Hi everyone,
Since I am replacing both worn camshafts on my 2012 model along with the worn cam chain and sprockets which I discovered while I was checking valve clearances before doing the MCM mod, I am also considering putting in the 351 kit since the bike has 14K miles.

I have done all of the free mods with the recommended jetting (DJ kit, 128 main, 38 pilot, DJ spring, needle, washers, and a Kouba air screw) for the lidless setup with an FMF Q4 slip on.

Is the MCM mod still beneficial with the 351cc kit? I am looking for power that is manageable in the low to mid range as most of my riding will be in the sand near sea level or eastern PA rocky, root strewn trails up to about 2000 feet.

 

I think Bill Blue has said the 351 and MCM are not compatible.
Years ago, others have did the combined approach with unclear results. I think one was member David R
Some theorize this should be a great combination, but we don't have examples of clear success. Would you like to be the pioneer?

 

Actually the response in the Marcelino thread was that the cam retiming didn't really do anything for the big bores from what everyone experienced. So no need to do it on the big bores.

 

Thank you for your responses. I will probably install the top end parts and run it for a while because I am tired of working on it and would like to get the darn thing dirty! At some point after that, I'll check the compression and make the decision about the piston and bore (or overbore).

 

Well here we go again. Here are the facts that anyone can find in the MCM thread:

MCM works perfectly fine on the 351 as was proven by two members - David R and jhoffy22.

jhoffy22 did all mods (351, MCM, etc etc) all at once. Ran his bike about a week then his engine failed.

David R ran his 351+MCM hard, and for quite a while.. Unfortunately, his assessments are contradictory and disjointed - to me, alcoholism is indicated. Also, he had modded his slipon to exhaust thru a 3/4 inch copper pipe !

Bill Blue verified very early on that piston-to-valve clearance on the 351 kit is the same as stock.

When Jhoffy22's engine failed, the blame game (for his event) was in overdrive here - with little rational thought used, everything was blamed. And Bill started telling peeps to not do the MCM on his kits - which is to be expected, and anyone in his position would do the same if the reputation of their kit were at stake. There are no actual mechanical problems with running the MCM on the 351..

MCM is a volumetric efficiency enhancement offered by the cams, valves, intake port and the rest of the intake tract. It "does what it does" regardless of the size of the piston below it - meaning a significant boost in TRQ production from off idle to 6500 rpm, and a tiny loss of TRQ above 6500 rpm.

IMO, whether to MCM a 351 or not is a simple equation. Anyone that is going to keep the stock header on their 351 might as well MCM it also - as the stock header cripples 351 power above 6500 rpm anyway. Likewise, anyone that is going to keep the lid on their airbox might as well MCM their 351 as the restricted airflow will not allow free breathing in the upper RPM band.
Those that are going to run a lidless airbox and a full performance exhaust system probably should not MCM their 351 - this is in order to give the kit the best opportunity to make biggy power in the upper 1/2 rpm range. Such a "setup" most likely produces the best overall power production possible with the 351 kit.


There is no denying that a MCM'd 351 is going to be a raging TRQ monster regardless of any other mods, or even no additional mods (except main jet of course)..

 

Just quoting what was said by Marcelino. Hadn't seen anything to the contrary.

His first post edits at the bottom. No doubt it can be done, just the question if it is worth doing on the bigger bores. Of course it's not too difficult to do to see how it does and undo if it doesn't perform to expectations.

If something changed I guess I missed it. By the way Hot Rod had a good article about intake exhaust overlap this month.

 

If Marcellino were still around, he would be forced to explain, and likely have to change, his opinion on this.. He did no testing on anything but his 250 and his opinion on effectiveness on larger displacements would need to be backed up with an explanation of why a larger draw through the ports (with a 351) would not benefit from the retiming even more than the 250 does.

IMO, he simply thought that the BB kits already make more TRQ in the area that the MCM enhances and would be superfluous..

We need a member that will dyno before and after MCM on a 351 to prove the point...

 

It really wasn't his opinion. It was more about input from riders, "seat of the pants". Seemed like consensus was the big bores didn't benefit enough to make it worth it. Since I have not done the big bore I have no opinion in it, just what was put out in the past.

You are right, it will take someone willing to spend the money on the dyno work to prove whatever happens. Until then it behooves a rider, who can't foot the bill for the dyno, to do the big bore and ride it a while to get some feel for the performance, set some benchmarks for comparison, then do the cam retiming and run against the benchmarks. I very likely will do that when I do the 300 kit, exhaust, and flat slide. Worst case scenario I feel it isn't worth it and set back to stock, but I wouldn't bet on it happening. Need to go back and read the Hot Rod article again.

It looks like Bill Blue is or has looked at the cam timing and clearances and is changing tune on it?

 

Which manufacturer makes the piston for the BB 351 kit?

 

We have no (coherent or otherwise) seat-of-the-pants/dynobutt assessments, no actual data, no nothing when it comes to 351 vs 351+MCM power differences.. All we have is the data proving effectiveness, (along with years of positive dynobutt assessments), on a stockbore+MCM..

I suggest you (klx678) prove me wrong on this, and support your (Quote: It was more about input from riders, "seat of the pants". ) statement regarding MCM effectiveness on the 351..

Ever since Jhoffy22's incident years ago, Bill does not recommend the MCM with his kits. I can't state this more clearly than that. If he is changing his "tune" on this subject, as you allude to, please inform us as to his "new tune" regarding the MCM..

Oldboy, call Bill to get the piston manufacturer - I forgot who makes it for him.. Also, have Bill verify, again, that his 351 kit carries the same piston-to-valve clearances as a stockbore.