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So I wanted to bring up the topic of HEI distributors with electronic spark advance.
First some background.
When we talk about performance ignitions and in respect to MECHANICAL advance, it's always suggested to go with the larger cap distributor to prevent potential spark scatter. So if the terminals are farther spaced, it's less likely for the rotor to accidentally arc to the previous or next terminal. Since the advance is mechanical in nature, the rotor should in theory be point to the terminal when the coil is fired.
However, with electronic distributors this is not the case. Typical GM installations call for 2, 4, or 6 degrees of "initial" advance in the distributor, and since the rotor is locked this means that the rotor will be somewhere between two terminals when the coil fires. This is why in theory you cannot have more than 45 degrees of advance in an electronic HEI distributor (when installed with zero initial advance), since at 45* the rotor is in the dead center between two plug terminals.
So, my brain started thinking of what is better for electronic spark advance, small cap vs large cap.
My thinking is that the larger the cap the farther (in distance) for the same number of crank degrees the rotor will be from the terminal, so the spark has to jump a gap that could be an inch or larger. So I'm leaning towards the move from large to small cap HEI being done to preserve spark energy and prevent misfires.
But then, it doesn't explain why the Corvette L98 kept the large cap distributor on TPI through 1991.
There also seems to be a debate regarding coil efficiency for the in-cap coil of the large cap HEI and the remote coil. Some folks feel that the in-cap coil can be more efficient since no additional coil wire is required, although others argue that the packaging limits coil size and output.
The overwhelming majority of articles written about small vs large cap distributors are focused on MECHANICAL advance, which doesn't apply to this discussion. This is purely a topic of electronic spark advance on locked distributors, as implemented in factory TPI and TBI cars.
Re: A conversation about electronic HEI distributors
It's interesting an subject... I guess I'll give my anecdotal perspective as I recently did some stuff on my large cap HEI setup...
For the coil...
I recently swapped over to an external coil and definitely saw a non-trivial improvement in performance and driveability. I don't know if it was necessarily a function of the location change from in-cap to remote as much as it was going from a ~38kV Accel coil to a 60kV Pertronix. Both coils were brand new, so there's no age-related factor in the improvement. Not sure if the coil in the cap gets hot over time and impacts resistivity of the coil windings? I have my external coil thermally isolated with rubber mounts and it's definitely much cooler to the touch than the rocker cover temperature, probably also thanks to the convective cooling off the fins. Picture below.
For the ignition timing...
Very interesting discovery when I saw this thread regarding "built-in" timing advance/retard with different modules... my stuff starts at post 53...
Since even back in the Jurassic period, I had thought the spark latency table differences between the Corvette and Camaro/Firebird calibrations only had to do with the diameter of the cap. But that thread showed me that it also has to do with the ignition module itself.
I also disagree that the gap between rotor tip and distributor terminal is large. If you take a large HEI cap and turn it upside and put a rotor into it, you'll see that the cap is actually very small, on the order of a typical spark plug gap.
The tip velocity of the rotor is obviously faster in a large cap than a small cap, so that probably has some effect on the spark timing calculations as well. Maybe the ignition module has that factor built into it? In which case the spark latency table in the calibration only reflects the performance of the module? I don't know...
I've never run a small cap, so I can't say whether one or the other is better. But I can say that the performance of my large cap is very, very good with the above items sorted out.
Re: A conversation about electronic HEI distributors
Interesting to note that the last gm performance vehicle with a factory equipped "distributor" and cap/rotor was the lt1/ opti. the spacing of the opti cap terminals is still large, like the 84-91 l98 large cap hei was. And at the time they were the cream of the crop at gm. Makes sense the f body didn't get as much power as the c4's did, so they used the small cap. Something about the large cap hei had to have been better for gm to not only keep it in the c4/l98 but use a different coil polarity for it.
Look at the early tbi, Cadillac stuff. They all used a different coil polarity. Do some catalog searches for the old accel hei super coils and you'll notice the Cadillac and tbi large cap coil was not the same as the 84-91 c4. In fact the literature spelled out clearly. " Will not fit 81-91 Corvette. If you recall the last c3 crossfire better also used the same thing. Wonder why gm held onto it and the 7 pin hei module for the c4 and ditched the exact configuration for everything else?
