Compression Ratio

[landon]

Curious what Compression pistons people are using here in Colorado? How high can you go with our available 91 octane gas? Daily driver at 6000 ft in Glenwood springs area... 

 

I am looking at getting a new engine built to replace my current 225k mile engine. This one was built in Houston years ago with 9.3:1 so certainly has not been an issue driving in the mountains. I'm thinking 9.5:1 minimum or maybe higher if possible?

May go with a 284 cam and stock ignition setup. Will be making trips to west coast with it so curious about running too high CR here in the mountains but running into problems down low?

 

Thanks!    

 

[jimk]

Landon,

The maximum reasonable compression ratio at your elevation and 91 octane is about the same as you would select at seal level with higher octane fuel.

The octane at our elevation is lower because for the c.r.,, the cylinder  pressure at ignition time is lower and the fuel  doesn't have the same tendency to to auto ignite.  So lower octane is sufficient.  Turbo engines today auto retard timing for the lower octane.  They would benefit from higher octane fuel.

Jim

[landon]

OK, I'm thinking about this wrong then. I thought because of the lower cylinder pressures you could run higher CR.

Octane levels I've seen at sea level are 92-93. I recall my engine builder in Houston (25 years ago...) using 9.5:1 pistons and relieving the CR down to 9.3:1 because that would work well with the available 92 octane fuel at sea level. Conventional ignition system.

 

I see people going with 9.5 and 9.8:1 in a number of the builds described on the FAQ. One builder I am looking at mentions 9.8 and I had a a similar recommendation from a builder in Denver a while back. This wants to be a smooth street engine with mild upgrades...  so not sure how much is too much?

 

Thanks,