The Stovebolt Site Forums The Shop Area General Truck Talk vacuum advance

I'd like to see a Mallory Unilite on the side of my 261 but for some reason the 6 cyclinder Unilite does not have vacuum advance.

What will I be sacrificing if I go for it?

The infamous ripoff Waynemfgco had them with vacuum advance but I can't find anywhere else. Can anyone help?

I'm runnung the Mallory unit w/o an advance, you will need to advance the base timing about 10-15 degrees to offset the lack of the vacuum advance.
If you don't, it'll backfire badly, especially when the engine is cold. As long as it's setup correctly there shouldn't be any sacrifice in performance. I would suggest using a 3rli Hyfire spark control with this distributor to get the most out of the coil, you will be able to eliminate the ballast with this addition.

Thanks for your reply. Advancing the timing will increase RPM. Can idle RPM be set back down where it was to start with via carburetor adjustment?

Yes, thats what I had to do. Mine still idles a little high but I'm running a hot cam and dual Holleys. Try to find a setting that your engine is happy at.

Thanks again, but if you only need to advance the timing and can then bring the idle speed back down with carb adjustments, there would be no such thing as electronic distributors with vacuum advance. The advance assy ain't free, so there must be some payback to having it on a vehicle. That is information I would like to understand.

The vacuum advance helps provide the advance at low rpm when the mechanical advance does't operate. Being electronic or points doesn't have any bearing on the need for a vacuum advance, the vacuum advance allows for less base timing for easier starting. Performance distributors forgo the vacuum unit and rely on a quicker pickup of the mechanical advance, but this requires more base timing at idle/low rpm. The result is more power in the mid=high rpm range.
Modern engines use the computer to control the advance/timing.

Vacuum advance is basically for fuel mileage.

All mechanical advance is sometimes used for racing applications, but many engine builders recommend the use of vacuum advance to save the engine from detonation. If you have the same total advance on mechanical only as you have on vacuum and mechanical you will have detonation at full throttle pulling hills at too high a gear which the vacuum advance would prevent because the vacuum would drop and you would have less advance. When the load decreases, the vacuum will build back up as would the advance, giving better economy.

The vacuum advance only works at partial throttle, and drops off as the rpm increases. At full throttle the mechanical advance is taking care of the advance that is required. By the way, I'm not running a vacuum advance and have the base timing set at 15 degrees BTDC. I pull up hills in overdrive with no detonation and little drop in rpm.

This site indicates vacuum advance and mechanical advance are additive at all rpms.

That link exlpians better what I was trying to say, it is additive at all rpm, but the vacuum is not adding any at low vacuum readings that occure at high rpm. It actually retards the timing at the higher rpm-the mechanical advance is all that is required. I checked the link with an engine performance manual and an ASE study guide-they both state that the vacuum advance primarily is to offset lean feul mixtures at low rpm/high engine load.

It does not retard timing at high rpms, just at high loads with open throttle. Once you reach cruise speed, even if it is at high RPM, and are no longer at a high throttle opening, the vacuum returns and the timing advances beyond the mechanical limit. This does not apply to racing, as then you are mostly under load and WOT, not cruising. Boats are happy with full mechanical because they are, in effect always going uphill.

What I meant to say was that the vacuum advance will retard at wot due to insufficient vacuum to operate it, not that it won't operate at high rpm. According to my manuals a vacuum advance requires about 3.6 inches of vacuum to operate, at wot the average engine drops to about 2.3 inches. As i stated earlier though, I don't have a vacuum advance, only mechanical, and my truck runs great without it. No detonation, and I have plenty of power through the entire rpm range. I do admit that this thread has me thinking that I need to bone up on this subject more, I have an ASE recertification test on engine performance coming up later this year.

This has been a great discussion for learning. The advantange of a distributor is always glossed over when it is mentioned elsewhere. I've been itching to buy a Mallory Unilite but unless I find one with vacuum I feel compelled to buy one of the modified units from Jim Carter.

Thanks.

Tom Langdon at The Stovebolt Engine Parts Co. sells a very nice HEI dist. set up for the 235/261's. It works slick and with the round coil looks very original. I have one on my 261 with dual carbs, a little cam and Fenton headers. It works very well. I opened my plug gap to .50.