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Please Help with Fuel Requirements

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Posted by: Brian

I have done a few searches, but have not come up with quite the info I was looking for. I am sure the information is out there, but I just don't understand much of the techno-speak about fuels and their effects. I propose a fairly simple question and hopefully can get a fairly simple answer in real-world terms about fuel requirements.

First, some background on my situation. Assuming that my two-stroke is jetted properly (i feel quite confident that it is), and has no mechanical problems, I'd like some advice on what type of fuel and mix I should be using to obtain optimum performance. The machine in question is a KTM 250 SX. The engine has a modified cylinder head by Clay at CM Machine. I asked Clay to cut the head for 'cleaner' power on the low-end, and that he did. I also requested that the bike be able to run on pump gas, but I think he may have gone a little too far, as the engine will ping slightly under certain load conditions. I am able to jet rich enough to make the noise mostly go away, but performance suffers greatly in the way of sputtering and loading up.

I am looking to experiment with some higher quality fuel to try and eliminate the slight ping, while maintaining clean jetting. My dealer carries several oxygenated fuels (VP U-4 and U-2). If I understand correctly, these are not what I want. They help in performance, but don't do much in the way of preventing detonation. He also carries several varieties of un-oxygenated fuels(Motoman 113 and VP C-12). I am under the impression that this is what I need to be running, but am not sure? I also don't know if I should run these straight, or mix them with a percentage of pump gas to obtain the right properties? Assuming again that I am properly jetted, what effects can I expect these fuels to have on jetting? Should I go one step leaner across the board (pilot, needle, clip pos., main?) I am hoping I can run my current settings and eliminate any detonation I am experiencing, and then proceed to fine tune jetting accordingly?

Any help at all is greatly appreciated, and I'd like to apologize in advance for any incorrect assumptions or generalizations I may have made. Thanks
Brian


Posted by: Brian

I got some advice from the guys on KTMTalk and bought some VP C-12 non-oxy fuel and mixed it 50/50 with a fresh batch of Chevron premium pump fuel, 40:1 Silkolene Comp-2 premix. I did some initial testing at a local place and the race gas seems to do everything I wanted it too, plus it smells good. Now I can start jetting to take full advantage of the better fuel.


Posted by: Masterphil

(As non-techno-speaking as possible):
I'll see if I can take this one, Rich. Even though it is in the wrong forum... BTW, next time do a search username "Rich Rohrich".

By far the BIGGEST plus to using race fuel is it's chemical consistency. Pump fuel can vary by HUGE ammounts chemically. This has an effect on the distillation curve, which is very important to consistent jetting at the edge. Race fuel is chemically identical every can. This is very important because it means that the distillation curve will be identical every batch. This is the reason it can be jetted so sharply, so safely.

What you are doing is not optimizing performance by mixing it 50/50. You are doing the same thing as mixing a fine wine with mad dog 20-20. All you are doing is enough to prevent detonation. When you mix race fuel with pump fuel, you are comprimising the consistency, and the jetting reliability. Don't try to jet it sharp if you are using a mix of anything with pump fuel.

The Oxy race fuels have a high enough octane rating to prevent detonation, and if you truly are looking for peak power out of your motor, oxy fuels are one step closer to getting you there. VP-C12 has a distillation curve that will work quite well in a 250 2-stroke motor. You'll likely find that you can go at least a full step leaner across the board. I know I'm running leaner jetting than most in my 250SX, and I contribute most of that to the race fuel I'm running. Here's why:

Distillation Curves
Dr. Dave Redszus - Refinery values in Norther Illinois during summer.
10% - 158.0
50% - 250.0
90% - 374.0
End point - 437.0

VPC12
10%-131
50%-194
90%-228
EP-233

Take a look at the temperatures required to completely vaporise pump fuel. Now look at the VP-C12 numbers. Much lower than the numbers for pump fuel. That's why pump fuel works well in cars, they have much longer intake tracts than a 250MX bike, and more time to vaporise the fuel. A much larger percentage of the VPC12 will vaporise and burn, making the jetting richer than it was with pump fuel. The race fuel will also spooge less in most cases. This is because you dont have near as much leaving the combustion chamber unburnt and forming carbon/spooge all in your pipe. Ideally, you want to look for a 10% point in the 125-150 range and an end point in the 210-240 range to get good vaporization in our bikes.

I think I just about covered the "easy stuff". This fuel issue gets so much deeper than it appears on the surface and I don't know nearly as much as that Rich charachter.

What jetting are your running in your 250SX?

FWIW, I've had really good success running Renegade 102 in my 250SX. It's also got a good distillation curve, but not quite as good as the C12 you're using.

Once you start to get that thing jetted crisp, you'll never go back to pump fuel. Oh, don't even mention that smell. It's the nastiest, loveliest, worst, best smell ever.

An Oxy fuel I'd like to get my hands on is some TT101.
TT101
10%-157
50%-205
90%-217
2.7% Oxygen!
And it's unleaded!
That's gotta be some HOT stuff. (figuratively speaking)


Posted by: Rich Rohrich

Quote:
Originally Posted by Brian
My dealer carries several oxygenated fuels (VP U-4 and U-2). If I understand correctly, these are not what I want. They help in performance, but don't do much in the way of preventing detonation.


Brian - Unless your engine is running loads of cranking compression or has some bizzare pipe tuning/transfer timing the extra octane from U2 (MON 101 ) and it's lead content should more than cover any detonatation issues and it should make more power than straight C12.

If you aren't really concerned about optimizing the setup and just want to stop the knock then 50/50 C12 & pump fuel should do the trick, but you'll sort of be defeating the purpose of modding the engine in the first place.


Posted by: Brian

Cool. Thanks guys. The 50/50 C-12 mix seems a hair crisper than regular stuff, but the main thing I was after was stopping some very slight pinging. I'd like to try some straight U-2 next time, but how does that effect jetting? Wow, that stuff is expensive though!


Posted by: Masterphil

Since the fuel already contains oxygen, the bike will run leaner. When you get the C12 dialed in, increase the jet sizes one step across the board and go from there.



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