Compression ratios & altitude

bones · Post by **bones** » Sun Oct 06, 2002 7:03 pm

When everyone is speaking of and giving recommendations of compression ratios, what altitude are the recommendations referenced to? I understand it's recommended to run higher compression at higher altitudes. I live and drive mostly at 4200 to 7000 feet and planning on 9.0:1. Is this too conservative for this altitude? Berg says 8.19 at this altitude is max but I had a 2 liter with .120 deck and it sucked at this altitude> your thoughts are appreciated.

JohnConnolly · Post by **JohnConnolly** » Sun Oct 06, 2002 11:12 pm

Actually, it depends on where you buy your gas.

if you buy at low elevations, you can bump your CR. The problem is the gas companies sell a lower octane fuel at high elevations (super here at 4200' is 89, and I recall at 10K it was 85 or 83). Engines can use more CR IF the octane was the same, but since you can't easily change CRs the gas companies just give lower octane fuels.

John

Mitch · Post by **Mitch** » Mon Oct 07, 2002 5:22 am

I live above 6000 ft. Low Octane is 85, med 89 and high is 91. I run 7:7/1 with on a 110 cam and dual Kads. Im sure I could bump it up to over 8:0/1 with no problems. If you drive your car down to sea level you mite have problems if you bump it to high.

At sea level air is 14.70 lbf/sq in, at 6552 ft its 11.53 lbf/sq in. You take 142/14.70=9.56, then you take the 9.56*11.53=111.26 psi. Thats a range of 78-111 psi for good compression at 6552 ft above sea level. All this with no pocket protector!

[This message has been edited by Mitch (edited 10-07-2002).]

Dexter · Post by **Dexter** » Mon Oct 07, 2002 9:06 am

Yeah right, Mitch, we all know you wear a pocket protector

jmillheiser · Post by **jmillheiser** » Mon Oct 07, 2002 5:55 pm

i also live at 6000ft and regular unleaded is 85, mid grade is 87, and premium is 91. i wouldn't run 85 octane in anything air cooled even at 6000ft. id run at least 87 in a stock bug at this altitude and 91 in any beefed up ACVW engines.

as for compression you can run higher compression with the same octane gas at higher altitude. it helps to offset the effects of the thinner air. and believe me ACVWs do suffer up here. at the Bug-In at Bandimere i saw a lot of 1776-1915cc cars with dual carbs, exhaust etc. running 17 and 18 second ETs and stroker motors running high 15s and 16s. i believe the fast run of the event was a 9.54@148mph by one of the pro-mod cars(not sure who, all i know was the car was black).

Mitch · Post by **Mitch** » Tue Oct 08, 2002 4:50 pm

You add about a secound to ET's above a mile high!

kdanie · Post by **kdanie** » Tue Oct 08, 2002 5:17 pm

Bones, Your 2.0l would probably run much better with less deck height than the 1/8" you had. Take it to .045" and I would bet you would see a considerable improvement unless you are using semi-hemi heads (this should spark some debate).
I have considered moving to Flagstaff AZ (7000+ ft.) in a couple of years but I realized I would have to turbocharge all my cars so I could get decent performance (could be fun....).
ken

Mitch · Post by **Mitch** » Wed Oct 09, 2002 5:23 am

My house is around 6500ft above sea level and Im going to turbo my next engine.