You hear time and time again that hei distributers cannot handle very high rpm. I don't know where this originated, perhaps by the competetive aftemarket ignition manufacturers.
I have used them ever since 1974 when the first came out and never have had a problem up to 6000 rpm, at least that I know of. In fact, for both reliability and performance they would be my first choice. Can't see spending big bucks for some of this aftermarket stuff. The only problems I've ever had with hei was:
1 Replacing the reluctor on a few in the very early years. Never have had to replace a module.
2 Replace pins for the advance weights that were almost worn thru.

Not being educated in electronics, it is my understanding that the GM hei modules used a Motorola chip (have a schematic somewhere) and that the components can switch very rapidly (milliseconds).

So my questions are:

1 Is there really a realistic rpm shortfall to the GM hei unit?
2 If so, is the GM module the limiting factor in poor rpm performance?
3 If not the module, then what are the rpm limiting factors involved?

Thanks in advance.

 

There are some rpm limitations in the HEI design, but hardly below 6K. Once you're talking high rpm, 6500 and up, you run into the fact that one coil has to fire 8 cylinders. There is just not very much time for the coil to charge up between firing.

You can help it by switching to a capacitive discharge system (MSD, et.al). Since they hit the primary with several 100 volts instead of just 12 the charge time is less. The best solution, of course, is to go to one coil per cylinder which is what GM did with the LS motors.

 

Never had a problem and the upgraded parts are cheap!!!

 

I had an article on HEI in a late 80's Circle track magazine that said to always use GM or NAPA modules, that the others had less high-rpm reliability. It talked about making sure the shaft was tight up and down. Other than what is discussed above, it should be prettty reliable at speed.

 

My only comment is that when switching from points to HEI on any decent street engine, the starting and running was MUCH improved....

even with same but modified wires to fit the HEI cap....

methinks one could **** in the carb and a HEI would fire it....assuming it was Crown going down, that is....

 

I use a 1975 HEI stock distributor firing the engine in one of my 68 Camaros. Cheap and reliable taken right off the shelve of my A-1 Cardone distributor inventory. I like it because the coil is housed in the cap and just one wire services all that's necessary the for the box stock 454 LS7 transplant...that pushes it to low 11`s and shifts near 7K RPM`s...I don't have bragging rights with fancy parts hanging in the engine bay at the local cruises but then again I don't need to raise the hood as it works just fine the way it is, all stock including the HEI...

 

the old timer speed shop mechanics here say hei is good to 7k and dont waste $ on the gimicks. one actually used a scope to prove the piont, i only ever had one problem, one nite i worked late and was rushing to get to the bar, my truck started and died, threw a module in and was tipping elbows in minutes

 

Years ago, I had a rich running carb, and only noticeable at idle THEN and ONLY then did I notice a MSD box seemed to smooth out the idle....

once running I noticed NOTHING.....and it was a Pontiac 455 Qjet engine....

 

Thats my experience also, seems the MSD pats the ole HEI on the back and say "we got this".Really noticable on high duration engines with fat idle mixtures. I ran a MALLORY once with that dumbass $100 module in it. Sitting on the side of the road with one of those bad is a day of riding wasted!! I try to build fast reliable HP with as many factory replacement parts as possible. That way if your miles from home theres a cheap replacement as close as advance or the zone. Then once home order the priemium part.

 

What?

 

The MSD compliments the HEI in hotter stronger cammed engines.

 

I was going to try to answer each question separately, but the answers kept overlapping, so I'll just try to offer a hodgepodge paragraph to catch a few key elements.
Every part on a car or engine has pros and cons. They could be cost, packaging (size), durability/robustness, performance, bling factor, etc.
Regarding the RPM question, the laws of physics obviously have to be obeyed, and the main issues here are the ignition coil and the internal magnetic reluctance pickup sensor. It takes time to store energy into a system, and the coil spec choice (by Delco-Remy) was meant to cover the spark energy needs and RPM range of the production GM engines at the time. The coil in an HEI system supplies full energy (the key element in an ignition system, rather than voltage as seen in a lot of marketing advertising) from idle on up to the torque peak of most engines (in the 3600 RPM range). (As a comparison, a points system essentially starts to taper off in energy delivery soon after you release the key from the start position. ) The HEI system will start to taper off in energy delivery after that RPM range, but voltage ability (needed to initiate the arc) remains unaffected. Given that the torque is dropping after its peak (indicating a drop in cylinder pressure), the engine will require less kV to arcover the gap, resulting in a reduction of energy needed for combustion initiation. In the event that the distributor would be required to provide more energy at higher RPM, a slightly lower inductance coil could be installed. This would provide less energy at low RPMs (due to the reduction in inductance/storage capability), but the higher RPM energy storage would be higher due to the coil being able to charge quicker. Essentially, there's no free lunch with most things in life. But like most things on our car, we can taylor the part to fit our needs if we want to.

