Aviation fuel?

**motorbyclist** · 7th February 2010, 11:19

Originally Posted by Jay GTI

In act I've been told that it's there for the dirt riders, not tried it myself, but 2-smokes apparently run quite nicely on a mix of normal fuel and avgas, jetted to suit obviously.

yeah talking to a lot of the big bore 4 stroke guys (including the guy i bought my bike off) they run a 50:50 mix of 91 and avgas.... I just run BP98 as anything lower and the bike doesn't idle well... and I'm running upwards of 12.5:1 compression ratio

**YellowDog** · 7th February 2010, 11:37

Originally Posted by marty

If it's Avgas, it's Low Lead,(110LL) and will be green or blue/purple.

Don't be a dick - it will NOT make your bike run any better - in fact, it will more than likely damage it if you do not tune the engine to run on it.

Last time I used 2 litres of avgas (and I have access to unlimited supply of it) in a tank topped with 15l of 96, the pipes ran grey for the next 250kms. There was no increase in performance.

Clearly it is best avoided then.

A few years back I had an old Porsche 928 and it had a very fancy electronic management system that would dynamically adjust the timing accordingly.

I used to stick 98RON in it and get somewhere between 10% & 15% further on it. This is not BS because it was a set 550km journey I was doing every two weeks on a 115kph cruise control. I also used one of those magnetic fuel pipe gismos that is supposed to make the fuel behave in a less volotile way and increase efficiency (that probably was BS).

I suspect that some of the older big engined bikes might have a similar sort of management system and be able to take some benefit from the higher octane.

But the moral of this story is, unless you know your bike can cope with the higher octane, avoid like the plague!

**motorbyclist** · 7th February 2010, 11:52

Originally Posted by YellowDog

I used to stick 98RON in it and get somewhere between 10% & 15% further on it.!

I've found several FXR150 all got similar economy gains (and a noticeable bit more go) on BP98, and my old VFR400 got a 10% improvement on it without any noticable performance gain (all compared against BP91)

**quickbuck** · 7th February 2010, 12:04

Originally Posted by YellowDog

But the moral of this story is, unless you know your bike can cope with the higher octane, avoid like the plague!

It's not so much "Coping with higher Octane", it is more of, is it necessary to run a fuel with a higher octane figure.

As we pay more for it, then it may not actually be worth it.

Yes, your Porsche would have run more economical on a fuel with a higher octane. No disputing that.
The ignition would have gone to Full Advance, as it didn't detect any knocking.... then you required less fuel to make the same power (Very roughly speaking mind).
In fact many bikes will actually run more economical too, if the rider buttons off the throttle in the cruse... Well, actually it doesn't matter what the octane of the fuel you use makes that work!

Some quick research here......

**Jantar** · 7th February 2010, 14:23

If you consider most piston driven aviation engines (lycoming, continental, etc) are designed to run at around 2500 rpm, avgas is also designed to be a slow burning fuel to give optimal performance. Then look at the Rotax aviation engines (912, 914, etc) that run at 5500 rpm, and the manufacturer recommends normal mogas, not av gas. The service interval on Rotax engines running avgas is 50 hours as opposed to 100 hrs on mogas.

Now which type of engine is closer to yours? The one that runs at 2500 rpm or the one that runs at 5500 rpm?

**Flip** · 7th February 2010, 17:56

Originally Posted by Jantar

If you consider most piston driven aviation engines (lycoming, continental, etc) are designed to run at around 2500 rpm, avgas is also designed to be a slow burning fuel to give optimal performance. Then look at the Rotax aviation engines (912, 914, etc) that run at 5500 rpm, and the manufacturer recommends normal mogas, not av gas. The service interval on Rotax engines running avgas is 50 hours as opposed to 100 hrs on mogas.

Now which type of engine is closer to yours? The one that runs at 2500 rpm or the one that runs at 5500 rpm?

Avgas goes back to the second world war when a bunch of nutters were putting 2000 hp 16 liter turbocharged V12 engines in planes, and they needed very high quality, leaded, high octane fuel to run on. The specification and regulations on piston engine air craft fuel goes back to WW2.

It amazes me how many people think octane and calorific value is the same thing. Octane is the fuels ability not to ignite under compression, however as the octane goes up the flame front speed of the fuel drops. Taken to the extreme if you run race gas in a low compression vintage motor the fuel burns too slow and is still burning as it leaves the cylinder and heats up the exhaust because of the radiant energy from the still burning fuel.

Avgas is a narrow cut fuel. It has bugger all small volatile molecules. These small molecules in pump fuel are ignited by the spark first and greatly increase ability of the fuel to be ignited and burnt by a spark. Some motors with a average ignition system just won't run well on Avgas, they spit, backfire and just run badly.

