Mechanical advance distributor

[magsnubby]

I have a Mallory Unilite mechanical advance distributor in my T-Bucket (stock GM 350 crate engine). I got the timing set fairly close on it when i installed the distributor but i need to put the light on it. How do i set the timing? Same as a vacuum advance?

[narlee]

Same way.

[moparforlife]

All vacuum advance distributors have mechanical advance, so it's the same way. The vacuum advance is added to the mechanical advance to aide in driveability and fuel mileage.

[Homespun91]

I have ** never ** found any dial-up timing light to be accurate, including Snap-On's versions.

There may be one out there somewhere.

In response to the original question- when setting the timing with a vacuum advance distributor, nearly all OE instructions will tell you to remove the vac. advance line & plug it. (There are a couple of weird exceptions.) So in that sense, you set the timing identically for your distributor as well.

As 440 noted, you are concerned with three basic aspects of your timing. (1) initial; (2) centrifugal (and total); and (3) the RATE of advance.

When discussing total timing, that normally refers to the initial timing plus centrifugal; vacuum advance is ignored until the end. Again, as 440 said, having 36 degrees "all in" by 2600 rpm is considerably different than 36 degrees "all in" by 3600 rpm. The weight of the vehicle, gearing, clutch/converter, torque curve of the engine, desired quality of fuel, etc. all play a major role. You didn't say what SBC crate engine it is, but a starting point for a light vehicle such as yours, running on 87 octane, might be about 12 deg. initial & 24 centrifugal, all in by about 2800- 3000 rpm.

There are great gains in power & response to be made from setting ignition curves correctly...or lost power from incorrect settings. The nice thing about it is that it doesn't cost much of anything $$-wise.

[holeshot]

MAGSNUBBY...well there is 1 differance, you don't have to remove the vacume hose and block it. but i assume you knew that. and when using the timing light, keep the R.P.M.s under 600 or the mechanical advance will kick-in...POP.

[29nash]

Originally Posted by magsnubby

I have a Mallory Unilite mechanical advance distributor in my T-Bucket (stock GM 350 crate engine). I got the timing set fairly close on it when i installed the distributor but i need to put the light on it. How do i set the timing? Same as a vacuum advance?

Yes

[Homespun91]

Originally Posted by 440roadrunner

This was so well said, I hope you don't mind my editing it and repeating it.

Hah! Well, my wife & kids think I'm an idiot, so I'll cherish that comment for a few weeks.

Keeping on the same theme, engines that are under greater loads such as hauling, towing, and so forth, or are in heavier chassis, may want a slower advance rate...and, possibly, a different amount of total timing.

Combustion chamber design plays a part in all this too. Some designs won't tolerate as much timing as others (all else being equal); for example the 351C 4V closed chamber can tolerate (and likes) more timing than the various pre-Vortec SBC designs. Another way to look at it is that, often, modern chamber designs don't need as much timing to produce the same amount of power.

That's why playing around with this stuff can be so rewarding.

[Hnstray]

Very nice commentary by BOTH 440roadrunner and Homespun91......very useful info for all of us. Thanks!

Ray

[magsnubby]

Originally Posted by Hnstray

......very useful info for all of us. Thanks!

Yep. Thanks guys.