What octane spins your wheels?

[Hitcher]

I was reading a thread about the retail availability of avgas (or similar) and was wondering what fuel most people used already and whether anybody used octane-boosting additives (orange syringes or other products).

[Motoracer]

I just run on 96-98 for the race bike and the road bike during summer. In winter, I'd run my road bike on 91 due to cooler temperatures.

I'd recommend Gull 97 out of all of them for road and amature track use (stock engines).

[Motu]

Doesn't matter with any of my bikes,nearest available pump.Some of my cars need to use BP Ultimate and one had to have an octane booster,but it has to be a quality booster,not STP type crap...plus 4 or plus 8 I used to use in the old Falcon.

[James Deuce]

The FIAT pinks on 91. No such issue on 98.

The Yamaha runs fine on 91.

[riffer]

The Yamaha runs fine on 91.

I was told when I got the Fizzer that it runs on 91, so that's what I use.

What would happen if I used 96 on it, apart from it costing more to run?

[jrandom]

What would happen if I used 96 on it, apart from it costing more to run?

As far as I've heard, nothing different, apart from indeed having it cost more to run. IANAC, but I've heard stuff about higher-octane fuels being 'slower burning' and therefore...? What? Is it implied that somehow the fuel will fail to combust quickly enough to create maximum cylinder pressure during the combustion cycle? What am I missing?

All I really know is, if you've got high compression, you need a fuel that withstands predetonation. Hence high octane in motorcycle engines. I just can't see how the 13.5:1 compression in the FXR can work well with the same octane ratio as the about-half-that compression in, say, my Ford Laser. That 150cc single cylinder is making the same power:capacity as the engine in a McLaren F1, and nobody would seriously consider tuning *that* engine to run on 91 octane, would they?

I swear the FXR runs better on Mobil 98. No observable difference between 91 and 96 though.

[Slingshot]

This is interesting, it's from the how stuff works website:

The octane rating of gasoline tells you how much the fuel can be compressed before it spontaneously ignites. When gas ignites by compression rather than because of the spark from the spark plug, it causes knocking in the engine. Knocking can damage an engine, so it is not something you want to have happening. Lower-octane gas (like "regular" 87-octane gasoline) can handle the least amount of compression before igniting.

[Hitcher]

I'd always thought that a higher octane fuel burnt more efficiently and effectively and, although more expensive, should improve your fuel economy. That could well be an urban legend promulgated by oil company employees...

[k14]

I'd always thought that a higher octane fuel burnt more efficiently and effectively and, although more expensive, should improve your fuel economy. That could well be an urban legend promulgated by oil company employees...

Dont think i have heard that before, but slingshot is right. The reason you use higher octane petrols is to stop knocking, which severly screws up the engine. I can definately feel the difference between 91 and 96 on my CBR, even at idle. It just runs a whole lot smoother on 96.

[jrandom]

I'd always thought that a higher octane fuel burnt more efficiently and effectively

Oh phooey. I suppose I could go and Google it, or even go find a chemistry reference somewhere, but I'm too bloody lazy so here's the pseudoscience blithering that I can remember off the top of my head.

AFAIK the octane rating really just refers to the ratio of hydrocarbons affecting the volatility (?) of the fuel (I *told* you I wasn't a chemist), the whole numbering thing is a ratio to straight iso-octane, which has a certain tendency (or not) to go bang under pressure. That gets numbered an even 100, and all other petrochemical compounds and mixtures thereof get numbered in relation to that. Lower numbers mean less resistance and therefore more likelihood to ignite uncontrollably under high pressure (ie, 'knocking'), higher numbers have greater resistance and are more likely to burn smoothly under high pressure, creating a nice even wave of ignition out from the firing point in a cylinder and therefore a useful pressure ramp.

Not having a smooth pressure curve in the cylinder is destructive - low-octane fuel can be like burning overfine gunpowder in a rifle (or whatever) chamber, causes mechanical failure from pressure spikes.

In any case all of the above means that SO LONG AS the octane rating of your fuel doesn't put it in danger of uncontrolled pressure ignition within the cylinder, it should make no difference. Bigger octane numbers give more safety margin for higher-compression engines.

Of course the *other* point is that the octane rating is but a sliver of a measurement in terms of all the factors that go into modern refined petrol fuels, and that 98 octane stuff could have a number of other relevant attributes.

Naturally all of the above could be completely wrong.

[slob]

I read somewhere that they use a standardised single cylinder engine with a variable compression ratio mechanism to measure the anti-knocking properties of a given brew of gas.

Apparently it's not just the compression ratio of the engine, the cylinder head design also determines whether it requires higher octane or not. Ignition timing would also play a part I imagine...

From personal experience, 91's worked fine on all bikes I've had (GN125, ZXR400, FZR600, KLX650, TL1000S, FZR400) although when I tried mobil's 98 on the TL it used to stall all the time when cold.

[What?]

Naturally all of the above could be completely wrong.

Nah. You put it well. Spot on, in fact.

[wkid_one]

All the scientific guff was posted in a previous thread

[XRNR]

I like AVgas cause its still got lead in it, and I am used to seeing how my bike is running by looking at the color of the pipe.

I can't tell anything from looking at the modern sooty black pipes.

Maybe I need re-educating like my lack of knowledge about modern speedo drives coming off the gearbox.

[Motu]

You heard about those water cooled bikes? Stupid idea,none of my bikes are water cooled - I'm not stupid y'know,I'll wait a few years an see how they stand up.

Yeah,I miss seeing a nice grey pipe and knowing I've got it spot on.Doing a valve grind on a car you would get this sweet taste in the back of your mouth - didn't find out till later that it was lead....no wonder little kids used to eat lead based paint.