Core XK's Garage

[07JeepXK]

I get those numbers running 87 with my 4"SL and 285s and no engine tune.

Haha I get those numbers with my 305/70 MT/R's

[06JeepXKHEMI]

Its 100% cause of the snorkel lowering air temps being a true cold air intake

[lekmedm]

Haha I get those numbers with my 305/70 MT/R's

I think I've mentioned this somewhere in another thread, but, at least for me, the question of which octane to run and possible tunes (if applicable) is a rather multifaceted one. It's one thing to run various octanes and engine tunes for power and performance reasons, but when some guys throw in what kind of MPGs their getting, I think we need to take a step back and reassess...

While it's common to talk about MPGs and their improvement with octane and tunes (or how they go down the crapper with lifts and added weight), the bottom line for the discussion is really money. I ask, what's the difference between 18mpg running on 87 octane and 18mpg running on 91 octane with a tune? Well, it's still 18mpg, but the difference is the cost of the fuel. I once did some calculating and came up with something like this (for my commute to work):

$27 (to fill half the tank) / 138 miles (my round-trip commute) = $0.1957 per driven mile

So, yesterday I paid $3.29 for 87 octane. It costs me about $0.20 to drive 1 mile, on average. Premium (91) at the same station is $3.59, so it would have cost about $29.50 to fill up. That translates to $0.2141 per driven mile. While this calculation does not take in to account driving styles (burnouts!) or conditions (city, hwy, traffic...), I think it provides a better comparison across setups. So, when the question is asked, "is the 4.7L more fuel efficient than the 5.7L with MDS?" or "which tune (and octane) should I run for better mileage?", I think it's better to just ask "which setup saves me the most money?" So at roughly 1.5 cents per mile difference between running 87 and 91, across 10,000 miles (yearly?) that's $150.

So, at least for me, I'm not looking for power gains, since the XK is my daily driver. I am happy with the stock performance, so I look to save some $$$ where I can. Running a higher octane does not work out for me financially.

I once did a calculation to compare the cost of running my XK vs my wife's turbocharged Subaru Outback. You would think there would be a big difference, but since the turbo requires premium fuel, the costs are almost the same!

[06JeepXKHEMI]

Personal preference is obviously the main underlying point. Im never satisfied with the stock performance haha and to me $150 per year to run a higher octane and more performance is well worth it. Reason I mentioned mpgs is because when you factor in the extra weight from tires, lifts, skids and other accessories it says a lot on how much tune helped with daily driving and maintained a more reasonable cost to the owner. Mpg are one of my biggest points with tunes, almost every tune Ive used has improved it while increasing performance

[07JeepXK]

I have to run the 91 performance tune on my jeep because the stupid dealer decided to do a flash on the computer years back without telling me and it was tuned with the Flashpaq already and it messed everything up. If I try to go back the the stock tune the transmission shifts all messed up

[Core XK]

Well my real concern was not MPG or money. My concern is what is best for the engine? Will using the manufactor recommended octane (89) pay off in the end? Or am I wasting money filling it up with Plus instead of Regular. I know premium is a no no unless your tuned for it. Which I will not be tuning it anytime soon.

Thankfully I am willing to put out money if it will increase the life of the XK. I am firm on keeping this truck until it dies. Keeping it properly maintained is number 1 on my list.

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[06JeepXKHEMI]

It wont help or hurt you, even if you did tune it. Running cleaner gas and no off brand cheapo **** will help, but running the 89 or 91 really wont help or hurt the life of the engine itself, just take care of it and dont rag on it like I do and it should run forever!

[lekmedm]

The octane you run has to do with engine timing and compression. The hemi is capable of adjusting the timing to some degree on its own, which is why you can run 87 or 89 octane. It doesn't hurt the engine. You will not get any benefit running 91 if you don't install a tune, but it won't hurt it.

[Matt]

87 can be run as long as there's no pinging. Pinging (pre detonation) will damage the engines. Every engine varies, so one person not having pinging issues does not mean you won't have pinging issues. There's also other factors that influence the likelihood of pinging to occur- higher ambient temps, cooling system efficiency, engine load (towing or heavy throttle), etc.

Facts aside here's my opinion:
If your vehicle is running well on 89, and you're not putting a lot if miles on it (ie not much cost savings), stick with 89.

[Core XK]

Well the reason I bring this up is because I am going from 12 miles a day to 46. So I may switch back to 87.

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