[Schurkey]

I have searched, and I found a chart posted on this site for HEI centrifugal advance curves based on using different center plates with weights marked "139".

What I want to know is: What are the part numbers (and sources) for the various center plates?

That chart I found is at http://216.178.81.108/forums/showthr...=Advance+Curve :

Centerplate=======Weights=======Dist.Degrees
368=================139=============10======
406=================139=============12======
349=================139=============15+=====
371=================139==============9======
489=================139==============14=====
375=================139==============13+====
364=================139==============12=====
397=================139==============12=====
370=================139==============12=====
346=================139==============11=====
396=================139==============11.5===
398=================139==============10.25==
383=================139==============12=====

Weights are easy: Any NAPA store can get you the 139 weights: Order DP115. Weight pivot pins are DP109, the little plastic pivot bushings are DP112.

[Schurkey]

Played around with the Sun machine today. Made an interesting discovery.

I'm working on an Olds HEI, but it's the same rotation as Pontiac, so the results should be the same. No guarantees on Clockwise rotation distributors--I'll screw with them later. (Chevy, Cadillac, Buick are CW. Olds and Pontiac are CCW)

I gutted the centrifugal weights and center plates out of my "core" distributors. For today's post, I used the same set of weights (130) and springs (aftermarket lightweight stuff) but I installed a small variety of center plates. I put each center plate in with the numbers "up", and then re-installed each one with the numbers "down".

Using the same weights and springs, the center plates made a major difference in the overall advance curve--just as you would expect from the table in the first post. Both the amount of advance, and the RPM of start-of-advance, and full advance, changed.

Here's the scary part. In every case, when the center plate was installed with the numbers "down", the advance curve would seem to be "normal". It would begin advancing similar to the others, it would seem to quit advancing similar to the others. You'd think it was just another advance curve. BUT!!! after the advance "seemed" to be all in, the advance held pretty steady until the RPM doubled, AND THEN IT ADVANCED A BUNCH MORE. So with my lightweight springs, I was getting full advance at about 2000 engine RPM, give or take a few hundred. And it would hold steady at that figure--perhaps even retard a degree or two because of the pickup coil effect--until about 3500--4000 engine RPM. From 4K to 8K it was into the secondary advance curve which might add another 10 to 20 degrees!

If you didn't check the mechanical advance ABOVE 4K, you'd never know this extra advance was happening. Think a bunch more "mystery" advance could cause engine problems at six grand? I do!!

On each of the center plates I have, one edge of the center plate has a smooth curve, while the other edge has a "dogleg" curve. The weights must act on the dogleg curve, NOT on the smooth, gradual curve.

You might want to verify how your center plate is installed, if you're running an HEI.

I'm still looking for part numbers or a source for the center plates.

I ordered some 139 weights from NAPA yesterday. They claimed there are only four left, and "they have to go to the manufacturer" to get them 'cause the warehouses don't have any. The only way this makes sense to me is that the weights are discontinued and I got the last ones. I hope I'm wrong.

[BVR421]

what happened with the numbers up? Did the advance stay steady? I always thought the numbers went down on a CCW dist?
What fun! having a dist machine to play with in the winter!

[Schurkey]

I tried 5 different center plates. Each one, with the numbers "UP" resulted in a curve that had a definite "end"--it advanced so far and then stopped. Each one, with the numbers "DOWN" had a "primary" curve that looked fairly normal, stopped advancing at a reasonable RPM, but then had a "secondary" curve that started above 3500 RPM and continued as high as 8K.

[head #96]

I've noticed this too while checking timing with the plates facing down. Curve will stop at my target 32 deg. than as revs increased timing would slowly increase untill I chickened out. Then I read an article by Rocky R. about this and flipped the center plate and timing curve stopped at desired rpm.

Glad to see some research to back this thou.

[P@blo]

In what arrangement were the center plates of factory HEI units , numbers up or down ?. Also would the use of the light/weak aftermarket springs allow the weights to reach an over advanced position as the springs that these weights/cam were intended for were heavy rate. Great post

[form406]

great post considering i was just studying the three distributors i have..the weights on both the stock pontiacs obviously have more mass than the rebuilt aftermarket i have in the car...im wondering what the best curve i can do at home would be for my over cammed 406...timing in fast or later...im not running vacuum advance, but im going to start toying with that again in relation to the distributor... at this point if i did run vac. adv. i would just want around 5 degrees adv. at idle...as far as i know...current initial is 14 btdc..ive tried lower but it runs best like this so far...i know my set-up isnt matching but this is what i have to work with for now...

