87 octane vs. 91 octane

[OneTomcat]

I have 3 tunes, 87 performance, 87 tow and 91 performance. I know that running the 91 tune with 87 octane is not a good thing but what about the other way. Specifically, any harm done running the 87 tow tune with 91 octane while towing or carrying heavy load in bed?

[Power Hungry]

Absolutely no harm at all. Running high octane fuel with low octane programs is perfectly acceptable, especially in warmer climates. In cold climates, I'd stick to low octane fuels since then tend to burn a little faster and provide a little more power on those chilly days.

Enjoy!

[Kwattro]

I run 91 or 93 on my 87 canned tow tune. It's my understanding that it's an extra margin of protection towing my 8500# trailer on those hot days in the hills.

However I'm worried about running 91 when the supercharger is installed, it's near impossible to get anything higher in some of the more remote areas we camp. I put my faith in Bill's tuning to keep the motor together haha.

[Power Hungry]

It's all a matter of timing. With proper tuning, you could run 87 Octane but of course you'd be losing a bit of the benefit of the S/C, although not as much as you'd think. As long as we keep things a little conservative, you'll be in fine shape.

[goracer]

How do you calculate any benefit or loss using 87 at lower mpg but costs less versus 91 which costs more but is a very slight increase in mpg? I beleive it's currently a $.25 difference per gallon.

[Power Hungry]

All fuel calculations of that type are handled on a cost per mile type calculation.

The following examples are actual receipts from our recent trip to Orlando. The fill-up stations were the exact same stations for both fills, only one going down and one coming back. The 2.8 mile distance discrepancy is because we stopped for a bite on the trip back but for all intents and purposes it was the same trip distance. The program we ran in the truck didn't change and it was an 87 Octane based program.

87 Octane - 2.299 per Gal.
Gallons Consumed - 17.133 Gallons
Total Fuel Cost - $39.39 (yeah, I'm kooky about my fuel totals!)
Distance Traveled - 316.3 miles
Average Fuel Economy - 18.461
Cost per Mile - 12.453 cents

91 Octane - $2.469 per Gal.
Gallons Consumed - 16.606 Gallons
Total Fuel Cost - $41.00
Distance Traveled - 319.1
Average Fuel Economy - 19.216
Cost per Mile - 12.849 cents

As you can see, there was a modest economy benefit by running 91 octane on the return trip. However, even though I used less fuel, the additional cost of the fuel actually raised the cost per mile so it didn't make it cost effective. I'm sure if I'd have put an 89 or 91 octane program in the truck, we'd have seen a more significant change in economy and it may have proven worthwhile to run the 91 Octane. That test is for another day, though.

And before someone jumps in, yes I didn't take into account ambient temps, humidity, traffic, headwind, or any other environmental factor that may have had some affect on the economy outcome. But then again, life isn't a constant, controlled environment so I don't that makes the test values any less valid.

I hope this helps.

[goracer]

Thank you (I think I double posted this question in another thread). I'm not sure what the purpose of you using 91 without changing the tune was but those calculations were exactly what I was looking for. What i'm trying to find out the cost per mile and which tune is more cost effective as you put it. +/- a few HP is not as important as +/- mpg with fuel being over $3/gal.

Can a level 3 tune be done with 87 octane? What differences would be the result versus a level 3 tune iwith 91?

[Longshot270]

You can run 87 on the level 3 BUT you have to lower the timing. I think when I was playing with it, I dropped it 2.5 to 3 on the programmer. I dont think it made much of a difference though. I also tried it with the canned 93 so Im not sure how a custom tune will respond (probably same). Easy way to know how much to adjust, change timing .5 for every octane difference.