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.

[jambo101]

Well said 427chev,i think most people fall into the marketing hype of naming the higher octanes with such colorful names (Super,deluxe,Premium,Gold etc)as to make 87 octane seem inferior,so they only fill up with that superior high octane and talk themselves into believing that they are somehow getting more power and better fuel economy without ever doing any meaningful measurements of whats really going on .

[DerFlosser]

Originally Posted by 427chev

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.

What amazes me about bulletin boards is that they somehow produce these attitudes where people assume that other people are oblivious and what they say is the gospel. As cool as you might be, I also have an engineering degree to my credit...and 2 other degrees to boot but that doesn't mean shit. Engine dynamics are pretty involved but fuel is a fairly easy thing to understand. Octane, as you know, refers to a fuel's tendancy to explode rather than burn. A low octane fuel will will burn more violently, releasing it's power/energy sooner than a higher octane fuel while a high octane fuel is more stable, and burns more slowly or energizes at a slower rate. Higher octane fuel can result in a smoother start and idle(more stable and efficient energy). As far as gas mileage goes....there are thousands of opinions and real life proof that higher octane fuels can deliver better mpg's. For example....here is a blurb from good ol' Tony...

http://www.fuelsaving.info/fuels.htm