Sometimes I hear read conflicting information from multiple sources that I...

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Sometimes I hear/read conflicting information from multiple sources that I consider knowledgeable and reliable for correct information. So I'd like to get a solid answer. At what amount of compression should you convert to race gas? I've heard everything from 160psi to 180psi. Can we narrow it down a little? Ken OConnor Terry Preston Mike Pozzuto Jason Krokenberger Dion Braud

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  • It's not a specific compression that dictates that. There are two factors that affect fuel choice;

    First, the speed at which the squish band compresses the charge into the center of the chamber. The "tighter" the squish or wider the squish area, the more energy is imparted into the charge as the piston rises. The maximum speed is calculated during the design stage of a head rechamber and the desired squish area ratio and squish thickness designed in.

    Second, the total amount of heat (energy) imparted on the charge at peak pressure. NORMALLY cranking compression is a great indication of this number but a better indication is an measured corrected compression ratio and overstuffing calculation to figure out total energy. Factors also include crankcase stuffing (which is generally fixed), altitude, and jetting (which is adjusted independently).

    For example, you can have an engine with 135 psi which detonates violently on pump gas. Some engines are happy at 170 psi on 93 octane.

    What can be said is generalizations which have to be taken as just those! My upper limit for pump gas (93 octane) is 165 psi. My upper limit for mixed race/pump is 175 psi. The upper threshold for gasoline depends on the intended use. If you are drag racing (short bursts of heat) and NOT hot-lapping you can get away with a lot more compression at the upper end of race gas.

  • It is dependent upon exhaust port timing and UCCR.

    http://www.macdizzy.com/compress.htm

    That is a good chart for the TRX250r. Blaster has a large bore to displacement so must use lower psi to octane but that will give you an idea of how compression psi relates to octane.

  • That should give you a good answer for your question. A motor can be built to run on 93 octane fuel with anywhere from 120psi up to 180psi.

  • Here's a little reading for you..

    http://www.csgnetwork.com/compcalc.html

    To calculate the pounds per square inch (psi) from the compression ratio, one needs the compression ratio and the atmospheric pressure measurement. Then, the first number in the compression ratio is multiplied by the atmospheric pressure, then divided by the second number in the ratio.

    For example, if the atmospheric pressure is 14.7 psi and the compression ratio is 11:1, the equation to solve for the psi is (14.7*11)/1.

    Therefore, the answer is 161.7 psi.

    The atmospheric pressure can vary, but the standard atmosphere produces 14.696 psi at sea level.

    For more exact calculations, the altitude should be considered.

  • Nobody has mentioned ignition timing. That is a whole other factor to this as well

  • Ignition timing is dictated by the chamber, UCCR and RPM

  • It's definitely still a factor tho. If someone is on the borderline of needing race gas/pump gas mix, and add a few degrees of timing you will find out real quick you ran out of octane

  • You are absolutely right. Sometimes even 1 degree can ruin a day depending on the weather conditions and how close the motor is to the det limit.

  • Excellent input guys. I'm not going to go into great detail on this subject and will keep it simple with a couple of facts. When we refer to “pump gas”, what are we talking about?

    E10 is the most commonly dispensed fuel at the station. This fuel absorbs water from the air and has a shelf life of about 90 days. It's garbage after that.

    When was the fuel purchased, winter or summer? They're different blends designed to do different things. The bottom line is they're not designed to do anything your ATV engine needs. Pump gas is designed to run in road vehicles that run at 185 degrees F with an average RPM of 2000 – 3000. If that's how you ride, have at the pumps. An octane number is nothing more than a reference to the fuels ability to resist detonation during combustion. The higher the number, the more resistance. Race fuel isn't going to make you go faster unless you need it. An engine that detonates on pump fuel needs a mix or strait race fuel.

    A stock Blaster will run fine on pump fuel as long as it's fresh and you're not beating the dog out of the engine.

    Our “pump fuel” builds will run fine on fresh premium fuel but I'm leaving a little bit on the table.

    Everyone thinks “race fuel” is expensive. It's dirt cheap in my book. Five gallons of VP110 will run you about $75. Mix that with five gallons of 93 octane for a total cost of $86 for 10 gallons of fuel. That's $8.60 per gallon for fuel that's going to work in the mud, sand, hills, and when the engine is under the most stress. How much fuel do you use per ride? I'm guessing less than two gallons. That's a grand total of $17.20 to have the time of your life and not worry about destroying a very expensive component of you quad. Now think about what a quality top end rebuild is going to cost. It's a no brainer and the piece of mind is priceless.

    At best, pump fuel is inconsistent and you never know what you're getting. I run it in my 2016 Kodiak but it's always fresh, stabilized, and I ride like an old man. I found 145# of static compression on Sunoco 94 octane was the limit when I could still beat the shit out of a Blaster. Static compression of 160# would always detonate without a 50/50 mix. I always ran straight 110 in my static 180# builds. Keep in mind, these were Blaster engines. My general rule of thumb is ANY modern two stroke motocross bike requires a mix of 50/50 race and pump fuel or you're asking for trouble. This turned out a bit longer than I expected.

  • Great answer. Thanks for taking the time to write it. Think I'm gonna start mixing some race gas in there.

    Do you happen to know the shelf life of race gas?

  • As long as the container remains sealed and kept in a cool (out of the sun) spot. You're looking at a couple of years.

  • I run Sunoco Optima 95 race fuel in my 250R and recommend the same to all my Customers that run stock or slightly higher comp. Anything else you are foolish bot to higher on the octain to be safe.

  • Pump gas is complete shit? Hydroscopic,unstable and faze separating shit!

  • I have a gas station nearby that has 110 at the pump for about $10 a gallon and there's a VP gas station downtown that sells race gas by the 5gal. Think I remember seeing different types/grades to choose from tho. I'll have to look at them next time I swing in there to see what they are. I was gonna grab a 5 once but remember not knowing which to get.

  • Is it Cam 2 at the station?

  • I don't recall. The pump is at a sunoco right across from a race track. Just know I've seen the 110 on it.

  • Let me know.

  • Will do. I'll have to look at the stuff at VP too. They weren't very organized. Just a bunch of random barrels of fuel in the candy isle lol

  • Dion Braud That's great if all of your customers are located in certain parts of the Northeast. The fuel isn't available anywhere else. http://www.sunocoracefuels.com/fuel-finder?city=&s tate=&zip=06239&fid=cat_optima&range=300

  • Yeah its limited to up here but any Race fuel in that rang is good as long as it promotes good oil suspension. Some of the fuels don't like to hold premixed oil in suspension and could cause separation issues. I don't like pump fuels for any small engines especially carbureted ones.

  • Chris Eldridge don't buy race fuel out of a pump, it's garbage, it's not a fuel that's sold in large quantity an sits in the tank for along time. If your gonna buy race fuel buy VP in 5 gallon cans, I buy VP 110 for 55 dollars for 5 gallon can, I use 2 cans per race weekend, my blaster will drink 5 gallons in a 55 mile race, some races end up close to 70 miles. Plus my sons drr uses 2 gallon a race. I tried pump race fuel an never will use it for anything more then starting a bonfire again

  • To me ignition timing is part of tuning an reading your plugs, as Jason said, changes in weather will require changes in timing as well as jetting ECT. I normally will do 1 or 2 plug chops at each race an evaluate jetting, an timing. An adjust as necessary. Most of the tracks I race at I've raced there before an I keep a log book of my tuning at every track. Most of the time I can go back to same settings, as long as weather is close