Re: A conversation about electronic HEI distributors
Originally Posted by Bill Chase
Interesting to note that the last gm performance vehicle with a factory equipped "distributor" and cap/rotor was the lt1/ opti. the spacing of the opti cap terminals is still large, like the 84-91 l98 large cap hei was. And at the time they were the cream of the crop at gm. Makes sense the f body didn't get as much power as the c4's did, so they used the small cap. Something about the large cap hei had to have been better for gm to not only keep it in the c4/l98 but use a different coil polarity for it.
Look at the early tbi, Cadillac stuff. They all used a different coil polarity. Do some catalog searches for the old accel hei super coils and you'll notice the Cadillac and tbi large cap coil was not the same as the 84-91 c4. In fact the literature spelled out clearly. " Will not fit 81-91 Corvette. If you recall the last c3 crossfire better also used the same thing. Wonder why gm held onto it and the 7 pin hei module for the c4 and ditched the exact configuration for everything else?
That's the big question, why did GM keep the large cap for the C4.. All other data seems to indicate the smaller cap would be better for the reasons mentioned in the article below (https://www.streetmusclemag.com/tech...onfigurations/). Also, look at aftermarket Holley (i.e hyperspark) and MSD distributors, all small cap. Also look at Mercruiser from the mid 80s onward, also small cap and these are boats that operate up to 6500rpm constantly.
There is no question that the spark has to jump a larger gap in the large cap vs small cap electronic distributor.
Some interesting things to note, both electronic versions are phased so that the rotor is 20* behind the pickup.
I just don't see any evidence to support the large cap being better in any way, again, in regards to ELECTRONIC timing control. You're going to lose spark energy jumping a larger gap inside the cap.
Now, what IS important that I think a lot of people overlook is ensuring your initial advance is in a good spot based on your target advance at peak torque RPM. You want the rotor to really be aiming at the terminal whatever your advance is at peak torque, so you should set your initial advance appropriately to achieve that.
Re: A conversation about electronic HEI distributors
Originally Posted by ULTM8Z
I also disagree that the gap between rotor tip and distributor terminal is large. If you take a large HEI cap and turn it upside and put a rotor into it, you'll see that the cap is actually very small, on the order of a typical spark plug gap.
I'm not talking about the distance between the rotor and the terminal when the rotor is aiming at it, but rather at any time before or after the terminal.
Remember, it's ELECTRONIC advance. On a larger circle you have a larger distance (in inches) between terminals, so when the rotor is 20* BTDC (in crank degrees) it is farther away in inches on a large cap hei than it would be on a small cap. As mentioned in the articles previously linked, a lot of spark energy is lost jumping that large gap.
But there is a reasonable argument in regards to coil performance and lack of a coil wire.
Re: A conversation about electronic HEI distributors
Distance as you guys are stating it is kind of irrelevant. It's in degrees of rotation. And a circle has 360° regardless of how big or small it is. So distance is unchanged. Now how far out from center you change the radius could change speed or time it takes for the pointer to hit a tower of the cap. But still a larger circle is typically more accurate and precise when it comes to precisely locating a degree. Most of the time a larger circle can be more accurately marked and laid out, especially considering the available electronics of the late 70's when the large cap hei was modified for efi use (I know it wasn't available until 80 or so on triple c cars, but promise it was developed 5 years before it saw use in a production vehicle)
Now it could be the accuracy of the larger cap and 7 pin hei was what was best for the ecu power and code available at the time, and it took a while for the ecu power to catch up to a point where use of the small cap stuff was feasible? And by the time it did gm was already past it and doing wasted spark stuff, the sbc just kept it out of reasons of economy. My point is they didn't make decisions based on performance so much as make decisions based on economics.
