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Discussion Starter · #1 ·
Looking for a way to improve the Chevy HEI. After using these units since the seventies, I was not aware of the apparent shortcomings in the design until reading the posts here for the GM HEI conversion into the 258 (which I've been running in 2 jeeps for a while with very good results over the ORIGINAL Duraspark system). I thought my tach was broken when I got my first 258 powered jeep about 3 years ago- it just wouldn't go over 3000 rpms even with all new components in the ignition system and a new Weber carb. Bolted in a GM HEI and Oh-what a difference- it's alive! Now I might be getting ionization, ozone buildup and crossfiring with these distributors, but they are a great improvement over the ORIGINAL Duraspark system. I am not close minded enough to not go back and try out the Duraspark upgrade that TeamRush has so kindly brought to our attention- I am gathering the parts for this upgrade now. My question is, what can be done to improve the performance of the SB Chevy HEI? I understand that the MSD Cap-a Dapt (MSD 8420) is a first step in reducing some of the undesirable characteristics of the HEI. Can the Ford TFI coil be used with the Cap-a -Dapt? Is the Cap-a -Dapt vented like the Ford cap, or should it be modified with a venting system. Does MSD make a Cap-a Dapt to fit the GM six cylinder HEI, so that those jeepers with 258s who have already invested in the HEI ( and maybe gotten rid of their Duraspark systems) can maximize the performance of their investment. Understand that carrying the GM HEI to CDI with an MSD module is not as easy as with the plugin Duraspark system, but it can be done. Has anyone done this with a project writeup? OK, that's enough questions for one post- especially on this apparently touchy subject of chevy /ford ignition comparisons. Let me emphasize that I AM NOT trying to open a heated debate thread on the subject- just trying to get a maximum practical HEI system upgrade for those of us who run SB Chevies and to get the best performance out of the GM HEI for the 258.


6,784 Posts
MSD makes a half a dozen ways to upgrade the GM style HEI.
They have one kit that removes the coil from the cap, and that's one of the HUGE problems the GM HEI has. Take the coil out of the cap, and three HUGE problems go away real fast...
(Inductance false triggers are more than cut in half, EMI and RFI from the coil go away entirely)

MSD has the first totally redesigned ignition module for the GM HEI. It eliminates a ton of module problems, and adds a ton of saturation time to the coil.
(This is the module that DUI uses, straight from MSD in their $500 top of the line distributor)

(It shouldn't take a rocket scientist to figure that if the factory put a 10 Ga wire on the GM HEI, they thought a 10 Ga. was the absolute minimum they could get away with.
Then some guy that thinks he can outsmart the factory engineers connects it to a 16 or 18 Ga. wire, and sense the engine runs, he figures he's a total success.
WRONG! And then he professes to be an expert, and tries to talk all of his buddies into the "upgrade"..., Then somebody smells a buck, and suddenly, sense GM HEI are so plentiful, "Kits", with no more fore thought, planning or testing than the original "upgrade", start to appear, and your dollars start to leak away...
The module is starved for power, and the coil has no chance of saturating. Some guys never even took the ignition resistor out of the power supply line because the 'instructions' didn't say too!)

MSD sells high current relays for the power supply for the GM HEI.
Most people connect them to the existing wiring for the DuraSpark unit, and the Dura Spark uses THREE power supplies (one to the coil, and two to the module), and no one of the three can feed the GM HEI... (Maybe all three together?...)
The GM HEI is a large, inefficient power hog.

If you connect straight to the Ford wiring harness, then the GM HEI isn't getting enough current to do the job it was intended to do.
(That fact may explain why there is reduced cross firing and ground firing in GM HEI's transplanted to jeeps. The coil just isn't getting enough current to develop voltages that can weld the weights to the pins, but you also aren't getting anymore to the plugs than the stock system either. The smoothers running probably comes from the quicker advance curve, the towers being spaced a reasonable distance apart, and the upgraded plug wires required for the GM HEI upgrade... The same things we accomplish with the Motorcraft upgrades... For a lot less money and aggravation!)

