[Jay S]

In addition to some part throttle efficiency gains when running vacuum advance for the street, when the engine requires a lot of part throttle operation, the vacuum advance increases the temperature of the plug tip too keep the plug’s burnt clean. The plugs last longer and do not fowl as easy.

I don’t know how the timing curve was setup, but if your going to run no vacuum advance on the street it needs more initial and less mechanical advance. If the builder set the distributer up for running no vacuum advance you will very likely need to recurve the distributer for running vacuum.

[Cliff R]

If your car is street driven, even with beefed-up, modified and more powerful engine VA should be used. It's not manditory, but only positives things happen if used (correctly).

The problem we see is that even though it's 2023 and there is PLENTY of good information out there on the subject WAY too many folks just don't understand it or how it works. I've even read a little of that right here in this thread.

I'm not going to go over all of it again, but for sure your engine needs it if you spend any time at all street driving at light engine load. VA is a "load sensing" device, so it NEVER adds timing when it shouldn't. Any heavy or full throttle and it is NOT a player and it doesn't come back in at high RPM's whether hooked to a manifold or ported source as often mentioned by folks who simply don't understand it.

Where most get into trouble trying to employ VA is that they have "modified" the timing curve or moved the base timing up far beyond where the factory set them at. So adding a LOT of timing as most VA cans are set up to do advances the timing too far and they experience negative results. Plus (if done correctly) a good engine build optimizes compression, head flow and cylinder pressure with improved VE. This requires LESS timing and fuel than the other way around.

Sadly, even some very experienced tuners and engine builders don't get it, and beleive it or not some of the most successful companies building "high performance" engines for street driven vehicles don't use it either. They believe that simply bumping up the initital timing and using much lighter distributor springs provides adequate timing for the engine, which it doesn NOT. The engine is still MISSING at least 10-15 degrees if not a bit more at light engine load, cruising, and during "normal" driving scenarios.

What VA does is to allow you to burn a leaner mixture and make optimum power with less fuel. You get improved combustion which means less fuel consumption, less carbon build-up on pistons and in the exhaust system, and cooler running engines as well. Plus your wallet woln't be as light either. If you don't use it nothing terrible is going to happen to your engine, you are just going to miss a few nice benefits and at the top of that list is saving money........

[Chesters68]

Anybody run a locked distributor with a timing computer?

[Formulajones]

Anyone with EFI and timing control does

I've had the ability to lock a mechanical distributor while using an MSD box that has a timing start retard function, but I've never found the need to, nor see any advantage to it. Plus since most of my cars want anywhere from 34 to 38 total timing, they don't need to be, or even want to be idling with that much timing.

Much more beneficial to produce a timing curve with reasonable idle timing and then toss in the vacuum advance on top of that for best results on any street/strip car.

Now if you're talking about complete timing control through a computer program setup with a nice graph that can be manipulated on a laptop, there are some benefits to that, one being very precise timing control with precise changes in certain areas that you simply can't do with a mechanical advance. At this point wouldn't one just want to go the EFI route and control fueling along with it??

There are some options out there if you just want electronic timing control by itself while still using a carb, but the ones I've seen only allow changes on a cell phone and don't really interest me.

[leeklm]

Although my current setup does not use VA, add me to the list of those suggesting you should have it on a street car. What sold me years back was a 455 with too small/improper aftermarket cam installed. Factory HEI with full mechanical advance coming in by 3k- car would overheat on the highway with no VA. Connect VA with total timing of 48 degrees at cruise speed, car would cool right back down within 2 miles!

As mentioned here, VA was invented for good reasons!

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[steve25]

Basically in a nut shell what’s going on is that at part throttle conditions without the added timing from VA the air and fuel charge in the cylinder burns too slow to apply force to drive the piston, so not only is less power being made but when it does finally burn all it does is add heat, and if this added heat does not make for a hot running condition then it will increase the chances of detonation.

[Cliff R]

In almost all cases nothing really bad happens when you don't add timing at light engine load cruising other than it takes MORE fuel to get the job done. What simply happens is that you have to press your right foot down a little further and the carb by design richens thing up a tad.

I've NEVER once in all the years of doing this observed an overheating condition NOT using VA, or being able to cure an oveheating engine adding it. Have tried scores of times and what I found is that any potential improvements were superficial and not really attacking the root cause of the running hot/overheating condition right to start with.

I'm not saying here all the stories we here about someone adding timing at cruise, or even more common hooking up VA at idle using manifold vacuum instantly cures an overheating problem, I've just never once been able to duplicate any of that when working with troubled engines that were brought here with those issues.

As for overheating or running hot Pontiac engine builds, I have found a few common denominators associated with the ones I've worked on. A LOT of quench area is NOT your friend with that deal. Camshaft evens also play a role and never once have I had one of these engines run hot with factory cams in them, always something from the aftermarket and those were tight LSA with early closing intake events 9 times out of 10. Which makes sense as those cams degrade combustion efficiency (more overlap/lower engine vacuum) at idle at low RPM's.

Of course many were simply fundamental problems like missing the plate or plates under the water pump, or using a "rebuilt" pump with a stamped steel impeller vs the large cast iron one, shrouds missing, radiators needing flushed and cleaned, clutch fans not turning fast enough, etc.....