Distributor Curve for Stockish 383

What should I be targeting for a distributor curve for my stockish 67 Coronet 383?

Initial? Total? All in at what rpm?

It is a street car that we take down the track occasionally - my daughter drives it. Runs 15.0 in the quarter. Weighs 4000 lbs with me in it. Has the original 383 with 8.9:1 compression, Holley Street Dominator intake with a Holley 750 double pumper carb Cam is 0.474 lift and 233/240 duration (intake/exhaust) @ 0.050.

Suggestion on what to do with the distributor? Its a 1990 vintage Mopar Performance vacuum advance with Pertronix ignition.

Interested too

Us 67 Coronet owner Polaks need to stick together! LOL!

OK I'll jump in and take a stab at it... how about: initial 12 deg, full 36 deg, all in by 3000 (heavy car)...

You'll need to modify the slots to limit the total advance to 24 degrees. Definitely run vac advance, opinions vary between ported and full (although the vacuum can has to be all-in at idle for the timing and idle RPM to be stable). Not sure how much vacuum your cam will have especially with only 8.9:1 CR.

You could contact Don at FBO for advice (and buy one of his timing plates too).

I've attached a curve graph that I found online a while back, but I cannot verify if this is an actual factory curve. Even if it is, they were pretty conservative. But, at least, it's a starting point. Good luck.

Roger

Try attaching again. Be curious to see what is out there.

I'll post one from the specs in the Plymouth FSM.
I've noticed that Dodge and Plymouth sometimes spec'd the initial slightly differently.

initial timing is going to depend on how many degrees are in the distributor. the cam/advance plate will be stamped with a number to tell how many degrees are in the distributor. then a curve can be planned.

I'm going to type this before looking at Dr. Charles because I'm curious how similar or different we think about this.
What should I be targeting for a distributor curve for my stockish 67 Coronet 383?
It is a street car that we take down the track occasionally
It sounds like you're willing to tweak everything.
I *think* 1990 was still a Chrysler built distributor - which has the better advance mechanism - and the big block versions - should have a curve similar to the factory curves for the 4 bbl big blocks.

I'd start with 4 bbl and 440 high performance factory curves and factory initial as reference.
Then take your timing light and tach and measure what is on the car now.
12.5* BTC at 600 rpm was the Plymouth initial recommended. It should start advancing a little above idle rpm.
With the cam change, I would expect it will be happier (more powerful) in gear if it was around 15 - 16* initial. The catch being whether it can be idled at 600 rpm - doubt it.

Around 1600 rpm the advance should slow down. The second (higher rpm portion) of the advance should be long and slow. This is critical when using vacuum advance.
Compared with the factory 383 4bbl timing, the MP distributor will probably have a quicker secondary curve - and look like it stops advancing around 3000 rpm. That's probably fine for what you're doing. A little higher might be better if the car was used for long trips.
Mopar Performance recommended 38* BTC for production head 383. I'd consider that max unless testing at the track suggests otherwise.

Attached are three images:
'67 383 4 bbl and 2 bbl timing specs. (Interesting to note the 2bbl combustion doesn't gain efficiency at higher rpms)
'67 440 Hi Performance engine timing curves. (Use the non-CAP for reference)
Mopar Performance timing recommedations from the instructions. (Notice they recommned setting the distributor based on timing at 2800 rpm - close to all in. We can discuss why they did this later, but it is a good reference point for your 'all in'. If you see your distributor is still advancing past 2800 rpm, then a litle less than 3** is probably safe. If you can, see what rpm advance appears to stop. Be careful of course).

Interesting. We're close!

If anything, the engine will need a little more timing at idle because of the low compression and sporty cam (ie overlap).

I would not use Don's plates because they limit maximum advance,
Then the whole curve has to be shifted up - and that messes up the part throttle timing with vacuum advance.

I have one of those plates and I seem to remember that it effectively shortened the slots on the inside? (Moves the weights out and then engages the plate).
So the total advance is still limited to the number of degrees labeled on the plate but it adds some tension so the curve shifts up.
Or I could be completely wrong don't feel like pulling the distributor and disassembling to find out.

Anyhow I doubt that cam will be happy idling at 600 rpm too. So if he's starting into the centrifugal advance at whatever the idle is, it will never be stable (more idle speed, more advance, more idle speed etc.... or less idle speed, less advance, less idle speed...)

My 451 (272@.050 mushroom) has a reasonably stable idle at 1050 rpm, 22 initial, 36 total, all in by 3000 or so. Could use even more initial but hot starter kickback begins around 24.
Only has 7" vacuum at idle - but it's manual everything.

Sounds like the OP should just start at a reasonable point like we suggested, and fine-tune from there!

https://www.forabodiesonly.com/mopar/threads/distributor-advance.229240/ Scroll thru there is a good chart about how much to shorten the slots. I usually go for .375 with just the light spring without vacuum advance. Set for 34 to 38 total without vacuum is a good place to start, the vacuum advance acts up if run with this set up.

.0154" of travel in the slot is one distributor degree. about .012" of travel for the vacuum advance arm is one distributor degree. the distributors in my street cars use both the light and heavy springs (i stage the curve similar to factory), reduced centrifugal degrees vs factory stock, and sometimes I limit the vacuum advance travel.

I can run 41 degrees total on 93 gas on a stock low compression 72HP 440. Then you get those engines that have carbon build up or burn oil and like to ping.

But does it make more power at 41 than at 36?

I timed it by ear and found it ran good at 41, after that it falls off if I don't run 93 it will ping. I do run a 4 pin GM HEI module.

The vacuum acting up is a clue.
I'd bet the slots are welded on the outside.

The chart is helpful but its best to measure what the engine is seeing. Timing is lost in the electronics, and as Charles and I were discussion, its possible that timing at idle measured is not the initial.

Back before we got so smart with the internet and magazine articles, and racing dynos and flow benches, when guys actually took their cars to the track and actually worked on them and tried stuff, they would sometimes see improved et's with 40° to 44°.

And sometimes they actually didn't see improvements, and sometimes they actually cracked top ring lands and other unpleasant things that come with detonation (which may be inaudible at full throttle...)

Having MORE information to work with is rarely a bad thing...

Timing does not break ring, detonation does. Two different thing.

So you were curious as to the OPs et difference was between 36 and 41°. I guess that was not clear.

don't forget, there are different advance cans and they can be adjusted with a 3/32" allen key.
your gas mileage will thank you for sorting this out.

TRUTH ^^^^^^^^

Definitely... but sort out the centrifugal advance first - THEN add vacuum advance and adjust for light-throttle cruise without surging.

I don't know what attitude your at but unless it is below 1000 FT MSL I would shoot for between 14 to 16 degree BTDC at idle and have all the advance in by 2000 RPM limit the advance to 34 to 36 max,
Set that up first with no vacuum advance and go drive it and then if you want to try the vacuum advance hook it up later so you can judge the results from both independently

Down here in Southeast Texas, when I still had my stock worn out 383 in my Charger, it really liked 22 initial, 40 total all in @ 2000rpm, 7 more on vac advance. ported vacuum