Stock GM HEI Distributor
 

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.

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.

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.

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…

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

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…

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

Terrible little package of junk.

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

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.

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.

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.

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.

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…

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.

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.

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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Good stuff.

Well...everyone has an opinion.

Some of them are just plain wrong.

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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I like the second one in your list as a good starting point. Of course every engine is a little different. It seems in general, the old urban legend of having all the mechanical advance in by 3000-3200 just isn't ideal with the low quality fuel we have at the pump now. I have been limiting the vacuum advance to 10-12 degrees on modern pump gas. I realize that's not ideal for best economy but I am not willing to run on the edge of detonation.

Most of the engines I've had over the years I limited to 8-10 degrees vacuum, for a combo of reasons, but mostly for 'cushion/safety'. It's what they like.

Most of the engines also liked a max of 32 total, even then aluminum head ones. I think the current engine I have in GTO w e-heads is a total of 34 WOT, but it's SEFI. (never ran a low-SCR engine though).

I've run 16-18 initial on almost everything, with mini starters, make up the rest with centrifugal, all in by 3200. I run 91-93 exclusively.

Of course elevation plays a part, I'm in the sea level - 1200' range.

With iron heads and a 150-175 shot I didn't need to pull out any timing even. Plugs proved it.

Ever engine's need is different for the most part, but all the ones I've built from the early-80s up have been fine with the above.


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Well the iriginal posters question was about an HEI, nowhere in there did he say anything about a points distributor, so no need really to extoll the virtues of how they work as far as his original question.

One must first fully understand the engineering behind why these things function the way they do in their intended application before someone goes modifying things for a different end result, good or bad.

That said if you believe it is in fact the springs only that limit an HEI… well then I’m really just not going to waste my time trying to teach the theory of operation on it. I’d suggest a search and about 5 hours of seat time on this site on the subject. The info is clearly there alright.ill save you some time though…. The wrong spring too far left or too far right CAN affect it sure enough , but it’s in fact the advance cam itself that does it as originally intended and not the advance pin as on the points distributors and even that is a different topic for a later date too lengthy for discussion here today.

About the best info I’ve read on this particular thread so far that I believe is post number 15, 17, and for sure 19.

Car came with a cheap aftermarket HEI which I replaced with another cheap (Cardone) distributor. I understand timing and the benefits of advance (mechanical and vacuum) and realized that for a stock 350 it’s not going to be an earth moving experience to allow earlier engagement of timing advance. A standard points distributor and coil would be preferred but the wiring has been changed and I’m really not inclined to re-wire for a points distributor. This HEI is about 1/4 inch from the firewall and it’s hard to even loosen the distributor hold down bolt. Saying all that to say I’m just going to stick with what I have and I thank you all for your knowledge, help, and suggestions.

Why would you argue with "that" guy about distributors ?

No worries.

We’re all here in some form or another to learn.

I a very broad , and I mean wide latitude , he’s right. However he’s not exactly sure as to why. Don’t take my word for it, it’s in his answer.

If you look at the thing as a complete engineered solution, or a total package if you will, you’ll see what I’m about to say is true.

We can take one of those distributors Kenth listed and I can supply you light enough springs that the weights override the center cam and dump a load of degrees extra over what Delco rated the distributor for in the first place, consequently I can find you some hood springs that won’t allow the weights to reach half of what Delco said it would.

So in that he’s right, but now you’ve gone completely outside both ends of the design. And now “you” become the engineer.

Everyone pretty much knows at least the ones that have been on this site for the last 25 years that the advance cam is the limiting factor on these HEI’s ….unless you do something goofy with the springs. And there are some on the list that the factory springs can’t seemingly control either, but those were actually limited by the engine rpm that the engine in question was designed as from the beginning. No one ever was supposed to turn a 4.3 Chevy V6 8000 rpm in a stock 1500 Chevy.

If you did GM would’ve considered you on your own. However you put that distributor in something that will turn 8000 , well your still on your own, you’ve now exceeded the original design parameters of the intended application.

I’ll not address the recommendation to disable the or (A) vac advance for a street driven car or even about 50% of the race cars I’ve been around as that idea or concept purely comes from one of two things or a combination of both….
Lack of understanding or lack of proper calibrated parts to begin with.
And I understand full well the implications of what I just said there cause there will be a useful example coming to illustrate if needed.

I’ve shown about 4 customers their distributor as they brought it to me on the machine without any modification other than their installed Moroso curve kit. All 4 demonstrated approx some 55+ crank degrees if centrifugal advance going exceedingly faster the more the rpm was cranked….

