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trx400ex
09-03-2001, 06:53 PM
hey guys ....i have a question about the this article. is what hes saying true about needing to run race gas on a any comppression 400ex?thanks here it is, btw i trust this guy and he knows his **** but i just dont know about this one


Jeff, I can do just about anything you want for the 400ex, especially for the mx racing, but the only problem we are going to have is with the choice of fuel. There is no way to build any power without upping the compression, even going big with lower compression still requires race fuel due to the added heat. The 400ex is a lot different than the 300ex, we can take the compression to the moon on them things and make them live, but the 400 is a much different, weaker design and you have to really maintain it, especially if you have it modified and it needs a steady diet of good high octane race gas, 112 octane or higher. You can build a 416cc 11.0:1 compression engine, but you are not racing woods, and then again Jeff, I tell my woods guys with that same engine to run race gas simply for added protection against detonation/preignition. The least slight detonation and this thing will pull head studs like crazy, it will seem like a simple leaking head gasket but what is actually happening is one of the cylinder studs is pulling the threads out of the cylinder-yuck! So now you see what we are up against, if it is a matter of not being able to get it, most places will sell the race gas at the track. We can build you a killer engine for mx, as my rider Aaron Slingluff just took first place at Loretta Lynns in Four Stoke B class, and that was with a 425cc engine, racing against MANY 440's, 426 YZF powered quads and a couple with the Suzuki DRZ engine. I can't take ALL the credit, he is also a very good rider. If you are interested in building a national mx enginje plan on spending no less than 2500 dollars and running race fuel. Thanks, Scott.

trx400ex
09-04-2001, 06:21 AM
anyone know if you have to run race gas with any compression on a modded 440

NVR2L8
09-04-2001, 07:21 AM
The general rule is that you can run pump premium fuel up to 11:1...any higher compression ratio and you need to run either a mixture of race gas, or just straight race gas. I modded my 400EX in May by boring it to a 416, and added an XR400 cam and FCR carb, and I run a 50/50 blend of race gas and premium fuel. Probably more octane than I need, but it provides protection against detonation and helps keep the motor running cool. BTW...after modding my engine, I could really tell an increase in heat. It's okay when just riding it normally, but it really heats up quick when I ride it hard.

09-04-2001, 01:17 PM
I think that guy failed to mention that the boy who won the B class had more than just a simple bore done on the motor and added a high comp piston.,,I know what it takes to make a bike like that a winner,,,,a lot of money and a lot of changes to the motor. I will be running a 10.8:1 JE piston in mine and plan on running pump gas,,,93 octane,,,anything higher,,,from what i read requires race gas. Most people WANT to go with race gas because they WANTED to go with a higher comp piston which both work together. I guess I'm one of the few that doesn't want to spend 4 dollars a gallon on race gas but still get a good strong running thumper engine. For one anything higher than a 9:1 piston is a higher compression piston cause the 9:1 is the stock comp. So basically you don't have to run race gas with a higher comp piston,,,UNLESS you go higher than 11:1,,understand now..

trx400ex
09-04-2001, 02:35 PM
he was trying to say that the amount of heat from other things will cause it to ping or whatever...i guess ill just go with ten and a half and then get a pingry oil tank

nakomis0
09-04-2001, 06:38 PM
Will someone please tell me how and why higher octane makes your engine run cooler? I heard it burns hotter and slower, you need the compression so it will ignite fast and not just sit there and burn. I have also seen a stock Kawasaki Tecate 4 burn a hole in his piston due to running 118 octane. anyone who knows I would really like to understand, this is the second time I have heard about it running cooler:huh

09-06-2001, 03:39 PM
Running race fuel is a good way to cut down on some of the heat on a 400. You do not HAVE to do it unless you up the compression but if you want it to run cooler, which sometimes results in more HP, then running race fuel will help. Do not expect to see some big difference in power by just adding race fuel because it will not happen. Most race fuels are better manufactured than pump gas which is better for your motor if you can afford to buy it all of the time.