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      06-23-2009, 04:56 PM   #1
dcstep
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Drives: '09 Cpe Silverstone FR 6MT
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Location: Colorado

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2009 M3  [4.00]
Octane vs. Altitude (blending for hp)

In several other threads I've talked about the problems of tuning my '09 M3 coupe at the altitude around Denver, generally 5100 to 5400 ft above sea level, so long as you stay out of the Rockies which are only about ten-miles away.

The graphs below all show my car with the Dinan throttle bodies installed, a Dinan chip and a Macht Schnell high performance air filter. The only difference is the fuel octane, which I vary by mixing 91 and 100-octane unleaded gasoline. The 91 is Shell and the 100 is Sunoco.

The dyno is a Dynojet showing SAE adjusted results, as follows:
  • Maroon Run is 100% 91-octane, yielding 340.17 hp
  • Green Run is 30/70 blend of 100/91-octane, yielding 345.74 hp
  • Red Run is 50/50 blend of 100/91-octane, yielding 338.65
  • Blue Run is back-to-back with the Red run, only using 5th gear instead of 4th to see if 5th was more efficient, which it was, recording 344.47 hp, around 6 hp better.

Clearly, the 30/70 blend is the way to go, which yields roughly 94-octane. The 50-50 blend is around 96-octane.

YMMV giantly depending on your altitude. 100-octane at sea level yields a 30-ish increase in hp. It's not so easy at altitude because our effective cylinder compression drops due to lower air density and the slower to ignite and burn nature of higher octane fuel just screws up the timing once the engine has reached its maximum advance, at least that's the prevailing theory around here.

So, to summarize, I gain roughly 6 hp by blending 100/91-octane 30/70. From that point, I lose 7 hp by upping the blend to 50/50 and a gain 6 hp due to the efficiency of 5th gear vs. 4th.

I've got a line on a custom ECU tune that may allow me to improve these numbers, a little, but that'll have to wait for a future report.

Dave
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Last edited by dcstep; 06-23-2009 at 10:52 PM.
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