Have a look at MSD's web site, and order a FREE catalog online!
(Did anyone miss the FREE part?)
MSD has more usable information in their catalog than most text books.
I have yet to find a single MSD product that doesn't live up to it's advertised specs.
I can't say that about ANY OTHER ignition products company, YET...

PerTronics is looking pretty good right now... But it will be at least a week before the PerTronics hardware is off of the test bench...
I fried one yesterday! Smoked it good!
I guess that's what happens when you dead ground any module when you aren't paying attention.... It died of user stupidity, not any design flaw by PerTronics...
Guess who the user was... /wwwthreads_images/icons/crazy.gif

Later guys, hope this helps, Aaron.

If Chris Columbus "Discovered" America (with 25 million already here), Can I Go "Discover" Florida?

Discussion Starter · #3 ·
Thank you, Team Rush. As I mentioned in the original post, it was very kind of you to provide the details of how to make the Duraspark system work to it's maximum potential in the 258 and AMC V8s. It was also kind of you to post MSDs URL for us. I went over their site briefly and did not see any info on the Cap-a Dapt, however. While I am waiting for my catalog arrive via snail mail, could anyone offer insight on my questions in the original post. There are more than a few HEIs out there in SB Chevies and if there is a conversion to install the superior Motorcraft Duraspark in a SB Chevy, I am unaware of it. If not, let's figure out how to make the HEI the best it can be. From TeamRush's insights, it would seem that the Cap-a Dapt would be a good starting point along with a remote (that's why I asked if the Ford TFI would work as a coil with the GM HEI) coil to remove the magnetic field. Then a CDI MSD module (Summit sells the MSD-6A for $131.95, or the MSD Super HEI kit for $188.95, which includes the MSD-6A, remote coil and mounting bracket, dust cover and coil wire, but it is an conventional oil filled coil). If anyone has gone this route or thinks that there may be a better way to go with the HEI, please post. TIA for all ideas.


Discussion Starter · #4 ·
going with the 6a and the external coil (the tfi is fine) will help eliminate a couple extra problems. first is getting the module out of the distributor. that was a very bad place to put it, as it has to put up with heat, vibration, and a contained electrical storm right over its head. the remote mounting of the coil has obvious benefits (one is getting it away from the module), and allows you the choice of a wide range of coils to use. make sure you use the msd rotor along with the cap as it is of a better material and will help prevent the weights from becoming welded, as well as grounding problems.

you will also want to make sure the stator and pole piece are in good condition and adjusted properly. the two thin wires on these are known to break (this is due to the vacuum advance pushing and pulling on them constantly) so watch for this. this includes checking the bushings (slop will cause poor or false triggers).

Good things come to those who do research!!

6,784 Posts
I must have missed something a while ago!
If you want to upgrade from GM HEI in a GM engine, It's really easy...
Use the HEI trigger to fire an MSD CDI module, and remove the coil from the cap so you can use a really powerful Ignition coil.

CDI kicks ass over any of the inductive ignitions.

If you want to go the low buck route, use a small GM points type distributor.
A company called PerTronics makes a points to electronic ignition upgrade that uses flying magnets to trigger the built in module. It's about $70 retail.
You can use one of the old can type coils, as is, or add a resistor to get the primary resistance up on one of the newer high output coils, and you don't need a module.

The PerTronics electronic conversion will fire the DuraSpark module, MSD module or can be used as a stand alone to trigger the coil from PerTronics, no module required.
The PerTronics coil does develop a fair amount of usable spark energy, and isn't a power hog doing it.

