Pump Premium vs. Av-Gas

[NWetRider]

Bike is a 99 CR250 used for woods. I am concerned about the pump gas because there is alcohol added to it all winter where I live. I can't see paying for race gas for a trail bike so it really isn't an option and mixing it with the alcohol premium isn't going to really do any good. Between pump or aviation fuel what is best? I have read all about avgas impacting throttle response but those discussions are all full of technobable and speculation. Is there any objective evedence of this? I don't know if a dyno would show it? Can anyone actually feel this effect? Will I need to change my jetting to try it? I don't really care if I gain or lose a couple horsepower, as long as it runs good.

I like how the aviation fuel can be stored a long time and it isn't too expensive plus I also have an airport near my house.

 

[Okiewan]

As long as you'll keep it pinned and ride at about 10,000 feet, yeah AV gas is great.

If you want an answer, you'll need to make an effort to understand the "technobable".

 

[Rich Rohrich]

I am new to 2 strokes and really don't think I can jet by feel.

The type of fuel you use isn't going to make a bit of difference untill you get comfortable with jetting by feel.

 

[NWetRider]

Okiewan - It is not a problem of understanding the technobable. I'm new to motorcycles but have been into mountain bikes long enough that I know the long technical discussions about the superiority of things such as certain materials, linkage designs, etc. often do not accurately reflect how things are in the real world. I am just asking if there is any objective evidence for any of it. Dyno results, some machine that measures throttle response, etc. Seeing data like this for different fuels (all on the same engine and properly jetted) would be helpful. If that doesn't exist then I guess I will have to go by anecdotes and the technical discussions alone. I wasn't meaning to put down what I have read so far but I was looking for a little more information before I choose a fuel.


Rich - I'm not as concerned about performance as I am about what the bad effects of the alcohol in the pump gas might be. Does it cause problems? It’s true I don't have the jetting thing figured out yet so I may have to try the pulling the pipe method. This could be very time consuming so I just want to decide on what fuel to use before so I only have to do it once. In reading on this site I saw that av gas is supposed to be better then pump gas with alcohol. But then I have seen lots of negatives about avgas also so I'm confused. I am not sure what I can get race gas for but it sounds like it is very expensive to use for trail riding. I never use all the power my bike has in the bush so it’s not really a performance issue. What do you recommend for someone who has a stock engine but can only get pump gas with alcohol?

 

[Rich Rohrich]

Rich - I'm not as concerned about performance as I am about what the bad effects of the alcohol in the pump gas might be. Does it cause problems?

None that aren't really performance related.

It’s true I don't have the jetting thing figured out yet so I may have to try the pulling the pipe method.

That "method" is really nothing more than an indicator of average exhaust gas temperature (EGT) over an extremely wide range of throttle openings. A lot of factors other than air/fuel ratio effect EGT which makes reading the pipe a total crap shoot except under very specific fixed conditions. Even then it's value is highly debateable. ;)



Just put pump gas in your bike, read up on the " ride and feel method of jetting", and jet your bike so it feels good to you. There is little point in messing with the fuel till jetting is second nature.

Here's a link that will help :
http://www.ericgorr.com/techarticles/carbtuning.html

 

[Buckholz]

Just put pump gas in your bike, read up on the " ride and feel method of jetting", and jet your bike so it feels good to you. There is little point in messing with the fuel till jetting is second nature.

Rich is dead on in his recommendation.

BTW, most of the KTM two strokes come with a jetting chart, that works pretty well. Not perfect, but pretty well, and as you get used to how the bike performs, and make small jetting changes around the recommended settings, you will develop a sense of jetting....

I run premium pump gas year round (yup, in the winter it has additives (alcohol) for smog requirements), use a quality 100% synthetic oil for powervalve engines, and, well, haven't had an engine failure in the last 10 years of racing as an expert/A rider. We race everywhere from 7,000' where we live, down to sea level and up to 10,000'+ on the Rocky Mtn Enduro Circuit.

 

[nephron]

From what my father told me the other day, the industry is phasing out AV-GAS in favor of Diesel. But hey, in the meantime you could get one of those I-CAT thingies to help out with the throttle response. ;)

Seriously, I think the objective data on throttle response and AV-GAS comes from fuel chemistry basics...namely (I believe, but this might be wrong) that fuels with higher proportion of aromatic hydrocarbon suffer flamefront (speed) issues. There is probably a lot more to it than that. AV-GAS also has a fairly high 10% point compared to other fuels---probably a safety issue in airplanes?

 

[mtk]

Av gas will never go away, unless all piston-powered aircraft go with it.

Jet fuel is only useful in turbine engines.