ponderings about airflow
 

Just been thinking lately. A klx 351 will put out about 28hp if I am correct with not much more to go with a carb, every bolt on in the book and some head work. Drz400's are around 38hp all corked from the factory. I figure 50cc shouldn't make that big a difference and wonder what the limkting factor really is with the klx. My only thoughs are either valve size and porting. What if another head was able to be swapped on? Imagine if the klx450r head would match up, would that bring more potential out of the 250/351 engine?

The dyno chart on dynojet's web site shows only 33hp for the stock DRZ400s.

http://www.dynojet.com/pdf/3109.pdf

Ride on
Brewster

I agree there has to be some major factor that limits the KLX 250 engine. It is just way down on hp/trq compared to everything around it. For instance, my research indicates it takes a modded KLX351 to match/slightly exceed the performance of a stock WR250R.

As far as I've read, porting and larger exhaust valves help. Of course with the addition of 101 cams. The best I think is realistically possible without doing crazy compression ratios is about 33hp, and that's on a perfect dyno day.

Once a carb, cams, headwork, and big bore installed, the best thing to due it change gearing for more low end, more high end etc. You can also try dropping weight off the bike.

If we're honest, I think pushing a KLX as far as one can with head work and such may be defeating the purpose of getting a KLX in the first place. Pumping a lot of time and money to max out the KLX may be better spent on a more serious dirt bike from the start. A bigger bore is the single biggest and best spent time and money on most motorcycle engines, and it appears the same on the KLX. The next item would be suspension tuning in the form of revalving and respringing according to rider weight and riding conditions, and that is often done on dirt bike bumping the 10K price range, so it's definitely not a waste of money.

Hey, I get the fun and challenge of pushing the limits on tweaking any motorized vehicle, but at a certain point I think reality has to set in. With a lot of disposable income and time...and another bike or two in the stable...the sky's the limit. On my KLX with an OEM 300 jug/piston, full suspension revalve, full airbox mods, and a full Muzzy exhaust, the only luxury item I went with is the 36mm pumper carb. I say "luxury item" because in truth it's not an absolutely required item like some of the other "uncorking" mods. But...I gotta say the carb has been such an impressive addition, that I couldn't imagine going back to the OEM CV unit.

When you get the KLX into the low to mid-20's hp range, the bike really does transform, and I don't think it's reflected in just raw dyno numbers. The smooth, tractable power delivery is so usable and pleasant and effective. Those wanting explosive power should really probably be lookiing at other bikes. The other side of this issue is that people use their bikes quite differently across the ownership spectrum, so it's impossible to dictate that one-size-fits-all.

As to the comparisons of other bikes like the WRR, I think some sell the KLX short. In December I rode with a guy for a few days in the desert areas of Big Bend Texas...a lot of rocky, rutty, loose terrain. He had a top drawer fuel programmer, full FMF exhaust, and airbox mod. While I may have been a little bit better rider than him, my bike got traction where his didn't and would pull him from a dead stop for quite awhile until we were near WOT, and he had the same gearing as me. I rode his bike, and frankly I'm not impressed with the powerband on these WRR's until they're well at midrange and up to WOT. Dyno charts aren't everything, as most of us well know.

I always thought of the wrr to be more of an mx type of bike, one that you ride to the track and do high flying jumps, and the klx more of a true dual sport/ trail bike

We own a newer KlX300R,and I performed the carb,airbox, and exhaust mods to unlock its' power. Like the Klx250, it's a whole different bike uncorked. TNC, I'm sure your bike has more power and response that our klx300 and would be a match for the wr. My klx250, at this point, would not be.

If you want 40+HP, you got yourself the wrong bike.

To be completely honest, if I only had 15hp I wouldn't want another bike. Nothing is quite so nimble unless a lot of money is spent. Just wondering so far as an engine capability standpoint. I'm more so trying to understand the real limiting factor with the engine.

OK, the real limiting factors...
The stroke is too long, you need a short stroke to turn 25,000rpms.
Then there is the head. The ports are too small, valves and springs too heavy, probably need another plug or two. The ignition system probably will not run all that, so put one of those in there. You will need to replace the cam drive with gears. Need some bearings for the cams. FI or DI would add another 10% power.
If you fix those and put a light weight piston in it then you may do get some power out of it.
With most things including engines, once you improve one system, there is simply another limiting factor that need to be addressed. So if money is no object, since most of us believe we put a man on the moon, getting 70+HP out it would not be much of a problem. And if that is not enough, there is always super charging and nitro.