HEI Upgrade for 71 C/20?

[jrc55]

I've got a nice straight '71 C/20 (Camper Special) with a 402BB that I inherited a few years back that I use mostly for light street use and the occasional lightweight hauling (very handy). Aside from already having fixed just about everything else on this vehicle, the block and carburetor have been rebuilt and the stock distributor upgraded with a Pertronix Ignitor, but it still doesn't seem like it's combusting as good as it should and the mileage is awful ( <10 ). It's been suggested that if I upgrade to an HEI ignition system, I'll see improvement. Now, I'm pretty handy with tools, wiring, and simple tune-ups, but I'm not a "gearhead" and not knowing anyone else with one of these I've got a few questions that I'm hoping someone would answer for me...

1. Has anyone else done this to this particular engine and what were the results and pitfalls to watch out for?
2. Will the bigger HEI distributor fit on the back of the block that close to the firewall? It's kinda of tight back there. I'd hate the have that problem after buying parts.
3. What else gets altered besides the distributor and tossing out the coil? Plugs? Gap? Ignition wires? More?
4. Exactly what do I need to go out and get so I can do this myself? Junkyard HEI? New stock? New aftermarket? Suggestions?

Any helpful comments or advice greatly appreciated.

Thanks in advance,
Jim Chapman
Portland, Ore.

[72C203503ONTHETREE]

Its an easy swap to do, I have done a lot of smnall blocks, but the big block uses the same distributor. Just remove the old stuff, make sure you now where to point the rotor for the new HEI. Make sure you run a full 12 volt wire to the batt terminal on the HEI, it will be marked "BATT", use a female spade connector. Use a 12 volt from a keyed power source on the fuse box. The other terminal right next to it is for the tach lead, if you have a tach. The plug behind the BATT and tach plugs is for the wires that come from the module on the base. Thats all there is to wiring it. I havent seen a big block in the older trucks yet, so I dont know how much room you have. As far as plug gaps, try about .45, factory settings for the 75 up HEI used to be .60 but the .45 has been found out to be better. Try both and see what works best, see if it misses. I would get a new or good used aftermarket HEI, you can get a brand new performance HEI from e bay for les than 100 buks, cap, rotor, and coil included. You will need new plug wires since the distributors terminals are like the spark plugs ends.

[MACKL]

Jim,
I can't help you with the fit of the distributor, you should be able to make a couple of measurments to determine if it will fit. There is a couple of important items to consider. The standard ignition works off of low voltage, usually less than 10 volts. If you run a full 12 volts, it will burn the points up real fast. Most vehicle achieve this by running a resistor wire to the postitive side of the coil, this knocks the voltage down to the require range. (Note, there may also be a wire running from the starter "R" terminal to the coil, it bypasses the resistor to give the ignition system an extra kick when starting.) When you convert, you will have to run a new wire to supply 12.66 volts to the "Bat" terminal of the HEI distibutor. This wire should have power any time the key is in the run AND start possition, but not the "ACC" possition. This one wire will complete the requirments for the primary side of the ignition system, now on to the secondary system.
The HEI distributor produces up to 40kv (that is 40,000 volts). Your old spark plug wires will not handle that much voltage. Change them and don't buy cheap-o wires, they will not last either. Pay close attention to your wire routing, heat is the worst enemy of the wire. Most likely, they will not fit in the existing wire retainers so you will have to upgrade them or at least use cable ties.
On your spark plugs, be carefull, the head design determines the type of spark plug you need to use, (ring seat or tapered seat, and the reach of the plug). You can not change that part. The advantage of the HEI system is that it can utilize a larger gap. (.045 vs .035 in most cases.) You should be able to re-adjust your old ones. The last factor in spark plugs are the heat range. (The temperature the plug runs at.) The range is determined by the distance the heat has to travel to the head to be cooled, it has no effect on the actual spark. One big misconception is a hotter plug will give you a hotter spark. Not true, it will give you a hotter spark plug. If the plug is too hot your electroids will be white and not last as long. Some of the plugs produced today can give you better performance but not because of the heat!
When purchasing your distributor, check it carefully. On a junk yard distributor, check the shaft for side play at the top and bottom bushings. Next look at the pick up coil wires. The go from the round coil of wires around the shaft to the module. That is the weekest part of the system. If brittle, they can be replaced for about $15.00. Next look at the vacuum advance. The diaphram will go bad, check it for its ability to hold vacuum. (Suck on it)
The HEI coil is in the cap and can be checked for excessive restance with an ohm meter. In most cases, this has a very low failure rate. The coil has 2 wires that go to the box on the side of the cap where the module lead connects. There should also be a ground strap that goes from the coil mounting screws to the box on the side of the cap. The coil will burn up without it. There is a lot to check on this, but could save you money if you are careful in your purchase.
If you purchase a quality new distributor, most of this will be done for you, but if you have to piece together all of the parts, don't forget the coil ground strap. I personally like to stay with the GM system, but there are good arguments for most of the top brands.
This is an upgrade that will help make your truck more maintance and trouble free, I would do the upgrade.
I hope this help...Kelly

