If the manual says 89 octane, you should use 89 or higher octane gas. Don't go by Premium or Midgrade labels until you read the octane rating. Some places have 87 or 89 midgrade, some have 92 or 93 or even 94 premium.
For maximum performance with minimal knocking, use the recommended gasoline: 89 octance. You can try 87 or lower if you want, but it is likely performance will be suffer (the engine is avoiding knock) and if you hear audible knocking, you should go definitely go back to 89 or higher.
There is no difference between the 87 and the 92 octane gas, the only thing that the 92 will do is prevent knocking better than the 87
The difference is is that the higher octance gas has a higher "flash point" than the lower octane gas. That is a critical and important difference. The octane in a gas is a measure of how hot and bothered (compressed) a gas can get before it ignites. When you run a high-compression NA engine (like the IS, although the IS does not have a
really high compression engine) or a forced-induction engine (turbo or super-charged) you really want all the octane you can get. If the engine is knocking or is trying to prevent knock (via on-board systems in newer cars or aftermarket devices like MSD or J&S) it is retarding the timing. If you (or the engine) are retarding the timing, you are not getting the maximum power out of the engine.
It's up to you what gas to put in. But 89 octane is in the manual for a reason. Lexus doesn't get kickbacks from Chevron.
with the compression ratio modern car uses, you usually don't have to worry about knocking under regular driving condition. The only engines you will have to worry about are the force induction engines.
Modern cars don't all have the same compression ratios. The higher the performance figures for a given size displacement engine, the higher the compression ratio is likely to be when normally aspirated. However, many modern cars have their own knock sensors and can retard timing when knock occurs.
And another thing, ever wonder what race gas is? It's gas with a very high octane rating, usually around 104. And octane boosters? Race gas allows engines to run with more advanced timing and/or more boost without knocking and pinging. In case some of you didn't know, knocking and pinging are signs of premature detonation (the gas has ignited due to compression before the spark has lit). This is bad. When bad enough, it causes engines to explode. In fact, in rotary engines, one simple ping is enough to blow apex seals...
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Tony
'01 Spectra Blue
'94 Turbo Miata
[This message has been edited by webmaster (edited September 04, 2000).]
