How many miles on your Jeep? On dino oil changes, they break in noticeably so at about 5K miles, making a pretty remarkable difference in seat-of-the-pants feel.

 

On 35K miles so well & truly broken in by now, but I know exactly what you mean about the sudden increase in power around the 5K mark (mine woke up overnight at about 3K).

The difference I noticed on the freeway yesterday was a noticable increase in torque and the ability to pull strongly up hills at 70MPH in 6th, however I just drove home from work late tonight on deserted suburban streets so I was able to give the JK a bit of a work out. From standstill with my foot down it is noticably quicker through the gears and races towards the redline, and there seems to be much less throttle body/drive by wire hesitation from idle (all JK drivers will be familiar with this; you know how when you pull out into traffic and stomp your foot down on the gas from idle at standstill and nothing much happens for a second or two until the fancy electronics get their act together)

When I've used this tank up I'll put in regular low-octane gas and see how big the difference is. If it's only going to cost me an extra 15 cents a litre (60 cents a gallon) over low-octane unleaded to get all this extra performance (or only 5 or 6 cents a litre over the mid-octane premium) then I'll gladly pay it from now on. On a side note judging by the dash readout (I won't know for sure until I empty the tank & then do a manual calculation) I seem to be getting at least an extra 2 or 3 MPG higway running the high-octane premium gas as well.

 

You say the engine detonates on regular. The computer can't compensate for it.
It doesn't detonate on mid-grade. The computer can compensate for it.
It gets better performance on the high-grade because the computer no longer has to compensate for detonation.

You should be looking for the reason your engine doesn't like regular gasoline.

 

Yep, what you've said is exactly what I was thinking originally. When the knock sensor detects detonation on the unstable low-octane gas the computer retards the timing to stop this ocurring with a resultant drop in power. The only problem here is that the computer doesn't start to take action and retard timing until 2000RPM (the reason why Chrysler states in the owner's manual that spark knock at low RPMs on regular gas is normal) however I've noticed these power and torque gains running high-octane premium right from 1000RPM.

I guess the real issue is why the JK motors suffer detonation on regular gas; mine's done it from day one and though the dealer reluctantly agreed to run compression tests and full diagnostics, etc he shrugged his shoulders and admitted that they all do it and thus always recommends mid-octane gas. At first I didn't believe him (I hadn't begun looking at this forum back then and didn't know it's a common issue with the 3.8L) so he decided to humor me by letting me drive two of their demonstrators (one straight off the transporter with 17KM/10 miles) on the clock & the other with 380KM/235 miles) and sure enough, they both rattled worse than mine does when I run low-octane (particularly the brand new one with 10 miles on it) and suffered even more of a power drop than mine when they hit 2000RPM & the computer started pulling the timing back. I've heard that oil-burning JK motors detonate badly due to the build-up of carbon deposits from the burnt oil decreasing the volume of the combustion chamber and effectively increasing the compression ratio thus causing the unstable low-octane fuel to ignite early resulting in detonation, however mine's not an oil burner.

Anyway, enjoying the power gains and better mileage of the premium so I'm not too worried (I asked a mechanic a while back of any negative effects of running the premium on a motor supposedly designed purely for low-octane. All he could think of is that premium will burn a little hotter and thus could be an issue if I was running a really lean tune (apparantly Superchips etc lean the tune right out) which by itself will burn fuel hotter and I was operating at sustained WOT and high RPMs, however this would be more an issue with old-school iron heads that tend get hot and don't dissapate the heat into the coolant as well as alloy heads, and didn't have the hardened valve seats that modern heads designed to run on unleaded have.)

 

Agreed. Ive swapped jeeps on a trail with a buddy who has a tj and his 4.0 felt so sluggish compared to my good ole minivan motor, and im running 35s on 4.10s.