GM HEI large cap to small cap swap

It's in a fuel injected application and I matched up wire colors from one distributor to the next, that should be it, right?

Bobzilla should know.....but he's in lincoln currently.

I've got conflicting information from different web sites and actual people, so I thought I'd fall back on you guys.

Unfortunately my swaps until this point have always been taking out a chevy engine and replacing it with something better so I'm a bit out of my element.

Make sure you have the right distributor gear for the cam, if I remember right the small cap ones are in roller cam engines, the large ones are for the older ones.

Bobzilla converted his tpi large cap hei over to the small cap hei when he did the miniram intake on his c4, but I'm not familiar with the specifics.

I think he had to run a different dizzy, coil, cap, and rotor though(don't quote me on this though)

should work.

worst case, you will be out a module.. but those are cheap..

now that i think about it- which wires did you "match up"? from what i remember, the wiring between the coil and module is a self contained harness, so you really only need to hook the power and tach wires to the coil and maybe one wire from the ecm to the module.

Most small cap dist were set up for knock control. There should be an extra control box the goes with it. Hopefully you got the whole harness. There isn't much to it. Your ECM may need to interface as well. What engine are you working on? If it's a small block chev, then yes you need the right dist gear. Cars had roller cams starting in 1986, trucks in 1996. Not sure about the 4.3 V6

Gearheadotaku wrote: Most small cap dist were set up for knock control. There should be an extra control box the goes with it. Hopefully you got the whole harness. There isn't much to it. Your ECM may need to interface as well. What engine are you working on? If it's a small block chev, then yes you need the right dist gear. Cars had roller cams starting in 1986, trucks in 1996. Not sure about the 4.3 V6

Corvettes might have had roller cams in 86 (or not), but the other cars got them in 87 with the trucks getting them in 96.

It's a crossfire injected SBC going into a toyota pickup.

I didn't have room to run a large cap HEI comfortably due to firewall issues, so I swapped in the small cap unit to gain some clearance. I can't get it spark yet and I'm pretty sure the PCM is wired up according to the pinouts, +12v constant and grounds where it needs them.

If I can get it to spark a few times, I'll swap the dizzy gear over, as it stands now I may need to make the big cap distributor work.

The plug I swapped over was the 4 pin one leading to the distributor, the wire color matched up. The small 2 prong connector goes to the remote coil and I have clean hot power going to the coil.

Squeezed the large cap HEI back in there and I've got some fire... intermittently. I did just twist the wires together for connections to the dist, so I'm guessing it's a quality of connection issue there.

Now I'm going to see if I can find tests for the pickup and coil on the small one. There is a new module in there, but it's autozone brand, so it could be no good either.

novaderrik wrote:

Gearheadotaku wrote: Most small cap dist were set up for knock control. There should be an extra control box the goes with it. Hopefully you got the whole harness. There isn't much to it. Your ECM may need to interface as well. What engine are you working on? If it's a small block chev, then yes you need the right dist gear. Cars had roller cams starting in 1986, trucks in 1996. Not sure about the 4.3 V6

Corvettes might have had roller cams in 86 (or not), but the other cars got them in 87 with the trucks getting them in 96.

You're right it was 1987, center bolt valve cover engines.

Tested both distributor gears while I had them out, both were magnetic, so they should be interchangeable.

Did some more research, the 8 pin HEI modules I'm using are fully independent of the PCM (found a really good page somewhere on using them with megasquirt). The new module I was using tested bad, as did the next 3 they pulled off the shelf in autozone and checked before before I left I've swapped the module out again and still no spark.

The pickup in the distributor had the proper resistance, even while flexing wires. I'm kinda at a loss as to what to look at next.

psychic_mechanic wrote: Tested both distributor gears while I had them out, both were magnetic, so they should be interchangeable. Did some more research, the 8 pin HEI modules I'm using are fully independent of the PCM (found a really good page somewhere on using them with megasquirt). The new module I was using tested bad, as did the next 3 they pulled off the shelf in autozone and checked before before I left I've swapped the module out again and still no spark. The pickup in the distributor had the proper resistance, even while flexing wires. I'm kinda at a loss as to what to look at next.

why would a factory distributor gear be non-magnetic? they used either steel or cast iron gears, and a magnet will stick to either of them.

I thought the non-roller cams had bronze gears, I could be wrong or just have that backwards.

I did get it to fire with another new module finally.

Now I just need to address the little things like finding an adjustable fuel pressure regulator.

There's actually a jumper harness available to swap between the two. I just got a pigtail from Napa and made my own.