What type of octane I should use???

[07WYarisRS]

Originally Posted by SophieSleeps

^ someone who knows his stuff.
I hate it when people use high octane gas in their cars and claim they "feel" a difference. I feel bad for them.

I don't necessarily agree about higher octane = loss of power and fuel efficiency. What is your reasoning behind that?

Again that's directed more towards NON fuel injected cars/engines.
A fuel that has a high flashpoint will not ignite as quickly and in a engine that can't compensate by adjusting the timming or fuel delivery that's going to cause a slightly rich mixture as it takes longer to ignite. And we all know the leaner your engine runs the more HP it can make. A sign that an engine is getting more fuel then it needs is carbon build up and even black smoke from the exhaust. This unburnt fuel kills the cat convertors quickly and reduces HO even more over time by choking off exhaust ports and build up in ring lands causing premature wear and blocked oil flow.

With an EFI system the car can make the adjustments needed to correct the fuel/air mixture to some extent.
This is why most Etest places recommend NOT using high octane fuel...

I've been building racing ATV/ motorcycle engines for years and have tested several different fuels and oil on the dyno as well as CHT and EGT gauges (cylinder head temp & exhaust gas temp).
If you set up a carbureted engine to run at WOT at say 1250 degrees on 87 octane, then run that same engine on 94 it's going to drop the egt slightly like 1210-1230 and that just means that you are missing out on HP. In most cases I am able to drop down a size or two on the main jet to gain back the HP and still be at safe operating EGTs.

The computer in a efi car will do the same to compensate by adjusting the fuel trims to lean it out and correct the fuel mixture to restore the hp.

Even at cruise speeds when the A/F mixture is cycling back and forth between too rich and too lean you are missing out on some HP.

Toyota's new ECU programming and use of wideband O2 sensors allow for a much more accurate air/fuel mixture. Instead of cycling back and forth between rich and lean like the old systems they are able to maintain a much closer mixture to stoich with out bouncing back and forth from one extreme to the next.

But like I said no matter what octane you use in your car the ECU/injectors etc will deliver a mixture as close as possible to Stoich and regardless what octane is used the engine with a perfect fuel air mixture and correct timing will not knock.

The only time a person needs to worry about knock and may want to think about using a higher octane is if knock is occuring at steady speeds under normal loads. Many times a higher octane is not even needed, you can just switch brands of fuel. If a higher octane is needed go up 2 points. eg 87-89. There is never any need to jump from 87-91-94 unless you have bumped up the compression an extra 50 psi. An engine like ours that is designed to run 87 and is suffering from detonation will not need more then 89 if it does you have a serious problem.

Occasionally our engines will experiance light ping/knock (spark knock/detonation) a bit under heavy loads but this is no cause for concern, it simply means our engines are operating efficiently.

[SophieSleeps]

Originally Posted by 07WYarisRS

Again that's directed more towards NON fuel injected cars/engines.
A fuel that has a high flashpoint will not ignite as quickly and in a engine that can't compensate by adjusting the timming or fuel delivery that's going to cause a slightly rich mixture as it takes longer to ignite. And we all know the leaner your engine runs the more HP it can make. A sign that an engine is getting more fuel then it needs is carbon build up and even black smoke from the exhaust. This unburnt fuel kills the cat convertors quickly and reduces HO even more over time by choking off exhaust ports and build up in ring lands causing premature wear and blocked oil flow.

With an EFI system the car can make the adjustments needed to correct the fuel/air mixture to some extent.
This is why most Etest places recommend NOT using high octane fuel...

I've been building racing ATV/ motorcycle engines for years and have tested several different fuels and oil on the dyno as well as CHT and EGT gauges (cylinder head temp & exhaust gas temp).
If you set up a carbureted engine to run at WOT at say 1250 degrees on 87 octane, then run that same engine on 94 it's going to drop the egt slightly like 1210-1230 and that just means that you are missing out on HP. In most cases I am able to drop down a size or two on the main jet to gain back the HP and still be at safe operating EGTs.

The computer in a efi car will do the same to compensate by adjusting the fuel trims to lean it out and correct the fuel mixture to restore the hp.

Even at cruise speeds when the A/F mixture is cycling back and forth between too rich and too lean you are missing out on some HP.

Toyota's new ECU programming and use of wideband O2 sensors allow for a much more accurate air/fuel mixture. Instead of cycling back and forth between rich and lean like the old systems they are able to maintain a much closer mixture to stoich with out bouncing back and forth from one extreme to the next.

But like I said no matter what octane you use in your car the ECU/injectors etc will deliver a mixture as close as possible to Stoich and regardless what octane is used the engine with a perfect fuel air mixture and correct timing will not knock.

The only time a person needs to worry about knock and may want to think about using a higher octane is if knock is occuring at steady speeds under normal loads. Many times a higher octane is not even needed, you can just switch brands of fuel. If a higher octane is needed go up 2 points. eg 87-89. There is never any need to jump from 87-91-94 unless you have bumped up the compression an extra 50 psi. An engine like ours that is designed to run 87 and is suffering from detonation will not need more then 89 if it does you have a serious problem.

Occasionally our engines will experiance light ping/knock (spark knock/detonation) a bit under heavy loads but this is no cause for concern, it simply means our engines are operating efficiently.

That's a mouthful. I didn't know you were referring to carb'd cars with that comment.

[sroberts]

Here in Denver, CO we have 85,87 and 91 octane. Since we are at a higher altitude it is recommended by the dealer to run the 85. You only need to run higher octane on higher compression engines.

[dbaxter]

Dont think you should be running 100 octane, I only use that when going to the dragstrip in my SS or GTO, just less chances of running on the low octane tables and less chance of knock.

I have been running the it on the lowest regular octane at any of the stations. Truthfully not careing as long as it was cheap. Runs great and getting the advertised 32 mph city. I am sure yanking the spare and putting in the CIA will help more then anything else, header and tuning also.

[riceboy]

Just put in the cheap stuff and drive...

[mikeukrainetz]

Lower octane gas will fire with less compression, if its misfiring or pinging the gas is lighting up well before TDC, higher octane will bring the flashpoint back up to prevent this. Ive found this really only happens on older vehicles where the timing (among everything else) is old and worn. Carbon buildup can cause this as well. High compression engines (60's-70's and less so 80's and rebuilt hi-po engines) require higher octane to prevent detonation and literal meltdown. What I dont really understand is in the scheme of things the Yaris is a high compression engine - it runs at 10.5:1 and can run on 87 octane... my 70 chev with a built 350 runs 10.25:1 and I cant get away with anything less than 91... it doesnt have aluminum heads but I never understood why one works and the other doesnt....