Petrol.

[TwoSeven]

I get the feeling there is a lot of people that think that high octane is good - without actually understanding what it means to bike engines.

Here is how I understand it (might be very basic, but it kinda gives the idea).

Petrol is made from distilling crude oil to remove various chemicals that make it up. Some of these chemicals form a vapour at low temperatures and others at a higher temperature.

Two of these chemicals are heptane (that forms a vapour at a very low temperature - and thus burns easily) and iso-octane (turns to vapour at a higher temperature and doesnt burn very well).

You could think of octane as a chemical thats mixed with heptane in order to prevent it from burning. So you could say that heptane has a rating of zero (it will burn), and octane has a rating of 100 (it wont burn) - at a given temperature.

Normal petrol is generally 13% heptane and 87% octane - which gives a fuel with octane rating of 87. In new zealand, its 9% heptane (91% octane) for 91, and you can figure out the other brands (this isnt actually correct but its a good way of thinking of it).

When you compress fuel, its temperature is increased. The more you compress it, the hotter it will get. Eventually it will get so hot as to self ignite (remember you have been mixing an oxygen supply with it before hand).

The amount of compression you can use is dictated by the temperature of the engine and the octane rating of the fuel. The higher the octane rating the more the fuel can be compressed before it will self ignite. A spark plug simply exists to allow you to controll the point in time that the temperature hits that ignition point (or to make it go bang when the piston is at the ideal point for maximum power transmission).

The more compression (of a given amount of fuel) you have, the greater the force that is applied to the top of the piston when the fuel ignites. The greater the force, the more power you have.

Now this is all good if you are raising the compression on your engine (and your engine is designed to handle a higher compression - it may go bang if its not). Raise the compression, put in higher octane fuel, run the engine hotter, get more power = good.

However, if you put higher octane fuel in and 'dont' raise the compression, you lose power and other things will occur.

The first thing that will happen is your fuel wont naturaly ignite when the pistons are at their optimum position for maximum power transfer, so you will lose some power in overcoming/making up for that. You will be relying on the plugs to make up the extra effort - which may result in installing the wrong plug.

The second thing is that if the fuel is not hot enough, it wont burn properly, so you get a residue (think of it like burning a wet piece of paper, some of it will burn, but not all). So you get fuel wastage (that petrol smell from a cold engine).

And, the tips of the spark plugs need to be at a minimum temperature before they will self clean (burn off that residue). If they cant - the plugs will cold foul (a clear residue forms that insulates the plug and weakens the spark). If they foul too badly, the plug will not fire at a hot enough temp to ignite your fuel, so the bike wont start (most likely flood). In NZ i have noticed on hondas you need a fully charged battery (14v) to get over this could fouling problem.

Finally, if you change the timing - the point at when the fuel ignites - and the piston is in the wrong place - you end up with a flame front that is either going to wack into the piston coming towards it, or have to chase the piston as it moves away - either way - lost power.

When you lower the compression the opposite happens, the fuel will go bang before the piston gets to a usefull place. I think this is known as pre-ignition - I dont suspect its good for the engine.

So on a motorcycle you get better performance if you change the fuel and the plug type to work with its compression, than if you just up the octane and throw every thing else out of wack.

At least thats how I understand it - so if its not in the ballpark, please feel free to correct me.

[Blakamin]

Thanks for that!... i will file it in my head and now go on a quest for compression

[kerryg]

The more compression (of a given amount of fuel) you have, the greater the force that is applied to the top of the piston when the fuel ignites. The greater the force, the more power you have.

Now this is all good if you are raising the compression on your engine (and your engine is designed to handle a higher compression - it may go bang if its not). Raise the compression, put in higher octane fuel, run the engine hotter, get more power = good.

That's really good information. I've got a GSXR100 engine running 12.2:1 compression. Do you reckon a higher octane fuel (eg race fuel) would give me any benefits? Pretty much academic at present but I'm interested to know for future reference

[Mongoose]

Could some one wot knows about these things tell me summit(Marty maybe) but is AvGas a "dryer" fuel than Auto Gas? Have heard it said before but have not found anyone that can confirm or otherwise.

[vifferman]

if you get caught walking out onto the apron with a gas can you will get in the shit. any one ever used tolulene, i have a cousin who sells race fuels and lubes, he does all the WRC cars when they come over here, and some of the privateers us this tolulene stuff mixed with regular pump gas, its ment to beef up the octane i fare bit, but i hear its tough on rubber parts.

Yup, that (and xylene) is exactly what the gas companies add to petrol to artificially boost the octane rating. It's cheap, it's nasty and the mixture of the two (xylene+toluene) is sold as paintbrush cleaner. Leaves nice sooty deposits in your engine, and is what was repsonsible for all the leaky fuel systems and engine fires when they removed (most of the) lead from petrol. Much more toxic than lead ever was proven to be.

[Motu]

Well,it's kinda dry when it's a gas,and kinda wet when it's a liquid....

[Mongoose]

Well,it's kinda dry when it's a gas,and kinda wet when it's a liquid....

Ahhhhh, duh, silly me.

[TwoSeven]

That's really good information. I've got a GSXR100 engine running 12.2:1 compression. Do you reckon a higher octane fuel (eg race fuel) would give me any benefits? Pretty much academic at present but I'm interested to know for future reference

Not sure, the only way you could find out is to run it up on a dyno for both types of fuel and see what happens. But i'm not really keen in trying out exotic fuels on my road bike. Pump fuel yes, but anything else - no


As to the dryness of the fuel as someone else asked - i'm not really sure what is meant. I believe it has something to do with the aromatics that are added to the fuel and their lubrication capability (or lack of).

