Ignition timing v/s elevation = rule of thumb?

[NIsaacs]

Like the topic says, is there a generic rule about this with any given engine build? I know there are a lot of factors that influence ignition timing, but once a build is established, with all the factors of that build including gas octane.

On my Ford puller I have the total timing set at 37* and never change it. Should I be? My normal pulling elevation averages 4500 feet. Am I leaving power on the table if pull at 2000 feet or 6000 without an adjustment.

I noticed at the Powell, Wyo. pull (4500') that all the west coast trucks got beat by the locals. These are good tough pro mod trucks, but they pull about 500' average. Did the elevation beat them?

If there is only 10-15 hp lost that's probably not enough to matter, but if there is 25-30 lost, that does. Is that much possible or no?

Nick

[machoneman]

Wallace has about the best free calculators around. While they do have one on hp loss from altitude, I'm not sure they have a timing versus altitude calculator.

http://wallaceracing.com/braking-hp.php

IIRC, Pro Stock (and other) racers hated to go to high altitude tracks due to the need to recalibrate, a lot, their carb'ed cars. But, I don't recall some local Pro racers back then (DSS Racing) changing timing. Instead, they had to fool with the air bleeds a lot to compensate. Interesting too is that all the nitro fuel cars then and even now plug in higher c/r pistons solely for these races to avoid losing too much hp and upsetting their carefully calibrated clutch/timer settings. 

[KMcCullah]

There is no rule of thumb that I'm aware of. Just speaking from my own personal seat of my pants experience here.  In thinner air you have a little less cylinder pressure which causes the air/fuel mixture to burn slightly slower. So advancing the ignition timing a little at higher altitudes seems to help me. At 4500' with C4-G iron heads I found 44 total advance worked good with 100LL avgas and 10.4:1 SCR. I like avgas because it's designed to burn in old airplanes at 12000' elevations. Nice and mellow, no pinging. But with Blair's Pro-Ports, the combustion chamber is a hell of a lot more efficient so I have not been past 34-35 degrees with 11.6:1 SCR and 50/50 Sunoco 104 and ethanol free 91. But without any dyno data, I don't really have a clue. Just a few trophies.  I hope to have some dyno time soon.   

[Falcon67]

My 302 is locked at 36, the 351C in the Mustang at 38.  DA here runs 4000+ in summer, DA at the Bracket Finals was around 900'. No adjustments.

[NIsaacs]

Okay, thanks. I will keep doing what I have been doing, which is nothing I was just looking for some free hp!

Nick