HEI pulling timing. Any solutions?

[Tuner]

Those of you who continue to insist the .... "retardation of the timing with a magnetic pick up ... is caused by distortion of the AC sine wave generated by the pick up & the distortion increases with RPM" .... has anything to do with the pickup, I ask you this ... do you have, or have you ever used, a proper fast enough oscilloscope to identify the characteristics of the delay, and measure the delay time from trigger signal into the amplifier to spark discharge?

Have you ever triggered the same module or amplifier with different magnetic triggers, HEI, Mopar, Ford, Bosch, Lucas, etc, and found different triggers cause a different amount of retard? In other words, do you actually know from empirical testing you have personally conducted, or are you just repeating interneticisms ??

It's common knowledge HEI type distributors with their magnetic pick ups, pull timing at high rpm.
My question is, what exactly is responsible? Is it the actual module, or the magnetic pickup that's responsible?
Is there solution to the issue?
Prepping the car for this coming season, I figured I'd try fix it, if fixable. The system is a marelli ignition. It was factory fitted in many popular Italian cars of the 70s & 80s. It runs the same module as your standard U.S. 4 pin HEI systems.

What kind of car and distributor is this you have? A few years ago I repaired a Marelli distributor for a Fiat Dino that someone had modified with weaker "all in at 2500" springs that the Ferrari V6 did not like at all. Fortunately the car's owner has a service manual that has distributor advance specifications and I was able to find springs that could be modified to fit and provide the correct advance curve.

A prudent person would assume Ferrari knows what is best for their engine, which for the Dino in that particular Fiat totals at 5500 RPM.

This Ferrari V6 Marelli dual point distributor weights are inside a drum or ring with compression springs in the outside of the weights pushing against the inside of the ring, as opposed to the more familiar extension spring systems used in most other distributors.

As with extension spring advance mechanism, the compression spring system can be calibrated to counteract the slew rate retard in the electronics.

Anybody with experience with the early '70s Accel BEI ignition knows those distributors used the same type of compression spring advance mechanism. Bill Jenkins mentioned modifying them in his book "The Chevrolet Racing Engine".

[ClassAct]

I wouldn't say the emphasis is emissions reduction for the applications we're discussing, rather an improvement in overall efficiency, and a better running engine. But yeah, the price of admission and the learning curve are definitely considerations. Just seems like a car seeing any amount of street use the juice would be worth the squeeze. And a fellow in your situation, capable of showing before and after results to your customers, it would be a win win situation.

I agree but its HARD to get guys to get on the dyno and spend the time to do it. Everyone just wants to make pull after pull without actually looking at the data and making tuning decisions. I think some of the dyno shows do a disservice because what they show in an hour or so really takes at least a full day and that's with 3 or 4 or maybe more guys working on it.

[AC sports]

This too is a FIAT marelli ignition system running HEI as standard

[BILL-C]

Thecurve does not retard when proper GM advance weights and center is used.GM designed in compensation for this with the OEM HEI diz

My shop has run many dozens of the chevy crate 602 circle track engines equipped with their newest HEI distributors. The timing retards just like everyone other one does with stock type module. NO magic there.

[barnym17]

The real solution is using a system that triggers based on previous cylinder firing like many boxes do now.But barring that the Bill Jenkins solution of having no stops for the centrifigal weights with some pretty stiff springs will work.He compensated for the electronics slew rate by allowing the timing to advance an extra 1 degree or so per 1000 rpm thru the engines rev range over his targeted setting.

[F-BIRD'88]

Thecurve does not retard when proper GM advance weights and center is used.GM designed in compensation for this with the OEM HEI diz

My shop has run many dozens of the chevy crate 602 circle track engines equipped with their newest HEI distributors. The timing retards just like everyone other one does with stock type module. NO magic there.

By how much at 6000rpm? At what rpm does the mechanical advance curve stop advancing?

[AC sports]

I've been reading the Jenkins solution....which I'm pretty sure he came up with in the early 80s. I can see it working, but very fiddly and time consuming to set up. I guess I'm going to give it a go since I need to stay period in modifications.

[Cris]

I though I would add a little background information to this discussion. I am not a HEI expert, but I have had to dig into the circuit a bit when mine went on the fritz.

This chip (at least according to the data sheet) is used on aftermarket (Delco) HEI ignitions. I would guess there are other chips available, but this data sheet actually shows their chip in an HEI ignition set-up and has specifications for delay. The delay is from the reluctor signal to the current in the primary of the coil.....so you would have to add some delay for the reluctor and secondary winding of the coil. I was a little surprised at the amount of delay (lag) in the set-up. The typical number is 60 uSec, which at 6000 RPM would be 2 degrees. (Using 6000 RPMs = 36,000 degrees/Sec, or 27.8 uSec per degree.) The max number is double that.


https://www.nxp.com/docs/en/data-sheet/ ... DS9670.pdf

[Tuner]

I though I would add a little background information to this discussion. I am not a HEI expert, but I have had to dig into the circuit a bit when mine went on the fritz.

This chip (at least according to the data sheet) is used on aftermarket (Delco) HEI ignitions. I would guess there are other chips available, but this data sheet actually shows their chip in an HEI ignition set-up and has specifications for delay. The delay is from the reluctor signal to the current in the primary of the coil.....so you would have to add some delay for the reluctor and secondary winding of the coil. I was a little surprised at the amount of delay (lag) in the set-up. The typical number is 60 uSec, which at 6000 RPM would be 2 degrees. (Using 6000 RPMs = 36,000 degrees/Sec, or 27.8 uSec per degree.) The max number is double that.


https://www.nxp.com/docs/en/data-sheet/ ... DS9670.pdf

Awesome, thanks for that link to the data sheet. However, that is only the zero cross detector and preamp chip, so the specified delay does not include the Darlington transistor driver that switches the 12 V high amperage coil current, which is slower than the preamp and accounts for the additional delay to result in the observed approximately 1 degree per thousand RPM.