Hell the tooling for the original large cap hei was made in the late 60's early 70's and by 1990 was probably worn out, yet the small cap stuff was newer, so it remained in use, and along with it development had marched on with the 8 pin hei modules. Lots of reasons they did what they did I'm sure.. but even if you look at the last production gm sbc the l31 vortec, it's cap used a larger diameter cap than the small cap hei did.
Now having said all that, the hei, big or small was never designed for high rpm, and had an acceptable amount of inaccuracy for the time. Anyone making serious power with over 6500 rpm is probably using a more modern ignition setup, the factory stuff dang sure did.
Last edited by Bill Chase; 07-28-2021 at 05:57 AM.
Re: A conversation about electronic HEI distributors
Originally Posted by Bill Chase
Distance as you guys are stating it is kind of irrelevant. It's in degrees of rotation. And a circle has 360° regardless of how big or small it is. So distance is unchanged. Now how far out from center you change the radius could change speed or time it takes for the pointer to hit a tower of the cap.
That's not what we are talking about. It's the physical distance in inches from the rotor to the terminal. The larger the gap the spark has to jump, the more energy is required. Electrical resistance.
Re: A conversation about electronic HEI distributors
Originally Posted by anesthes
That's not what we are talking about. It's the physical distance in inches from the rotor to the terminal. The larger the gap the spark has to jump, the more energy is required. Electrical resistance.
-- Joe
curious Joe, have you drilled a hole in the caps of both small and big hei and verified the rotor to plug wire tower gap is bigger on the large cap stuff? reason i ask is, the holley dual sync i bought took a crappola, and for now im just going to put hei on it. i have the oem unit from my 91c4, but also have a small cap unit with coil from an early 93-94 ish c1500 i can use either. have you swapped from big to little or little to big and noticed any good or bad results? my 383 has a hard redline of 6200 rpm so i doubt i will tax either unit to badly. its going to have to work until i can afford a hall effect crank trigger, i'll use a vortec distributor for cam sync and some d581 coils eventually, but for now.
also, have you ever used msd 6 series box with the large cap 7 pin hei? i can actually wire the magnetic pickup into the msd, and use it for crank reference to the efi, and still trigger it via the efi bypassing tghe hei module.. or i can just wire the hei straight to the holley. unsure which setup would work best.
Re: A conversation about electronic HEI distributors
Originally Posted by Bill Chase
curious Joe, have you drilled a hole in the caps of both small and big hei and verified the rotor to plug wire tower gap is bigger on the large cap stuff? reason i ask is, the holley dual sync i bought took a crappola, and for now im just going to put hei on it. i have the oem unit from my 91c4, but also have a small cap unit with coil from an early 93-94 ish c1500 i can use either. have you swapped from big to little or little to big and noticed any good or bad results? my 383 has a hard redline of 6200 rpm so i doubt i will tax either unit to badly. its going to have to work until i can afford a hall effect crank trigger, i'll use a vortec distributor for cam sync and some d581 coils eventually, but for now.
also, have you ever used msd 6 series box with the large cap 7 pin hei? i can actually wire the magnetic pickup into the msd, and use it for crank reference to the efi, and still trigger it via the efi bypassing tghe hei module.. or i can just wire the hei straight to the holley. unsure which setup would work best.
Hey Bill,
I have not but the actual terminal under the cap that the rotor arcs to is a greater distance (in inches, not degrees) from the rotor vs the small cap.
I have used a 6al with the 7 pin HEI large cap in a 87+ y-body, but it was still computer controlled - the MSD just went inline with the coil.
You can wire the pickup direct to the MSD (or to your EFI controller) and skip the module. That's what a lot of Holley guys do.
My thunderbolt ignition recently died on one of my boats, and in a pinch I converted it to mechanical HEI (using a marine DUI distributor). It's a large cap. I did see that a lot of conversions are using the small cap HEI with the electronic module. The "marine" electronic module doesn't need an ECM, as it's built in advance curve is sufficient for marine applications (basically +27* by 2,000 RPM). Probably also a good drag race module, but I do prefer computer control.
I'm in the middle of building a new shop, so all my stuff is in storage. I'm going to be converting my 68 convertible over to the Holley EFI over the winter, still on the fence about using large cap, small cap, or the holley Hyperspark distributor.
