to limit total timing advance with a lockout of some sort. Right now I have really good response at trail speeds, but on road accel. kinda sucks when I get above 2500 rpms.

 

Here's a couple pics, Mine is set to limit total to around 34 Degrees if I'm not mistaken.

 

I just completed converting an even fire distributor to an odd fire for my 225 a couple a weeks ago. I used the accel adjustable advance. My initial timing is set at 7* BTDC with a mechanical adv of 15* for total crank advance of 22*. With the accel adjustment screwed all the way counter clockwise then back in 4-1/2 turns my vacuum plus mechanical total crank reads about 37*. This is using ported vacuum and reading the advance at near 2800rpm. I took my son-in-law for a show & tell (he is a member here) with a semi power shift to second it broke loose the right rear tire (33's) with 2.73 gears on dry asphalt on straight road. I had not tried to do this intentionally due T-90 tranny (third one) and I was not expecting this kind of power torque. This was my first attempt and checking the power range since the HEI change. I will leave averything where it is now as I have good power, good cruise (smooth) engine runs very cool and no ping. I am very pleased with the HEI and now my son-in-law wants one for his cj6.