[Longhorn Man]

Lots of HEI info on the FAQ board...
In all reality, your electronic conversion, if in good shape, should be fine and any benifits you get in the swap to HEI would be minimal...onless your dist is on its way out.

[glock35ipsc]

I swapped out Dad's 72 C-20 402 big block (stock for this truck) with a HEI, fit just fine, no clearance problems at all.

The first thing we noticed right away is that it started immediately, I mean like RIGHT NOW, when the key it turned. Runs a helluva lot better too!

[Blue68]

Don't expect much better gas mileage as a byproduct of the HEI. Just expect less maintenance and perhaps some noticable better running.

On the wire front, you can reuse the existing plug outlet on your firewall bulkhead connector by pulling the white (at least I think resistor wires were white) 20 guage resistor wire and replacing it with a purple (that's the color for non-resistor) 16 guage wire. You can try to reuse the spade that is in the bulk head or buy replacements from a parts store. I rewired most of my underhood using wires from Napa and connectors from Pep Boys and some from Mad Electronics. Painless was too expensive for what was under the hood so I rolled my own.

[68 Suburban]

My truck jumped 2+ miles per gallon with just and HEI and new re-gap plugs.

[jrc55]

Thanks all for the suggestions. This is good information. I already converted to a full 12V supply for the Pertronix, so that much is done.

Any opinions about going with new stock, junkyard stock, or aftermarket?
And, since this is retrofit, what later models are compatible so I know what to ask for?
Also, I'm seeing a lot of comments about the vacuum diaphragm and weight curving the advance. Is there some straightforward way to make sure this is close without turning this into an Automotive Science project?

[GMC Jim]

As Longhorn Man said... there is good info on HEI's in the FAQ's.....Also if the replacement HEI was removed from a 'California Emmisions" vehicle...performance will not likely to be as good as with your old 'points" dist... So; just any old HEI will not give the best performance.... The distributor advance curve on the HEI should match as close as possible to the engine and distributor specs of the engine the HEI will be installed on.....Jim

[busterwivell]

I didn't notice any change in gas mileage with my 402. I changed the wires and plugs at the same time. Had no problem with clearance at the firewall. I used Autolite #404, gapped at 45. I bought a complete HEI from NAPA...and I kept blowing modules. Took it back and got another and had the same problem. My friend owns a junkyard and he gave me a used one to try. It's still in there a year later without any problems.

[GMC Jim]

The electronic module is a critical part of the HEI distributor. This module is located opposite the vacuum advance unit and is secured by 2 screws. Quote from GM 1978 Service Manual-- " When replacing the module a liberal coating of special silicone grease MUST be applied to the metal housing surface on which the module will be installed. If this grease is not applied, the module will not cool properly, which can cause the module to malfuntion. "
This grease can be obtained from a GM dealer or from an electronic supply facility ( including Radio Shack.) Electronic parts suppliers refer to this grease as ' heat sink compound " which is used to conduct heat to the mounting of various electonic components; mostly those I.C.'s associated with power supply components.
I would suggest that one should remove the module from any used HEI unit; clean the module and its mounting surface; re-apply fresh silicone grease.......Jim

[busterwivell]

You can get the grease in a tube from NAPA. One tube should last a lifetime.

[jrc55]

Thanks all for the comments.
Jim