Adjustable Vacuum Advance

Smart people are using MVA. Period. GM cars used it. Not sure if Chrys did. My GTO left the showroom floor idling with 26* of timing [ 6* initial + 20* added with MVA ]. 10.75 CR, with a mild factory cam, around 200* @ 050.

Ideas change as more knowledge is gained. Thinner piston rings are used today comparted to yesteryear, just one example.

I am not going to spend all day trying to prove to non-believers what experts/engineers proved long ago.
So just a few examples:
- From D. Vizard. Author, race driver, engine builder, author of 30+ books & 3000+ technical articles. In one of his carb books [ think about why ign is discussed in a carb book...]. "The optimum idle advance is typically about 35-40* for a short cammed street engine & [ though not commonly realized ] as much as 50* for a street/strip engine."
- By DV, PHR magazine Nov 04. :"At idle & low speed operation, the amount of adv reqd to effectively burn the fuel & air entering the engine can be as much as 50-55*. This is handled by the vac adv: a function many hotrodders believe is not needed because their favourite drag racer does not use it. By taking the time to hook up MVA you can get a big cam to idle as it were about 20* less than it really is...."
- Mopar Muscle Oct/15. "..raised the init timing from 15 to 25*.....There was a noticeably better off idle & low speed throttle response & another inch of vacuum at idle. The added init timing raised the idle rpm..". [ The idle rpm increase occurred because the engine made more HP from the extra timing; the extra vac was because the engine burned the mixture more efficiently. While the timing increase was done statically, vac adv could have been used & the same result would have been achieved ]
- PHR 8/04. "With vac adv hooked up...carb & ign allowed a very smooth 650 rpm idle". [ 350 Chebby making 485 hp ]
- Super Chevy 11/16: "..you should consider hooking the vac adv to straight manifold vacuum. This is counter to what you may have been taught in high school auto shop, but it is actually a very good idea.....That simple change [ adding 10* of MVA improved idle vac a solid 2"....& the engine no longer felt like it was struggling against the c'ter."
- PHR 12/07: "Just to give an up front idea what this was worth [ hooking up MVA ], the engine's minimum idle speed WITHOUT the aid of MVA was a lopey 900 rpm. With MVA, it was 780 rpm with only about half the lope."

More on MVA, scroll down to post #6:
www.hotrodders.com/forum/vacuum-advance-hooked-up-directly-manifold-bad-47495.html