Ol Ron & Mechanical Advance

[Chris Nelson]

WEll, Ol Ron, I did some homework on what you advised about mechanical advance Versus Vacuum advance and guess what. YOU WERE RIGHT! I spoke with the tech at both Pertronics and Mallory and they both agreed that it could improve gas consumption at a cruise speed to use a vacuum advance. They both thought that mechanical advances were fine for street use, but that you could gain a bit of mileage with the vacuum style. The only thing they might have disagreed with was whether or not the cylinders walls were getting washed down. They thought that was stretching it somewhat unless the carb was out of adjustment or the spark was weak, but they did agree with you about the increase in mileage. I learned something. Thanks

 

[36tbird]

OK, so how does it work? I mean, I know how vacuum advance works as well as mechanical (centrifugal) advance. My understanding is that you need to set up for the total of all three advances, 1. initial, 2. mechanical and 3. vacuum. What does the vacuum do while you are going down the road at cruise to improve mileage? Does it react quicker than mechanical to slight changes in demand? Thanks, Lou

 

[Bruce Lancaster]

The issue is that significantly more advance is needed at part throttle cruise than at full throttle. It is not really "economy" versus "power', it is just plain an issue of efficiency...without compensation for different engine loads, your cruise advance is late and you are blowing power out the exhaust. No less an authority than Smokey Yunick (think of Ron but with better funding for flow benches and dynos! Another real thinker!) had another insight...he said the perpetual fouling problems experienced by racecars during pit stops and yellow-flag laps was because of the lack of vac advance...they foul because they are running retarded at part throttle.
Ford never fully got it right for the flathead...the '49-53 distributor had good part throttle curve from its 2-way vac can, but full throttle venturi depression usually gave full throttle advance somewhere between slightly weak and totallyabsent.
The early distributors, especially those with the 11A advance unit, had very nice full throttle performance, but the Mallory design vac brake compensation was a weak approach (same system used on pre-Mr. Gasket/China aftermarket Mallory) and on modern gas allowing brake to be backed off all the way it disappears entirely.
The old Chevy distributor that Ron develops from is not only a good basic design, the aftermarket produces lots of modifying parts for both its centrifugal and its vac curve.

 

[Ol' Ron]

During cruise conditions the carburetor is producing a lean mixture. This takes longer to burn, so more advance is necessary to burn all the fuel. However, this advance and cause detonation under WOT, so using Vacuum we can reduce the timing to a manageable amount. The stock Flathead has a very low compression and may not benefit as much. The 42-8 distributor in good condition provides an almost perfect advance curve under all driving conditions. It also uses manifold vac, so the type and number of carbs doesn't matter. The only commercial dostributor that meet these same requirements is the MSD. The SBC distributor uses the same advance mechanism and can be Taylored to any advance curve.

Yes I kind of stretchered the cylinder washing, but it makes you think.

 

[36tbird]

Well, in my feeble brain, this appears to be another egg/chicken, horse/cart debate. But, I think those are healthy in that they promote thought and eventually understanding on any given subject.

 

[BUBBAS IGNITION]

Interesting discussion;

I would add some verbage as well.
Its very common ( i will use the chevrolet v8 , forgive me in advance ) to see a chevy distributor with a 24 degree mechanical advance ( limited by a slot and bushing ) and a vacuum unit that also supplies 14 degree (limited internally) and a intial setting of 10 degrees as well.
If you add them all up it would total 48 degrees and we all know this would be way too much advance.
Now let say the maker wanted the engine to have a total working advance of 36 degrees ( which is pretty posible) It could be a variation of many factors all in the same day of driving.
First of all you wont ever have full vacuum at wot and you wont ever have full mechanical advance at full vacuum (light load ) conditions,
so one posiblility is to have 10 intial ( used for starting , 12 engine advance at 2000 rpm and and 14 at light throttle from the vacuum. Total advance is 36 degrees which is just what we wanted.
Open the throttle ( loosing vacuum) and lose 4 degrees of advance from the vacuum unit and gain 4 degrees in engine speed with the mechanical ( rpm) advance.
Thats the perfect world we would like to have on all gasoline engines.Each compliments and enhances the other with a overlap of functions etc.

Now here in Indiana ( during sprint week this week ) they just pull the rpm to 3000 rpm and set them at 36 degrees with a locked advance and a magneto, and run them wide open all night ........

 

[Ol' Ron]

What Richard and I are trying to do is find the optimum timing for HIS engine. The cam, compression and other factors, prevent these numbers from use on say a stock engine with it's mild cam low compression and open combustion chamber. However we feel once we get the hang of it, we might be able to come up with a plan or chart that might help others. We don't sell anything and all the info is free.

 

[Bruce Lancaster]

One thing about this discussion...new people souping a flathead think ALUMINUM and CAM. Gotta have lots of finned aluminum, gotta have a whump-whump-whump at idle.
Reality 101: Half the stuff at least you do actually hurts performance under normal driving when full throttle and RPM can't be used, and lots of the aluminum does very little for you...sometimes less than the stock iron you just tossed in the dumpster.
Your timing curve when it is changed from bad or just OK to just right gives you a kick you can feel at all speeds and conditions, not just on the odd occasion when you have a clear road and no cops in sight. Proper timing is a BIG part of throttle response as well as power at all speeds and loads.
By the way, IMHO, the '42-8 distributor is probably the best bolt-on ignition, the Chevy '57-73 (approximately) and its MSD clone is the easy one to play with, with easy provisions to alter rate and total of centrifugal and vac devices and plenty of aftermarket springs and widgets for said tuning. The big, ugly HEI is also actually a very good device with almost as much tuning stuff available as the early one. It may be the most durable and reliable ignition system ever made...