Home Forums Gear Hunting Trucks and Four-Wheel Drives Higher octane gas

How does higher octane gas improve how an engine performs, as in temperature and mpg?
Does it have more btu's per gallon and therefore more energy; even though it burns hotter, which isn't good for your engine under heavy load?
Would using higher octane gas to boost power harm your engine if you did alot of heavy towing in really hilly country, where alot of engine heat could build up?

Seems like I read somewhere that it burns hotter and is actually counterproductive somehow.

Unless you have a high compression ratio, higher octane does nothing for you except cost you more money.

More energy is simple answer.

If you're buying gas at gas station I don't believe there's enough difference to cause problems. It you were running av gas or racing fuel that's a different story.

I've found that in my truck going up and over the Rockies at 10,000 feet plus running premium gives a slight boost in performance. Mileage differences are difficult to measure in those extreme conditions and I don't do it on a regular basis. That's the only time I've ever bothered to pay extra for premium.

It is worth it? Maybe.

Increased engine heat is not a problem with higher octane gas?

Actually, I believe it runs cooler, and is certainly more difficult to ignite.
That is it's advantage in a high perf engine. It resists detonation.

Sorry, I can't answer that with any degree of expertise.

A high compression engine can produce more power for it's weight/displacement but it requires higher octane fuel to prevent detonation/knocking/premature-ignition. Aviation fuel is much higher octane because airplane engines(non-jet) need to produce more power for their weight so are higher compression.

Wiki has a good description.
http://en.wikipedia.org/wiki/Octane_rating

Octane ratings are not indicators of the energy content of fuels. (See section 4 of this page and heating value). It is only a measure of the fuel's tendency to burn in a controlled manner, rather than exploding in an uncontrolled manner. Where the octane number is raised by blending in ethanol, energy content per volume is reduced. Ethanol BTUs can be compared with gasoline BTUs in heat of combustion tables.

Higher octane gasoline does not have higher energy density, it is just more difficult to ignite. The reason a particular vehicle may require high(er) octane fuel is because of a higher compression ratio and subsequently an increased likelihood for predetonation (knock). Many vehicles are now equipped with knock sensors to accommodate this but if your vehicle was designed for regular you are wasting money by filling up with premium. Elevation does play a role, which is why at sea level premium has an octane rating of 93 and at elevation you'll usually see 90 or 91 for premium.

http://en.wikipedia.org/wiki/Octane_rating#Effects_of_octane_rating

It's not much of a problem these days to go from 5k to sea level, but older cars would knock like crazy if you went downhill a couple k. We'd be tuned for high elevation at home and would have to buy high octane when visiting in Portland or Seattle.

OK, if it's harder to ignite and runs cooler, it would be better for an engine towing a heavy trailer at higher elevations or towing alot fo weight on a long trip.
"An engine" being my stock Tundra 5.7 liter.

I do remember reading somewhere several years ago that there could be as much as a 300 degree differnce in combustion chamber temps between regular and premium...does that sound about right?

Higher octane gas burns slower. Lower octane gas burns faster, but can detonate with too much compression and timing. The more modern cars have knock sensors along with other sensors that will reduce timing when lower octane fuel is used.

The performance of the vehicle is mainly affected by the lowering of the timing. Using the higher octane fuel would allow the vehicle to run more timing advance and the vehicle would respond accordingly.

Cars today run a lot more compression with the same amount of octane (lower) than cars of the mid 70's, 80's, and part of the 90's. They are able to do this with a number of advancements especially in the computer controlled area.

Some other advancements would be how the engine is cooled in the head area. For example, in the mid 90's GM came out with the LT1 engine for the Corvette, Camaro, and Caprice. The Corvette and Camaro had aluminum heads and the Caprice had iron heads. The engines were very much alike other than valve lift, but the compression was higher on the aluminum head engines due to them staying cooler.

Edited to add: I meant that GM could run a higher compression ratio with the aluminum heads over the iron heads.

I believe you are over thinking it. Run what the manual calls for. Running anything else is a waste.

Then you don't think that when towing a heavy load over long distances the engine would run cooler with higher octane gas because the engine is built to compensate?

long bob touched on it, unless your engine has the ability or in the old days you could adjust the distributor to advance the timing you get NOTHING from high octane. the timing must advance to see a benefit. a high compression engine requires higher octane, one can back off the timing with a high compression engine but they are defeating the whole reason for having high compression and at some point if the octane isn't enough the engine will not even start. I had this happen when a guy sold me on the idea that 305 small chamber heads would work great on my 350 chevy. what I needed was better flowing heads not more compression. now days engines are much higher compression but this is because of electronics and probably cam grinds to deal with it.

I know this is directed at K1500, but he is right about what the manual calls for. There are rare situations that the manual "originally" called for low octane pump gas and would benefit from higher octane gas.

