The Stovebolt Site Forums The Shop Area Engine Shop Distributor for 302

I'm trying to choose a distributor to use on my 302. The stock one is a Delco 1112386, which has no vacuum advance. For street use I think I'd like to have one with vacuum advance, but I'm having trouble figuring out the best choice.

The specs of interest appear to be Cam Angle(dwell?), the degrees of max. centrifugal advance and the RPM at which it occurs, the vacuum reading at which vacuum advance starts, the vacuum reading at max advance and the degrees of maximum advance.

For traditional single-point distributors Cam Angle appears to be available in three ranges: 28 - 35 degrees, 31 - 37 degrees and 38 - 45 degrees. Max centrifugal advance ranges from 12 degrees @ 1000 rpm to 17 @ 1800 rpm or 18 @ 1700 rpm. Beginning of vacuum advance ranges from 4-6" to 7-8.5". Full advance vacuum ranges from 7.5" to 18.5. Max. advance seem to be limited to 7 and 10 degrees.

Anyone have any advice in this area, and if possible, the reasons for the choice?

Thanks

Which "302"? If you're referring to the late-60's Z-28 Camaro V-8, I believe the original distributor was a dual-point unit without a vacuum advance designed primarily for high-performance operation with little attention to fuel economy.

If you're referring to the GMC 302 6-cylinder, I can't comment, as my exposure to that engine has been very limited.

If you're going to use the Camaro as a muscle car like it was intended to be, stick with the stock dual point setup and don't worry about gas mileage. If you're going to make it a "Granny's Grocery-Getter" street engine, drop in a mid-80's HEI distributor with a ported vacuum advance. The original 302 had an extremely radical camshaft, and didn't like to idle below 1,500 RPM or so. Getting any kind of low-speed performance or gas mileage was just about a lost cause, regardless of the type of distributor.
Jerry

Doh! Left out the important details.

It's a GMC 302 six out of '53 big truck, that I'm putting in '52 Chevy 1-ton. Transmission will be a Clark 280VO, where top gear is 0.8 overdrive. Rear end going from 5:14 to 4:10 when I find a suitable donor, or 4:57 at the highest. The goal is to cruise at 60mph with 2000rpm (on the flat).

Why not a HEI, would be an easy fix and provide tons of spark or better yet Langdon's has their mini HEI and Tom knows all the specs. Check out Langdon's Stovebolt.com

Jim

+1 on the HEI! It's a pretty easy conversion, assuming the dimensions of the mounting are similar to the 235/261 engines. I machined a HEI distributor for an exact fit to a 261 recently, including a custom-made hold down bracket, and that approach works a lot better than trying to adapt the wobbly bracket from the rotating-housing original distributor with the vacuum advance. Be sure you use a big enough wire from the ignition switch to the dizzy, without any resistors in the circuit.
Jerry

Jerry, I also machine the 250/292 HEI to fit deep enough into the oil pump and then make a split ring hold down like Speedway sells for mags. Is yours easier to make? Also run a 10 ga wire from battery through relay to terminal on HEI cap. Use original 16 ga ignition wire to activate relay.

I also substitute the shaft from a V-8 distributor for the 2-piece 6-cylinder shaft, cut it off to the correct length for the 235/261 engine, and file the correct size and shape tang for the oil pump drive. It's a pretty easy conversion to attach the 6-lobe upper part to the shaft by either welding or brazing. The 2-piece shaft has a nasty habit of introducing a wobble if the housing is not centered perfectly in the block. In some cases, it's necessary to install a sleeve over the distributor housing to assure the distributor stays exactly centered in the hole in the block, then machine the OD for a close slip fit.
Jerry

Here is something that some of you might like to play with and still use the stock distributor. It uses the stock points to trigger a GM 4 pin HEI module.
Points triggered HEI
The original condensor is removed or at least the wire taken loose and tucked out of the way. A stock coil with ballast resistor can be used or an aftermarket coil that is designed to be used without a resistor. Some of those do not have one side of the primary and the secondary coils tied together like the stock point type coil does. That is what the optional ground is for on the side of the coil drawing
This setup still uses the points, but because there is very little current flowing through them, they should last forever. The point dwell angle is not an issue with this setup.
I found this on another forum and with a few changes, wired it up on the bench as you see it. Spinning the dist by hand, had a single plug snapping away.
Another guy on the forum also wired one up on the bench and made the observation that the coil fires when the points close, rather than when they open. This may alter the rotor phasing a little, so it is something that would have to be checked. His plan is to try it on a Fnord Flathead.
Here is a very interesting web page that shows wiring up the HEI module to stock inexpensive late Chevy Crankshaft position sensors for the trigger. The main reason I included this is because it shows the same wiring for the ballast and stock coil. It's very near the bottom.
Crank Trigger HEI
The page is actually for small engines and tractors, but don't let that stop you from looking at it. Toward the bottom they talk about using Chrysler and fnord electronic ignition modules. The problem with this crank trigger is that there is no mechanical or vacuum advance, but it would be a good start for a fuel injection setup.

A stock 302 distributor has 14 degrees(28 crankshaft} of distributor mechanical advance at around 3200 engine rpm.With the initial 5 degrees,a total of around 32-34 degrees.A 235 Chevy distributor will drop right in, or the vacuum advance unit from a 235 will work on the stock 302 distributor.I believe a 235 dist.has a little more than 14 degrees of mechanical advance,probably not an issue for casual driving of a stock engine so long as it doesn't ping.
Be carefull on those HEI distributors used in older GM 6's with a hold down clamp like a V-8.Since the stock set up uses a kind of floating clamping,the original machining of the distributor pad wasn't critical.When clamping down the HEI,do check to make sure the dist tang doesn't push down on the oil pump drive more than necessary which might cause a siezure of the oil pump.It has happened before from what I hear.
Richard,that HEI module using points is interesting.My modified GMC 302 has a multi spark CD ignition box trigged by the stock points,and yes the points last a long time.But electronic switching of the coil is most likely more accurate especially since many old distributors are worn.

Good points, Tony! Part of the setup on the HEI distributors I modify is to machine away the entire original mounting flange, and fabricate a hold down fixture with a pinch bolt clamp. Then the position of the pump drive can be adjusted up or down for the best fit, ditto with the drive gear/camshaft fit.

I also check the centrifugal advance curve on my distributor tester, and make whatever changes are necessary to either the total advance or the spring rate. Using the stock distributor to trigger an electronic module is a good idea, as long as wear or wobble issues are dealt with first. If a 4-pin GM module is used, be sure and mount it on a BIG aluminum heat sink with a good ground. Those things ground the coil through the bare metal base plate, and the power transistors develop a lot of heat. Use dielectric grease between the module and the heat sink.
Jerry

The heat issue is something to be aware of, but let me add this.
The guy that posted this was using a big MSD Blaster coil and no ballast. He didn't have a spark plug on the coil wire and was letting it arc about two inches from a coil wire to its ground. He reported that the module was getting very hot.
I used a stock Chevy coil, a ballast resistor and a .035" plug gap. My module wasn't warm after spinning the dist for maybe 2-3 minutes.