[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.

[Sun Tuned]

It sure can be worth it.

You are correct, see where you are first, then make a change and recheck.

A lot of, but not all Pontiacs distributors are in the neighborhood for amount of advance, but the “Hood Springs” they shipped with need to be addressed.

Along with some limitation of the vac advance unit for sure.

Fairly reasonable gains can in fact be had at much of anything under 3500 rpm, especially at cruise.

[Sun Tuned]

There is an easy 25 hp increase under there off the brake or clutch up to arpund 3000 or so when you get it right.

That’s really nice cause that’s where you’re going to spend 90% or better of your driving anyhow. So it’s very useable.

[PontiacHO]

Good to hear. It makes digging that HEI cap out of there seem worth it. HEI isn’t a easy fit on first generation Firebirds…

[PontiacHO]

Well scratch that…. The rotor button screw rounded off in this cheap offshore HEI distributor. For some reason the screw that would break loose was torqued to 75 lb ft. The garage is in the full sun so it’s probably 110 degrees in there. Tired of standing on my head. Sending the Moroso advance kit back. This car fights me so much that I’m almost at the point of crying uncle and slapping it on FB Marketplace. Venting here but I thought this would be an enjoyable hobby?…

[65 Lamnas]

Originally Posted by PontiacHO

Well scratch that…. The rotor button screw rounded off in this cheap offshore HEI distributor. For some reason the screw that would break loose was torqued to 75 lb ft. The garage is in the full sun so it’s probably 110 degrees in there. Tired of standing on my head. Sending the Moroso advance kit back. This car fights me so much that I’m almost at the point of crying uncle and slapping it on FB Marketplace. Venting here but I thought this would be an enjoyable hobby?…

If it’s in the budget, you can get one of these and enjoy setting up the curve with the hood shut.

www.progressionignition.com

[PontiacHO]

Those are nice but for my cobbled together 350….not needed. Just thought I could do some fine tuning with what I had. Slapping it back together and forget about it…

[Sun Tuned]

Best move you could make is sending the Moroso curve kit back where it originated from….

Terrible little package of junk.

[PontiacHO]

Like I was taught 50+ years ago by my very practical Dad, “don’t mess with it if it ain’t broke”….

[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.

[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.

PS- GET A FACTORY GM VINTAGE HEI UNIT. I had to put a new module in mine, and also check the CAPACITOR, and pickup coil for fraying of wires in the distributor. I had one that had a leaking cap too.

[PontiacHO]

Thanks. It’s buttoned back up. It runs smooth and is actually better than when I started on this journey 2 years ago after refreshing the cam-valvetrain and having the heads rebuilt.

[Sun Tuned]

The springs only control the rpm of the start of the advance and the rate with which it advances.

They have absolutely nothing to do with the amount of advance, that function is solely controlled by the particular advance cam that is installed upon the distributor in question at the time.

Some 17 different advance cams were used for all Pontiac HEI applications and of that , 4 are of particularly good interest. The rest are rather so so. You have to watch these advance cams and worthy of note are the ones that allow an excessive amount or extra advance beyond what the springs will allow. By that I mean no matter what cam you use if the springs are so light(such as the hair thin copper colored ones in the mr gasket/moroso kit, excessive amounts of advance will be present as the center cam will be overcome by the springs that arent anywhere close to the correct tension and at that point the advance cam can no longer do its job as intended. We call this excessive override and it is in fact a very real issue using too light of springs.

There is a chart (2 actually), one that I compiled many years back and a second one that our very own Rocky Rotella compiled about some 6 mounths later that both show this info I am speaking of.

As far as it bears on this conversation since the Moroso/mr gasket kits were brought up, let me step out of cue here and address this one more time.

There is a factory GM weights and center cam are going to be the best ypu will lay your hands on ever, period. There is a steel is heat treated and they will ring like a crystal glass when dropped on the floor. As such they are very durable…

The aftermarket curve kits mentioned suffer two very serious issues right off the bat straight out of the package, and are why folks in the know avoid them like Ebola .

First they are of mild steel construction. Under operational conditions inside the cap they will rust and powder up far worse and much faster than the originals., which in case you were wondering is why the GM pieces are heat treated.

Second, unless they changed something the design of the weights are such that they could become hung up on the tips of each other upon deployment or retraction a slight amount if the weight hole and weight pin interface wasn’t ideal. Seen this happen multiple times.

A third unmentioned issue is the higher the rom you turn it the farther the advance keeps going. This is due solely to the design of the advance cam itself in these kits.


No telling how many have been sold over the years but I’ll tell you this I’ve filled 2 5 gallon Berryman carb cleaner buckets with them over the years correction the poor advance from these things.

I would avoid them if it were me and I will not use them here for these specific reasons.

[PontiacHO]

Not that I’ll revisit this (with this HEI anyway) what advance kit is preferred? My plan was to use the medium springs and not use the Moroso weights. Summit has a “recurved” HEI that looks interesting. For my stock 350, that’s probably overkill…

[Kenth]

Here are some Pontiac HEI distributors that i have found useful as is.
Chose initial for your engines needs, vacuum advance unit and your good to go.
Distributor rpm´s and degrees, double these #´s at crank.

[GTO-relic]

Originally Posted by Sun Tuned

The springs only control the rpm of the start of the advance and the rate with which it advances..

WRONG- with an old points small cap distributor they do, because a points distributor has a limiter stud/bushing in the top of it- but not with an HEI- because an HEI HAS NO LIMITER BUSHING. with an HEI the springs also control the total timing at higher rpm. I tried all 3 sets of springs from the kit, weak-medium-heavy, and also the factory springs, back to back- with timing light hooked up. Every time I put a weaker spring in, the car had more total timing at high rpm.
A street Pontiac with 9 to 9.5 compression needs all the initial advance it can get. the high initial timing setting with strong springs does that. why set timing to 9 to 12 degrees BTDC, when you can just dial in 20 and BANG IT'S ALREADY THERE the moment you hit it, because it's in the initial. NO LAG TIME- ZERO LAG TIME.
then the centrifugal puts in 16 more degrees advance as well. forget the vacuum advance if you're using an HEI, you don't need it.

[HWYSTR455]

Actually, the center cam & weights limit total timing on a stock HEI dizzy.

So it is true that the springs only control when advance starts and rate of curve.

Some aftermarket advance kits have bushings you slip over the weight post to control total, but it's been a while since I've looked at them.

Setting initial timing over a certain amount can cause cranking issues, and pretty sure 20 degrees the starter would struggle.

Most iron head Pontiac engine usually like 32 degrees total, and many of the aftermarket weight/spring kits can go to like 46-50 degrees, because they are generically based of a chebbie.

For a street cruiser, you want vacuum advance. And you want an adjustable can. Some of the aftermarket vacuum can kits come with a limiter cam to manage total vacuum advance, and that you want too.

If you go full time vacuum to the can, it can help with idle and idle cooling.



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[vertigto]

Good stuff.

[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.


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