Just filled up the tank with 91 octane premium fuel

[427chev]

Originally Posted by DerFlosser

I am not disagreeing with what you are saying. My point is that it's possible for the car to start smoother due to the higher octane fuel because, even though you may build engines and see drastically higher compression motors, 10.5:1 is still a "high enough" compression ratio to see benefits at start and idle condition utilizing a better grade fuel. I am not saying to run the car with the higher octane fuel...I am merely saying that it is a good possibility that the feeling is real in regards to the smoother start.

Nothing you said makes sense.

First of all, small chambers (inherent to a small displacement engine) are more efficient than larger ones because flame propagation occurs over a smaller area. That's the main reason that motorcycle engines are able to run such lofty compression ratios.

Secondly, the Yaris is one of the few engines that offers DIRECT INJECTION - which cools the intake charge and as discussed in the following link:

http://www.greencarcongress.com/2009...portfolio.html

10.5:1 is not a high compression ratio for a modern, direct injected, small displacement engine. The 2.4 liter, direct injected GM engine discussed in the link above runs an 11.4:1 CR!

Hence, the 10.5:1 compression ratio in the Yaris engine (where each cylinder represents a mere ~ 0.375 liters and where direct injection is involved) isn't comparable to an engine that utlizes port injection and larger cylinders (particularly the bore dimension).

What amazes me about these bulletin boards is that people voice opinions without having any REAL understanding of the subject matter, most of which boils down to advanced engineering principals.

There is absolutely, positively NO EMPIRICAL EVIDENCE supporting the notion that any modern engine that has a recommended fuel octane rating of 87 PON can, in any way, benefit from a gasoline of a higher octane rating. If anything, empirical evidence suggests the opposite, since the slightly lower burn rate of the higher octane fuel may adversely impact the intended timing of the complete combustion process, relative to the position of the piston and the valves.