Reaplacing Factory Distributor with After Market Curiosity / Questions

Curious, I have a 1987 Corvette that I had a new engine built for. Since I am in the process of changing the Pick-up Coil & the Distributor is out I decided to Web shop around for possibly replacing the Distributor at some point. Maybe not right now but in the near future. For those of you who have checked out my ride know that it is a Gen1 350ci with nearly 400hp. Running the HEI & checking various sites & my Summit Racing & Jeg's sites I run mostly across HEI Distributor's with Vacuum Advance. A few questions come to mind:

1) Who among you have installed one of these types of Distributor's & 2) where would one connect the Vacuum Advance? Example Jeg's Performance HEI #40002K.

2) What advantage is there in having a Vacuum Advance vs non Vacuum Adance?

3) So many that I see show & look to be External Coil Distributor's but state used on 1987-95 GM cars, incl. GM Performance Parts Distributor (#809-1103952). My limited knowledge I have not seen anything like these on L98 motor's. Would this be for a different 350 motor that Chevy produced? or just advantage in having a stronger Coil set-up?

4) Summit Racing lists this: Davis Unified Ignition 12620BL - Davis Unified Ignition GM Computer-Controlled Distributors, has anyone installed one of these & how does it perform?

5) Would just going with a "Rebuilt Unit" just be the best way to go? Keep in mind I have 10.5:1 Flat Top Racing Pistons, 1.6 Stainless Roller Rocker's & other mods that make my engine very much non-stock: Cardone Industries 100-301869 - A1 Cardone Remanufactured Distributors.

As I said I am just web shopping right now & gathering information for down the road. Any comments or suggestions would be greatly appreciated. Thanks.  
Gregorio Lonewolf

1 and 2...  TPI motors are FULL adaptive electronic spark timing control...    hooking up a mechanical advance with a vacuum advance would be stepping backward 30 years..     there is a detonation sensor on the side of the block.. at least once during each key on cycle.. the ECM kicks the timing forward till it can hear a tiny amount of detonation..  then it backs it off a step..  this is the adaptive.. so it can learn how good the fuel is you have poured in the tank..  with a conventional vacuum advance distributer.. its just there..   the ignition timing can probably upward of 45 degrees of advance..  buick v6 motors can do 48 degrees of advance at idle..  just because you step on the brake and drop it in drive.. from about 15 degrees in park.   nothing changed.. just stepped on the brake and pulled it into drive..  how are you going to match that with a vacuum advance distributer.


later GM motors used a small cap HEI with a remote coil..  gm engineers found they did not have to have such a HUGE cap with terminals so far apart to prevent spark scatter inside the distributer..  so they phased them out..  actually the small cap HEI were less expensive to manufacture.. as only the stem needed to be cast.. the base plate on the housing was die cut sheet and crimped into place..  the reluctor was attached to the shaft but it used less parts and used die cut steel teeth and a conventional ceramic magnet.. instead of a formed reluctor..   only problem.. the ceramic magnet inside the reluctor cracked and caused random ghost patterns in the magnetic field... this caused a LOT of problems..   some of the larger parts rebuilders have actually replaced the original style delco stamped steel reluctors on the shaft with a aftermarket design that was very much like the earlier large cap HEI.. and they work great...   the sein wave from the pick up coil is so much cleaner for the module to handle..    there are adaptor harnesses for the small cap HEI to work in a big cap installation..    if you just want a remote coil..  you can get a remote coil adaptor for your cap..   easily done..


4... i was not aware that summit or davis offered a Computer controlled distributer.. i wonder if it uses the screw down cap the corvette HEI uses.. or does yours use latches like a conventional chevy..


5..    rebuilt units are OK.. they don't come with the 4 wire harness. to the ECM harness...    use care in removing yours as the strain reliefs like to snap..   not all the remans get a NEW pick up coil...  sometimes they don't get a new module..

sometimes they don't get a new cap harness..

use the phone.. ..  ring.. ring..  

Great information... Answered so many questions & knowledge gained. This may help other's who are interested & have similar questions or thoughts. On #4, most are latch style whereas mine is screw down using a 1/4 inch socket. On #5 it was a matter of curiosity, you killed that notion. I was further curious since I noticed that there were 2 types of Distributor available, small & large HEI, mine being the later variety. Of which I am glad that mine is. So much going on work wise & w/ the wife lack of time to complete the Coil replacement & waiting for the new Tach Filter I ordered to arrive in the next few days. I'll give you a ring once I am back working on the Vette. BTW, I had seen an adaptor for external coil usage on my type of HEI claiming much higher voltage ratings etc. etc. Not interested right now as I just want to be able to get this thing started & running. THANKS.  
Gregorio Lonewolf

to complete this thread...   this is the large cap HEI L98 distributer.. that uses the 4 screws to hold the cap down..



one will notice that the steel plate over the module.. this is to prevent HIGH voltage from the tip of the rotor from effecting the electronic spark control.  i have had a hard time lately finding good EST harness section that are missing from the above picture...

this is the module end of the connector shown on the distributer above...  U518



this is the connector that plugs into the cap...



notice the black wire..  that connects to the brass terminal with the screw through it as shown just above..

this is the ground strap that fits in the cap directly under the coil...



this is the in the cap HEI coil...



notice the black wire with the ring terminal. this should be on the same screw that the hooked end of the ground strap is on..

this grounds not just the ignition coil.

it is the SPARK return path...    

yes...  the high voltage spark that runs down the wires and across the spark plug gap has to go somewhere to create a complete circuit...   spark plug gap..  heads.. intake.. distributer hold down clamp, distributer housing..   the screw holding the condenser bracket. the black wire to the cap. the ground strap.. through the laminations.. if not painted..  to the black wire on the ring terminal into the coil to create a complete circuit..

guess what.. some racers have used enough sealant on race engines that the heads are insulated from the block.. so they have started running a 3 ring wiring harness .. one wire on each head and one on the distributer.. this shortens the spark return path..  saves it needing to go through the intake, hold down clamp and then the housing..  

this is one of the various GM small cap HEI reluctors..  notice the cracks..



this is the V6 version of the reman distributer.. notice the different reluctor..





lets go back to the coil...

see the circular pattern in the side...  thats high voltage leakage..  if you had the hood open and could see the coil in a really dark area..  you could see little purple flashes across that area of the coil...  





oh.. and just for kicks.. this is an HEI coil.. that somebody forgot to swap the ground strap over from the old cap during a tune up...



let me tell you .. when this flash over happens.. you are going to hear it..  its like a shot gun going off under the hood..

notice the burned away wire insulation..  and the burn marks on the frame of the coil...

if you look carefully at this blown up coil picture.. you will notice that the installed went as far as removing the paint from the corner of the laminations of the coil to get a good ground connection.. but.. they forgot the ground strap...  

a LOOSE condenser hold down screw will also have the same effect.