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Teach me about large/small cap distributors

Mastiff

Mastiff

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I'm trying to surf up info on this, just for my own knowledge. It seems like large cap distributors were pre-TBI? Thing is, I have a large cap distributor on my TBI setup. It has no vacuum going to it, and it runs the TBI just fine. Were there factory setups like this, or do I have an aftermarket oddity? I remember swapping the distributor when I went to TBI, but I can't remember why I had to. What does the TBI need besides the different module?
 
If I remember right, there were a few early TBI setups that used the big cap with a different module. But I've never seen one myself. I was told something about the pulse signal that the normal big cap dist couldn't produce correctly for TBI :dunno:
 
They did come on a few models. But I don't think ever a truck with fuel injection?

What's differant Spark Latency of the EST module and it's programmed into the chip. Can change timing quite a bit like aftermarket modules do!
 
eagle mark said:
They did come on a few models. But I don't think ever a truck with fuel injection?
Click to expand...

TBI? If I looked at a stock 1991 K5, would it have a large coil in cap distributor?

What's differant Spark Latency of the EST module and it's programmed into the chip. Can change timing quite a bit like aftermarket modules do!
Click to expand...

Yeah, I'm running Dynamic EFI, which is mostly GM TBI hardware. Just wondering why the whole distributor needs to change when going from carb, instead of just the module?
 
Mastiff said:
TBI? If I looked at a stock 1991 K5, would it have a large coil in cap distributor?



Yeah, I'm running Dynamic EFI, which is mostly GM TBI hardware. Just wondering why the whole distributor needs to change when going from carb, instead of just the module?
Click to expand...


My 91 has a small cap distributor from the factory.
 
Wonder WTF I have going then... large coil in cap with TBI timing feedback and no vacuum/mech advance. :dunno:
 
Probably out of a Camaro or? They did make them, just not in our TBI trucks, as Cuervo said Small Cap external coil.

Better design, look into cap and rotor and see why it can't cross fire.
 
Mastiff said:
Wonder WTF I have going then... large coil in cap with TBI timing feedback and no vacuum/mech advance. :dunno:
Click to expand...

TPI engines from IIRC 85-87 came with large cap HEI with coil in cap, 88-later used small cap HEI with external coil.
 
4X4HIGH said:
TPI engines from IIRC 85-87 came with large cap HEI with coil in cap, 88-later used small cap HEI with external coil.
Click to expand...

They had TPI in 1985? Mine's running with a 7747 TBI. Did you mean TBI? Like maybe a car application?
 
No, cars had TPI back then. Corvettes started crossfire even earlier, but no idea what they used for distributor. Uncommon enough I'm sure you don't have to worry about the x-fire stuff.

I should check my '87 to see what it's got, I'll trust it is small cap. Don't understand why they didn't go all one size at one time across all the lines, but typical GM I guess.
 
Mastiff said:
They had TPI in 1985? Mine's running with a 7747 TBI. Did you mean TBI? Like maybe a car application?
Click to expand...

TPI started in 1985. I know for a fact that 1985 and 1986 used the large coil in cap HEI and maybe it was 1987 that was the changeover to small cap HEI with externally mounted coil. I know that ALL TBI trucks used the small cap HEI.
 
GM made the large cap to get bigger spaces between the firing points to avoid misfire when they started running more and more spark advance.

Then the Large Cap HEI was to big as cars got smaller so the new design of the small cap HEI external Coil was built. This gave them the couple inches needed for engine placement to firewall. In the meantime they used what they had... large cap... while small caps were being built into everything.

Like I said earlier it is a superior design to regular cap and rotor and cross fire is always an issue when running high spark advance. So they are degree a little to help but the real problem solver was the cap and rotor and the way it was channeled to avoid a cross fire to wrong cylinder.