To all those folk who say their std motors run better on avgas, well sorry but "bollox". If I am wrong about this well so is Mr Honda, Mr Suzuki and Mr Yamaha who only spends about the GDP of a small country making sure his motors run as well as they possibly can on pump gas. I guess you know more than they do.

**Expert** · 13th February 2010, 17:59

Avgas is high octane because planes fly in a low oxygen atmosphere so you have to take the oxygen with you in the fuel. It's for Slow revving, big bore, low compression engines.
It works when mixed with pump gas to stop pinking in dirt bikes or bikes working hard with unreliable cooling, like going slowly in mud but revving hard, but I've had better results mixing methanol with petrol in these circumstances though.

**gatch** · 13th February 2010, 23:47

Interesting seeing some of the myths of avgas coming out.

Should run your lawn mowers on it, you'll shave 20 seconds a lap in your back yard hahahahahahaha.

**avgas** · 14th February 2010, 00:09

DO IT!!!!!
RG ran fucking lovely on up to about 80% mix of the good ol av' - but thats strokers for you, simple engines.
Dunno if I would run injected engines on the stuff - but good ol' carbied anything runs sweet on the stuff. Be careful though as the stuff has the most bizzare aroma - and it very addictive when burnt.
GB 1 (425) ran stonking on about 50% mix, any higher and I gained smoothness, but lost speed, and less and she went back to her knocking self. GB 2 (400) hated more than 30% mix, GB 3 (500) was the same.
ZXR 400 loved anything over 30% - but then again that was race carbs, SP motor and rumbling Laser ZXR9 pipe. So it basically ate lightning and shat thunder.
Ran the TS185 on 100% once, sounded funny. Like and old warbird.
Since I am now banned from buying the stuff from a previous source would be good to know if I can get it at Kumeu. But do it to a bike that isn't worth the coin of course.
Still my most favourite smell in the mornings. Av' from the pipe

**FROSTY:Me** · 14th February 2010, 10:11

adding to the thread a little. Modern bike (post 95 at a guess)on avgas --waste of time effort and money.
run an older bike on a 50/50 mix and you do see gains. But its because the bikes were desighned to be run on leaded fuel.
As for the discussion re how "bad" the lead is for you/the enviroment I agree but its also a crock of shit.
The fuel we currently run is not exactly good for you

**Pussy** · 14th February 2010, 11:36

Originally Posted by Flip

Avgas goes back to the second world war when a bunch of nutters were putting 2000 hp 16 liter turbocharged V12 engines in planes, and they needed very high quality, leaded, high octane fuel to run on. The specification and regulations on piston engine air craft fuel goes back to WW2.

It amazes me how many people think octane and calorific value is the same thing. Octane is the fuels ability not to ignite under compression, however as the octane goes up the flame front speed of the fuel drops. Taken to the extreme if you run race gas in a low compression vintage motor the fuel burns too slow and is still burning as it leaves the cylinder and heats up the exhaust because of the radiant energy from the still burning fuel.

Avgas is a narrow cut fuel. It has bugger all small volatile molecules. These small molecules in pump fuel are ignited by the spark first and greatly increase ability of the fuel to be ignited and burnt by a spark. Some motors with a average ignition system just won't run well on Avgas, they spit, backfire and just run badly.

To all those folk who say their std motors run better on avgas, well sorry but "bollox". If I am wrong about this well so is Mr Honda, Mr Suzuki and Mr Yamaha who only spends about the GDP of a small country making sure his motors run as well as they possibly can on pump gas. I guess you know more than they do.

This man is on to it.
Judging by the mis-informed opinions of some... Jet A1 would be even better... after all, it is jet fuel!

**Kickaha** · 14th February 2010, 12:03

Originally Posted by Pussy

Judging by the mis-informed opinions of some... Jet A1 would be even better... after all, it is jet fuel!

I doubt there'd be to many motors that would run on it, we used to have an old heater that probably would though

**Pussy** · 14th February 2010, 12:24

Originally Posted by Flip

Taken to the extreme if you run race gas in a low compression vintage motor the fuel burns too slow and is still burning as it leaves the cylinder and heats up the exhaust because of the radiant energy from the still burning fuel.

...also the main reason why avgas is VERY good at burnung exhaust valves.
Aero engines have sodium filled valve stems to cool the valves (ones running a high bmep).
I wouldn't even think about exposing my sem fiddy's titanium exhaust valves to avgas

**Expert** · 14th February 2010, 12:54

Generally, apart from vintage stuff (low compression, slow revving) if your bike runs better on avgas then you have a problem with your bike that running on avgas is masking ie. your ignition is too far advanced, compression is too high etc. You'de be better off tuning the engine to run properly on pump gas like it's intended.
The benzene put in unleaded fuel is arguably worse for you than the lead it replaced.