I don’t have the luxury of wondering how far they’ll go, I’ve watched it and fixed the correction for them mor times than I care to count.

I still don't understand why so many are stuck on HEIs, they suck. Guess people are just lazy, they think it's an 'easy' fix because it's 1-wire.

So you go from 60's tech to 70's tech, um, we are in the next CENTURY, step it up for cryin' out loud.

Modern dizzys have the springs/weights/curve all laid out for you, RTFM.

For that matter, just go to a programmable. Many allow curve tuning direct from you friggin' cell phone. Just break outside of the box! Stuck!

Yeah, losing my patience I guess. Trying to make old tech work like new tech is like futile. Spend your' energy on other stuff, move on, get with the program.

Simple systems of an engine, fuel & spark, just treat like individual systems. You setup a flexible fuel delivery or ignition system, it will last you your life time. Doesn't matter what the 'pump' is, those systems can manage whatever you throw at it. Why keep doing it all over & over again!?!

Everybody keeps doin' chit with 'pusher pumps' etc, blows me away. Build a complete, flexible fuel system and be done with it.

'I want it to look stock'. Ok, well, then don't complain when your chit falls down at the strip, duh.

Can't have it both ways, at least without spend a ton of money, and for what? So a factory mustang/camaro/vette can crush you're chit at a stop light on Saturday night? Embarrass your date?

Have to quit regurgitating the same 'ole same 'ole, just come out an say it. If people can't take it let them figure it out on their' own...


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Honestly I figured that it was just easier to buy one of the many RTR distributors on the market vs dealing with whatever version of HEI the previous owner swapped into my car and I’m happy that I did. Super easy to follow the directions and build a curve with the supplied parts and then verify with my timing light, one afternoon worth of work and moved on to with my life.

There are other ways to look at it. Taking a simple task and making it complicated with electronics ect is kind of backwards. Electronics will always fail at some point. More failure points.
Once you get a distributors curve set up its done and will work just about forever. Its also part of the deal with working on old cars.
Your progression distributors will not get you any more power than a properly set up standard unit.
Simple task, usually best getting it done with something simple.

HEIs are electronic, are you suggesting to go to a points dizzy? That's backwards IMO.

'Electronics' are much better than they were 10, 20 years ago, and at some point you will have to put some type of trust into them. Do you own a new(er) car?

If you're making 600hp, for example, you can't run a mechanical pump.

I think the RTR dizzies are a good solution. Reliable, easy to setup, small cap, and you can run it on your next 3 builds not having to worry about needing an ignition system. 3 wires, two go to coil one goes to engine ground. It's not rocket science.



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If you're so much into the next century then why even run a gas engine or Pontiac at all- electric cars can smoke any Pontiac ever made. Watch what you're asking for, you just may get it someday when they BAN gasoline cars for hobbyists altogether. This is a nostalgia based hobby- wake up and smell the coffee. You don't need all that high buck bling for the average street car making 400-600 HP level on pump gas, an HEI would easily handle it. If you were running NHRA Pro Stock or NASCAR then yes, you'd need it. But then you wouldn't even be posting here at all, and not even running a Pontiac. The "next century tech" you speak of DON'T RUN Pontiacs ! So you're carving out one little thing you like to tinker with, and saying the whole hobby should upgrade to it. Where is all that stuff made ? Mostly in China, Mexico. How good can it really be. Where will it all end up someday ? In the schitcan. But those old SD421, SD455, RAIV, RAV heads will be like bricks of gold 50 years from now. I'm saying this respectfully...you can soup up a fishing pole to the latest, greatest too, but do you really have to, and you're not gaining much in reality. It's still just a damn fishing pole- in 1800, or 2026. Is the shrinking drag racing scene with Pontiacs really worth all that high tech whiz bang stuff in the car, when the problem REALLY IS the tracks are all closing, changing to only 1/8th mile, and the participation is on a steady long decline. In that respect, for a street car or mild race car, I'd stick with a free GM HEI and make it work. They sure as hell don't suck, there's many strong running cars out there with HEI's in them- they powered all the 1975-79 Trans Ams that are now being gobbled up, restored, sought after. You're investing in something that has a zero or negative ROI in the long run with electronic upgrades, that in 5-10 years are obsolete and worthless with zero collector value- in a hobby that is 95% about OEM nostalgia, collector value. The number of true Pontiac hard core racers today is probably less than 5% of the entire hobby, compared to the street cars out there running HEI's and still running points ignition. Racers may be 1% of the entire hobby. Get a reality check the tracks are not filled with Pontiacs racing lately- and there's almost no tracks left at all. Where ya goin' in the long run with all that ?

