GregD

Assuming that Superchips did their tuning at sea level, it should be fine to run the 93 tune with 91 octane gas in Denver or higher altitudes. In Denver, the cylinder pressures should be about 17% lower than sea level since the air pressure is that much lower.

Atl JK

Yeah, one crazy thing about the increase in altitude is that I'm also getting a nice increase in MPG. Not the big decrease in power that I expected either. Not until I hit the mtns at least...

Foo

So are you going to change your name to DEN JK? yep, I'm in the airline business.

Atl JK

I should.

JPop

I saw that you had mentioned that before and I did a little math to figure out how that could happen and also how the motor reacts to less air. On the motor side, the MAP sensor is going to get you the same amount of air pushed through the cylinders whether your at sea level or 5,000 feet. The only difference would be at WOT where at altitude you wouldn't be able to get the same amount of air in. The advantage is that you're moving a vehicle with a large drag coefficient through thinner air. The thinner air is going to decrease your drag by about 20%, or at 70mph and 5,000 feet about 40lbs less.

JPop

It's not going to make any difference (my apologies to those I gave a different answer to previously) running at altitude as the MAP sensor is what's going to decide what the appropriate fuel mixture is, not the existing air density of the locale. The volume of air needed at sea level or 5,000 feet remains the same to run at 2500rpm and the only thing that would change would be the throttle body blade position to adjust the amount of negative pressure in the intake manifold.

GregD

But your peak cylinder pressures, which occur at wide open throttle, will be about 17% less. My understanding is that peak cylinder pressures assuming the rest of the tune remains the same, determine the octane requirements of the fuel.

JPop

Because the 3.8L JK engine is a MAP system everything is calculated from manifold air (or absolute) pressure. At WOT/Open Loop I'm unsure as to how the fuel needs are calculated, and if there is an assumed amount of air density. I will check next week and see what I can find out as everything on SC's forum has no indication that a drop in octane is required at altitude.

GregD

My experience has been with other cars that have had a tendency to ping at sea level seem to have no problems with the same octane gas at altitude. I've also read that peak cylinder pressures are the primary determinant of the octane requirement.

With a MAP sensor the amount of fuel should be matched to the amount of air going into the engine. If there is a default map for WOT, the engine would run very rich at altitude; at that point the readings from the oxygen sensors should signal the computer to reduce the amount of fuel, and correct the mixture. But I don't think any of this matters for the octane requirement since it won't impact the peak cylinder pressures, which will go down in direct proportion to the density of the air ingested into the engine.

I'm not an expert on the subject, just somewhat knowledgeable, but it's my understanding of the reason why.

oregon signs

Do you guys leave the flashpaq plugged in or put it away.