In my small block,
I used the small chevy points type distributor, (common as nails and dirt cheap)
Added a Chrysler style reluctor and pickup coil, (easy to adapt and $15)
Used a MSD Cap-A-Dapt with phased rotor, (about $35 the first time, and $18 for replacements, gives an adjustable rotor and the wide ford style vented cap)
Used a MSD 6AL module, (around $170)
(Could have used the HEI or DuraSpark modules for a lot less... They don't care how you trigger them, but wouldn't have been CDI)
Used a Japanese coil with really cool coil connector, ($12)
Used MSD blue plug wires, (around $35)
Used Autolite plugs, (buck each)

See attachment....

Sorry, I missed the point the first time...
Does this help clear things up....


If Chris Columbus "Discovered" America (with 25 million already here), Can I Go "Discover" Florida?



Discussion Starter · #6 ·
it looks like you built yourself an accel distributor/wwwthreads_images/icons/tongue.gif. a long time ago i bought one for my ford (had points, and the price was right at the time), and what did i get: a dodge distributor with a ford gear and a hei module strapped to the outside, go figure. i could have done the same thing for a third of what i paid for that and had money to spend on something else.

those pertronix kits work well. we use to sell them to a lot of guys doing resto work and wanted to stay with an original appearance ignition system. only one extra wire to hide (a little loom goes a long way).

Good things come to those who do research!!

Discussion Starter · #7 ·
Thank you Aaron and Blackjack. I plan to stay with the basic GM HEI distributor (and Aaron, I know you don't care for it, but it has always served me well) and upgrade it to CDI, TFI external coil and Cap-a-Dapt. Where do you get the replacement Cap-a-Dapt cap for $18? Is it the same as the Ford Cap? I've only seen it listed as a package with the base. Thanks for providing info on the Pertronix point distributor conversion as well as the setup you use in your SB Chevy. When you get a chance, please provide us with ID #s of the components you used to build this unit. One note, although I've personally never experienced a GM HEI module failure, I've always carried a complete spare HEI distributor and an additional module in each vehicle (I've got this thing about getting back home!). Being prepared for a failure may be the reason I've never had one.


6,784 Posts
No harm, No foul-- don't sweat it.
You didn't even begin to make it personal, so no problem.
If you honestly want help, I'll do everything I can. You said from the start you didn't want trouble, so we're cool.

It's no secret I don't like GM HEI's. I do recommend them over points, and you are more than welcome to use what ever you are comfortable with.
I don't know of anyone that makes a Cap-A-Dapt product for the GM HEI.
The cap spacing is plenty good on a GM HEI if you get a good rotor that will stand up to the voltage, and get the coil out of the cap.
MSD sells that kit very reasonably, and really good caps and rotors to boot.
Someone needs to make a distributor cap and rotor for the GM HEI that gets the rotor up off of the distributor shaft, and you could do that without increasing the size if you moved the coil out too... Kill two birds with one stone...
(MSD, are you listening?...)

The parts I used are stock 76 Chrysler New Yorker pickup coil and reluctor.
You have to use a Dremmel tool or round 'rat tail' file to open up the reluctor enough to fit over the stock Delco points cam lobes, but that just takes time.
I can buy both pieces for under $20 here at the local Auto Jerks...
I just sent step by step to a couple of guys with pictures, if you want them, email me with an email address that can accept pictures, and I'll forward the stuff to you.
It's a pretty big file, so if you have WinZip, let me know that too...

[email protected]


Here is one for you bud!
I have a GM HEI module screwed under the distributor housing on my jeep!
It's my 'Limp Home' backup...
I figure if I'm prepared, it won't happen...

I talked about that in some earlier posts. I'm a large fan of the GM HEI module if you get it out of the distributor, in the open air, and in the ground plane where every little bit of RFI and EMI kicks the sh*t out of it...
I hook the tester to it every time I do a tune up and make sure it's still working.
It's a Borg-Warner a customer left here in a GM HEI when we built a real ignition system for him, and I figure, "Waste not, want not."

See Ya All, Aaron.

If Chris Columbus "Discovered" America (with 25 million already here), Can I Go "Discover" Florida?