Here's a picture of cap, don't have one of rotor but it too has channels to direct spark.

gm-cap-rotor-channel.jpg
 
Found some old pics I took during my TBI conversion. Looks like I have a 4-pin connector and a power wire going to the distributor, and another connector from the base up to the cap. Anyone seen anything like this before? Can you remind me what the connections are for at the cap? If I recall, there's the button in the middle, and two come in the connector.

FigA2-Underhood_kit.jpg


FigE6-New_distrib_installed.jpg
 
Okay, found this on another thread. Looks like I have the four wires to the module, then the power lead just wraps around to go up to the cap. The other connection is the tach. Make sense?

EDIT: I brushed up on my cap connections. The original HEI had a total of 5 pins (3 on the inner connector and 2 on the outer), with +12 coming in to coil+ and also tapping off to go to the module. Three wires went from the distrib base to the cap: +/- coil and +12 from ignition. With the TBI distrib, only two wires come from the base, and these go to the same place in the cap. It looks like the guy who made my harness used the 3-wire connector as a means to get +12 ignition back to the ECM (pin A6 perhaps). The 4-connections at the module don't care about where the coil is located.

Interesting to note that the module for the TBI (diagram below) requires no +12.

attachment.php
 
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Mastiff said:
Okay, found this on another thread. Looks like I have the four wires to the module, then the power lead just wraps around to go up to the cap. The other connection is the tach. Make sense?
Click to expand...
Yes, makes sense. The 12 volt power going up to cap is for the ignition coil power!

Mastiff said:
EDIT: I brushed up on my cap connections. The original HEI had a total of 5 pins (3 on the inner connector and 2 on the outer), with +12 coming in to coil+ and also tapping off to go to the module. Three wires went from the distrib base to the cap: +/- coil and +12 from ignition. With the TBI distrib, only two wires come from the base, and these go to the same place in the cap. It looks like the guy who made my harness used the 3-wire connector as a means to get +12 ignition back to the ECM (pin A6 perhaps). The 4-connections at the module don't care about where the coil is located.
Click to expand...
There's so many modules and ways to convert, who knows? Don't worry about reverse engineering it, just follow the wiring for the Small Cap. See pic...
attachment.php


Mastiff said:
Interesting to note that the module for the TBI (diagram below) requires no +12.
Click to expand...
Right! The module works off of a 5 volt reference that comes in through the By-Pass wire. When dis-connected there is no ECM timing control, only what timing is built into the module and you set timing to 0* with by-pass dis-connected.

So how can you get any timing with no power? Well the Pick-Up coil generates it's own power to module and module has an amplifier inside.

You'll see in diagrams that the external Ignition Coil get's it's 12 volt power feed directly from fuse source to generate spark plug spark! Has nothing to do with EST/ICM module except being told when to turn on and off spark!

It's a very complicated module! Here's what is going on inside.
attachment.php


gm ign module.jpg

HEImodules.jpg
 
This is from the Chevythunder web page:

1985 and 1986 engines used the flat tappet hydraulic cam and lifters, if you pull the distributor from this earlier engine be sure you plan on using it on a 1986 and earlier block. This is the familiar large cap HEI distributor. In 1987 the engine block went to roller cam and lifters and the distributor is smaller with remote coil. The use of the roller cam meant that a special metal was used in the distributor gear to make it compatible with the steel billet cam. If you have a 1987 and later block, be sure to use this distributor. If you plan to use the earlier distributor in a later block, you will have to change the distributor gear.
 
7Deuce said:
This is from the Chevythunder web page:

1985 and 1986 engines used the flat tappet hydraulic cam and lifters, if you pull the distributor from this earlier engine be sure you plan on using it on a 1986 and earlier block. This is the familiar large cap HEI distributor. In 1987 the engine block went to roller cam and lifters and the distributor is smaller with remote coil. The use of the roller cam meant that a special metal was used in the distributor gear to make it compatible with the steel billet cam. If you have a 1987 and later block, be sure to use this distributor. If you plan to use the earlier distributor in a later block, you will have to change the distributor gear.
Click to expand...

Does this logic apply if you are planning to use a later model distributor in an earlier block?
 
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