MC18 to MC21 porting compared

**Thu Jan 28, 2010 9:48 am » