Thread: pump gas

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techinspector1: Thanks for the explanation... as for pump gas we get up here, 91 octane premium is as high as it goes. Thinking about cranking pressure, I decided to play with some math:

At sea level typical atmospheric pressure is 14.7psi or 29.9"Hg (at 32*F). Typical pressure I see here in the summer is < 24"Hg which amounts to 11.8psi(?) but at higher temperature ~80*F

At 14.7psi ambient air pressure a 160psi cylinder pressure would come from 10.88:1 compression (I think this would be dynamic compression). For 11.8psi air pressure (high altitude) you get 160psi cylinder pressure from 13.56:1 compression (!)

I need to find out how to account for pressure conversion at more realistic temperatures... there are flaws in the math from the different temperatures but I think the general point is it takes more compression to get 160psi at lower air pressure.

At any rate I guess this begs the question is it the DCR that matters or the actual cylinder pressure that matters? If cylinder pressure is the key, it seems higher DCR can be used at lower atm pressure (i.e. high altitude).

Am I thinking right on this?

Of course, if I want to drive to Kansas I might run into trouble but I'm looking to get the most out of a bracket car that always runs at high altitude on 91 octane.... thinking out loud.

I'll try some more research on this and let y'all know if I learn anything else...

-Chris