Tweak timing curve or leave as-is?

[Secret Chimp]

I recently cracked open my distributor (which had been curved by FBO for 19 degrees all in by 3100 for some inexplicable reason) to find it still appeared to have the stock light/heavy spring combination.

I took the heavy spring out and found that put me with timing all-in by 1800 rpm. That's on the bottom end of the stall range on my car (318/727), so I didn't consider it a problem. I want my timing to be all-in before I get up into my 2100-2300 cruise range. The car drove fine down the road and under load.

However I accidentally set my initial with my vacuum hooked up, so I had an actual initial way too low. After setting the initial to 15 to meet the conventional 34 degree wisdom for small blocks (15+19=34), it pinged under load. Not just at low RPM, but I tried running it up lightly past the mechanical all-in point and giving it gas and it still pinged. It wasn't horrible but it was bad enough.

I backed it off to about 12 and now it seems happy in all ranges. Is 31-32 degrees total fine from a power and economy standpoint? Or would it be worthwhile to try installing a single Mr. Gasket 925B spring in the empty slot to see if I can slow the advance down a few hundred rpm? I drive the thing a lot for an old car and use 87 for $$$ sake.

 

[440roadrunner]

Seems to me you answered your own question. You sped up the advance rate, and now it pings. You need "more spring."

 

[Hemi Rebel]

Whats been done to the engine?

 

[Secret Chimp]

Just a 6900 cam, standard Performer intake and MSD 6AL ignition on an older MP distributor. The engine doesn't really wake up and go until around 2500 rpm, I don't know if that's the cam or what.

 

[RustyRatRod]

Not surprising it would ping under load rather easily. That cam only has 204/214 degrees duartion @.050" ground on a 112. That's a good street cam. It should have great bottom end with the right curve. It sounds like what you've done is go from one end of the spectrum (not enough advance) to the other (too much advance). Are you using the vacuum advance? If so, does the vacuum canister have a hex shaped nose right behind the vacuum nipple? If it does, it's adjustable. I always throw two light springs in the distributor and limit my vacuum advance. Works good everytime with a mild setup.

 

[Secret Chimp]

It is adjustable. I think it pulls about 8 degrees or so. Is it best to get that set up so it's pulling max just at the edge of a typical highway cruising vacuum? I haven't checked where it's at since I got it back from FBO.

 

[Meep-Meep]

The one heavy spring doesn't really come in to play until high RPM because it's slotted. It's good to have that last bit of tension in the last 2 degrees so the weights don't bounce. And this is out of the DC book.. Seems to me all in by 3100 may be a little late but it also depends on A/F mixture. Your cam sounds like a great choice for a cruiser with stock drive train.

For vacuum advance tuning you really need to drive around with a vacuum gauge Teed in on the ported port and get an idea what is happening while at different conditions. This will give you an idea of throttle position under different loads. Then go back and adjust the vacuum can with a timing light and a hand pump and test drive again. Keep the tools with you for road side adjustments.

If you have too much compression for the available octane then you are fighting a loosing battle. The best solution is to adjust the compression for the fuel used then tweak the timing.

 

[Secret Chimp]

I threw in a single 925B spring to supplement the stock light spring and I'm almost there. I have a tank of 89 in the car now, and it'll ping just barely (like you can hear a half dozen individual pings) at WOT on up to 5k and sometimes uphill under heavier throttle under 2k. I also backed in the vacuum advance so it starts to activate by 11 inches of vacuum instead of around 14.
I didn't have a chance to check my curve again, but if I have it topping close to or just above 2100 I'll call it done and see what 92 or a degree or two less of advance does. Hopefully premium will do it because the car feels extra tuff set up this way now.