I think it was a recent version of the Mazda RX8 that would be one. They had advertised a certain HP rating that was only capable with higher octane fuel, IIRC. They had to update the manual, maybe some settlements to the owners, stickers, etc....

Now, if your truck calls for the higher octane fuel then you would benefit from more power and ability to tow heavily in the hills, but I cannot think of any modern trucks that do require the higher octane fuel. Outside of maybe the Raptors or something, but I doubt you would be towing with one of those.

I don't think you would notice any cooling benefit from the higher octane fuel. It would be a waste and may actually hurt your performance if your truck calls for lower octane fuel. Remember that I said higher octane fuel burns more slowly? That means that lower octane fuel burns more quickly.

What does that mean? Well, an engine can benefit power wise from the quickest burning fuel you can get away with. This has been a somewhat secret racer's trick. I actually race an aluminum head LT1 in Stock Eliminator and an iron head LT1 in Super Stock. These are the versions of the engines that I mentioned in my post before.

The factory compression of the aluminum head is 10.4 to 1. When we get through bringing the chamber volumes, piston installed height, head gaskets, etc....into the tightest of tolerances then it is nearly 11.5 to 1. The iron head ends up about 11.25 to 1.

In both instances, we make a little more power on the dyno with VP C25 which is 113 octane, but the engines accelerate down the track better with VP C11 which is 104 octane. This is great because C11 is $12/gal and C25 is $25/gal.

How does that apply to you? It may show you an example how an engine is under an extended load will perform better with a quicker burning fuel. In a total performance basis we try to run a fuel that is just barely enough octane to keep the engine from detonation.

Not in any example of octane that I have given do I feel it would make your truck run cooler or hotter. I think your truck would perform better with a lower octane and the cooling would be best handled by fresh coolant and a transmission/engine oil cooler.

I think that higher octane is needed when called for by the manufacture of the truck you drive. I don't suppose it would hurt anything other than your wallet, but I don't think it will help. In addition, the truck ECM will probably have to 'relearn' itself when switching octane.

We are at the pinnacle of truck performance. It's not like the old days when you had to constantly adjust and tweak. All that being said, you may be right about the higher octane being better, I'm not a mechanic.

Rather than buying high octane, I would try to buy ethanol free fuel. THAT will make a noticeable difference in mileage and performance, based on personal experience. At some stations, the premium fuel is ethanol free. I think this is why some think high octane is better. They put in ethanol free premium after running regular test with ethanol and they get a bit more power and mileage. It's the ethanol, not the octane.

Thanks guys, this has turned up some interesting info for a layperson like me. I probably should have mentioned in the beginning that in my Tundra, Toyota recommends regular gas and not high octane. I tow quite a bit with it and I just wondered if high octane would be better for the engine.

I also can't pass up the opportunity to say that the Tundra would probably blow the doors off a Raptor.

K1500, I know what you are saying about non-ethanol premium as compared to regular pump gas. Last week a friend was running his John Deere 420 dozer and I put non-ethanol premium in it and he could tell the difference right away. He commented on the difference in power being really noticeable...but his gas (regular, with ethanol) had been in the tank for a couple months at least and so was "old".

To no one in particular. If you have access to Avgas....don't. It is higher octane, but it is made for high altitude and low RPM's. It does not do that great in high reving street or race vehicles.

All the other stuff about useing what the owners manual says is spot on. Some premium in the gas stations does not sell real fast either, it can quite often be older and contaminated compared to the "cheap" stuff that sells out all the time.

Up here most high test or 91 oct is ethinol free, so the question is, do you get enough extra mileage running 91 at 60 cents or so extra per/gal as opposed to the up to 10% ethinol low test to make the mileage increase viable??
Personally I think not, but have not tried.

Bu the way it is very easy to test for ethinol just google it, I use a small test tube because one thing you don't want in a 46 T-crate is ethinol

If Toyota recommends 87 octane run it. You have lots of power

My Tacoma's rated horsepower is with 92 octane. I run nothing but 87 in it, no difference in milage (I calculated all my milage for a long time when the rig was new), at 10 horsepower difference which is not worth the added cost of the fuel to me. The variable timing adjusts for the octane and keeps it from pinging.

the tacoma retards the timing for lower octane and advances timing for higher octane fuels. I'm not sure, but my hunch is it is adjusting all the time to advance the timing as much as possible without pinging.

If you do not have computer controlled timing, you need to run a fuel with high enough octance you dont get detonation or pinging (try pulling loads up hill) which is very bad for your engine. Running higher octane than you need does not make more power unless you have computer controlled timing.


At 2000 rpm cruise, I dont think the 2 or 3 extra horsepower difference you might get from more advanced timing and higher octane fuel is worth much for milage. Mayeb if you are towing heavy loads up hills, but I never bother with it.

Higher octance fuel is harder to burn. It's not "better" if your engine is not setup for it, or cannot take advantage of it with computer controlled timing.