Re: A conversation about electronic HEI distributors
Originally Posted by anesthes
Hey Bill,
I have not but the actual terminal under the cap that the rotor arcs to is a greater distance (in inches, not degrees) from the rotor vs the small cap.
I have used a 6al with the 7 pin HEI large cap in a 87+ y-body, but it was still computer controlled - the MSD just went inline with the coil.
You can wire the pickup direct to the MSD (or to your EFI controller) and skip the module. That's what a lot of Holley guys do.
My thunderbolt ignition recently died on one of my boats, and in a pinch I converted it to mechanical HEI (using a marine DUI distributor). It's a large cap. I did see that a lot of conversions are using the small cap HEI with the electronic module. The "marine" electronic module doesn't need an ECM, as it's built in advance curve is sufficient for marine applications (basically +27* by 2,000 RPM). Probably also a good drag race module, but I do prefer computer control.
I'm in the middle of building a new shop, so all my stuff is in storage. I'm going to be converting my 68 convertible over to the Holley EFI over the winter, still on the fence about using large cap, small cap, or the holley Hyperspark distributor.
just do a crank trigger and vortec distributor, then when you are ready to go cnp youre all set, and dont buy the tpi harness, get the universal mpfi harness, its the same money, but already has the main harness for each coil bank. but can still be used with a distributor. i have to add 8 wires to the tpi harness to convert it , plus they are old stock, they dont have the 4 pin can wiring, or the power tap harness, the universal does. or use any ls harness. more or less has the same features. if you use any holley hyperspark, dual sync etc you must use an ignition box. the dual sync is actually good, just dont use it to distribute spark. if used for cam and crank signal they seem to last. the ionized air created when using them to distribute spark has caused quite a few failures, plus rust and corrosion. if i had it to do over again id just do the efi connection 58x reluctor and cover with built in 1x cam signal, then use a modified hei to drive the oil pump. the only time 58x starts to become troublesome is ultra high rpm high power stuff, they tend to lose sync as the crank flexes etc. at that point most go to a 4x crank trigger.
as a side note i actually have the efi connection 24x reluctor, sensor, and holley aluminum vortec cover. and the msd 1x cam sync oil pump drive that is plug and play with holley 24x harness. here for the sbe l98 turbo project i have planned. only drawback is it cant be used with a double roller timing set. spoke to a few in the know and the gm set sold on the 602/604 crate engines is actually very good, and for most hydraulic roller stuff with less than 500 over the nose spring pressure will last a long time. plus its actually slightly less rotating mass. i wound up finding a wrecked k3500 with good 7.4 vortec and abandoned the project for now. ive got a complete terminator x, as well as us shift quick 4, and good 4l60e too. may end up selling it off. so check with me before you go buying new, i just want what i paid, and i purchased 12 months before the recent price hikes took effect. have reciepts for all of it, 95% is bnib, a few items were purchased ebay. but still brand new open box. nice looking foundation Joe, hope youre doing a 12-14 foot ceiling so you can put in a 2 post lift later? they can be found used around here for 1200-1600. and usually run on a dryer/stove style single phase plug.
Re: A conversation about electronic HEI distributors
Originally Posted by Bill Chase
just do a crank trigger and vortec distributor, then when you are ready to go cnp youre all set, and dont buy the tpi harness, get the universal mpfi harness, its the same money, but already has the main harness for each coil bank. but can still be used with a distributor. i have to add 8 wires to the tpi harness to convert it , plus they are old stock, they dont have the 4 pin can wiring, or the power tap harness, the universal does. or use any ls harness. more or less has the same features. if you use any holley hyperspark, dual sync etc you must use an ignition box. the dual sync is actually good, just dont use it to distribute spark. if used for cam and crank signal they seem to last. the ionized air created when using them to distribute spark has caused quite a few failures, plus rust and corrosion. if i had it to do over again id just do the efi connection 58x reluctor and cover with built in 1x cam signal, then use a modified hei to drive the oil pump. the only time 58x starts to become troublesome is ultra high rpm high power stuff, they tend to lose sync as the crank flexes etc. at that point most go to a 4x crank trigger.