This is going longer than I anticipated. I'll submit this before it times out, and then edit it with more content later.

Edit: The next item of discussion is the pickup sensor (similarly used in many aftermarket distributors). Again, due to the laws of physics, there is a phenomenon known as magnetic switching lagtime. It takes a somewhat constant time for the pickup signal to swap polarity. This is seen as a retard delay over RPM, as the electrical part of the pickup signal doesn't change at exactly the same time as the mechanical parts in the distributor move past each other. Fortunately, this isn't a problem at lower RPMs, as the mechanical advance is designed to add a touch of extra advance to compensate for the signal lag. It is only after the mechanical advance has maxed out that the lag shows up in the actual spark timing delivered to the plug. The lag (IIRC) is about one degree per thousand RPM, so the net effect is about a three degree loss of advance at 6000 RPM, assuming the mechanical advance was all in at 3000 RPM. There's a couple ways around this issue, but that's a topic for another time. While a variable reluctance (VR) sensor has this minor drawback, it is very reliable compared to other pickup designs, and a key reason it is used by so many manufacturers. Another advantage of a VR sensor is the change in signal voltage over RPM. This varying voltage allows the module to use varying threshold levels to initiate dwell start and stop points. This allows the module to maximize the energy stored into the coil, while simultaneously minimizing undesired heating of the module electronics.

 

I've never had trouble with them. I used one in my old 427 for many years with 2 plates worth of N20 and 7500 rpm. Never missed once!

JIM

 

For most applications, no. At 6000 RPM, the ignition system is switching at 400 Hz. That's not that big of a deal, even for 70's era electronics. As some posters have noted, the system is giving good service at higher RPMs than 6000. My main concern, or bit of curiosity, is what the VR sensor's output is at seven or eight thousand RPM. The sensor voltage (an interestingly shaped AC signal) gets bigger with RPM, and at some point the voltage will try to break down the diffusion boundaries of the integrated circuit inside the module. This obviously wouldn't be healthy for the module, but I don't know what that RPM point is.
There's an interesting phenomenon that happens at high RPMs. It's frequently mentioned that there isn't time to charge the coil at higher RPMs, hence the plugs see a drop in energy delivery. (Remember, energy is the key item. Voltage is reasonably easy to supply.) If you put an oscilloscope on an HEI at high RPM, you'll notice that the main problem frequently isn't a lack of time to charge the coil, but a lack of time to discharge it. Because the ignition events are happening so fast, the coil frequently doesn't have time to discharge its total energy before another dwell time starts. This causes an amusing waveform where the coil gets charged up, but only has time to partially discharge, and then gets charged up again. (It's like half emptying a bucket, and then filling it right back up again. You've got a full bucket each time, but you're only getting to use half the water.) This situation generally assures that the coil always has sufficient energy to create the voltage needed to arc over the plug gap, but the system never has time (at high RPM) to deliver the long spark time potentially available, given the energy content in the coil at the time of ignition.

 

I went from GM HEI to MSD HEI, didn't notice a difference. Other than the rev limiter which has been a good thing.

 

69 427, wouldn't a box allow the bucket to drain completely with multiple spark?

 

They don't like to talk about it, but all multiple spark systems switch to single spark at higher (over ~3000) rpm.

 

Stock HEI at 7500 rpm? I knew your engines didn't obey the laws of physics, but.... daym!

 

Im looking to replace my HEI. Is the Accel High Performance HEI Replacement Distributors a quality unit or is it cheap junk like the other budget distributors?
Thanks

 

what is wrong with your gm hei? accel has a fancy sticker so it must be better