[GTO-relic]

Originally Posted by PontiacHO

Been wondering if changing the mechanical advance springs with lighter springs on my stock 350 with a HEI distributor would make any noticeable improvement just for cruising? I assume that the first thing to do is see how much timing and determine when (RPMs) that the timing is all in. Most everything I’m seeing is recommending 36 degrees all in by 2,600 to 2,800 RPM. Comments and experiences are appreciated.

NO- DON'T DO IT. Here's why:
the springs on an HEI are also the advance limiter mechanism, if you put the weakest springs in, it will advance too far, cause a ping, and when you back off timing to remove the ping, it will have no power and be a dog- because you won't have enough initial timing for strong tip in and idle/off idle. that car will stumble if flat boarded because the timing is too far back in the lower rpm range to take the throttle opening and fuel.

HERE'S WHAT TO DO: buy a Mr. Gasket etc. advance kit FOR HEI. Put the heavy weights in, and the STRONGEST SPRINGS- not the weakest. Start the car and set initial timing at 20 degrees BTDC with vacuum advance disconnected. Yes 20 BTDC. I use a timing tape. When you then rev the motor higher with the timing light attached, you should see 36 degrees total- the strongest springs limit additional advance to 16 degrees centrifugal, at least on my car they did.

20 INITIAL + 16 CENTRIFUGAL = 36 TOTAL
Leave vacuum advance disconnected and PLUGGED. Don't use it- You no longer need it.

setting total with a factory HEI is tricky. It was an emissions distributor for reliability, low maintenance- never meant to be a performance distributor. that's what you have to do, the car will run brutally strong then. I can be idling along in 2nd or 3rd (M20 4 speed) and flat board the gas, it will blow the tires away. depending on your static compression you may/may not have to run premium fuel all the time- 93 octane, or better yet 91-92 octane non-ethanol. to be honest you should only run premium non-ethanol in any old car you don't drive daily with a carb, and anything else with a carburetor for that matter, even power equipment. or you'll be cleaning carbs out yearly.

my 455 is 9.3 compression with a McKellar #10 cam, and until I did the above, could not get the car to run right across the power band. Now it does run great. If you want to use less initial timing, try the medium springs, but then you'll have to RETARD the initial timing, and the medium springs will advance MORE than 16 degrees. I tried the lightest springs and NO DICE- even though lightest always worked great in an original points distributor converted to Mallory Unilite. that's because a points unit has an advance limiter bushing to stop timing no matter what springs you use, an HEI does NOT have a advance limiter bushing. the strength of the springs is the advance limiter on an HEI- that's why the factory had such strong springs on it to begin with. the relationship between initial and total timing on an HEI from the factory is fkd up. that's 1 more reason why the 1975-79 Pontiac 350-400-455 cars were dogs from the factory. then again their compression was so low they wouldn't ping on any fuel. but they made like 200HP. those cars only had 8:1 or lower compression.

the heaviest springs in the Mr. Gasket kit, are still weaker than the factory GM HEI springs, on the unit I have.

[Schurkey]

Originally Posted by GTO-relic

HERE'S WHAT TO DO: buy a Mr. Gasket etc. advance kit FOR HEI. Put the heavy weights in, and the STRONGEST SPRINGS- not the weakest. Start the car and set initial timing at 20 degrees BTDC with vacuum advance disconnected....

20 INITIAL + 16 CENTRIFUGAL = 36 TOTAL
Leave vacuum advance disconnected and PLUGGED. Don't use it- You no longer need it.

Well...everyone has an opinion.

Some of them are just plain wrong.

[HWYSTR455]

It's easy to map your' existing advance curve with timing marks and a timing light. Just create a 2 axis chart on graph paper with timing on the left, vertical column have timing, from say 0-50 in 5 degree increments, and the bottom axis RPM from 0-6000 in 500 increments.

Hold the engine a each RPM point and read the timing with a light, and put a dot on the chart. Keep going until you have the lower axis completed, then connect the dots with a line.

This of course is done w the vacuum can plugged off.

Once done, at idle, you can use a hand vacuum pump on the vacuum can and slowly pump, going a half inch of vacuum at a time, and read the timing increase with the timing light.

EDIT: You can create a graph for that too, left vertical column do timing, and bottom column do inches.

Simple. No dizzy machine needed.


.