Because the big difference between a points distributor, and an HEI, is the point distributor has a centrifugal advance limiter bushing in it, and the HEI does NOT. What limits centrifugal advance in an HEI is the very heavy advance springs- the springs serve 2 purposes, control the RATE of advance, but also the LIMIT of the advance. On a points distributor, the springs only control the RATE of the advance. The points unit will advance until it hits the bushing stop, so the centrifugal advance timing added at high rpm will always be the same no matter what spring is used, it'll just hit the stop at a different rpm. The factory HEI with OEM strong springs are so strong, the centrifugal advance will never reach full advance point within the rpm limits of the engine. If you use the weakest or mid-level springs in a kit, it advances TOO FAR, there is no stop bushing. If you use the factory springs, it doesn't advance far enough. So you have to use a slightly lighter spring than stock, and bump the base timing up to 20 degrees to compensate, and get 36 degrees total timing in within the rpm limits of the engine. Once you realize what you're up against there, it's an easy fix. Drop the strongest kit springs in, with the kit weights, bump the base timing up to 20 degrees, leave the vacuum advance off, and forget it. You have 36 degrees total and the car is a rocket. It's actually easy as hell. I've been working on Pontiacs 45 years now- the best tweaks are usually the simplest, the hard part is understanding the problem first.

37 years ago...SuperStock SD455- tweaked distributor, fuel pressure- 597HP, 543TQ, with a Qjet, GM block, crank, rods, heads, Nash intake. RIP Don Kennedy...
You don't need "next century" high tech....why not just take that exact same 1989 motor, put a Victor on it, with an 850 Holley, and set of E-heads, or KRE, DCI, Tiger, etc. heads...what would the distributor, stock block engine make then- with the same ignition. It doesn't take a genius rocket scientist to realize, it'd be banging on 650-700HP.

https://www.hotrod.com/features/supe...-982-1434-68-1

"Reducing the spark-plug gap is one way to check whether there's an ignition problem. With an effective ignition system, drag engines typically make the most power with .050- to .070-inch plug gaps. When the plug gap was narrowed from the .055-inch run up to this point to .040, the engine picked up another eight hp on the top end. More significant, the engine gained another 35 lbs.-ft. of torque at 4750 rpm, and the overall curve fell off much more slowly on each side of the torque peak (2B). At this point, Kip's crew found that the timing kept "bouncing around" and that the distributor rotor was not correctly phased. Kennedy was using an Accel single point distributor triggered by an MSD-7, not having had time to perform the custom mods required to adapt a state-of-the-art crank trigger to the "nonstandard" Poncho motor. As an interim fix, Kip corrected the rotor phasing and erratic timing problem by sectioning and welding the advance plate to the shaft. This eliminated the vacuum advance curve, which is not really required on a drag engine that launches and runs at rpm. Repeatability returned to the extent that small timing changes could be accurately evaluated.

So far, all tests had been run with 36 degrees total timing. Now, with the plugs again gapped at .055 inch, a series of runs were made, starting at 32 degrees and advancing the timing in two-degree increments per test until the power fell off. At 32 degrees, the engine was down 64 hp and 42 Ibs.-ft. from the best previous run, plus the horsepower and torque peaks declined by 250 rpm to 6000 and 4500 rpm, respectively (2C). Power increased as the timing was advanced until, at 42 degrees, the engine's output zoomed to 581.2 hp at 6500, and 552.4 Ibs.-ft. at 4750 (2D)! At 44 degrees, power fell off dramatically, and the test was terminated early to avoid detonation damage."

Well, I am a hot rodder, car hobbyist, and I will do me, personally don't care how others do it themselves. But I am interested in everyone's approach, and respect other's opinion. There's really no 'right' or 'wrong' way.

I don't give a crap about ROI, but, one thing to consider is Restomods and Pro-touring cars, which usually have a much higher sale point than a factory stock configuration, with possibly an exception to 100% factory correct "investor grade" cars. But those you don't drive...

Modern engine swaps are also very popular now, and one reason some do that is the perception of 'reliability'. Those cars too are demanding considerable money (ROI).

Racing, well, there too, categories such as F.A.S.T. and other similar forms are growing, and new tracks are opening. It's arguably more popular now than it was in the 80s-90s or even later. Once closed tracks are making a comeback too.

Let me also point out that the article you posted they used an aftermarket distributor and an MSD box, along with a number of other non-Pontiac OEM parts, including a Holley carb, larger fuel lines, and an ELECTRIC pump.

Let me suggest you further educate yourself on timing curves and how an HEI works, since it's obvious there's a gap there, pun intended.



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