Thread: Higher compression ratio's OK at high alt.?

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I was reading a car mag. and a reader from Denver asked about building a motor with higher compression than usual for a daily driver and was wondering if that was doable. The magazine responded that if the motor stays in the high elevation area, like Denver, that he should be fine all the way up to 11 to 1 compression. I thought that seemed pretty high but I don't answer tech questions for a magazine either. They also told him to run a smaller carb. than usual because the throttle response would be better with the small carb./high elevation. Does anyone agree with any of this or have experiance with making street cars go fast in the Rockey Mountain states? I live in Utah and this of huge interest to me while I'm trying to build a 327 for daily driver use in an El Camino.

Many years ago we race in Denver a number of times for Divisional Meets. The high altitude made the tune up a challenge, I do remember that!!!! Can't comment on the compression thing as whenever we were out there the car was running on race gas.... It's hard to get enough air in the engine to make it run correctly at altitude.... Bandimere used to have a "tune up reccomendations for the Mile High" section on their website, might want to check there....

When elko69 was making his inquiry about determining his Dynamic CR after the motor was built I ran across this article.

http://mgaguru.com/mgtech/power/pp105.htm

Cylinder pressures are directly related to CR and atmospheric pressure.
CR is also a function of atmospheric pressure as a denominator.

So, the lower the atmospheric pressure, the higher the resulting CR.
Although the theory is sound, I do question the fact that in this example, 190psi cylinder pressure yields only 6.5:1 CR

The article states:

"It all goes back to PV=nRT."

(At sea level atmospheric pressure is 14.7 PSI)

When the gauge pressure reading is 190psi, the absolute pressure is 204.7psi.
P1=204.7
P2=14.7
V2/V1 = effective compression ratio

V2/V1 = (204.7/14.7)^0.71 = 6.5/1
SURPRISE !!!

The "1" is the total volume above the piston at top of stroke.
The "6.5" is the effective piston displacement plus the final top volume
Effective displacement is then only 5.5 parts of that 6.5 total.

From practical experience of living there and racing there (Denver/Fort Collins), I can tell you increased compression according to your cam selection is always a plus. A bit more lead in timing on stock motors was always a plus too. The biggest concern was getting air into the motor. Good manifold selection was critical and never reducing the size of the carb, rather work on the booster for sensitivity and getting the best selection for dropping the booster down as far as possible into the venturi. On injected motors we had to watch how close the nozzle size and pulse widths for best A/F ratios. It's all a matter of air in that altitude. Velocity being more critical than size and volume such as manifolds and induction.

Originally Posted by nitrowarrior

From practical experience of living there and racing there (Denver/Fort Collins), I can tell you increased compression according to your cam selection is always a plus. A bit more lead in timing on stock motors was always a plus too. The biggest concern was getting air into the motor. Good manifold selection was critical and never reducing the size of the carb, rather work on the booster for sensitivity and getting the best selection for dropping the booster down as far as possible into the venturi. On injected motors we had to watch how close the nozzle size and pulse widths for best A/F ratios. It's all a matter of air in that altitude. Velocity being more critical than size and volume such as manifolds and induction.

Very well written.