I'm seeing a co-mingling of two different topics here. Low octane fuel and absolute mileage, both as related to a high performance (boosted) application.
FIRST:
Low octane. Bad idea if you are running a boosted setup. Period. Run higher octane fuel to account for the increased compression to prevent detonation. This is so fundamental AND NECESSARY that the dollar/penny difference between 87 and 91~93 octane (depending on what you have in your area) should have a common and consistent response from any and ALL of us who are giving counsel relative fuel choice.
SECOND:
Mileage optimization. Basically speaking, the fact that a vehicle is boosted in and of itself DOESN'T necessarily make the vehicle "inefficient", or subject to certain poor mileage. The OPPOSITE can also be true, depending on how EFFICIENT your setup is + how you are driving your vehicle. A boosted vehicle usually has a lower compression UNTIL the boost is being generated. At the low compression driving end of things, it is perfectly possible that your mods and flow/combustion efficiency is HIGHER than a normally aspirated vehicle being driven the same way. EVEN under low or moderate loading and boosting you may STILL get better mileage than you would in a normally aspirated application. It's about HOW WELL your vehicle combustion is occurring for any given setup.
Case in point: My Supra has run it's BEST MILEAGE at points in it's life where I was at HIGHER BOOST and POWER level capable setup, but due to all the mods which contributed to efficiency of flow and reduction of power conversion losses, I achieved 27.5 mpg on a long, full tank drive on the highway SPECIFICALLY targeting mileage as my objective. That same setup UNDER HARD, BOOSTED DRIVING would quickly drop down to around 20mpg or even below that mark. STILL not bad for a 300+ hp car. (NOTE: The Supra NEVER achieved 27.5mpg even as a STOCK vehicle... so although I had INCREASED horsepower substantially from about 230 to 300+, I had BETTER mileage POTENTIAL, just NOT when I drove it FOR the power... but it was still pretty efficient overall.)
At the end of the day, it IS possible to have BOTH power and efficiency (mileage) but NOT necessarily AT THE SAME TIME. Sure, there may be RELATIVE EFFICIENCY throughout the performance envelope, but when HIGH POWER OUTPUT is the primary objective, you are ALWAYS going to have to accept a reduction in absolute mileage.
It's all about how effectively your vehicle is setup and converts the fuel to propulsion energy. The key is, are you driving with a gentle foot or a heavy foot???
If you are STILL getting poor mileage when driving relatively "gently" and/or on open, highway driving with steady state smooth cruising, then your vehicle is inherently inefficient. That's a whole different issue from what we are discussing altogether.
All that having been said, if you are looking for POWER, then overall, you will HAVE to accept the trade-off of mileage for that extra "ooomph"!
-crisp
by crispone » Wed Aug 31, 2011 3:00 pm 
BTW where is festus when you need him 
