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Fuels and Octane Ratings
What's it mean in HP??
Question: Will my bike make better power with higher octane fuel?
(Only for sure if it's knocking with lower octane fuel)

Dare I disagree with the Gordon Jennings's and Kevin Cameron's the world??? Well">

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Fuels and Octane Ratings
What's it mean in HP??
Question: Will my bike make better power with higher octane fuel?
(Only for sure if it's knocking with lower octane fuel)

Dare I disagree with the Gordon Jennings's and Kevin Cameron's the world??? Well, I'm feeling a bit cocky today...

There IS a difference in fuels - aside from the octane rating.

Different fuel blends burn at different rates under the same temperatures and pressures. Absolute fact.

Peak cylinder pressure, for best power output, MUST be timed to occur just after the top of the stroke.

To tune for maximum power, you would like to use a fuel that burns at the quickest possible rate - without "knock" (there - that covers pre-ignition and detonation) so you can initiate burning at the latest possible time and still produce peak cylinder pressure at around TDC. That way, the rising cylinder pressure pushes back down on the top of the rising piston for the shortest period of time - decreasing power lost there.
A more specific location for peak cylinder pressure would be ~15 degrees after TDC. There is some variance from that figure - but not more than a degree or so in an engine using gasoline.  When the ignition timing is correct, power output will be highest for that rpm and throttle position.
(thanks, Al Cline!)

If you look at the octane rating as only an indicator of what the fuel was designed for, you'd guess that a 120 octane fuel was probably designed for an engine that was prone to "knock" - like an older American V8 with relatively poor cooling and high compression. That fuel will be blended to burn at controllable rate that matches the high pressures and temperatures present AND resist "knock". A good match for those V8 engine conditions.

Burn Rates- That's the ticket!!
If you take that same fuel that worked well in the above V8, and run it in an engine, like a cbr900, with it's lower cranking compression and lower combustion chamber temps, it will, without a doubt, burn much too slowly at those lower temps and pressures and reach maximum cylinder pressure too far after TDC for best power.

Things generally burn slower when they are cooler and vice versa. Peak cylinder pressure will occur much past TDC - decreasing the power produced if you keep the same ignition timing.

You can advance ignition timing to try to recover power, but that will cause the air/fuel mixture to burn earlier in the crank stroke and spend, percentage wise, more of the energy produced by the expanding, burning mixture, pushing back down, trying to prevent the piston from rising up to the power stroke - robbing power.

If you MUST use a slow burning fuel, which USUALLY has a high octane rating, advancing the ignition timing will lessen the power loss, but the best results are usually obtained with the quickest burning fuel obtainable, that, of course, doesn't "knock".

All other factors being the same, except for burn rate - use the quickest burning fuel that doesn't "knock", light the spark in the middle of the combustion chamber, adjust ignition timing to reach peak cylinder pressure ~TDC and keep your mixture correct. When the ignition timing is correct, the engine will make best power for that fuel.

There is a difference in the burn rates of different brands of fuels that are available. Some compliment one engine and some compliment the existing tuning of a different type of engine. Our Supersport YZF750 was really responsive to different pump fuels and liked a different type of race fuel (same brand of fuel) than our gsxr750 Supersport bike. Something on the order of 1%-2% power difference.

Generally, the cbr900's are extremely power sensitive to different fuels. It has to do with the basic combustion design.

Does using a fuel with higher octane numbers automatically make more power?
Not unless they are preventing "knock".

My vehicle runs fine and doesn't "ping" on "regular" fuel, but, it's a little "peppier" with "premium" fuel. What should I use?
If you are wanting the extra power - use "premium" fuel - if you are saving money? Use "regular" fuel.
As long as it doesn't "ping" all is well, as far as generally accepted......

Is there a difference in standard street pump premiums?
Yes. ~1%-2% power output. Try a few and use the brand that works best in your bike. Or, you can bring your bike in and I can charge you a lot of money to test fuels for you.

Is there a difference in additive packages between different brands of fuel - even if they have the same "octane" rating?
Yep! There actually is. Some of the detergent packages are patented. I know for a fact that Chevron's Techron (techroline?) was patented. Years ago, when they first released it, they really loaded up the fuel with it. If your 4 stroke motorcycle jetting was too rich - it would actually, as an "oily solvent" with a flashpoint of ~1500f, build up on the insulator of the spark plug, unless you cleared it out every once in a while - it would actually foul!
That was many years ago and all is well now!
Oh - it really would remove minor fuel injector deposits, too!

Energy content?
There is a really SMALL difference in different pump premiums - depending on the fuel recipe - I'd suspect a  insignificant difference - like .01% power difference. Relative importance between different standard fuels? Much less than being off by 1/4 main jet.

Octane boosters?  

If they prevent "knock" in you vehicle, they WILL help produce more power, but if the engine is NOT knocking? No significant / cost effective, happy results. "Not a "buy" at this time." There ARE compounds that will improve power, yes, but they weren't  down at Pep Boys and Grand Auto when we bought all of them in 1999.
If you make a compound that works, we are available to do confidential testing. (As far as testing, I dearly wish that there was an available, cost effective fuel compound that did work!)

Is there a difference in 100-105 octane race fuels as compared to street pump premium when used in a motorcycle engine?
Yes. Some of the best WILL ADD, without a doubt, repeatably, no question about it, 3%-4% power improvement (under 2.6% oxygen content and without nitrobenzene or related compounds). Some of the 100-105 octane race fuels that were not designed for high revving, low compression engines don't work very well at all - making roughly the same power as pump premium.
Now.... that was 2002 info -
2004? Even more power from fuels - and still at the ever popular USA $17 to $22 a gallon range!

Have a doubt? Get some Nutec or Elf race fuel. You will feel it.

And yes, there are some odd compounds in race fuels. One of the above fuels left a brown carb deposit. Wouldn't wash out with gasoline, solvent, carb cleaner, 409, Simple Green, Fantastic or soap and water. Weird. Weirdest - it DID dissolve and drip away in liquid form after being doused with an aerosol can of "Off" mosquito repellant that we had left over from the Brainerd AMA National. It was a waxy deposit.......

Factory Pro in one of the only motorcycle Fuel Injection AND Injector matching facilities in the US - We do both and more design dynamometers that deliver .2 hp meaningful resolution and repeatability.

We can supply matched sets of injectors for better fuel control and increased power. We can supply up to 1% tolerance injector sets when available.

As far as what GJ and KC write, I've grew up reading and rereading everything that they both wrote. I ported an SR500 cylinder head just like GJ did in a magazine project. As I was doing it, I just knew I could spend more time and be more careful and end up with a small port that flowed more than his did. But, that's another "experience" (experience is what you get when you don't get what you want.) Actually, it did work OK. Ask some day. I think that it's still in my home garage, next to the Suzuki Rotary engine oil.....

I just wish they would tell you what did work, rather that explaining why something they didn't actually test themselves (like octane rating) doesn't work. It gets perpetuated throughout the industry - causing some nut (me) to spend hours explaining how things work today!!

Best wishes and happy 2001!


PS - MTBE is a disagreeable compound. Think about what it does to that nice flexible organic rubber (melts) - It is bad for rubber and bad for you. Keep your hands out of gas.

Octane numbers explained

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