[Shadowjack]

Rocky Rotella went through all this a couple of years ago and there were a couple of articles in HPP or somewhere about it. I'd lay odds that's where your original chart came from. Lot of discussions here and on the CP board about his conclusions about turning the center plate upside down as well.

[Rocky Rotella]

I planned on staying away from this topic since it had drawn much controversy in the past, but those that had the differences have since worked them out. I have also been asked by two different PY members to weigh in on the subject since it follows what we had found and reported in the May 2004 issue of HPP magazine. So please view that the following as only my opinion based on my findings. However anyone might choose to modify their HEI is at their discretion.

The chart in the initial post was provided by Sun Tuned several years back. A quick search on his name will reveal his HEI expertise. His chart reflects advance curves when the centerplate installed numbers down. Our chart that appeared in the article reflects maximum advance with the centerplate installed numbers up.

GM originally installed the centerplates of CCW rotating distributors with the numbers down. And this could be considered the “correct” way. But we, like Schurkey, found that reinstalling the centerplate so its numbers face upward can oftentimes provide a usable advance curve. It may be considered “wrong”, but it offers another significant benefit. The flat portion of the centerplate acts as a positive stop preventing any additional advance at high rpm.

We have seen several centerplates (when installed numbers down and combined with moderate tension springs) provide a specific amount of mechanical advance around 3000 rpm. And as engine rpm increases towards 5000, mechanical advance stays steady. But certain centerplates can advance as much as 6 additional degrees when engine speed exceeds roughly 5000 rpm. Imagine the catastrophic results that could occur from 30 degrees of MECHANICAL advance under heavy-load at high rpm if a respective engine is pushing the envelope of compression on pump gas. Not to mention the lost power from negative piston work as spark occurs earlier than necessary.

The over-advance conditions (that occurred past about 5000 rpm) were likely not an issue on stock engines since most of those with factory-installed HEIs were unlikely to rev much past 4500-4800. And some of those centerplates that suffer from this are listed in Sun Tuned’s chart with a “+” after the advance amount. One way to positively prevent additional advance is to weld or braze the advance slot limiting the timer core’s travel. Jim Hand’s book offers a clear illustration of this on page 115. This will allow the use of virtually any centerplate that provides more maximum advance than the desired amount.

So in the end, run the combination of weights and centerplate that your car seems to perform best with. Just be sure to keep an open mind while testing and listen intently for signs of detonation. And if you find it uncomfortable to tinker and test a variety of advance curves, Sun Tuned offers a recurving service. Simply send him your current HEI and he’ll send it back matched to your combination and ready to drop in.

[Junkyard Dog]

Does Suntuned still rebuild HEIs?

Is there any good info out there on rebuilding your own (bushings and all)?

[Rocky Rotella]

JYD, I know that Sun Tuned was adding shop area onto his house specifically for this. So as far as I know he is still doing it. Try contacting him at [email protected]

[Sun Tuned]

Was building new shop, then sold house to friend/employee and then bought another house also without shop space. Still remodeling for the wife and kids, and am underway for new detatched shop. Likely done in march/april. Not doing any dist now. Too many other irons in fire at present. Honey-do's, hunting season, work, etc.

Could braze the slot, or if lacking oxy/acetelene, could use a fillister head screw with diameter reduced to reach desired amount of advance. Used as a stop for the adv plate itself. Both work well but you gotta watch timing bounce as pins or plate slap up against the stop if using really light springs. Usually see this on circle track cars where there is constant on/off throttle application at high rpm. Probably be fine on street car though.

If you're gonna use a metal stop of sorts, then shape of the adv cam is really not a big factor insofar as fast ramp or slow ramp, because the metal stop is whats going to limit the total advance. However, if that being the case, the adv cam with the fattest ramp will usually be the best to use. The fatter the ramp the more gentle or gradual the adv rate will be and if you can get one that will span nice and evenly for say 1000-1500 rpm versus the number of centrifugal degrees you're looking for, depending on the application, you'll then make the most power.

[P@blo]

Great info Rocky and Sun tuned. I guess since my center plate #'s face down I better take out the one light aftermarket spring and put both med. rate springs. At least until I can do the brazing outlined in Jim hand's book this summer.

[form406]

for me this is one of the better posts as i have not addressed this dept too much...