Mitsu engineers' choice of static compression ratio for this engine represents a good compromise for pump gasoline with a small frame turbo, which delivers peak torque in regions where the engine is particularly knock prone. Where fuel octane is the limiting factor, more power is realized by using an SCR that allows a better ignition map than an SCR that requires retarded timing to accommodate the fuel. Just a couple of degrees of ignition retard at critical points in the curve are enough to more than offset any theoretical benefits of a higher SCR. Then again, the shorter rod/stroke ratio (1.5 vs. 1.7) of a 2.3 tends to be just slightly more forgiving, but exactly how much more is difficult to quantify. There's nothing insensible about playing it safe.

As per my recollection, BMW's 2.0L F1 engine cranked out over 1000whp with octane limited fuel and an SCR of around 8:1.

 

Just thought it should be mentioned, I saw the Skyactiv mentioned earlier. It is a direct injection motor, takes on a whole different set of principles.

 

Yeah we got one in the family and I couldn't believe that thing runs on 87. I wish some of that weird cupped piston magic was applicable to us. I'd love an E85 build with a decent 91 octane backup plan.

 

The 'magic' lies in the direct injection itself. There can't be any preignition when the fuel isn't there yet. We won't ever have that luxury.

 

If plan is for pump gas only, stick in the 8.5-9:1 range. I wouldn't go lower, and stock compression is 8.8:1 which is proven to work well.