Fuel Questions

[johnny5]

I currently run on 91 octane pump gas. The question I have is whether or not it is good to run pump gas in my cr250. In california they put a lot of crap in the fuel and I just don't know if it is okay to run (ex. ethenol blends). Is aviation fuel okay to run? Is there an alternative to VP racing blends? The prices are outrageous? If you have any fuel suggestions let me know. Thanks!

 

[JST122]

It is fine to run pump gas in your CR, you should just keep it 91 Octane or higher. Most manufacturers will just recommend that you are consistent with the type of fuel so if you use race fuel you should always run on race fuel rather than switching back and forth between race and pump fuel. For the average rider you are not going to gain huge, useable power gains or advantages from race fuel in a stock bike. My recommendation is to save your money and run the pump gas it certainly does not hurt your bike and as long as you run 91 or higher the bike will run well and you wont experience a lot of pinging or bogging.

A cheaper alternative to raising the octance of your fuel is to buy Octane Boosters, but again the average guy is not going to notice a difference between pump and race gas, especially in a stock bike.

Check out this article at Motocross action it provides some good info: http://www.motocrossactionmag.com/m...0&tier=3&nid=DE77783E28A14A15A42E7DD583701C93

 

[robwbright]

As Rich Rohrich said (paraphrase):

AVGAS is almost always a better choice than pump gas and never as good as race gas . . .

In my RM 125 AVGAS served me well - I had better throttle response than 93 octane pump and it seemed to produce slightly more power. However, I am switching to race gas this year.

If you shop around you can probably find a cheaper race gas alternative. The stuff I think I'm going with is $4.75/gallon. (Renegade Racing Fuels in Kentucky) Eric and Rich also say its ok to mix race gas with pump or AVGAS 50/50 or 60/40 to save a bit of $$$. Just experiment with it.

 

[tyesai]

Or if it runs fine and isn't detenotaing on 91 octane just run it. Otherwise you are just pissing away money. I guess if you are racing and are hardcore you might want to consider race gas. Search around, you will find tons of info.

 

[robwbright]

Fuel in General:

http://www.ericgorr.com/techarticles/Fuel_Basics.htm

AVGAS:
http://dirtrider.net/forums3/printthread.php?t=115202

 

[JST122]

Johnny5, in my opinion you should steer clear of avgas in your bike, both the article I sent in the above link as well as the articles robwbright sent will both tell you that running avgas is not recommended.

 

[robwbright]

Actually, the articles I sent say that AVGas is almost always better than pump gas. It has a lower 90% evaporation point, which apparently means it will burn more thoroughly. However, it is far from perfect as a fuel for MX, because similarly to pump fuel, it is designed for engines running at 3000-4000 rpm as opposed to 10,000+rpm.

My experience showed better throttle response and slightly more power (seat of the pants test) than 93 octane pump gas. To get the right results I had to raise the needle clip two clicks.

Your results may vary. . .

 

[robwbright]

Oh yeah - pump gas apparently tends to be inconsistent and variable from delivery to delivery. AVgas is a more consistent mix, and therefore, easier to jet consistently.

All of this info was learned from Rich Rohrich and the other guys here.

 

[robwbright]

Oh yeah again ;)

If you read here long enough, you'll find that Motocross Action is not noted for good, reliable information . . .

 

[JST122]

robwbright take a read through your articles again. A direct quote from the first article you provided is, "It's best to stay away from AvGas for your bike. We'll go into the specifics of AvGas in the next installment. Keep in mind that octane requirement is lower at high altitude and high humidity. An engine that ran fine at 10,000 feet could very easily detonate at sea-level, or a sudden drop in humidity on a hot day can cause knocking that never appeared before."

The first article later goes on to state, "We'll go into the details of comparing race gas to find the right fuel for your application, and we'll also tell you why AVGAS is a poor choice for your race bike..."

In your second article, it states, "So it sounds like Avgas is really bad for our purposes, and for the most part it is, but given the sorry state of pump fuel today, Avgas is looking better all the time."

It later goes on to state, "If you don't need the additional octane that 100LL provides (AVgas), then MTBE based pump premium (especially Amoco) will tend to provide better throttle than Avgas assuming you have any jetting skill."

robwbright, if in your experience you feel that MXA is not a source of good reliable information, I can respect that, it is your opinion. However, I can tell you that the article they provided is based in large part on facts and from there they have provided their objective based opinions.

