KX af project or not?...

[cnrcpla]

When they hit 9800k they are at their limit. You can only make them go so fast before you sacrifice engine reliability. That's why 4ts blow more offend than 2ts, they push more rpms than they are necessarily built to handle. You could make them push farther and make power beyond it, but you would run the risk of over stressing a lot of engine components, even for the simple 2t.

[Stusmoke]

That makes sense. And I've noticed the 125s do seem to sign off a little earlier than the 250Fs in reviews. Good Ole 2 strokes

[TMKIWI]

All to do with port timing stu.
In a MX bike you want tractable power so motor is designed to only rev to 9k.
Put the same engine in a road racer or kart with different port timing and you will pull alot more revs and have alot more power.
It just moves the horsepower curve up the rev range which is not suitable for dirt bikes.

[Stusmoke]

All to do with port timing stu.
In a MX bike you want tractable power so motor is designed to only rev to 9k.
Put the same engine in a road racer or kart with different port timing and you will pull alot more revs and have alot more power.
It just moves the horsepower curve up the rev range which is not suitable for dirt bikes.

I was just going to mention that if it was a blowing up issue why can the super bikes rev up to 18 grand. Thanks

[cnrcpla]

If you were hitting 14k on a motocross bike wouldn't you be changing piston rings before every race? Not sure, but my logic is more rpms = faster engine wear then running less rpms. 

[msmola2002]

Because you are comparing apples to oranges. A short skirted short stroke 4 stroke, which only has a very hot event happening every second rotation.

[Stusmoke]

Ok I feel bad now I've sort of unintentionally thread stolen.

 

If you were hitting 14k on a motocross bike wouldn't you be changing piston rings before every race? Not sure, but my logic is more rpms = faster engine wear then running less rpms. 

Yes I know but motoGP bikes would undoubtedly be rebuilt after every race at those insane RPMs.

[cnrcpla]

True... I think I'll stick with 9k and a rebuild after 50 hours. 

Anyways, back on topic. Have you seen any nice KDX engines rearing their head? They seem like a fairly common bike, so an engine shouldn't be to hard to come by.

[nom de guerre]

^^^ I have seen a couple, may end up buying a complete bike if I have too. I am pretty much set on a KDX 200!

[cnrcpla]

Yeah, I have seen them floating around the internet for like $700 for the whole bike. Not to bad...

[2STROKEREVOLUTION]

Hey a quick and really unrelated question that I have: Why do the 250 smokers only make power up to roughly 9000 RPM but keep on revving until 13 grand? Not meaning to thread steal here just a random question I had that seemed silly to make a new thread over.

There are a lot of factors. As some have said, ridability is a major factor. I saw a custom streetbike with a KTM 250 motor pushing near 90hp. But that powerband would never hook up in the dirt. Second, 4-strokes have very short strokes compared to 2-strokes. This allows them to rev quicker since the piston doesn't have to move as far. Third, reliability, more rpm, more wear and higher stress on parts. Fourth, you need torque and rpm to produce power, if your motor can't breath at higher rpm torque drops off. At some point, torque drops off faster than rpm so your power falls off. Modifying the porting will move your torque up in the rpm range and you will be able to get more power up there, but you will sacrifice bottom end, and that leads to the bad ridability for dirt bikes. So I think you get it, you pick a balance between all the factors for your type of use.