View Full Version : Ignition Timing

06-02-2013, 04:21 PM
What are disadvantages to over advanced timing? was running my 440 in the 12-14 area bumped up to 16-18 for better throttle response just wonder if its ok to stay there or will it cause engine failures?

John Kunkel
06-02-2013, 05:18 PM
The numbers you're talking are initial advance, what's the total advance?

The downside to too much advance is detonation. Even if you don't hear detonation it can be present; reading the spark plugs is a good clue.

06-03-2013, 02:59 PM
Exactly. You have

1--initial advance, IE where it is that you set idle timing. "More" of this to a point helps low end throttle response. This was scaled back dramatically in the 70's "smog" era, even to AFTER TDC settings on some engines

2--The mechanical curve in the distributor. This is "how fast" it advances controlled by one or two springs, and "how much" it advances, controlled usually by the length of the slots in the mechanical mechanism. The amount of this advance is ADDED on top of initial

3--Vacuum, in most modern cars off "timed" or "ported" vacuum which means no vacuum at idle, and because of the nature of the port, vacuum also drops at WOT (open throttle.) This means vacuum is only active at part throttle, light loads, "at speed." It is for increased fuel mileage. Vacuum advance is ADDED to whatever initial + mechanical is at the time. Total of the three on some engines might be over 55 degrees "cruising" down the highway.

SO how do you add all this together?

A--Initial--this depends on the build up of the engine, compression, cam, etc. You need to play around some. GENERALLY, setting idle timing for highest vacuum reading on a warm engine, and double checking carb idle speed and mixture, then re-checking timing and vacuum reading, will give you a good place to start

B--mechanical curve. If you are trying to adjust initial, above, with a factory smog distributor (anything made after 1967) you WILL find that the advance curve is too long and too slow. You WILL be "over" advanced. Your ONLY alternatives are to either buy a performance distributor with a shorter/ adjustable mechanism, or to have your stocker "recurved". This is done by brazing/ welding up the distributor slots, and then filing/ grinding them to specific dimensions to limit advance, and, last, install weaker springs

C--Vacuum advance. These days, about the only thing you can do is to look around and find an earlier, "non" smog advance "can" or on a "street car" that you don't care too much about, do without it.

GENERALLY a performance dist. you want some 12-15 degrees initial on a "mild stocker" and as much as 20 initial on a hotter cammed engine. Some racers run a "locked" distributor with NO advance, and so "the timing" is set right at "total." A performance mechanical curve might be somewhere in the 16-22 degree range, now, I'm referring to CRANK degrees here. On my was 360, now 318, with a mild cam "about" the same as a 340 stick cam, I run about 15-17 initial, 20 "in the distributor" for about 35-37 total. Some engines, today's fuel, won't tolerate that much advance, and "like" more in the 34 range, or sometimes even less.

Most of the shop manuals will specify distributor specs in "distributor" degrees which is HALF of crank degrees. If for example you tear down a stock Mopar dist, the degrees advance is often stamped right on the (sometimes bottom) of the advance mechanism. This is DISTRIBUTOR degrees, so if it says 15, this is NOT a "nice short" performance curve, but rather THIRTY crank degrees!!!!

http://image.moparmusclemagazine.com/f/17054031/mopp_0301_08_z+mopar_electronic_ignition_system+ad vance_plate.jpg