MC 250 port timing...

[rpduc]

Ok, I'm in the process of total rebuild on my '05 MC 250. Right main bearing went...

So during disassembly I measured how my piston aligned with exhaust port. Top of piston .83mm above bottom of exhaust port. One .3mm base gasket. One might conclude they dropped the cyl. to increase compression on the MC. The parts fiche also shows a different head for the MC.

So based on the conventional wisdom expounded on this site I mentioned to the engine guy pressing my crank that I was gonna raise my cyl. To align the exhaust port and would likely need squish band correction.

Well... He said (emphatically) that aligning top of piston to bottom of exhaust port is WRONG... that piston should be aligned to bottom of transfer ports. Basically said bottom of exhaust port is irrelevant... He also mentioned that IF the piston is high on the transfer ports, correcting that will give me more top end... I got plenty of that now I just want a 'crisper' low to mid.

My questions:

- I didn't check piston related to transfer ports (and cyl. is at machinists presently) but why is the talk here about exhaust port and not transfers? Maybe bottom of trans and ex is equivalent on these motors?

- If all I'm worried about is low-mid grunt is there anything to be gained by messing with altering port timing via base gaskets? The way I understand it one would actually want to lower the cyl. to move timing towards more favorable low end...

- I will definitely get the squish corrected once this is back together so maybe that's sufficient?

- Does this sound like I'd maybe like a 300?

[Jakobi]

I just had a quick look at my 300 jug. The intakes and exhaust are both the same at BDC. Basically by changing the base gasket stacks you're changing the time and duration that the ports open/closed which will alter the engines performance and characteristics.

As it is running with the piston .8mm above the exhaust/intakes you have the engine set up to favour bottom end. Depending on the final compression ratio of the engine it may also be set to boost bottom or top. A higher CR will make better snap down low, where less CR will want to rev further.

Basically your mechanic was right, that if you raise the cylinder more you will be taking from the bottom and adding to the top. Moving the cylinder lower making the overlap greater than 0.8mm and you will start to see significant losses to top end so I wouldn't recommend that. If you haven't had the squish corrected and the compression ratio set I would look at doing that. It will improve engine efficiency and you'll get a gain across the board.

Other considerations. Does the MC model use a more aggressive CDI? 2k2 ignition? Have you tried altering the ignition timing? Flywheel weight to stop it racing off so fast? How many teeth on the rear sprocket? Are you using a povervalve cover to boost low end? Jetted on the richer side to improve torque?

[jhendr3702]

Originally Posted by Jakobi

I just had a quick look at my 300 jug. The intakes and exhaust are both the same at BDC. Basically by changing the base gasket stacks you're changing the time and duration that the ports open/closed which will alter the engines performance and characteristics.

As it is running with the piston .8mm above the exhaust/intakes you have the engine set up to favour bottom end. Depending on the final compression ratio of the engine it may also be set to boost bottom or top. A higher CR will make better snap down low, where less CR will want to rev further.

Basically you're mechanic was right, that if you raise the cylinder more you will be taking from the bottom and adding to the top. Moving the cylinder lower making the overlap greater than 0.8mm and you will start to see significant losses to top end so I wouldn't recommend that. If you haven't had the squish corrected and the compression ratio set I would look at doing that. It will improve engine efficiency and you'll get a gain across the board.

Other considerations. Does the MC model use a more aggressive CDI? 2k2 ignition? Have you tried altering the ignition timing? Flywheel weight to stop it racing off so fast? How many teeth on the rear sprocket? Are you using a povervalve cover to boost low end? Jetted on the richer side to improve torque?

i am on a mc250 also (2007).. i was going to change it to a 300 also.. someone on here brought up the more aggressive cdi. so is started working on the 250 . i actually added a fww to it, a large power valve cover and went back from a 50t to a 48t rear sprocket along with an 18" rear wheel and love it.. i never thought i would take a 250 over my 300 ( i still have both) it has plenty of low end and will rip on top.. plus the suspension is awesome...mine is a little on the rich side and leave it on rain mode (dual map)...

[Jakobi]

Originally Posted by jhendr3702

i am on a mc250 also (2007).. i was going to change it to a 300 also.. someone on here brought up the more aggressive cdi. so is started working on the 250 . i actually added a fww to it, a large power valve cover and went back from a 50t to a 48t rear sprocket along with an 18" rear wheel and love it.. i never thought i would take a 250 over my 300 ( i still have both) it has plenty of low end and will rip on top.. plus the suspension is awesome...mine is a little on the rich side and leave it on rain mode (dual map)...

You saying I nailed it in one??

On the topic, when I changed from the 300 top end to the 250 I went from having the ports timed flush to a slight overlap. This was mostly because the 250 cylinder was an S3 Race ported one which had the exhaust raised a touch so I could do this without losing anything on the top.

Like you I would choose a 250 over the 300 now. To the op, when you say you're looking for bottom end, where is the 250 letting you down?

[jhendr3702]

Originally Posted by Jakobi

You saying I nailed it in one??

On the topic, when I changed from the 300 top end to the 250 I went from having the ports timed flush to a slight overlap. This was mostly because the 250 cylinder was an S3 Race ported one which had the exhaust raised a touch so I could do this without losing anything on the top.

Like you I would choose a 250 over the 300 now. To the op, when you say you're looking for bottom end, where is the 250 letting you down?

yes sir... i am...

[Jakobi]

Originally Posted by jhendr3702

yes sir... i am...

