Long distance two stroke - what goes wrong?

[DrZero]

I've been discussing Best in The Desert series with some friends. As far as bikes go the move seems to be towards the 450cc motocross bikes (of course the Honda 650 is still a force). Less seen are either big bore two strokes or the latest 250cc 2-strokes.

For everything else I like to do I think the smoker is a better choice, but if I decided to run in the desert races would a 250cc smoker survive the 250 mile races that make up the BITD series.

I noticed that there is a couple guys from Mexico that run an YZ-250 in the SCORE series and seem to do very well.

I talked to the only pro-racer I know (car guy) and asked him what he thought about 2 vs. 4 stroke and his take was that it all comes down to cooling, but suspects the 4s are easier to keep cool, thus have longer life at speed.

I know that Kawi 500 was king of the desert in the 90s, but it was almost as purpose built and unweildy elsewhere as the Honda 650 that replaced it.

Among more versitile sized generally available bikes is the 4 stroke just better suited to long desert races?

(I see the 250s have their own class, so if the top speed is little less than the big 650s I don't see that as a huge problem, though I note that a 450 won the Baja 1000 this year)

 

[Rich Rohrich]

At some point it comes down to piston crown cooling. A 250 two-stroke piston crown sees 9000 firing cycles per minute at a sustained 9000 rpm. A four-stroke engine running that same rpm sees only 4500 firing cycles, along with 4500 cycles where the crown is essentially allowed to cool. This difference impacts a host of other things in the engine including piston expansion, piston to wall lubrication, cooling system efficiency, detonation and pre-ignition tolerance, etc.

While there are certainly other differences between the two and advantages to both styles of engines, in that type of environment piston crown cooling is the one issue that no one has found a good solution to over the long haul. The guys running KX500s did an amazing job of working around these difficult issues, and they deserved their place at the top all those years. Hard work and lots of broken engines paid off. :cool:

 

[VintageDirt]

My uneducated guess would be fuel economy is better on the 4 strokes.

 

[76GMC1500]

Make sure you're running a piston with 2 rings for better crown cooling.

Fuel is certainly an issue when running long distances. I only get an hour or 2 of riding out of 2 gallons of gas in more technical riding. Your fuel is going to burn up a lot quicker when you're running faster.

 

[2 strokes for life]

I know you can get 7 gallon tanks for the CR500 and the KX500(i would imagine). So that rules out the fule econnemy.

 

[VintageDirt]

I know you can get 7 gallon tanks for the CR500 and the KX500(i would imagine). So that rules out the fule econnemy.

I wouldn't call 7 gallons of fuel onboard an advantage. :cool:

 

[2 strokes for life]

Well it could still go far. Besides have you ever riden a XR650. They call it a PIG for a reason. It is very hard on gas.

 

[Farmer John]

I don't think the 250's hold up very well during a full BITD season. Everyone I know that runs a 250 uses 2-3 bikes a season.

Keep in mind the 500's were competetive up until Kawi told Destry & Shane they could no longer ride them at the end of '04. The #1 problem with the 500's is the head gasket. If you can get the coolant to stay in the motor all day long you will be competetive.

As far as millage, BITD has pit every 30-65 miles, so we have never needed more than 3.5 gallons per section. We do use a ZipTy 4.1 tank with a dry break, but the only time we ran out of gas was in '03 when I was pizzed & rode through a pit without stopping. Our KX500 with Eric's 540 kit & Falicon ballanced crank gets in the 19-20 mpg range. The Berkely Honda prepped XRpig50 that we race in Mexico gets the same. A friend of ours runs a 525 XC-W in BITD & he gets 16-18mpg.

Personally, given the choice of KX500 vs, well, anything with 2 wheels, for BITD, I will take a well sorted KX500. The thing we have found that seems to make our KX survive is getting it all set up so the motor never really works super hard. We run 15/40 gearing & have had the motor built & balanced. Balancing a 2t single is a trial & error kind of thing. You need to pick a RPM & have it balanced for that RPM.

 

[Farmer John]

btw, what class to intend on riding?

A 250 2t would be quite competetive in the Am. classes. We used to run 35+ex ('01 & 02 class champs), now 40+ex. #704