Generally, low octane (faster burning) fuel is used in engines with low static compression ratios. An engine with a high static compression ratio is more likely to pre-ignite the fuel-air charge -- by compression or hot-spot -- burning it before the spark plug does. Pre-ignition is the pinging sound an engine makes when the exploding fuel-air charge slams into the still rising piston on the compression stroke. High octane fuel burns slower and is less susceptible to compression or hot-spot ignition.

It would initially appear that the RDX K23-A1 with a static compression ratio of only 8.8-1 would funtion well on low octane. However, because of forced-induction the K23-A1 is both a low and high compression engine.

The formula to calculate forced-induction compression is: FCR= [(BP/14.7)+1]SCR
FCR - final compression ratio,
SCR - static compression ratio,
BP - boost pressure

The K23-A1 reaches max boost of 13.5 psi at 4500 rpm and is then reduced to 8 psi at 6500 rpm. So [(13.5/14.7)+1]8.8 yields a maximum FCR of 16.88 at 4500 rpm.

If on low octane, the engine management computer (via the knock sensor) then retards spark timing and enrichens fuel mixture to prevent pre-ignition. This reduces power and the less completely combusted fuel/air charge goes out the tailpipe, wasting fuel and possibly damaging the catalytic converter.

In the case of ChemicalLew's TL-S, his normally aspirated engine has a fairly high SCR of 11.0-1 and requires 91 octane. He made a reasonable effort to eliminate variables, and I think his finding are valid. The computer was protecting the engine at the expense of fuel.

 

Yeah it's ok, don't worry about my ability to speak the english language properly. I fully understand what a moot point is, I was pointing out that a mute point as was posted would have to be a point that cannot speak.

 

There is nothing wrong with mixing 93 with 87 to achieve the 90-91 Octane the RDX needs. Fuel burns based on your right foot habits. At the present time I am averaging anyware between 24.4 to 25.7 MPG in my RDX (Mostly Hwy to get work). Yes, I am driving between the speed of 60 - 65 MPH. The overall MPG since my purchase is 21.8MPG City and HWy.