"K-town" <jdu52580@carolina.stopspam.rr.com> wrote in message
news6cqd.7794$Mu3.802926@twister.southeast.rr.co m...
> 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
>
High octane fuel is for high compression engines, to prevent detonation
(ping). Your owner's manual will tell you what grade to use - probably 87
octane. Rather than going to a premium grade (which, it's true, often has
higher detergency), stick with the recommended octane. Some brands have more
detergents than others, and they advertise that. (I like Shell/Texaco and
Chevron.) I had a Nissan 300ZX that would get balky - the clutch would start
to feel grabby - if I ran 76 in it for a couple months but would straighten
right out about halfway through a tank of Texaco. My other cars never seem
to care.

We also have a Toyota Prius, and on the Prius newsgroup others have warned
me it won't run right and will even set the "check engine" light if premium
grades are used. (The Prius has what Toyota calls a "modified
Atkinson/Miller cycle" engine - it uses valve timing to tame the 13:1
expansion ratio to a variable compression ratio.)

Mike