911 sc dizzy vacuum advance part number

[scott.k]

Originally Posted by psalt

Scott,

The vacuum advance has little or no effect at idle. It is designed to add 5-10 degrees of timing at part throttle cruise. It is most active at around a 50-60 mph cruise when there is around 18 inches of vacuum.

Vacuum advance has no effect on measured performance. It is a load based system, light load it adds advance, full load, no added advance. You cannot compensate for it by increasing the static setting, you will have too much advance under WOT and it will ping like crazy.

If you drive your car 100,000 miles a year and fuel economy is your goal, buy a good used distributor, retard the timing to 3 BTDC, set the mixture to 55 duty cycle, run it on 87 octane, and put on a set of steelies with 165 tires at 45 psi, this will give you the lowest cost per mile.

Your Mopar description makes no sense to me. A locked distributor on a drag race car is a reliability issue, not a performance adder. The reason MSD makes electric retards is to get the motor to start, it will kick back if locked at 38 BTDC. There is little or no benefit to recurving a stock distributor with a stock lambda CIS, unless you are replacing a worn out distributor. The small runner motor cannot handle more than a few extra degrees of timing and bumping the static to 7 or 9 and running premium is about all you can hope for.

Paul, thanks for the help. I think a couple other people on the forum have used this ignition with a locked distributor already with carbs or some other setup that they had to change the timing with. I am basically just trying to find a solution/ fix for a crapped out vacuum advance (a part that is no longer made). So it adjusting/ twisting the dizzy just changes Idle advance, then yeah, I agree that it wouldt do anything for me. But if it effects the advance across the whole rpm range, then it could be very helpful, because I would just retard the timing with the MSD when I need to and not lock out the dizzy. I was thinking of running a couple extra degrees of advance across the rpm range and use 92+ octane, not a large gain, but a little.
I dont care about gas milage, to be honest, as I dont drive it every day, and dont put a ton of miles on it.

So just to clarify once more, sorry for my lack of knowledge, does turning the dizzy to adjust the timing create more advance across the whole rpm range? If so, I could get the stock, or with a couple degrees extra advance, without using the vacuum ad.

(this idea was used by a member in the link to the other thread I posted, and it sounds very smart to me).

[psalt]

I dont care about gas milage, to be honest, as I dont drive it every day, and dont put a ton of miles on it.

So just to clarify once more, sorry for my lack of knowledge, does turning the dizzy to adjust the timing create more advance across the whole rpm range? If so, I could get the stock, or with a couple degrees extra advance, without using the vacuum ad.

Scott,

If you don't care about gas mileage, you are wasting your time and money worrying about a vacuum advance.

Yes, turning the distributor and raising the initial timing effects the advance throughout the rpm range. The mechanical advance inside the distributor is speed related. If you start at 5 BTDC with a stock distributor that has a 10 degree (20 crank) mechanical advance, you wind up at 25 BTDC max. If you bump up the initial to 10 BTDC, you wind up at 30 BTDC max. The mechanical advance is speed related and the distributor does not know or care if the engine is at rest in neutral or under full load at 140 mph. But the engine does.

What you are missing is that vacuum advance is a load based system, it only adds advance under light load cruise when VE is low and the engine can handle the extra advance without detonation. Under load at WOT it adds nothing. You can't replace the extra 10 degrees of vacuum advance by bumping up the initial, you will have too much timing under load and your pistons may wind up like these:


What caused this engine damage

The 911 is a knock limited design and the later small port SC is detuned for 87 CLC fuel by severely retarding the max advance from the ideal. You may get away with one or two extra degrees with premium fuel and a cool climate, but everything else (mixture, compression, ring seal, blow by, cooling) has to be perfect. My advice would be to look for a used stock distributor for a stock engine, disassemble and clean it, and run it without the vacuum retard. Keep your current distributor as a spare, and if and when you modify the engine with carbs and cams, have your old one recurved or lock it out and use the MSD