Best Way to Pay for Gas in NM, Co. Utah ?
While in Florida last year, in many areas there was one price for paying with a credit card and another for cash. Sometimes the Cash price was 10 cents or more cheaper than credit card price. What...
Forum Discussion
GoPackGo wrote:
I've never seen 85 octane in the Midwest, but I think the reason you might find lower octane out west (higher elevations) is because as you gain altitude you need less octane.
Tim
absolutely correct.
At higher altitudes octane requirements go down. so if you see 85, 87, 89 or whatever, the lowest octane meets requirements for vehicles that specify 87, and the highest octane meets requirements for vehicles that require "premium". this holds true with cars with turbochargers as well. Boost is relative to ambient air pressure (altitude)
Much of Montana, Idaho, colorado, utah, wyoming is high enough that octane requirements are adjusted for altitude.
air pressure at sea level is about 14.7psi, 12.2psi at 5,000 ft, about 10psi at 10,000ft. so there is a 20% drop in atmospheric pressure between sea level and 5,000 ft and a 30% drop by 10,000ft.
and yes, there are major roads in Mt and Colorado and utah above 10,000ft and the majority of many of these states is above 3500ft. Hence the reason for lower octane requirements.
the effect is along the lines of reducing compression ratio. The max compression in the cylinder is air pressure x compression ratio. so reducing the ambient air pressure reduces cylinder max pressure.
For example, assume a 10:1 compression ratio and 100% volumetric effiency. at sea level the max cylinder pressure is 10x14.1=141 psi (I'm neglecting the effects of Boyle's gas law which says temperature will rise as well)
At 5,000 ft, max pressure is now 120psi instead of 150psi.
