Just filled up the tank with 91 octane premium fuel

[07WYarisRS]

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.

DETONATION...look it up
thats the ONLY reason to use a higher octane fuel...
thats why we have higher octane fuels, not because they can give better mileage or make the car run smoother or start easier... Fuels DONT WORK THAT WAY
Higher octane is available at the pumps because some engines are forced induction, some engines run high compression ratios like todays modern Super sport bikes and off road bikes. Even some of these engines running 12.5:1 compression only require 87 octane

I hear what your sayin...
I'm just sayin NO, its not possible to start easier. Besided the compression ratio there is compression rates and HEAT... you need to calculate the HEAT of combustion into that mix... sure the lil yota engine may be a higher compression ratio then many other auto engines but at 10.5:1 there is not enough heat and compression at start up to take advantage of the higher octanes higher flash point..
My cammed Suzuki GSX750F pushes 100 hp runs 11:1 compression and redline of 12,000rpm and only needs 87 octane... Do you really think the Yaris needs better fuel then my crotch rocket?

Like i said it all has to do with the fuels flash point, the lower the flash point the easier it will ignite, the higher the flash point the harder it is to ignite.
Thats why many bio deisel set up have a deisel start because the biomixtures like used engine oil of veggie oil often has a much higher flashpoint and is harder for the engines to start on them. they start on the diesel and then switch over to run on bio once warmed up.

With higher octane fuels they not only have a higher flash point making it harder to ignite but they also contain additives that eliminate or remove free radicals in the fuel that can promote detonation. While the actual burnt temp of the fuel may not be changed the flame front speeds can be altered or slowed to help eliminate detonation. If you engine has no detonation, then there is NO benifit to running higher octane fuel... NONE

people often think higher octane fuels burn hotter and faster then lower octane fuels... this is a myth, actualy the lower the octane the faster it ignites and can burn faster depending on the additive and free radicals in the fuel..
If you were to take a carbed engine with fixed timing and diid two runs on a dyno one with 87 and one with 91 octane fuels. the 87 will always produce more HP and higher EGT (exhaust gas temps) then higher octane fuels...

Do you know why it feels smoother on higher octane fuel... they call this soggy. When an engine is jetted too rich or running too rich it has a soggy feel to it. An engine that is running a little to rich will run awesome, but it lacks power. the richer it is the less power it makes. And ebgine running leaner produces more power, too lean and you start to run the risk of overheating and detonation and pre-ignition ( not even close to the same thing bTW)
When an engine is running efficiently it can have what is known as spark knock or PING under heavy loads, this is normal but the more efficiently the engine is burning the fuel and the more power its making the less smooth it can feel.
Thats why racers often say the engine is a little lazy or soggy feeling when running rich or loading up. This is usually caused by an eninge that only needs high octane pump fuel but is being run on race gas.