The vacuum advance that increases fuel economy ALSO increases part-throttle power. This is seen as better "throttle response". It's an efficiency improvement for part-throttle, low-load operation. Efficiency is good.
I believe a street car needs vacuum advance. Race cars, not at all. When you buy the MSD 8517 or 8548, you burn your bridges because vacuum advance cannot be added to the Pro Billet distributors. The ready to run distributors do have a vacuum advance. When you couple a quicker mechanical advance rate with vacuum advance, you can end up with too much total cruising advance. Most stock canisters have 14-18* of advance. 8-10* is usually what works best. 40-44* at cruise gives you better economy and cooling. If you have part throttle ping, it is because the vacuum advance isn't being pulled out fast enough. This is what usually turns people off to vacuum advance. The canister has a spring that opposes the vacuum advance, the engine must overcome that spring to add advance. Under high load or larger throttle openings, the vacuum drops off and the canister spring wins out. That is why I like to use the stock vacuum advance canisters, the stronger springs in those units make the vacuum advance go away faster. All of this is combination dependent, your requirements will differ. Rear gear and converter can make a big difference in your VA needs. Less load = more engine vacuum. You need to tune the VA to your needs.
Here is what the factory timing looked like on a late 60s GM high comp motor ( 10 to 1) with open chamber heads and a mild 4bb Cam.
Vacume advance is great. If anything at least your advancing your vacume. So why was it mazada rotory engine cars had an advance vacume can and a retard can vacume?
If you need or want more low end off the line vacuum advance helps on street strip and even some race cars unless you have a real BIG cam. I ran a vacuum operated fuel pressure regulator years ago as well. At high vacuum it reduced fuel pressure to 3 psi then at no vac it would go up to 7. Seemed to save gas...didn't work at track as it didn't recover from 3 psi at the starting line when you nailed it.
Im not sure it was for emissions. The rotary had an oil line to the bottom of the carb to help keep the rings lubed up.
Rotary engines are poor performers for HC emissions. Retarding spark, in conjunction with the thermal reactor at the exit of the exhaust port, burns the HC making the engine acceptably "clean" for the emissions regulations of the era. Bad for fuel economy, though. Seal lube was a problem with rotary engines. They burn oil by design.
I have a 1000 cfm holley without ported vacuum. I am running advance off intake to distrbutor. I have been bumping the timing up slowly or otherwise with timing light my timing retards when i rev it up off idle only. This is a streer/strip build 455. I haven't checked it to see where the timing is but im sure it'sat least 38 deg or more. Off idle is better now, but i am tempted to adv it more. Am i entering a danger zone? I was tempted also to find carb with ported vacuum. I am running single plenum intake.
Does that 38* include the vacuum advance? With VA, and light load cruise, timing can reach mid to high 40's and the engine will tolerate it. At WOT, any advance from the vacuum advance will disappear. What matters is what the timing is at WOT once all the mechanical advance is in. You can't guess at ignition timing, you MUST know what it is. If you don't it may cost you power at the least, or engine damage at the most. With vacuum advance hooked to manifold vacuum, the timing may appear to retard if vacuum drops off idle. Opening the throttle quickly will drop vacuum. The vacuum advance canister has a spring that OPPOSES vacuum pull. Engine vacuum has to overcome that spring to advance the timing. When vacuum drops off, the spring wins out and pulls out any advance the vacuum created.
http://www.v8buick.com/index.php?threads/power-timing-your-buick-v8.63475/ Remove the vacuum advance, plug, and forget it. (For Now). Work with the initial + the mechanical advance. Measure that as you increase the RPM. What you want to do is set the total WOT timing. It can be difficult with stock weight springs which sometimes delay full mechanical advance to as much as 4600 RPM. You can't be free revving the engine that high in Park/Neutral. It isn't good for the engine, and your neighbors will not like it. What you need is a set of light springs that let the weights fly all the way out at a manageable lower RPM. Then you can set the maximum WOT timing, and play with spring combinations to get it in at 2500-3000 RPM. Once you get that squared away, you turn your attention to the vacuum advance. Until then, leave it plugged.
Definitely listen carefully to what Larry is saying he helped me out of one heck of a mess with my timing! I read his power timing thread over 20 times and followed his instructions that he gave me to a T and now my motor is running mint!! Best of luck to you!