The two articles you provided to back your point of view that avgas is good to use; actually denounce it as a higher performance alternative to pump fuel. So I am confused at what point you are attempting to convey? So basically what it comes down to is that AVgas is not formulated correctly for our motocross and off road riding applications. If you are really set on getting extra performance out of the fuel that you run and you think at your skill level you will benefit from the net gain in performance, then just spend the additional money and buy some race gas rather than try to cheap it with avgas that really is not providing any benefit to our application.

 

[robwbright]

Ok this is going to be long, but anyway . . .

As to MXA, I haven't read it very much in several years. I wasn't clear in what I said. What I meant was that if you read around here long enough you'll find that the general opinion here of the persons I have come to consider experts is that MXA is not a good source for reliable information. I've come across numerous articles here making fun of some of the "information" in MXA, and most of what I've read makes MXA looks pretty inaccurate.

My point was not to pick on you or pick a fight. Before signing on here, I would have thought MXA was a reliable source, and the article you posted looks pretty good generally. However, one needs to be careful when dealing with information coming from sources that may be tainted by the influence of $$$. That's the conclusion I've drawn from reading here. You can get plenty of good, accurate and FREE information here from people like Eric - He's done over 40,000 cylinders (along with mine) and really knows what he's talking about.

As to the AVgas question:

First, a correction: I was wrong - I should have said it's better than pump fuels with ALCOHOL (i.e. ethanol). Apparently most of the gas stations do use ethanol, but you can find some premiums without it (Amoco is recommended on here).

Second, when Rich says, "It's best to stay away from AVgas" I'm pretty sure what he's getting at is that race fuel is ALWAYS better. Most people asking about AVgas are debating pump fuel cost vs. race fuel cost and looking for a middle ground.

What I'm getting at is that AVgas is better than many pump fuels.

This topic has been covered seemingly hundreds of times here. I've asked numerous questions about it and talked to Eric a couple times. I'm pretty sure I've got the general idea here. The point is not that AVgas is ideal - it's that AVgas is usually better than your local (ethanol) pump gas because of consistency and (I think because of) a somewhat better distillation curve. As noted, ethanol alcohol in pump gas sucks - AVgas doesn't have any.

For example, example distillation curves are approx. as follows:

Pump gas:
10% evap = 150 degrees
50% evap = 250 degrees
90% evap = 375 degrees
end point = 425 degrees.

AVgas:
10% = 150
50% = 210
90% = 275
end point = 325

Race fuel:
10% = 125
50% = 210
90% = 225
end point = 260

(Now that I have a built, high compression motor, the race fuel I am switching to is:
10% = 152
50% = 205
90% = 222
end point = 250

It doesn't take a chemist to see that AVgas - while not ideal -is closer to race gas in its evaporation/distillation properties than pump gas is. I suppose this is why I got better results with AVgas than my local pump gas, even though it is certainly not race gas. However, I can get AVgas for $3.50/gall and some race fuels run from $4.50-$12.00/gall.

And oh yeah - you can store AVgas for long periods of time - apparently even leave it in the tank and carb over the winter (in my father-in-law's experience). My karting father in law tells me that numerous high level 2 stroke karting titles have been won using AVgas.

Rich once stated:

Unless your bike is knocking BADLY there isn't any power to be had, and the throttle response will likely be about the same or worse unless you are running a high alcohol percentage pump fuel. The upside is the AvGas will likely be more consistent and easier to jet, but from a pure performance standpoint it's of no real value.

Maybe my bike's compression was high enough that I needed the extra octane from AVgas. Or perhaps my local gas station has really crappy premium. Nevertheless, I DID get better throttle response and a bit more power with the AVgas. It certainly does smell cleaner and more pure. Anyway, I am not alone in my results with AVgas compared to pump.

Some other quotes from other articles here:

From njrealtor55
my 2 cents... av gas is better quality than most of the pump gas you will find out there.

From "fishhead"
Good read. The 90% point for the current crop of reformulated pump fuels is significanly higher than that of 100ll at this point in time. About 325F to 350F depending on winter or summer grade per Chevron's published info.. The endpoint published by Chevron for 100ll is about 275F and pump fuel is about 400F-425F.

