Quote:
Originally Posted by curvedglass
P235/70/15 BFG T/A's
No idea on the timing.
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235/70r15 @ 745 revs per mile with a 3.07 rear=2,475RPM@65MPH.
Same tire with 3.73's=3,010RPM@65MPH.
in other words, you would be turning roughly the same RPM you do now at 85MPH with the 3.07's @ 65MPH with 3.73's.
Quote:
Originally Posted by TJ's Chevy
You want no more than 36 degrees no load so I'd bump it up to 12 and see what happens, the engine may be low enough on compression that 14-16 may not be an issue. Try 12 then 14 and if 14 is good I'd stop there and be sure to run high octane. lol
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36º is only a very rough "rule of thumb" and does not include the extra provided by vacuum advance under low load conditions (and some stuff will make max power with timing in the low 30's, while other engines will want 40+). There are also different mechanical advance curves for different years/applications, so you can't really make too much of an assumption based purely on initial timing.
Quote:
Originally Posted by curvedglass
I ran out to the parking garage at lunchtime. I set it to just about 14. Seeing how much it was off yesterday I don't think I have to worry about pings. I drove it around the garage one time and goosed it pretty good. No pinging. Felt just a least bit better at the pedal. Have to see what the ride home brings. It is great having the corporate parking garage for lunch forays. I have done detailing prior to cruise nights, tire rotations, wiring. Everything but an oil change. Security did come out one day - I had my Buick wagon to work and was adjusting the vacuum switch for my 200-R4 kick down. Ended up frying the meats with a little too much smoke. Said he heard the squealing!!
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I can't speak for your exact combo, but this is what I did the 250 in the truck I'm currently driving. I only run 87 octane.
I had an HEI I put in that was frozen solid with rust, so the timing was locked at wherever you set it to all the time. I tried 36º and it did fine most of the time, but would knock with a lot of load and throttle opening at lower speeds. I backed it down to 32º and it only did it at very low speeds and WOT under heavy load. It was early in the year and cooler outside at the time.
I finally replaced that with a working points distributor a little bit later. I found a good one in my pile that had 24º of mechanical advance and some typically stiff springs that made the curve lazy. I found some very light aftermarket springs in a SBC distributor I had laying around and installed those.
I wanted to start out at 36º total, so I set the initial to 12º and checked the curve. It came in quickly and already had 28º by 1,800RPM (all in at 3,000).
IIRC this did pretty good overall (can't remember if I could make it ping or not, though).
Point wear eventually increased the timing to something like 38-40º and it would ping under heavy load going up highway hills. It was around 95º at the time and the engine runs somewhat warm at highway speed (something like 190º instead of 180º). The engine was also rebuilt at some point in time, so any pitiful quench it once had has no doubt been taken away by rebuilder pistons and a thick composite head gasket.
I backed it down to 8º initial/32º total as a very safe tune that I can run WOT under any condition and not worry about detonation. Fuel economy didn't appear to change, although I think it made a little more power with more timing.
I don't know what the curve for your HEI is, but IIRC most have 20º of mechanical advance, so 12-16º initial is probably about right. If the curve is slow you still might not have very much advance where you are running it, so check it and see what it's like there. An advance spring kit is only a few $ from Summit or whoever.
Hope this helps.