Hard to say how much R&D Kawasaki put into the S version after so many years of building the 300 etc

On the one hand, I'm sure they'd love to one-up the competition by having a 35hp or more street legal 250 enduro, but they may save the R&D for other bikes.

Or they may want you to buy a KLX450

 

You might have a look at the intake boot off of the motocross bikes. The design is for higher performance using 250cc. I'm sure the factories have been testing for years which one works best. Iirc Moto tessari came out with different intakes.

 

If they would jump through the hoops to have them factory street legal in North America I'd bite.

 

Depending on how it is installed it can. If it was truly straight from air box to carb throat that is best. If it is angled at all, that is a kink versus the smoother S turn of the stock tube. If you look at exhausts you seldom see a kink in the pipe, rather a smooth flowing curve. Same with two strokes, a high quality expansion chamber is either vacuum formed for smooth transitions or if it is made up of cones the cones are cut and rotated in very small slices in the bends. The less costly pipes will have more kinked turns.

So if the straight Gates tube is angled at either end it could easily be causing more turbulence. Fluid flow likes smooth flowing bends, not kinks.

In addition, there can be a self fulfilling prophesy too. There have been exhausts that sport riders have installed that they swear run better because of the big hit they feel. What they don't realize is the pipe hurt their lower mid range so bad that when it hits it makes the rider think the bike is stronger. Is this the case here? I don't know. Without something to prove actual increase it is purely subjective.

 

All very true of course.

So far, all I know is it ran different on the test run; seemingly worse - and then went back to normal after I switched it back to stock boot.

At least it did 'something', now I just need to try and find out what.

 

I've seen some bikes around and online now and then with a bulge halfway between the carb and air box. An idea that could be sound. Otherwise I am not familiar with MXers, there aren't many to see typically though there are some around, and adapting a boot could be a big headache.

My XR200's boot had a round flat spot on top. Evidently to be used for an emissions connection on some model at one time or another. I froze it in water with the flat spot just below the surface so I could drill that out with a hole saw and add a small K&N filter with a 1" spigot. It seemed to not necessarily be a gain. Maybe it threw off the ram effect of the stock boot.

So mods are always a 50/50 proposition I guess

 

You are right, trying. That's half the battle, figuring out what and why is the other part. If there was a way to get the path totally straight that would be the trick. How much gain can be had might be questionable.

I did some calculations a few weeks ago on the snorkel. I measured the opening of the KDX snorkel and calculated the opening area. Then I measured and calculated the largest part of the bell opening on the carb. They were nearly equal. Only problem with the snorkel is the bending of the air flow. Same is actually true of the air box lid when the snorkel is pulled out completely. The sharp edge creates turbulence as air enters. I'm figuring the reason why taking the lid entirely off is better than just removing the snorkel is because the sheer size of the area opened is so much greater than the engine requirement that the turbulence around edges and such will not be significant enough to affect the performance.

One point on the intake tube. I believe it has a bit of taper up to the carb, that would cause the air flow to speed up as it flows into the carb. Same deal as the reason the carb has a big bell on the end going to a smaller throat.

I honestly think until the carb is significantly larger the air boot/air box won't affect the flow much. The lid and snorkel are another story. I think they may. I think Marcelino showed that on his dyno runs doing with and without air lid.

I also think anything much bigger than the 36 used by many here will be overkill until the engine is cammed and ported to suit. That will also make it peaky, because you are essentially duplicating the KX250F engine design. Harder to ride and less likely to run a lot of miles reliably.

There was a thread on the ADV rider site heralding the CCM 450 adventure bike. They seem to think it is going to be something special in performance. Fact is CCM cut down performance to the horsepower level of a DRz400 in search of reliability for long street riding. The big plus for the bike is the adventure type fairing, more comfortable seat and such. Kind of a smaller KLR650, which ain't all bad if the price is right. But it isn't going to have CCM supermoto/off road performance, because the engine will not last.

Makes me think of what on of the hot rod magazines pointed out. To make an engine get more horsepower out of the same displacement one has to raise the revs - considering the cam timing and head flow is optimized. I think cam timing optimization is the reason the Marcelino mod works for the 250 and not for the bigger bores. The bigger bores might need different cam lobe profiles to suit the increased displacement. With the OEM cam the Marcelino timing change is not any better at doing the job than the more advanced timing. But I'm not an expert in that area, just thinking about what I have learned to date.

And I also like "talking out loud" too. To make one think.

 

There are other concerns that come into play at times - like the shock or other parts. Air boxes sometimes make the intake boot have to "flow" around the shock. Problem is if it cannot maintain a round shape they will go to a larger D or oval shape (the flat of the D being in the inside of the bend, keeping up the intake velocity) They will look like a bulge, but usually the actual area of the cross section is designed to maintain the size of the boot. As the shape flattens it has to elongate in one plane to maintain cross sectional area, otherwise the flow is restricted.

In a side note, I think considering space available, the 250 intake tract is fairly well designed, as big an air box as possible and a good boot design round all the way through with minimal directional change.

There just isn't any way to get sport bike quality air box performance within the confines of a dual sport. It is a constant trade off.
More talking out loud....

 

Pleasant read klx678.
In the vein of thinking out loud. I hope we can research and test simple mods, (perhaps 1/2 hp at a time), in order to create a recipe that creates power levels close to those of a BB kit on a stock bike. And do so for far less money than the kit costs. This "recipe" would also have the benefit of preparing the engine for maximum benefit if/when a BB kit were to be installed.

 

So, in summary, if my a/f ratio did go rich, it's either because the kinks rather than bends in the Gates hose connections slowed flow/increased vacuum OR because flow increased drawing more fuel out of the jet(s)..



I imagine I can jet for it once I get around to playing with it again, but I'll still have to somehow measure performance afterwards

One poster here a few years ago swore his bike ran worse when he put the snorkel in backwards..(!) Go figure. I think he test rode it twice to verify and got the same results both times. The lower end of the snorkel is cut at an angle IIRC and ...maybe the angle with the backwards snorkel threw off flow enough to be noticed? If so then it'd seem the snorkel is right on the edge of providing enough air?

Note that KDX riders routinely take their snorkel out. You just don't see any hop ups on cars or motorcycles where they restrict the intake tract to gain power. Once in a while someone opts a forum that taking the lid off etc just requires a larger jet by itself and doesn't actually do anything-but that's counter to elementary hot rodding isn't it?

A carb larger than 36mm would be less likely to run a lot of miles reliably only if you were riding the bike harder? This 36mm pulls so well down low with my 14/48 gearing, I don't see the 40mm falling on its face. Worse will be starting from scratch with the jetting-I've ordered extra sizes in everything for the 40mm since it takes 2 mos to get here I can't effectively tune if I order jetting a little at a time.


The math may initially add up for snorkel area vs air box & carb size until you factor in demand. What about when the engine's asking for 20 liters a second at 10K rpm, and has only a very small fraction of a second to get it each cycle depending on cam duration?

How large an opening would you need to flow 20 liters of water per second at 14.7 psi? I suppose there's a formula for fluid flow somewhere online, but I'd bet you'd need an opening a wee but larger than the snorkel hole in the lid.

Previously I measured 2. 1875 OD at the air box spigot and 2.125 at the stock CV carb; the TM36 is listed at 2.125 OD as well. There may be taper built into the boot.

FMF is selling this doo-dad you insert into your boot for better flow-only $100 !