Distributor cams - what's the difference

[Donald H.]

I have a few delco distributors, non corvette, and I'm setting one up for my 65 El Camino L79 engine that I just put in the car. The correct distributor for a 65 Chevelle/El Camino L79 is the 1111071, but those are pretty rare so I got a 1111075 which was used on the 250/300 HP engine.

I've been testing this 075 distributor with various springs to get the curve as close as possible to the L79 specs. The 66 service manual list the centrifugal advance as '0' at 900 RPM, 15 degrees at 1500 RPM and 30 degrees at 5100 RPM. I can closely match these with the distributor I have except mine peaks at 24 degrees at 3000RPM and does not advance any more up to 5100.

I assume the purpose of the distributor cam (pictured) slot is to limits the amount the weights can swing out at advanced RPMs. So I assume that if the cam has a slightly longer slot then I would get more advance.

The distributors I currently have all have different numbers on the cams. The one pictured is 520. The others are 532, 724, and 526.

I could take them all apart and measure the slots, but I'm trying to avoid that.

Is there any source that can explain the differences?

Thanks,

Don

[James G.]

Re: Distributor cams - what's the difference

I am of the opinion that distributors were originally set up to allow the least knowledgeable person to use a vehicle without damaging it-
Unless there is a particular reason you are trying to match the original specs - you will probably get a better running, better accelerating vehicle with a more performance oriented advance curve in the distributor.

Check out Duke's postings on the subject of distributor advance.

IMHO Full advance at 5100 is kind of lazy
AS I am typically all in by 3000 and have an advance can fully deployed at idle.

Oh forgot - yes the slot limits the mech advance. there are long slot and shorter slotted advance cams.

[David M.]

Re: Distributor cams - what's the difference

Agreed. Modern tuning specs are better.
You can use two different spring and weight combos to achieve a specific curve. Its a myth they need to be matched sets.
The weighs and stops are all different. That makes a custom curve attainable.

Are you using a vacuum can?

Try 12-14* of base timing.(Adjust the A/F at the same time using as vac gauge. You want the strongest vac signal possible at curb idle through adjusting timing and AF).

Another 8-10* on vacuum. If no vac can then bring in the cent earlier.

All in at 3-3500 for a small block 327 for a total of 40*ish. Small blocks tend to like even more total.
All depends on your set up....cam, compression, carb, trans, rear gear. Every engine is different.

Basic rule is... give it as much timing as it will tolerate with your favorite gas typical ambient conditions and driving habits.
This is where a Sun distributer machine and a dyno shine when super tuning.

[Chris T.]

Re: Distributor cams - what's the difference

The 532 (1970240 R/B 1969763) was originally used in the 1967-68 Z28 302 Camaro. It is considered a long slot and was rumored to have a different cam profile to prevent point bounce at high rpm. Many drag racers started using it before electronic ignition changed the game. Can't help you with any reference to crank shaft degree changes when switching cams.

[Duke W.]

Re: Distributor cams - what's the difference

It's generally accepted that the last two digits of the number is the amount of advance that is allowed by the slot. I've seen both 732 and 532, but don't know the difference. I started a thread on this a few months ago, but it didn't yield any more information.

The L-79 spark advance map is rather ill-conceived... a lazy centrifugal curve and overly aggressive vacuum advance. My tuning seminar offers suggested starting points for all Corvette engine families. Also, search for threads stated by me and read the "L-79 distributor blue print/overhaul thread.

I can give you a recommendation assuming a REAL L-79 m, but need to know your fuel octane and the engine's compression ratio.

Duke