93 octane tune anyone?

[blefevre]

Is anyone running a 93 octane tune while using the 91 octane gas we get here? I want to buy that MarkD chip for cheap but am worried it *could* knock. If I buy directly from Mark he said he would exchange it if I did get knocking, but this used one is much cheaper so I might just pick it up.

The theory of course is that there is less dense air in Denver, therefore the cars are naturally less resistant to knock. 91 octane should be close to 93 octane at elevation. Anyone have experience?

[Roland H]

gasoline theoretically gains 0.8 octane per 1000 feet iirc. so 91 is really more like a 95. obviously depends on quality of the fuel and the DA that day, but a 93 tune should be pretty safe I would assume.

[Vince30]

^I did not know that, Thank you.

[Roland H]

yeah, at your altitude I'd run what the suggested minimum is though.

[Vince30]

I figured that much out, but its still cool to know.

[joshw02]

Is anyone running a 93 octane tune while using the 91 octane gas we get here? I want to buy that MarkD chip for cheap but am worried it *could* knock. If I buy directly from Mark he said he would exchange it if I did get knocking, but this used one is much cheaper so I might just pick it up.

The theory of course is that there is less dense air in Denver, therefore the cars are naturally less resistant to knock. 91 octane should be close to 93 octane at elevation. Anyone have experience?

No problem. I ran the Mark D 93 chip on 91 without any issues.

[blefevre]

Perfect! Thanks!

[u3b3rg33k]

gasoline theoretically gains 0.8 octane per 1000 feet iirc. so 91 is really more like a 95. obviously depends on quality of the fuel and the DA that day, but a 93 tune should be pretty safe I would assume.

Not exactly. Lower MAP = lower cylinder pressure = lower octane requirements. The octane rating of the fuel doesn't change.

[Roland H]

yes, sorry. I meant to state that it would be comparative to running 95 at sea level. Did the calculations a few years back.

[Bimmerman325i]

I approve of this thread, and have nothing to add that hasn't already been said.

If 85 in an 89/91 tuned car at altitude works perfectly for 17 years, a 93 tune on 91 at altitude will be just fine. What was the name of the bus driver?

[Bishop]

Wait.... wut


















Someone gimme the run down on this octane stuff?...

[Bimmerman325i]

Octane is irrelevant; the important variable is ambient air density.

At sea level, the air has a very well known specific density. Let's call it X. To make power, you combust air with fuel. Maximum power is at a specific air:fuel ratio, which is dependent somewhat on the octane rating of the fuel. The higher the octane, the more you can play with timing before detonation or pre-ignition. This translates to more power.

However, as you gain in altitude, the air becomes less and less dense. Here at >1 mi high, we have roughly 82% the air density of sea level. This is why we have much less power as well- less air to burn a given mass of fuel.

Whereas at sea level you can only vary the fuel part of the air/fuel ratio, at altitude you have varied the air component as well. Less air means the fuel octane does not need to be as high to maintain the same detonation and timing characteristics. A given octane rating, say 85, will act much like a 90-91 octane rating here at 1mi high, and a 91 as a 95+. This is why at altitude you can get away with running a much lower octane rating than the manufacturer specified without problems....in most cars that don't compensate for barometric pressure (i.e. it measures/calculates density).

That help?

[u3b3rg33k]

What's the highest octane available up there in Boulder? I assume the folks with altitude compensating engines (aka turbos) have a different view...

[Bimmerman325i]

91 or E85.

FI cars are a totally different story.

[clarkkey594]

What's the highest octane available up there in Boulder? I assume the folks with altitude compensating engines (aka turbos) have a different view...

The increase in altitude still affects turbocharged engines but not nearly as badly as NA motors.