High Octane Gas?

philh wrote:

Turtle n Peeps wrote:

Technically, saying that premium has less BTU's is like saying I just took a leak in the ocean and now it's higher. Is it true? Ya, but.........:R.

More like adding to a 55gal drum :) I used to have the energy numbers, but they are long gone :(

Point being, it's the burn rate of premium that's an advantage to engines built for higher octane fuel, and to a lesser extent engines tuned to run it.

I'm confused? :h You were talking about BTU's and then in your last sentence you talk about burn rate. Those are two different things entirely.

Since this is a towing forum lets talk about fuel terms and what they do and mean to us as RV'ers (with a little talk about non-RVing fuels thrown in.)

BTU's; British Thermal Units:
A measure of heat units. What does this mean to us? The more BTU's a fuel has the better your mileage will be for a given engine. (This is why E10 will net us about 4 to 5% less mileage than straight gasoline.) This is a BIG reason a diesel engine gets around 30% better mileage than a gasoline engine. A fuels BTU's has NOTHING to do with the power it can put out. As a matter of a fact it's inverse proportional for every fuel I can think of. (less BTU's = more power).
(As an RVer, get the fuel with the highest BTU's if you want the best mileage; that would be good ol #2 diesel for you that don't know. Or straight gasoline for you with gasoline engines)

Burn rate: How fast any given fuel burns. Usually measured in milliseconds. As stated, burn rate has next to nothing to do with the octane of the fuel. One can make a fuel with a high octane rating and slow burn rate; or the other way around. Fuel air ratio means WAY more on how fast or slow a fuel burns than the octane rating. A lean mixture burns slower than a rich one. Burn rate of a fuel is also mutually exclusive of the amount of power an engine puts out. IOW's a very slow burning fuel can make a lot of power and a very fast burning fuel can make a smaller amount of power.

Octane Rating: A figure indicating the antiknock properties of a fuel, based on a comparison with a mixture of isooctane and heptane.
Octane is mutually exclusive of the amount of BTU's a fuel has or the amount power the fuel can produce.
What can octane do for us RV'ers? That is a super complex question now days. In the old days of distributors and carburetors you would just run the fuel that did not "ping" in your truck and you would be good to go. If your truck did not ping on regular that's what you would run. You would not pick up any mileage at all if you ran premium if that was the case.

Now days things are a LOT more complex. A computer decides up to a 1000 times a second whether the fuel is up to the task or not. It gets its info from knock sensors in the engine and intake air thermometers in the intake and many, many, many more sensors. From all this info it can advance the timing (Good for mileage) or give it more fuel (bad for mileage).



I know, I know, I know; some of you are asking why as an RV'er can't I just open my manual and just read what the OEM says to run and be done with it? Don't they know what's best? Welllllllll, sort of.

OEM does not know the conditions you tow in. They might say, "use regular fuel while towing." And this might be fine if you are towing in Denver when it's 70 degrees out. Or even in 2000 foot altitude in 40 degree weather.

But if you happen to tow at sea level and it's 100 degrees out your engine will likely drop timing out like a big dog (bad for performance and fuel mileage) and add fuel (also bad for fuel mileage) to prevent detonation. In this case premium would be a far better fuel to run despite what the OEM tells you.

The reverse is also possible. Ever wonder why towns like Denver sell only lower octane fuel when the OEM say's to use only "x" octane fuel? Well now you know! You don't need real high octane fuel waaaaay up there!


This is why there is so much debate about the octane of fuel to run on towing forums. We have people saying that they tow with X brand of truck and the OEM says it's recommended to run premium fuel while towing. But they say they run regular fuel while towing and it runs just fine!! Then there are people that say they tow with brand x truck and they run premium fuel and they pick up mileage over regular fuel.

How can they both be right? It's because they don't see the same conditions either one sees and the computer is making different decisions for different conditions.

The bottom line is this. It's best to use the fuel OEM recommends to use or if you really want to split hairs like some do, buy the fuel for the conditions you will be towing in. Regular conditions call for regular fuel. Harsh conditions call for a more premium fuel.

Or you can be like me and forget all this talk about gasoline and buy a diesel and jam #2 in it and get 12 MPG at 70 MPH. :B