Discussion Starter · #9 ·
Cap-A-Dapt for the HEI, like an empty cap with a coil wire connecting "post" on the top ??
I have one, bought from Jacob's.
It is simply a coil wire connecting post, bolted to a bras rod, which goes all the way inside the HEI cap, down to the small spring-loaded "thing", which transfers the voltage / current out to the rotor.
The brass rod is machined so that it has a "shoulder" on the top.
You drill a small hole in the top of the plastic cap, extend the threaded end up through the hole, and bolts on the coil wire connecting post.
Two "critical" dimensions: the length of the brass rod, and the hole has to be in the correct place (center of the cap ..)
*VERY* simple to make yourself ...

PerJ (From Norway)

<[email protected]>
Using self-discipline, see

6,784 Posts
*PerJ* wrote,
*Cap-A-Dapt for the HEI, like an empty cap with a coil wire connecting "post" on the top??
I have one, bought from Jacob's.*

Like I said before,

The Cap-A-Dapt MSD sells is for the small points Delco distributor, and adapts it to the larger Ford style distributor cap with brass terminals and cap vent.
This kit DOES require a MSD rotor, but stock Ford caps will fit it as replacements.

They also sell a heavy version of the stock Ford adaptor for you Motorcraft guys...
It will accept stock Ford rotors and Caps.

They do make an adaptor that fits any GM style HEI cap that takes the coil out of the cap.
It takes the place of the coil, an provides a WELL INSULATED high voltage tower for the coil wire to connect to. It uses standard spark plug towers, so no special coil wire is needed.

What I said was,
Someone needs to make a distributor cap and rotor for the GM HEI that moves the rotor up off of the distributor shaft, and farther away from the module and pickup coil.
By removing the coil from the cap, the dimensions of the stock cap could stay the same.
You would need an external coil, but GM HEI's need that anyway.

I don't care for Jacobs hardware, I don't like their prices, and I don't think much of their so called, 'customer service'.
Buy Jacobs at your own risk.


If Chris Columbus "Discovered" America (with 25 million already here), Can I Go "Discover" Florida?

Discussion Starter · #11 ·
Aaron, Thanks for all the info and for the email describing your SB Chevy ignition system. I've got a couple of old 327 point style distributors laying around (Mudwolf is THE packrat of the Carolinas) and will start grinding reluctors on the next rainy day! As far as the GM HEI goes, correct me where I'm wrong: Get the best quality HEI cap and rotor (MSD#8437 & 8462), move the coil out with MSD dust cover conversion #8401 ( is the Ford TFI coil the very best one to use here?) & move the module out of the distributor body ( best to go with a MSD 6A module to get CDI and keep a spare GM HEI module as a backup for limping home). Is it necessary or desirable to vent the cap, a la Ford? The spacing of the posts on the HEI cap is good, but the rotor needs to be moved up when MSD begins manufacturing the HEI Cap-A -Dapts late this summer ( of course, this depends on all of us SB Chevy lovers calling and emailing MSD to demand the product starting, uh, today!). Whose HEI advanvce curve kit do you prefer, and is an adjustable vacuum advance mechanism needed? Call MSD today, guys. We want GM 6 and 8 cylinder Cap-a-Dapts with adjustable rotors for proper phasing. If they have ???, tell them to call Aaron at TeamRush.


6,784 Posts

No answers here, just rambling commentary...

As for the best coil... That's all subjective...
Any stock application premium coil will work with the HEI conversion.
I don't see any problems with using the normal size canister type super coils, or MSD's blaster coils. They are all 12 volt coils, and the primary resistance difference isn't enough to talk about...
The late model TFI coil or GM two connector clone should work just as well.

The problem with using the stock GM HEI module to fire the coil is it just doesn't have big enough power switching capabilities.
The module just can't deliver enough current to the coil to do the job correctly at anything but idle, and the saturation times then are so large ANY coil should work. Off idle and through the RPM range is another story.

GM made a remote coil configuration for the HEI, and it's in all GM 4 cylinder engines, and some of the I-6 engines, if you want a true remote HEI coil to use.