as a side note i actually have the efi connection 24x reluctor, sensor, and holley aluminum vortec cover. and the msd 1x cam sync oil pump drive that is plug and play with holley 24x harness. here for the sbe l98 turbo project i have planned. only drawback is it cant be used with a double roller timing set. spoke to a few in the know and the gm set sold on the 602/604 crate engines is actually very good, and for most hydraulic roller stuff with less than 500 over the nose spring pressure will last a long time. plus its actually slightly less rotating mass. i wound up finding a wrecked k3500 with good 7.4 vortec and abandoned the project for now. ive got a complete terminator x, as well as us shift quick 4, and good 4l60e too. may end up selling it off. so check with me before you go buying new, i just want what i paid, and i purchased 12 months before the recent price hikes took effect. have reciepts for all of it, 95% is bnib, a few items were purchased ebay. but still brand new open box. nice looking foundation Joe, hope youre doing a 12-14 foot ceiling so you can put in a 2 post lift later? they can be found used around here for 1200-1600. and usually run on a dryer/stove style single phase plug.
I'm running a Holley sniper 4bbl, not Holley HP so everything is integrated.
I actually had second thoughts about converting the 68 over. It runs fine on the quadrajet and mechanical dizzy, and being a convertible it's just an ice cream cruiser anyway.
I bought my '79Z back that I sold 12 years ago. Been thinking about putting the 412" in that with the sniper and a small turbo, like a T76 or S475. The Sniper will support 650hp in a blow through application. The car was already set up as a drag car suspension wise, and I still have the enclosed race car trailer so might as well.
I saved my rotary 2 post lift from the other shop I tore down this spring, but I think I'm going to sell it and buy a pair of brand new lifts. I've had the rotary since 1999, and it works well but has it's quirks (like no arm locks, no rubber pads, etc). I can probably sell it for $900 and I'll get a new 2 post for about $2600 and a 4 post for the middle bay for about $4500. That way I can do oil changes quickly on the 4 post, and easily double stack the corvettes during the winter and leave the 2 post bay open for service. The third bay I'll probably keep my truck in or use for unfinished projects, etc.
7.4 is what a 454 ? I don't do much big block stuff anymore. When I used to play with the jet boats everything was big block, but I never really did anything with them in cars. I was actually surprised GM doesn't offer a big block engine in a HD truck. I just ordered a new 2023 sierra 2500HD and the only engine option was the 6.6 gas or 6.6 diesel. I went with the gas because I didn't want to deal with DEF fluid. $70k plus dealer fees..
Re: A conversation about electronic HEI distributors
Originally Posted by anesthes
So I wanted to bring up the topic of HEI distributors with electronic spark advance.
First some background.
When we talk about performance ignitions and in respect to MECHANICAL advance, it's always suggested to go with the larger cap distributor to prevent potential spark scatter. So if the terminals are farther spaced, it's less likely for the rotor to accidentally arc to the previous or next terminal. Since the advance is mechanical in nature, the rotor should in theory be point to the terminal when the coil is fired.
However, with electronic distributors this is not the case. Typical GM installations call for 2, 4, or 6 degrees of "initial" advance in the distributor, and since the rotor is locked this means that the rotor will be somewhere between two terminals when the coil fires. This is why in theory you cannot have more than 45 degrees of advance in an electronic HEI distributor (when installed with zero initial advance), since at 45* the rotor is in the dead center between two plug terminals.
So, my brain started thinking of what is better for electronic spark advance, small cap vs large cap.
My thinking is that the larger the cap the farther (in distance) for the same number of crank degrees the rotor will be from the terminal, so the spark has to jump a gap that could be an inch or larger. So I'm leaning towards the move from large to small cap HEI being done to preserve spark energy and prevent misfires.
But then, it doesn't explain why the Corvette L98 kept the large cap distributor on TPI through 1991.
There also seems to be a debate regarding coil efficiency for the in-cap coil of the large cap HEI and the remote coil. Some folks feel that the in-cap coil can be more efficient since no additional coil wire is required, although others argue that the packaging limits coil size and output.
