ASK THE MXPERTS: RACE GAS IS MUCH TOO RACY FOR YOU
RACE GAS IS MUCH TOO RACY FOR YOU
I own a 2018 KTM 250SX two-stroke and am running VP C12 in it, but I can’t seem to get it jetted properly. It doesn’t run well down low, and I wonder if you have some jetting advice for me. It is a stock engine with a Pro Circuit pipe. I read on the internet that I should be running MS109 or MRX02. What should I do?
Are you really running 100 percent C12 in your stock KTM 250SX engine? It doesn’t need that much octane, can’t use that much octane and will run ragged on that much octane. When it comes to horsepower alone, stock engines do not benefit from high-octane race gas. Pump gas and 112.5-octane C-12 produce the exact same power. Why? The pump gas, which has a 91-octane rating, is right at the base level of a stock engine’s needs. On the other hand, 112.5-octane C-12 burns too slowly and too cool for the stock compression. The conclusion? You are burning money in your stock KTM 250SX engine. The manufacturers have no intention of building stock engines that require expensive or exotic race fuels. They know that the average rider wants to fill up his gas cans at the corner filling station. Your bike, and every modern engine, was designed to run on the octane found at the pumps (and not just the pumps in Detroit, but also in Helsinki).
When would you need to run C12 in your 2018 KTM 250SX two-stroke? (1) When your engine has been modified with more advanced ignition timing, cranking pressure in excess of 200 psi, or the compression ratio has been raised with head mods. (2) When you experience detonation with the best available premium unleaded. If it pings, you need more octane. (3) When you find yourself away from your home track in deep sand or sticky mud, you should up the octane. (4) When your local gas station seems to be selling you fuel that isn’t actually premium fuel, but is instead a lower grade. MXA always suggests that you buy your fuel from the busiest gas station in town to ensure that they have fresh fuel in their underground tanks. (5) Most two-stroke bikes can run pump gas as long as the engine is stock; however, a few two-strokes are very sensitive to an exhaust pipe change, most notably the YZ250. But, even in this case, you still wouldn’t run straight VP C12 in the engine, but instead a blend of pump gas and C12 (starting with a 50/50 mix and working your way down to the minimum amount of C12 it takes to eliminate the pinging).
Don’t make the mistake of equating octane with horsepower. Octane, no matter how much, has little or nothing to do with horsepower. Octane does nothing more than improve a fuel’s ability to prevent detonation, pre-ignition or self-ignition. Fuel with too high of an octane rating will hurt the engine’s performance because it runs too cool, which means it will not burn completely before the piston is already out of range. It is the race fuel’s components and blend that create the horsepower, not the octane rating. High-octane fuel allows a tuner to go radical with the compression, ignition and porting, but, by itself, octane doesn’t add anything to the power quotient.
As for the VP fuels you could run, here is what they offer.
C12 is a non-oxygenated, leaded, 112.5-octane, two-stroke fuel. It sells for $78 (per 5 gallons). This is the go-to gas for any ported, milled, aircooled or modded two-stroke engine.
MS109 is an oxygenated, unleaded, 105-octane, four-stroke fuel. This unleaded race fuel has no business in a two-stroke. It sells for $87 (per 5 gallons).
MRX02 is an oxygenated, leaded, 104-octane, two-stroke fuel. It sells for $168 (per 5 gallons). If you had a high-strung 250SX engine instead of a stocker with a pipe, MRX02 would be the best-performing two-stroke motocross fuel to use. Because it is oxygenated, it delivers up to 7 percent more power than pump gas or non-oxygenated race fuel, but costs more than $30 a gallon.
T2 is a non-oxygenated, leaded, 105-octane two-stroke fuel that is already pre-mixed with two-stroke oil at 40:1. It sells for $60 (per 5 gallons of pre-mix).
U4.4 is an oxygenated, leaded, 108.5-octane two- and four-stroke fuel. Unlike MRX02, U4.4 is not legal in AMA racing but can be used in local racing. It does require larger main and pilot jets. It sells for $78 (per 5 gallons).
What should you do? The simplest solution is to run pump gasoline at KTM’s recommend 60:1 gas/oil ratio (10.67 ounces of oil to 5 gallons of gas). Start with the stock KTM jetting and go up or down one size on the main and pilot for your locale. The second option is to run pump gas with a percentage of C12 in it to compensate for the quality of the pump gas in your region. The third option is to buy T2, which is actually VP VPR race fuel pre-mixed with what we believe to be Maxima oil at 40:1. The fourth option is to run U4.4. It is a pump gas replacement fuel for both two-strokes and four-strokes. U4-4 will give you a big boost in power for around $15 a gallon. You will have to rejet. The fifth option is the expensive MRX02, but it is best left to serious tuners. The option that you should not consider, and never run, is straight 112.5-octane C12 in your stock engine.