Timing Question

[CaliforniaCobra]

I have a 393 Windsor Stroker. It has an MSD Digital 6 ignition and a Mallory vacum assited breakerless distributor. The vacum hose is connected to the timed port (upper) on a Holley 750.

With the vacum disconnected, I have 12 intial and 34 total time. When I connect the vacum the intial stays at 12 bit the total reads 45+. Am I reading this wrong? Any recomendations?

CaliforniaCobra
My Buildup

[Jim Dandy]

Jack Roush says "Set the advance curve to be as quick as possible before the engine begins to ping during acceleration." Bill Caroll says that 10 degrees intial is good but total ignition lead over 38 degrees "will be totally dangerous". I don't know what type of cam your running or the vacuum your pulling but if you don't see a change when its hooked up I'd check the vacuum at the carb. or the vacuum canister. 45 degrees sounds like alot of advance. Good Luck........Jim

[webebob]

It looks good to me!
Put the vac gage in the car where you can see it, and go drive. You will soon see why all is OK.
At high power settings, the vac drops and the vac advance goes completely away. Total timing comes back down.
The extra timing above normal total timing while cruising is what the vac advance is supposed to do. It is good for 2-3mpg. My timing tops 55 deg on the hyway cruising.
One thing you should check in a hi-perf car that has vac advance. With the engine off, distrib cap off, suck on the line going to the vac adv and see at what vac setting the vac advance starts moving inside the distributor. Then put everything back together and go drive with the vac gage hooked up where you can see it. Check under hard acceleration, as the rpms come up, that vacuum never comes up high enough to start advancing the vac advance. This would be real bad. Most people that have this problem, diconnect the vac advance. The solution I took was to hook a switch at the throttle that disconnected the vac adv at half throttle. Either way, having the advance pull on at full throttle is not OK.
Bob

[SCOBRAC]

How do you get on the fender to check the timing at highway speed to verify 55 degrees???

[SCOBRAC]

Seriously though.. John. 45 degrees is way too much how did you measure that an advance light or ? at some point 2800-3200 rpm you should see it stop. Regardless whether you use a mechanical or vacuum advance it should top out at 33-38 degrees..

[CaliforniaCobra]

Great responses! I have learned a lot more about timing than I expected too.

Basically, I have learned that it is not uncommon or really wrong to see such high numbers on a vacuum assisted configuration. I did test the distributor and found it finiky, sometimes releasing the extra vacuum and sometimes not.

Bottom line is that I will replace out the vacuum assisted advance for a pure mechanical one.

The best reading that I found so far came from an unknown source which I posted at the following address:
http://www.nekoweb.com/demo/timing.htm . Good reading and thanks for all of the help!

[webebob]

It isn't either/or. The vac Advance is added to the mechanical advance. It does add to total timing which is why you have to unplug it to do ANY timing. When your done timing the engine, you plug it back in and it adds to what you have. It also completely goes away under load. All (non computer controlled) engines with Vac Advance also have Mechanical Advance. Not visa versa.
45 degrees is too much total timing, but not too much total plus vac advance. But don't time your engine this way. Disconnect the vac advance to time your engine.
Bob

Originally posted by SCOBRAC
Seriously though.. John. 45 degrees is way too much how did you measure that an advance light or ? at some point 2800-3200 rpm you should see it stop. Regardless whether you use a mechanical or vacuum advance it should top out at 33-38 degrees..

[CaliforniaCobra]

All,

I need to clarify here.

Without the vacuum attached, timing is 12 degrees initial with a total of 34. With the vacuum attached, it goes up to 45+

Sorry for the confusion.