I've heard people say the benefits of the MCM are not as apparent on the big bore kits like the 331 and 351. I don't know if that's proven or not and why that would be the case. I'm just wondering if anybody has done the MCM on a 300 bore and if it gave similar results to the 250.

 

This has always been an interesting question to me - Cam phasing for performance is done on ALL DOHC engines - with predictable results for whatever settings are chosen.

Yet in here, changing from a 250, to a 300, to a 351, seems to mean you go from a normal engine to a "magical" one that is above the "rule of law, ICE physics, etc etc"...!


Valve-to-piston clearance issues are, always, the only concern when you play with cam phasing.. Which, in our case, is not an issue with 351 bb kits and surely is not an issue with the 300cc "kit" - as per the very few members that have actually run the 351 + MCM.



Now, rant over.. MCM will modify the TRQ production exactly as it does on the 250. There is absolutely nothing about a larger piston that changes what cam phasing does to TRQ production. What is different is the inherent TRQ production with a larger displacement engine. What you have to decide is if that is what you want - Normal "biggy" 300 TRQ + MCM TRQ is gonna make a crazy powerful bike below 6500 rpm with, perhaps, a top end that sheds power a tiny bit more than stock timings..

My KLX300R does unassisted power only wheelies in 1st and 2nd with 3rd gear needing a handlebar tug ( it is running stock header and "uncapped" stock silencer as well as a full FMF, powerbomb+Q4 AND a lidless CVK setup) - but for me, it needs more top end power.. So I'm researching how to do that with cam phasing..

To your point: I have never found any history, in here, of anyone running MCM+300cc. All I have found is speculation/opinions about such a "combo"..

 

I remember hearing that valve to piston clearance was not an issue. I believe we do have an "interference" motor but apparently the small amount of cam phasing is not a problem.

I was thinking more along the lines of a volumetric efficiency situation. Whereas the increased displacement through the existing port volume and valve size somehow negated the benefits of the retimed valve openings. Perhaps this is more drastic as displacement increases. Charge velocity and harmonics may be difficult to measure on our motors but must still be a factor.

Or perhaps it's just more difficult to tell the difference between 28 to 29 HP than it is to feel the difference between 21 to 22 HP.

Without getting too nerdy about it, I was hoping there was some prior experience whether pro or con. Maybe when i get ready to throw a 300 jug in it, I'll try it stock first, then MCM.

Either way, I think I'm going to press the KACR off the cam completely.

 

I agree on all counts.

As you know, the effect of a bigger piston (as opposed to a bigger piston+longer stroke) is to increase velocities and modify quench.
If our ports, valve sizes, and cam lobes, fall "short" in the care and feeding of a BB , wouldn't it have to be in the upper RPM range where gas passage demands are greatest ?

MCM increases VE below 6.5k - As you seem to know, this means an increase in the effective compression ratio below 6.5K.

In thought experiments, I seem to keep getting 250+MCM VE enhancements simply magnified with the increased velocities of the 300 piston as a "most likely outcome"..

 

Heck, if you have the time why not try it, you can always put it back if the performance should drop.

 

Well, if you do it when you install the "kit", you'll not know its' effects unless you go back in and undo it...

IMO: Based on the power of my '07 KLX300R (+ lidless CVK setup+ stock cam timing)and my MCM'd KLX250S, I am absolutely sure that, regardless of what you run for an exhaust system, MCM+300 will create a "monster". Stock header will create even more power below 7k rpm (I'm gonna call that combo "scary power") , FMF header+slipon will create more power above 6.5k rpm (I'm calling that "throttle control mandatory, 4k-redline, or you'll be in the hospital".)

 

That's pretty much what i thought. Most of my riding (around town) is below 7k. I would prefer to go into the motor 1 time for that job. I'm leaning towards including the MCM into the plan from the beginning. I'm sure the gains will be in the RPM range that best benefits my riding style.

BTW... I am running 40-140, lidless, stock header, DG-R slip-on

 

Oh, so you are! I assume you mean a #40 pilot, DJ140 and the DN0228 needle from the 2152 kit..? If so, I guess you know we've recently learned that, lidless without MCM, you should be running the DN0228@2N - as usual, but the stock slide spring instead of your DJ spring..?

My DG-R was stolen out of my truck - I really miss that "Bad Boy".. Your current power levels should be "same-as" Houstons4.. His Delkevic slipon also runs a smaller dia. midpipe..

You know, if you "lock down" the engine at TDC on the power stroke (all valves closed and cam gear timing marks lined up ) so that it won't move off of TDC while you work, you really can't "screw up" the MCM.. Just do one cam at a time and make'em look like KLX678's pictures.. Yank the spring off the KACR and you're done. If you do press it off, be sure to make a permanent scribe mark on the exhaust cam so that the KACR could be reinstalled.. If you or someone else ever takes it to 351, we may learn that you'll need to reinstall the KACR..

 

I agree with the KLXter on doing the MCM. It's like doing valve shimming. At first it looks intimidating with the cams and chain and all, but for me it was like one of those "big piece" child's first puzzle. Big pieces with one way to set it up. When doing the MCM if it isn't right, with the pictures that were provided by riders that have done it, you can easily see it.

Just make sure everything sets up like the pictures - crank at TDC compression, cams bolted in the holes as mentioned in the instructions, and marks lined up two teeth high - like the pictures. If it isn't right you will see it!