1998-Octane, 92+, 89, 87 - Real World results?

[emarkay]

I know there are knock sensors, adding spark retard (as acoustically detected, not sensed by fuel type, per old rumour) as needed to correct preignition/detonation, but on these Classics, what are some results of testing, comparing similar conditions with varying octane levels. Also, any non-MPG related experienced driveability concerns?

Thanks!
MRK

[rorafan]

IIRC, there were posts a few years back where some folks did some live testing with a scan tool. I *think* the consensus was that most could get away with 89 during cooler weather.

Found the thread (there are others too): aurorah.proboards.com/thread/12430

Of note -

aurorabrain:
89 in my 1997. I have seen no performance improvement at the track, or any KR on a scantool in up to 80 degree weather. However, there is little to no carbon in my engine, which can explain why this works for me. I may have a factory freak i suppose, although the engine is about to die any day now. My 4.0 has the same crankshaft end play that speedneon was talking about, except i noticed mine about 50k miles ago.

scottydl:
Short version - in the 2nd-gen and most cars out there, you are wasting money on anything over 87. In the 95-99 4.0L, 91+ is recommended for best results (that engine has higher compression) but many of us do fine with 89 and/or the occasional 87.



I usually run 91+ except during cold spells, which around here is anything below about 10F. Maybe overkill but I like knowing I'm using what the engineers spec'd for the car.

[human]

The '98 model I used to have used to get the same mileage on 89 octane as it did on 93, but it suffered a pretty big hit when I used 87 unless I added a bottle of octane booster, which I got from the Dollar Tree. I haven't done as extensive experimentation with my '95 that I have now, but it gets around 18 in town on 87 octane with a bottle of boost, same as the 98; however, it doesn't get as good on the highway as the 98 did. I really have to baby the '95 to get 26 mpg on the highway and the '98 would routinely give 28 or 29. Both cars had/have the Autobahn package, but the '98 model had California emissions.

[scottydl]

Keep in mind that if/when you drop octane, it takes the sensors some time to recognize the new fuel and adjust. I'd give it a couple tanks, otherwise yeah dropping from 91 to 87 in a Classic may be noticeable at first but would theoretically smooth out over time.

Octane booster is more of a gimmick than anything... every 1 "point" on those products (some are rated up to 5 I think) only gives you 0.1 added octane. So even on a +5 booster product, you are only getting 0.5 added octane. This is assuming you are adding to whatever the product bottle considers a "full" tank.

When I owned my '99 I committed to using only 91+ for six straight months, about in the middle of my ownership period. I noticed no difference in power, acceleration, or fuel economy during that "test" time period, compared to the 89-90 I typically used the rest of the 5+ years (65k-70k miles) that I owned the car. YMMV... literally!

[skatterbrane]

Aha! I did not know that there was different emission equipment on CA cars! I am on my 5th 98-99 Autobahn equipped Aurora. ALL of my previous ones easily got 28-30 mpg on the highway at 75-80mph. My current one is from CA, and I typically only get 25-26 mpg. Mystery solved.

One thing I will NEVER understand is if you get less PPM emissions, but use MORE gas, I would think the net yield of emissions would be better on a car that uses LESS gas even though the PPM readings are higher! This was even more apparent in the 1967-1975 years when cars went from say 20 mpg to 13 mpg because of progressively more emission equipment. A typical 1966 small block Ford, GM or Mopar V8 would get better gas mileage than a typical 1975 4 or 6 cyl.

[RCA1186]

I know when I used 87 the first time I definitely noticed a lack of power, then again it was only one tank, probably not enough time to adjust. Anyway I only drive the Aurora once or twice a week, so buying the 7-8 gallons every two weeks the price difference is negligible to me, I'll pay for the peace of mind haha.