93 Octane vs 91 Octane

[Suberman]

Originally Posted by Clipdat

It does "burn" slower in the sense that it doesn't explode prematurely before the full stroke of the piston has been reached. 91 is "harder" to combust versus 87. That's the reason why if you put 87 in a twin the ECU has to pull back the timing and not be so aggressive.

Flame front speed is the same. Combustion chamber pressure rise is slower. Anti knock additives don't produce as much combustion chamber pressure as gasoline, because the additives have less chemical energy per litre. That's why you can run higher compression ratios and more ignition advance. The extra power comes from the compression ratio and ignition timing changes.

Toyota recommends 93 and expressly rates their engine at 200 bhp only if 93 octane is used. I'ts in the handbook. So, for this engine you can get full power only with 93 (94 in Canada, I've not seen 93). At higher elevations it can make no difference because the engine can't breath enough air. At my locale my engine is down by around 10% from rated hp due to elevation.

[Suberman]

Originally Posted by Future

We did the test at QuebecFRS.com... Difference between 87, 91 and 94 octane on an FR-S...

Results: 87: 155hp, 91: 160hp, 94: 166hp

Source (french) : http://www.quebecfrs.com/forums/index.php?topic=2222.0

This is consistent with Toyotas own information which is in your handbook. At least it is in my BRZ handbook. Not the figures but the statement that rated power is only achieved while using 93 octane fuel.

[Mikem53]

Originally Posted by Mikeez

The real difference between both of those is that 91 contains more lead. Therefore, lead is a metal and takes longer to burn. Since it's a fine metal, however it will not affect much since 93 do contain traces. What you can do is when you travel outside your area. Add better gas, you can use whichever you want.

Lead? That was phased out completely in the early 90's.

[Suberman]

Originally Posted by Mikeez

The real difference between both of those is that 91 contains more lead. Therefore, lead is a metal and takes longer to burn. Since it's a fine metal, however it will not affect much since 93 do contain traces. What you can do is when you travel outside your area. Add better gas, you can use whichever you want.

"Lead" is taken to mean tetra ethyl lead, grinding up pencils and adding it to your tank won't help.

(Joke: pencils aren't lead either, calm down).

In the 1930's a clever American discovered that adding tetra ethyl lead to gasoline improved knock resistance markedly. The fact that this compound was also highly toxic was swept under the carpet. Tetra ethyl lead won the Battle of Britain. As soon as US avgas arrived in Britain in 1940 Rolls Royce turned up the supercharger pressure in the Merlin engines which then out powered the Daimler V12 in the Me 109 by over 10%.

By the time it became well known that tetra ethyl lead was so toxic the entire motor fleet required it, especially non-jet aircraft. It took over 40 years to ban the stuff. In fact, only the advent of catalytic converters that would be destroyed by lead contamination finally secured its ban.

Mind you, one of the replacements is also known to be toxic....