K-town wrote:
> Hello,
>
> Not being an engine expert, I figured I'd throw this question out to
> this NG. I have a 1990 Civic LX, 1.5L DPFI 5-speed, and usually I put
> regular unleaded (87 octane) in it. Every once in awhile I'll put mid-grade
> (89 octane) or premium (92 or 93 octane) in it because I heard that doing so
> will keep your injectors clean. (Is that true?) Once I really splurged and
> put 100 octane fuel in it (only 5 gallons) and it did run pretty good. One
> thing I noticed, is that if I run premium (93) in it several tanks in a row,
> it will actually not run quite as good. It's a very minuscule difference,
> but I can tell there is a slight decrease in performance. If I drop it back
> down to mid-grade or even back to regular on the next tank, it seems to go
> back to running better. Is this all in my head, or is it possible to have
> an "octane overkill" on such a small 4-cylinder engine?
>
> Thanx in advance,
>
> Jonathan

increasing octane effectively = slower flame front propagation speed.
that reduces detonation in combustion chambers with poor flow/mixing
configurations, sharp angles & extended reach. good mechanical design
addresses these problems pretty much completely, therefore low octane
can be used without detonation and with ignition timing matched
accordingly. using a high octane fuel in a low octane timed engine puts
too much of the combustion energy out of the exhaust because it's still
burning on the down stroke, not expanding and providing push for the
pistons.

that said, you may notice a slight increase in power for a mid grade
fuel in that motor without a detonation sensor. partly because, in the
absense of a sensor, the engine timing used has to be a "compromise"
range designed to be safe and partly because of fuel chemistry.

here's an interesting post that touches on the subject:
http://www.digest.net/bmw/archive/v9/msg04942.html

it seems to assume that ignition timing is optimal for each different
combustion characteristic of the different blends described. you can be
assured that gas companies use all the cheap options for their lowest
grade of gas.