I know when I brought my bike over from the uk years ago, I noticed a squeak coming from the engine (more like a metalic ring - very high pitched). I was told that it was caused by dry fuel due to the lack of additives. Never really bothered to ask.

[vifferman]

That's really good information. I've got a GSXR100 engine running 12.2:1 compression. Do you reckon a higher octane fuel (eg race fuel) would give me any benefits? Pretty much academic at present but I'm interested to know for future reference

Unless your bike has fuel injection and electronic engine management complete with all the sensors so that it can adjust for different fuels, it's unlikely that a higher octane fuel will give any benefit apart from reducing knock (pre-ignition). Despite what many people commonly believe, higher octane does not equate to more power, just the ability to run in a higher state of tune (advanced ignition, increased compression, perhaps leaner mixture), as the fuel ignites (i.e., the flamefront advances) more slowly than a lower octane fuel. The actual energy output as measured by a bomb calorimeter or similar is less for high octane fuels.

[slob]

From my experience on my previous bike (Suzuki TL1000S), running on 98 as opposed to 91 made it harder to start in the mornings. On the other hand, when I bought my XR600R, the guy who sold it to me instructed me to avoid using 91 and opt for 96 or 98 instead. Apparently the ancient engine used by the XR is prone to detonating (pre-igniting?) on 91. Goes to show that it's not only highly tuned engines that need higher octane fuel..

[Motu]

[QUOTE=slob detonating (pre-igniting?) [/QUOTE]

Detonation is post ignition,fuel will ignite spontaniously in a high pressure part of the combustion chamber after the sparkplug fires,the collision of the two flame fronts is what does the damage.Pre ignition is the fuel igniting before the spark plug fires,this is more likely to be from chamber deposits or plug electrodes.

It's all to do with combustion chamber design - they were designed for what fuels were available at the time - that fuels change means engines designed for a different fuel may give problems....some engines are more forgiving,but they may not be better,most likely not if they have some margin of error.

Alcohol is practicaly detonation free,speedway engines run 15:1 and if rules permit,even higher.LPG (Propane/butane) and CNG (methane) are also high octane fuels and can run higher compression ratios.Actualy all this talk about octane and toluene and stuff is misleading,they are all part of the hydrocarbon mix that makes up what we call petrol,they are all there all the time,it's just the ratios they are fiddling with - hey,let's add a bit more of this eh?,that should stop the buggers moaning.

Dry fuels? huh? they are all dry,if they are wet your bore will be washed and wear out pretty damn quick.Lead added some lubricating properties to the fuel,now we haven't got that,they may have to add some self lubrication to the fuel.Again,the engines were designed around a fuel with self lubricating properties,when removed some will give problems ,some not.

[kerryg]

Unless your bike has fuel injection and electronic engine management complete with all the sensors so that it can adjust for different fuels, it's unlikely that a higher octane fuel will give any benefit apart from reducing knock (pre-ignition)



My bike lacks these newfangled gimcracks and geegaws...so I guess I might as well stick with the normal juice


Not sure, the only way you could find out is to run it up on a dyno for both types of fuel and see what happens. But i'm not really keen in trying out exotic fuels on my road bike. Pump fuel yes, but anything else - no

It runs sweetly on pump gas and at this stage it's not important enough to me to pay for 2 dyno runs. Cheers for the input

[F5 Dave]

Also what has not been given much credence is the calorific value of the fuel. Methanol for example has an octane rating of 115, but a lousy calorific value, this is why you need to raise the compression to silly levels & drill the mains out ~ 2.3 time the size to get more fuel in & make use of hi comp.

Basically if your bike doesn’t knock when you load it up (throttle on up a hill in a high gear) then higher octane will likely be of only psychological benefit.

Many different Race gases can be bought overseas, these include oxygenated fuels & with new rules unleaded are becoming more popular.

IN NZ my understanding is they just repackage old AV as ‘Racegas’. At least this is what a friend (ex employee these days) from one of the companies in Seaview distribution plant told me & showed me the tanks we filled my can from.

It is heavily leaded & at least 105 octane. The more lead you add the less difference it makes. For planes you don’t want them destroying themselves esp. at high altitude so the petrol is made with a reasonable safety factor.

It is an interesting observation on my racing 50 (2 stroke) when NZ was changing from the old leaded Super 96 to unleaded I tried these 2 fuels & AV on the dyno in one sitting. I adjusted the jetting & timing to find optimum setting for each fuel.

The new unleaded was scary & suppressed the power detonating (was running silly high compression ratio. The Av gas ran nicely. But by far the best fuel was the leaded Super, almost a hp difference (which on a 50 is a LOT). Obviously the super burnt nicely, quickly enough for the revs & had a high calorific value. Having no more I turned my attention to tuning the bike as best I could for Av. I tried several different combinations of compression, head shape & squish designs, but all to no avail or improvement over my initial tests.

The Unleaded I avoided. My roadbike at the time (RZ350) blew up on the first decent trip on the new gas, having been tuned safely for super. Unfortunately this happened before my tests or I would have lowered the compression back to std knowing how bad the new fuel was. I believe subsequent batches were a lot better.

[Jay widda 150]

does anyone here have a diesel bike??? that would be fun to play with!!!

[vifferman]

does anyone here have a diesel bike??? that would be fun to play with!!!

No, but there was a home-made one in Hamilton, and the US military recently announced that with the conversion of their Kawasaki 650's to run on J8(?), they were now able to use one fuel for all vehicles.