As Rich mentioned,lead build up in a 2 stroke motor is not even a issue with AVGAS 100LL. If jetted correctly, head will be spotless,powervalves will have no excess carbon build-up. Piston crown will have no excessive build up from lead to cause concern. If you are going to run AVGAS take the time to re-jet, it is usually a must.

It isn't really a matter of octane it's a matter of matching the distillation curve to the application. If this post doesn't clear things up let me know and I'll try and get back to it later tonight.

. . . piston cooling isn't an issue for four-strokes but it is a major concern for high output two-strokes. The closer you get to the thermal limits of a particular engine design the more useful fuels with high end points (90-100% on the curve) become. The end point distillation temperature of these types of fuels is high enough that some of the fuel can make its way into the combustion chamber in liquid form. While this would normally prove to be a liability in most engines it could be useful when you are tuning to the ragged edge. The liquid fuel droplets entering the combustion chamber will leech some heat and help to cool the piston crown. In an engine that is pushing the thermal limits this can be the difference between a win and a holed piston. Some specialty fuels like Philips P45 are designed with a large jump at the end of the distillation curve to specifically provide this cooling effect. . .

An interesting side note here is the end point distillation temperature of 100 LL Avgas is high enough that some of the fuel can make its way into the combustion chamber in liquid form. As you start flirting with the thermal limits of a two-stroke engine Avgas can become much more useful.

I actually run my bike on 100LL av gas. It runs much cooler than on 91 octane pump, has a more consistent idle. It just seems to like it better. Also, the smell is awesome.

Depending on what type of AvGas is available to you it should stop the detonation up to a point.

I have been running 100 low lead av gas in my 00" husky 410. its starts eaiser ,better throttle response.thats the only gas this bake has seen i dont have any problems with it.it works greta for my bike.its alot cheaper than VP gas or cam2 fuel and gives the same results

In the end AvGas is still preferable to alcohol based pump gas especially in an open bike.

It will be a sad day for me when they stop making 100LL. As for fouling plugs, I have a B8EGV that has seen many new pistons come and go. Yes, I do carry a spare. But have not needed it in over 3 years, running 100LL!

I've run av 100 low lead for a couple of seasons in my 200exc with no problems. I notice just a tad less tendency to stall at low rpm's. I didnt notice much difference on the idle, but I tune mine to not stall while idling in gear with the clutch in. It will not ping, even flat out in 6th (long straight across a pasture at the last race high humidity and about 93degrees). I figure it doesnt cost that much more than reg gas and if it stops detonation, then its worth it. My 200 has never fouled a plug, and if its over 75 degrees, needs no choke, and starts 1st kick. I'll keep using it as long as it stays resonable. Good luck with your GG'

I struggled with 100LL for awhile. It just wouldn't give me the throttle response I was looking for no matter what I did. It is probably more consistant than pump gas though. And smells so damn good! <s>

I have run avgas LL110 with no problems. You will sacrifice a bit of throttle response and ability to obtain peak performance opposed to a proper race fuel, but the consistency of Avgas is far superior to that of the pump albeit not the cleanest buring fuel you will find.

If you are a serious racer looking for optimum performance, stay away...if you are looking for something more consistant than the pump, go for it.

Alot of guys will blow you off of Avgas, but I'd be willing to bet most of them are giving "I read it" advise and never used it. But again, it's not the best choice for a short intake motorbike. It was designed for a longer intake and running at consistant RPM's at high altitudes. More pains are taken at the refinery to keep it more consistant than pump fuel. But, there is stuff in it that doesn't suit our needs.

If you are torn between mixing a good fuel like C12 with pump fuel, and running Avgas...run the airplane fuel and save your money.

No problem using Av Gas, its better than the stuff they try to sell you at the pump. I have been using it for 23 years without a problem.Just make sure you use 100 low lead(blue in color)
OW43

 

[robwbright]

One other thing - I re-read the MXA article and I see zero discussion of evap/distill curve. They basically discuss nothing but octane, which is only a part of the picture. I suppose they're trying to keep the discussion simple for the "average" rider, but perhaps the average rider needs to know the whole story - I'm sure glad I learned some about it - and still am . . .

They also say they run pump fuel so they can recommend jetting to the "average racer". It appears to me that many or most of the "average racers" here use race gas. ;)