10:1 compression @ 6k feet

[Sport/Truck]

I have a question about running 10:1 at altitude (6k). I am using stock cast iron heads, and also want to run 87 octane gas. One shop said no, but another said yes- because it will act like lower compression IE 10:1 will act like 9:1 at this altitude. He also mentioned if I go to sea level I’ll need premium.
Is there any truth to this?
Thanks,
s/t

[woodridge]

No matter what the compression ratio, air density makes a big difference.

[special-K]

So it will run better up there.Is that bad?And better,yet,at lower altitude.Isn`t that better?I`d say 10:1 is the line between premium and regular.At higher altitude and lowered compression,you`d still want premium to run it best.The main thing is compensating with timing and air/fuel adjustment.I`d say iron heads at higher alt.is a good thing.

[timcos]

I have a reman 350 from Jasper. I run premium and I drive normally at 4300 to 5200 feet.

I do not know what my compression is.

[jacobs]

I run 11:1 in a 350 with 041 iron heads and 87 octane. My pickup goes from 2800 feet to 9000 feet evelation and on occasion up to 14,000 ft. At lower evelations, I do have to back off the timing or it rattles like a diesel. I presently have over 60,000 miles on this engine. My reason for high compression was to get maximum fuel economy...not for performance. My research has indicated fuel economy and hp is directly proportional to the compression ratio....within limits of course. So I'd say yes you can.

[jhow66]

How long a wing do you need to get your truck this high?

[jacobs]

Originally Posted by jhow66

How long a wing do you need to get your truck this high?

A long long one!

[Sport/Truck]

Thanks for the help. I wouldn’t mind running the 11:1 but still not sure about that. But glad someone has first had experience doing this. What cam are you running BTW? I’m also after the fuel economy (within reason ~ 15mpg) & performance. Are you using a q-jet carb?

The main reason to use the lower octane is the cost. I had a turbo car that only used premium and I live 25 miles from my work and it can get expensive in a hurry. I spent about 4k in fuel a year on one car and it uses 87!

If anyone else has some input please help.
s/t

[jacobs]

Originally Posted by Sport/Truck

Thanks for the help. I wouldn’t mind running the 11:1 but still not sure about that. But glad someone has first had experience doing this. What cam are you running BTW? I’m also after the fuel economy (within reason ~ 15mpg) & performance. Are you using a q-jet carb?

The main reason to use the lower octane is the cost. I had a turbo car that only used premium and I live 25 miles from my work and it can get expensive in a hurry. I spent about 4k in fuel a year on one car and it uses 87!

If anyone else has some input please help.
s/t

Actually I have two Chevy engines with 11:1....a 327, and a 350. The 327 has an old stock 350hp 327 camshaft (151), 2.02 cast iron heads, and a Quadrajet. This engine I built in 1965. The 350 in my pickup has a stock mild 1970 camshaft, 041 cast iron heads with 2.02 valves, and a Quadrajet. I built this engine in 2000. Because I ocassionally haul heavy loads, I didn't want anything but a very mild camshaft in the pickup. I personally wouln't build an engine with under 10:1 compression for 5000+ elevation. BTW in the last 25 years, I've never purchased any fuel other than 87 octane, it's not necessary.

[special-K]

Do it.The high compression requires less throttle to gain same power.Your carb meters fuel intake.This equals better economy...unless you have no control over that right foot. I think you can find a sweet spot on regular.And,you`ll be hangin`at an average altitude most of the time.It`s not like you drive from the beach to your mountain to get to work.

[powell72]

At 6K feet with aluminum heads you'd be just fine with 11:1. Aluminum Heads allow you to cheat alittle more compression on pump gas, especially at your altitude.