MCM is prob the only mod I haven’t done on my bike. I recall reading somewhere that it might not really be needed if you have a big bore. (Maybe in Marcelino’s original post on it?)

If my goal is torque, and plenty of it, and I don’t care about top speed at all...mid range, yes...but top speed can be crap that’s ok... Would the MCM be for me?

2014, EFI w/ejk, 331 BB, stock header, Q4, airbox lid sometimes off, sometimes with kdx snorkel depending where I’m riding. 13/47 gearing. Think that’s all the pertinent info.

 

i also remember that mcm + bigbore is not realy gonna do anything positive.

 

This is a subject that has not be resolved.

Through older posts / threads, ONE member (David R) did run with MCM + 351 extensively - proving that there is no problems with doing so.
Unfortunately, due to the strange components he had fabricated and bolted onto his exhaust, his pumper carb that was running his version of a "proper tune" on an airbox with a snorkel, etc etc, his results are only pertinent to him - no general determination of results is possible from his experience..

About all you can count on is the MCM data I charted. MCM will increase VE from off idle to 7000 rpm, with approximately same-as VE above - compared to stock timings. This positive volumetric efficiency enhancement will occur regardless of what size piston is in the bore..


All the above is assuming the fueling is correct for max power. Improper fueling curves destroy power production regardless of engine mods..
Too rich kills power as fast as too lean, but if failing at properly fueling your KLX is your only option, too rich is engine safe, too lean is not..lol



I have talked extensively about this combo in the past.
The below is strictly IMHO :
The stock intake and exhaust ports along with the stock valve sizes will all become overtaxed with a 351 slug trying to suck and blow thru them as revs rise past the midrange. Stock cam timings are optimized for a (approx) 6700 rpm trq peak - which is max VE... MCM lowers max VE to to around 5200 rpm - but maintains higher TRQ longer than stock timings. which indicates that valve size and port dimensions are optimized for midrange power !

So.... Do the MCM in order to further capitalize on power production in the RPM range that the valves and ports are flowing their best in - which is also the rpm range that is least likely to become overtaxed by the larger pistons' airflow demands..

It would seem a 351 + MCM + Full exhaust system + Lidless airbox (with lidless CVK setup) could set a new benchmark for BB KLX power..

 

Outrecording, since you are running the stock header on your 331, you are already giving up any hopes for upper midrange - top end power production - so the MCM makes even more sense in your case..

IMO, your stock cam timings are optimized for higher rpm power than your stock header can support, They are fighting each other and you are the loser..

I'm guessing that with a DJ148 - DJ150 main jet, needle at 1N, stock slide spring and lift port on a lidless airbox, (or a properly tuned EFI) , anyone that then adds the MCM to a 351, both the cam timings and the stock header will be working together - and they'll be in for one hell of a surprise ! Power from off idle that must be respected, not indiscriminately toyed with..

 

 

 

To maximize stockbore low end power, you use the stock header and the MCM. The 351 will overtax the stock header much sooner than a 250. Power loss (with a stockbore), from using the stock header, starts to show up, IIRC, in the 7000 rpm range.. How much potential power is lost from using a stock header on a 351 is debatable - it could very well be stifling a 351 starting at very low RPM's.. So your question about keeping the stock header or going with a FMF performance header cannot be answered - there is not enough data to even form an opinion..

It must be noted that (as far as I know) no one has run the MCM on the 331 BB kit. A call to verify that the kit maintains stock piston-to-valve clearances would be prudent. And extra care should be used to test the end result of the MCM before actually firing up the engine. Once fired up, any clearance issue will destroy your engine.

So while I feel it is assured that the MCM will give you a tremendous boost to off idle to 6000 rpm power, great care should be used in testing the MCM for safe engine operation..

 

 

You are absolutely correct - Long ago, Bill verified stock piston-to-valve clearances on the 351. Which further verifies the MCM is safe with the 351. You will also find out that, naturally, Bill discourages the use of MCM with his kits.. It's like, "Hey Bill, I want to monkey around with cam timings, whadda ya think ?".... What would you say if you were Bill ? LOL

What you may not know is the reason we have no data on BB's and the MCM - The "event" that occurred long ago whereby a member performed all mods all at once -including the 351 and MCM only to lose his engine to catastrophic failure. This had a profound effect on testing BB's with the MCM - ceasing all such testing.. I joined just in time to be able to read and analyze this event. It was hilarious/tragic/moronic how everyone blamed everything on his engine failure - including the MCM - when the actual fact was that he ran his bike "hard and fast" hundred(s) of miles with no issues, prior to failure ! This event was, thankfully, erased from the forum- the low-IQ mob mentality that permutated the forum surrounding this was disgusting - In all fairness, there were a few (back then) that tried to bring some realism to the discussion. Nonetheless, MCM + BB took a big "hit" with those peeps back then.. In recent years, I was able to have this member respond to a discussion I was having about the safety of MCM + BB. Verifying that he ran the bike for some hours - before the failure - verifying safe piston-to-valve clearances along with David R 's long running experiment..

I usually don't write novels - that's KLX678's purview - but you may have wanted to know some history of why the issue of BB+MCM has never been properly tested with usable performance results..