Ha! Choice! I'm going to have two beers to celebrate!

[Elvis74]

I install alwasy my cylinder with the thinnest gasket, 0,15mm.
And I modified once the cilinder head to get the right squish.

The wrong thinking of everybody is, to align the piston with the bottom of the ports, but when the piston is in bottom dead center, there is now pressure any more and no gas will flow, means a adge between teh port and the piston makes no difference.
After installing the cylinder the first time like this, I checked out that I reduced the loss of flushing.

[Jakobi]

Changing the deck height certainly does have an effect on the power curve. Even if nothing is happening at BDC any changes effect the time in the stroke at which the ports are effective. IE having the piston sit higher in the cylinder means that the exhaust port will open later, and be open for a shorter duration. This will give more back pressure and low end power, but as such won't flow as well higher into the rpm (someone correct me if I'm wrong - just using logic and pretending that my beer is a piston and the desk is the exhaust port).

From my rebuild thread:

Originally Posted by dmcca

So i got the cylidner on Friday arvo... just had a chance to map it out this morning and compare it to the last stock '11 EC250 i worked on...

Some obvious things about the S3 cylidner...
- it is the exact same basic cylidner as a stock unit, obviously cast from the same batch as the standard cylidners, it just has S3's logo on the side.
- its had some extra work done to the ports over the stock unit and the finish on the ports is MUCH better than the last stock one i had (which was terrible)
- the aux EX ports have been widened
- Main Ex port has been widened and raised to match the timing of the aux EX ports (stock is staggered with Main Ex lower)
- Main and Aux transfers have been widened
- Boost port has not been touched

All pretty standard stuff aimed at more top end power, most likely at the expense of bottom (as to be expected)...

I ran it through the sim software and got the following graph...



BLACK is the S3 cylidner with a 1.3mm base gasket stack
RED is the S3 cylidner with a 0.5mm base gasket stack
GREEN is the stock EC250 cylidner with a 0.5mm base gasket stack (how the last stock bike that i worked on was set up from the factory)

So the S3 produces significantly more power up top than the stocker due to the larger ports, but you pay the price in reduced bottom end. When i did the last one i faced the same issue and decided to leave the 0.5mm base gasket stack in order to retain some bottom... it was pretty much a half way measure and the final results were very very similar to the RED line on this graph (that person had asked for an increase in top end without a drop in bottom if possible).

So Jake the choice is yours...

- run a 1.3mm base gasket stack and have the port floor flush with the piston at BDC (remembering its only there for a fraction of a second)... this will give the strongest top end but less bottom/mid and the head will have to be shaved 0.8mm to get squish clearance to 1.25mm (current SQ with the 1.3 gasket stack is 2.05mm)

OR

- run a 0.5mm base gasket stack and have the piston a bit above the port floor at BDC (remembering its above the port floor for the rest of the crank rotation anyway)... this will give less up top but keep more down low. The head wont need to be shaved because the cylidner is 0.8mm lower (dome volume will need to be sorted out still though, yet to do the calcs on that)

Of course you can run anything else between 1.3 and 0.5 if you want and its intuitively obvious how the power will be affected.

Aside from the drop in bottom end power the real issue is how important is is to have the piston flush with the port floor at BDC anyway?? In my opinion (and there are many others way more qualified to answer this) the port floor issue is the lesser of two evils... for normal trail riding id rather retain some bottom/mid and live with the fact that my port floors arent flush for the ~0.0001 of a second they otherwise would be at 8000rpm.


Another issue with the EC250 is that the fact that the powervalve snaps open very fast causing a drop in the mid range with the larger exhaust port... if you could keep the same opening start point (about 4500) but extend the opening period out to 7000+ rpm (currently fully open by ~6000) it would work better with the bigger ports... this is obvious in this graph how the GREEN line doesnt drop at all at 6500 but the other two do... its timed to work with the smaller stock ports.

Plenty of food for thought.

[Jakobi]

And for the 300 its even more evident that dropping the cylinder too far will kill the top end..

Here is the last graph that Dave did up for Cols 2010 EC300 Euro

The Black line is the stock engine, in this case 2.5mm squish, 11.9:1 comp, 1.3mm base gasket stack, piston level with port floors at bdc

The Red line is the cylinder dropped by 0.7mm to give a squish of 1.8mm, no further changes other than what occurs by dropping cylinder.... comp increases, exh duration drops, transfer duration drops

The Green line is the cylindner dropped by 1mm to give a squish of 1.5mm, no other changes.

The Blue line is dropping cylinder a bit but then doing some basic mods to the exhaust port to bring the top end power back... best of both worlds... better bottom without the loss on top. You can take this much further to get better top end if you want.

[lonetree]

Originally Posted by Elvis74

I install alwasy my cylinder with the thinnest gasket, 0,15mm.
And I modified once the cilinder head to get the right squish.

The wrong thinking of everybody is, to align the piston with the bottom of the ports, but when the piston is in bottom dead center, there is now pressure any more and no gas will flow, means a adge between teh port and the piston makes no difference.
After installing the cylinder the first time like this, I checked out that I reduced the loss of flushing.

if i did that to my 250, which has 1.3mm of base gasket with the piston .63mm above the exhaust post ( and presumably the intake ports too), i would end up with a huge amount of piston above the port at bdc, and as jakobi's graphs show it kills the engine....
im not sure you should go with piston and ports flush on a 250, but for a 300 it seems to work...
Really all that doesnt matter as much as making sure the squish is right at the end of it all......