The MSD HEI module replacement doubles the current handling capabilities of the module, and keeps the timing and dwell from 'drifting' when the rpm goes up.
It's common for timing to drift as much as 12 degrees at 3500 rpm on a stock HEI module, and the dwell times go straight to hell.
MSD finally found a way to fix that.

Once the trigger is working correctly, and the module problems have been addressed, it's time for the coil in the cap problem...
Then a coil that puts out useable spark energy, instead of making lighting inside the cap....
Then there is a decent cap and rotor...

Does it sound like GM put this design out before it was tested?

WELL, They didn't. When the GM HEI came out, it was light years beyond the old points systems. A GM HEI would go 40 to 50,000 miles with out needing tuned up, and if someone says they got that, or get that out of points, they are lying or they are nuts.

With points, the tune starts degrading as soon as you turn the key, and rapidly turns to crap.
With so called, 'Breaker Less' ignitions, there is no physical contact switches, so the headaches breaker points caused were gone.

Now that we look back some 28 years cense the first HEI hit the street, we can find flaws.
The 1972 HEI prototypes, or the 1974 through 1976 HEI models that had no modules or sill used points to trigger the modules, or the 1976 to present HEI's... Pretty much unchanged other than additions for smog crap...
But ask anyone that switched from points to HEI... They will tell you the difference was night and day!

It's an old design. The coil was the latest thing when it cam out. Absolutely revolutionary.
Ford already had a better version in the works, but the technology to build the coils cost efficiently was still being developed, but GM used the existing technology to get the voltages up... and it was a vast improvement over the low outputs points systems were producing.

Now over the past 30 years, we have found out that sheer voltage isn't the name if the game. Spark duration and amperage levels are more important than huge voltage outputs...

Look over the new GM High Energy Ignition coils...
And the new Ford TFI Ignition coils...
The outputs are identical, the inputs are so close there really is nothing to talk about, the design and windings are the same, even terminal placement is the same...

Same for the modules,and both are using smaller distributor caps now that the raw voltages have been reduced to half what they were during the 'voltage wars'...

It's no secret I pick and choose what works best in testing, but I have the education and equipment to do that.
I am just trying to pass along updated information to everyone else.
The guys that are screaming, 'HEI ! HEI !' are living in the 70's.
Everyone is allowed to use what ever they believe in and are comfortable with, and I never once said that GM HEI wasn't a great upgrade over points...

My Ford conversion just brings the Jeep guys into the late 80's to early 90's...
MSD will bring you into the late 90's early 00's. (wondered when I was going to see that in print... Didn't think it was going to be me doing it though...)
And it's all off the shelf with no major modification to anything.
The most I said to do was change the distributor springs...

Just thinking out loud again...
It's on topic this time for once!!!/wwwthreads_images/icons/wink.gif

See ya, Aaron.

If Chris Columbus "Discovered" America (with 25 million already here), Can I Go "Discover" Florida?

Discussion Starter · #13 ·
Re: SB Chevy HEI Upgrade, remote module

Comment on 'removing the HEI module' to open air.... I have been running two SB Chevy's for over 20 years each, with the stock module moved outside the distributor - on an aluminum bracket, mounted to the firewall. It seems to work, just fine. Also, look at what Fiat did, factory stock, to their spider, around 1980, or so. We had one in the family for a long time, and it used the GM HEI module, mounted remotely on an aluminum heat-sink, on the inner fender-panel. Fiat used a pair of wires, almost 2-ft long, to connect it to the distributor.
My point - there is no reason for the HEI module to be in the distributor.
How is this for another 'odd'-one?... I've got an OLD Chevy, with a 265 V8, running on an ignition that includes the mopar reluctor and pickup, a HEI module (mounted remotely), a Mark-10 CD unit, and the original 55-56 cannister-coil. It's been this way since the parts were available, maybe around 77, or so.
Best wishes. bobH

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