Darlington transistors are recognized to be relatively slow because they are actually two transistors in one package that switch one after the other. This is probably why the Mopar Orange box and Ford Dura Spark boxes are so slow, 2 deg per 1000 RpM.

I have no idea why some of the gypo modules are so slow and why they are voltage sensitive, faster at higher volts and slower with lower voltage ... weird.

[Tuner]

I've been reading the Jenkins solution....which I'm pretty sure he came up with in the early 80s. I can see it working, but very fiddly and time consuming to set up. I guess I'm going to give it a go since I need to stay period in modifications.

What model of FIAT engine and what Marelli distributor is this, compression spring or extension spring. Is it a small diameter unit similar to the Bosch VW distributor? Can you show a picture of the advance mechanism, weights and springs?

[Cris]

Tuner, take a closer look at the data sheet. The delay they are specifying includes the darlington transistor (MJ10012), an 8mH coil, and some other bits and pieces. It is not just the IC. Note the delay is referenced to the schematic of Figure 1.

[AC sports]

The engine in focus is the classic fiat twincam as found in various 124/131 & Lancia models.
I've welded the stops to limit advance. It now has 15* static and 35* max. Measurements at crank degrees.
I've been re thinking of the Jenkins solution, and now suspect it might not work in this case.
His solution of removing the advance limiters and controlling the max advance with stiffer springs, doesnt address the electrics. Still searching....I do have a spare dizzy to play with though. Heres a pic of the advance mechanism.

[Tuner]

The engine in focus is the classic fiat twincam as found in various 124/131 & Lancia models.
I've welded the stops to limit advance. It now has 15* static and 35* max. Measurements at crank degrees.
I've been re thinking of the Jenkins solution, and now suspect it might not work in this case.
His solution of removing the advance limiters and controlling the max advance with stiffer springs, doesnt address the electrics. Still searching....I do have a spare dizzy to play with though. Heres a pic of the advance mechanism.

Bill Jenkins' example pictures are of a Chevy dual point distributor with Mallory high speed points which he was using with a points triggered Prestolite transistor amplifier, the same unit Mopar used for NASCAR Race Hemi engines, Richard Petty, et al.

He shows modified weights that are missing a lot of weight and a spring that obviously could not work, however gives the clue that large wire and few coils is the plan.

If you have two identical springs like the stronger one in the pic with only 4 coils and larger wire diameter, that setup might make a slow creeping advance rate.

Do you have access to a distributor machine?

To reduce the amount of advance travel, limit it in the retard direction, so the lever on the weight can move toward the center of the shaft all the way to the the toe of the lever, so the cam on the shaft is driving the weight lever at the tip, closest to the axis of rotation.

The lever is flat and the cam lobe on the shaft has a radius, so the lever rolls over the radius for several degrees near the maximum end of the travel. The spring tension and the shape of the radius control the amount of advance per incremental RPM increase.

I am not in favor of reducing the mass or weight of the pendulum part of the advance weight like Jenkins shows in the picture. I rather increase spring tension by means of larger spring wire diameter or smaller coil diameter, or alter the radius of the cam under the tip end of the lever. To leave the weight with more mass gives the mechanism more stability because the greater pendulous mass can better absorb torsional oscillations.

If you play with the weights, pull them apart with your fingers as the centrifugal force would and feel the spring tension while you observe how much the cam and shaft rotate with lever motion, I hope you can see what I mean.

The bottom of this page http://www.lucasinjection.com/FUEL%20CAMS.htm has a printable degree wheel you can enlarge and glue to a piece of cardboard, cracker box or ?, cut it out and make a wire pointer to measure the degrees of travel your mechanism has, Don't forget to include the retard.

Observed with a timing light on dyno. Also when free revved in neutral. If I reset high rpm timing for max power on the dyno, I get mid range detonation. Those 3-4 degrees make a difference.

You now have 20 crank degrees (or so), so 10 distributor degrees. I don't know what cams you have, so the RPM range, If you want 35 degrees at 6000 and 30 at 3000, this is going to bump the initial to some (which is probably OK) because slew rate is stealing that 1 deg. per 1000 while the mechanism is adding 2 per 1000, so it is actually advancing 2 (crank degrees) in the mechanism per 1000 to get 1 on the crank because the electronics is stealing 1 per 1000 RPM. (more or less).

You need to disassemble the mechanism and polish the shaft and the inside of the rotating pole piece for minimum friction. High temperature, high drip point, Molybdenum disc brake wheel bearing grease is what to use in distributors.

My local hardware store has stainless steel springs, 4" long, .250" OD, .046" wire diameter. To make strong enough springs for a curve like this in small Bosch or Japanese distributors (and HEIs) I cut pieces 6 or 8 or ? coils long and bend the end coils to make new loops the desired installed length between the pins in the mechanism with 3 or 4 or 5 active coils.

Sometimes in HEIs, depending on the weight and cam combination, 41 weight with a 375 cam, the big springs that come in MSD distributors out of the box, .046" WD x .280 CD, shorten and close the eyes to only a few thousandths installed preload ... are just right.

[AC sports]

Part of my problem I think is the high rpm the engine runs. Up to 8000 even though I try cap it at 7750

[Circlotron]

The retardation of the timing with a magnetic pick up ign is an electrical issue, not a mechanical one. It is caused by distortion of the AC sine wave generated by the pick up & the distortion increases with RPM.

Geoff, see also my post here -> viewtopic.php?p=963556#p963556