Topic: Octane

[chrisfranklin]

To save a little cash, I have been using 87 octane gas.  Recently, I started using 91 (93/94 where I'm from back east) Octane which is "Supreme" at $2.80+ for self serve in So Cal.  

I figured the jeep 4 and 6 cyls are designed to run on regular 87 and that running higher octane fuel means you are essentially throwing your money away.  But, for whatever reason (ECU Timing advance?), my 4 does seem to drive a bit better on 91 (89, mid-grade doesn't seem to perform any better than regular 87 on mine).  Anyway, my Jeep 4 cyl Revs better, has better low end torque, generally more fun around town.  Heck, enough to where I may not bother with any more power upgrades unless I do a lift, rear axle swap, lockers & 33" tire set up vs my current stock & 30's setup.  Of course, if I did a major suspension/axle change, then I'd probably get serious and swap or supercharge.    

You guys are probably wisely running regular. But, if you are drivng an upgraded 4 (perhaps with an open K&N sucking more warm engine air that can cause the ECU to retard timing) you may find it worthwhile to go with your area's high-octane fuel (91-94+ depending on altitude and other factors).  It's f#$%ing expensive, but it's made me a little more at peace with the current mild performance upgrade setup on my YJ 4 cyl.

[Bounty Hunter]

I've done quite a few power mods, I'll try a tank on my next fillup and see if I notice anything.

[happy024]

My Dad has always told me that high octane is only really worth it if you have a high compression engine. He has built many engines for the Hot rods he builds now in his retirement. So his advise to me was just to make sure to put in a good fuel system cleaner once a month and you'll have the same results but much cheaper.

just my 2 cents

[BlackYJ]

High octane gas is designed for high compression engines, it helps to reduce pinging.

[01amberfirewv]

I have to use high octane (91-93) or I can't get on the interstate (full throttle) with out knocking and pinging.  It has done it for as long as I can remember.  I bought the Jeep new and the stealership said to use high oct and if that didn't work they would do the full system clean for about $100.  At the time gas was much cheaper so it didn't bother me then but now... :evil:

[chrisfranklin]

Yeah, the 2.5 I don't think is in anyone's book as a high-compression engine.  But, I still notice a positive difference in drivability using 91 (my area's "high octane"). As I said, probably has something to do with warm intake air and ignition timing in my case  (of course, this ain't a scientific study, no dyno test or anything and my Jeep may have variations unknown to me that are outside of this board's general idea of a "control" Jeep 2.5 4cyl)  :)

Price for 20 gallons of self-serve 91 is about $6 US up from the price of the same in 87; I generally fill up 2-3 times a month.  So, yeah, you may find it worthwhile to go 87 + adding some kind of additional bottled fuel-additive if there is a savings there.

[chrisfranklin]

I looked up somethin on octane at a site called faq.org; it suggested that,

"An increase in ambient air temperature of 5.6C increases the octane
requirement of an engine by 0.44 - 0.54 MON [27,38]."

Well, the ambient air temperature, at least of intake air, on my 2.5 is probably not exactly cool.  I use a a 3 inch diameter intake tube and open K&N type filter located as far from the engine as possible, but still unshielded and inside the bay.  If I notice any performance increase using high octane, I'm guessing it's probably for this reason.

[lutzke21]

Did it state anything about altitude?

[chrisfranklin]

Didn't see anything on altitude.  My experience in Denver suggests that the high-octane gas sold there is rated at "89," I believe.  Fairly sure that higher altitude allows one to run a lower octane fuel.  

Whether altitude nulls the "engine heat increasing ambient air temperature and intake temperature and thus requiring one to use higher octane fuel for optimal performance," I'd say probably not.  Even at altitude, wouldn't be surprised if you'd benefit from running the highest octane fuel available.