Some thoughts on setting the distributor curve on a 350 SBC. Bear with me, this is how I learn...by explaining what I don't know in public!

 

In the quest for a better running engine, I've spent some time with my Edelbrock 1405 600 CFM carb. With some jetting, rod and spring changes, I feel I'm getting close to target. But in doing that, I'm now beginning to focus on the ignition side of the equation. My gear head Cousin laughs at me, telling me "You've got hidden power in that distributor, adjust it already!" So that's what I'm going to try to do

A quick review;

Engine; GMPP HO350, 330 Horse. 9.1:1 CR.

Carb; Edelbrock 1405 600CFM

Dist; Pertronix flamethrower billet w/Vac advance. Pertronix coil.

Fuel; PetroCan 94 octane

Engine runs very well from idle to WOT, however at wot, under certain conditions, pinging will occur, sometimes heavy. Problem; The initial timing was set only by ear, because I cannot get a light on my timing marks. They are buried by the PS bracket, and I cannot get enough angle to see in, due to the intrusion of my non stock rad shroud. But I feel that I'm pretty much in the ballpark with initial. Probably around 12* or so. I know, I know, you're probably shaking your head, but it is what it is, lol!

 

The Pertronix dist gives me 3 advance spring options to adjust the mech advance curve,

Copper; Start 1000 RPM, end 2000 RPM

Silver; 1500 - 3000 (in place from factory)

Bronze; 2000 - 5000.

 

And also gives me 3 mechanical advance degree limiting tab options,

Natural; 12 degree mech limit.

Blue dot; 16 degree

Red dot; 20 degree.

The distributor mechanical advance is said to deliver 24 degrees of max advance. So with applying different combinations of springs and limiters, one can "fine tune" the mech curve.

 

With my settings as is, I'll say I have an (estimated) 12 degrees initial, combined with 24 degrees of mechanical, for a total of 36 degrees all in at 3000 RPM. I'm not going to factor in the vacuum in this.

The car starts and idles well. Load at part throttle induces no pinging. But it will ping at WOT though to 5500. So where I feel I should go is to NOT retard timing, but to possibly LIMIT total advance. And then possibly QUICKEN the advance curve.

If I was getting idle to part throttle pinging, then without question, I'd retard the initial slightly...but it never pings there!

 

My plan is to add one copper spring to quicken the all in to around 2500.

And add the 20 degree limiter.

Check for pinging, performance gain or reduction.

Go from there.

 

Any thoughts on this.? Where do most low compression 350's like to be? Am I on the right track?

 

Cheers, Mark

 

 

just a thought to help with the tuning, use a timing tape and a fabbed marker of your choice.

as long as the engine is TDC , mount the pointer were you can see it .. then use a performance timing tape around your dampner.

you can mount it wherever it suites you.

AND you can't always assume the mark on the dampner is correct, look at the one on my 455, the yellow mark is true TDC, the factory one is at about the 8' position in the pic.. it has rotated on the hub. I will be replacing it in the near future.

I'm on the manifold port Randy.

The pinging is only happening at the upper end of WOT, under big load, pulling hard. And that would be under very low manifold vacuum. The Vac advance should be of no consequence there would it? 

I guess ideally, one would want to be on the threshold of detonation across all rpm's, idle to 5500. Or is that not necessarily the case?

and detonation does not always point solely to ignition.. also can indicate LEAN.. which you are covering already with your carb tuning.

GLHS60 wrote:

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I just realized what engine you are working with and 36 deg. is too much advance for that engine, 32 is the recommended total. Hypereutectic pistons are very sensitive to detonation and cracking is very likely so please reduce your timing right away!!

Thanks
Randy

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 Good point. Pinging is really hard on an engine and should be avoided.

Also, in my example I used 36 degrees because that was a fairly common total timing setting for older engines like I'm used to but many newer smallblocks with their improved fast burn combustion chambers make maximum power with less timing, and of course, less timing is also safer.

You're on the right track. Sounds like you need a bit more initial timing but your total sounds close. Do all your testing without the vacuum advance hooked up. You want a clean idle and crisp off idle determined by your initial advance. Then limit your total so you are a few degrees from pinging. Poor fuel quality and heat promote detonation so leave some cushion concerning total advance, eg. a few degrees away from detonation. After you have a satisfactory initial and total then it's vacuum advance time, a totally separate deal.

Thanks
Randy

Put the copper springs in today. Starts advancing at 1000 now, and is all in at 2000. Was 1500 -3500 before. Feels livelier. No new pinging down low. Still some up top.

Next I'll add the 20 degree limiter, see if it stops the pinging. If it does, then perhaps I'll incrementally add just a little more initial, until I'm just at the threshold of detonation.

I can say the engine is running way better than it was last week.

It's a good feeling when you can find hidden power with some adjustments.

Thanks for the input!

Glad it might be though of as useful, thanks!

Just got back from a run, and I now detect a slight hint of pinging under load, at or above 2000 rpm. Another thing worthy of note, is that with the total coming in so fast, is the exhaust note. It's become louder down low. I don't know if I like the Blaaat much. I have Thrush welded mufflers (really just Flowmaster 40 knockoffs), and it's now just above what would be deemed obnoxious. Maybe it's time for something quieter. 

When you think about it, the appearance of slight pinging down low, is really a good diagnostic tool. It confirms to me, what most logically saw as a simple case of excessive ignition advance.

Had I not installed a fast curve, I would have only ever seen detonation up high, at or above the old "all in" point of 3500. Now it appears on or after 2000.

Time to start dialing the initial back. And when there is zero detonation, I'm good! I then move onto understanding vacuum advance.

 

cheers'

Wow.....thanks for the clinic!

great work !