The overwhelming majority of articles written about small vs large cap distributors are focused on MECHANICAL advance, which doesn't apply to this discussion. This is purely a topic of electronic spark advance on locked distributors, as implemented in factory TPI and TBI cars.
-- Joe
I've tagged vorteciroc on this. He seems to a specialist in things GM electronics related. I'm interested in the electronic distributor in general as the weights, springs, vacuum cans and the etc gets a little old after a while.
Re: A conversation about electronic HEI distributors
HEI!
I am NOT that Old, LOL!
(way before my time).
If you guys are simply discussing the relationship between Firing-Accuracy and the Ignition Distribution Element (Rotor in this situation) Diameter...
Larger will always be more finely adjustable in terms of establishing (as close as possible of a mechanical relationship to) Firing-Accuracy.
Re: A conversation about electronic HEI distributors
Originally Posted by vorteciroc
HEI!
I am NOT that Old, LOL!
(way before my time).
If you guys are simply discussing the relationship between Firing-Accuracy and the Ignition Distribution Element (Rotor in this situation) Diameter...
Larger will always be more finely adjustable in terms of establishing (as close as possible of a mechanical relationship to) Firing-Accuracy.
Think 8" Degree-Wheel vs. 24" Degree-Wheel.
Well, I tried.
I'm before the days of HEI's. I was the guy who knew how to set up dual point distributors on the Mopar street racers I knew back in the day. Electronics were just coming of age.
I practically lived on the old Sunnen Distributor machine. (And still have access to one should the need arise).
Re: A conversation about electronic HEI distributors
Originally Posted by vorteciroc
HEI!
I am NOT that Old, LOL!
(way before my time).
If you guys are simply discussing the relationship between Firing-Accuracy and the Ignition Distribution Element (Rotor in this situation) Diameter...
Larger will always be more finely adjustable in terms of establishing (as close as possible of a mechanical relationship to) Firing-Accuracy.
Think 8" Degree-Wheel vs. 24" Degree-Wheel.
So my point was surrounding the distribution element (rotor). On an electric advance distributor, the rotor doesn't get any closer to the terminal when firing (like it would a mechanical). So my thoughts were simply that the distance (in inches) between the rotor and the plug terminal would be greater on a large cap than it would a small one, which would create more resistance as the spark would have to jump a larger gap.
So my theory was that a small cap would be better for this reason. However, I do like the coil packaging of the large cap.
Re: A conversation about electronic HEI distributors
Originally Posted by anesthes
So my point was surrounding the distribution element (rotor). On an electric advance distributor, the rotor doesn't get any closer to the terminal when firing (like it would a mechanical). So my thoughts were simply that the distance (in inches) between the rotor and the plug terminal would be greater on a large cap than it would a small one, which would create more resistance as the spark would have to jump a larger gap.
So my theory was that a small cap would be better for this reason. However, I do like the coil packaging of the large cap.
-- Joe
When discussing the distance between the Electrical Contacts for an Inductive Ignition (HEI) the Gap and Added Resistance is actually of Benefit (very similar to increased Spark-Plug Ground-Strap Gap).
On more Modern Capacity Discharge Ignition (MSD) the distance between the Electrical Contacts is of moot point.
(However the Spark-Plug Gap recommendations must still be followed... or the Ignition Coil Dwell/ saturation must be increased further).
Re: A conversation about electronic HEI distributors
Originally Posted by vorteciroc
When discussing the distance between the Electrical Contacts for an Inductive Ignition (HEI) the Gap and Added Resistance is actually of Benefit (very similar to increased Spark-Plug Ground-Strap Gap).
On more Modern Capacity Discharge Ignition (MSD) the distance between the Electrical Contacts is of moot point.
(However the Spark-Plug Gap recommendations must still be followed... or the Ignition Coil Dwell/ saturation must be increased further).
Interesting. So I guess for my Holley Sniper EFI it's either large cap or the Hyperspark.
04-07-2021, 09:18 AM
