Working with Lew to get a custom tune and he mentioned that the 6.2 requires 91 octane minimum so he only offers 1 tune for our trucks.

Looked in the manual and sure enough 91 minimum but will run on 87 with knock and reduced power and millage, says if you hear knock then go to 91.

I had no idea, should have read the manual I guess.

Did some data logging and sure enough the knock sensors are actively cutting the timing while driving.

Neal

 

I run 89, no issues; runs fine.

87 knocks/reduced power/sluggish.
93 performs no difference. It's just a waste of money.

 

I'll run this tank of 87 out and then fill with 93 then do some more logging to see if it stops the knock sensors from cutting timing.

Neal

 

The primary reason you dont really notice is because our modern engines have knock sensors. If that were not the case you would just have sever engine damage from the pre-detonation. So instead when you run fuel below the octane requirements needed for the higher compression engine the PCM just retards the timing which in turns kills your fuel economy, power, more unburned fuel rapidly decreasing the life of your catalytic converters, etc.

This is further exacerbated by our modern fuels with percentages of ethanol which degrades faster and quickly loses octane points as it ages and some of the components evaporate out of the fuels. Yes - you are saving $5 on a fill up (figuring 25 gallons at a .20 per gallon cost difference) by using regular instead of mid grade but with the reduction in economy and other negative issues its probably actually costing you more. Of course premium is even more running .40 to .50 more per gallon around here than regular. With the right tune though, and keeping your foot out of it as much as possible (not easy with a killer tune ) you can actually have a nice improvement in fuel economy that can come close to offsetting the additional costs of the premium fuel.

Of course other factors come into play as well. If your truck is over 100k miles you probably need to go ahead and replace your upstream O2 sensors, properly clean your MAF sensor, new AC Delco spark plugs, keep tires aired properly (even a little on the high side), can all add up to significant fuel economy savings.

Neal - I'd personally run at least mid-grade gas. If you have been running premium for a couple of tanks Lewis definitely needs to know that if you are getting him the logs with that fuel as you will pretty much need to keep running premium. He can adjust the trims and timings for what you plan on running but he needs to know this. Of course each time you change the tune the long term fuel trims all get reset and it takes a couple hundred miles for things to settle back in to a normalized state in your PCM.

Other things will also affect stuff like the tuneup items I mentioned and performance enhancing things like a cold air intake, exhaust mods like cat-back or full headers, any sort of forced induction, aftermarket cams, etc. I'm planning on getting Lewis to do a tune for mine too but I'm waiting until after I've done all my mods and tuneup stuff (much if it is now done) and then let things settle in for a couple of weeks.

 

Yes, I've read that these trucks have dual timing tables and that when running the lower grade fuels the PCM will shift to the lower table if the knock sensor sees the knock long enough.

Neal

 

I've been emailing Lew quite a bit and he has given me a game plan to get things right in prep for his tuning.

Thanks for the info MixMan, right now there are no plans for upgrades on the engine but who knows what the future will bring. I did talk with Lew about future upgrades and he said no problem just send him a log and he will update the tune.

Neal

 

Well I got the 87 out and filled with 93, drove it around a little while and then done some data logging. Consistently higher timing numbers with better acceleration feel and more spunk. I then installed the Diablosport 97 tune and instantly noticed the change, even better acceleration with shift points and shift firmness more to my liking, just a more fun truck to drive now.

I took a small road trip on a local hwy at a constant 60 to 65 MPH with the cruise set, at times the truck was pulling timing down to 2*. After the fuel change I took the same drive and noticed the truck was holding 19* to 22* consistently without cutting.

So on my truck at least running the correct fuel and the tune has changed the drive-ability of my truck drastically.

Next is to have Lew build a custom tune for my truck,

Neal

 

Get your long tube headers on there and then get the custom tune done

 

I asked Lew about adding headers, etc..... and if this would be covered within the year after getting the tune and he said it would so no issues with getting the tune and then the headers and mufflers.

Neal

 

When towing, the truck controls it's engine temp much better w/ 91 octane and recovers the temp after pulling a hill quicker. Since it's 115 degrees here, we only use 91 now.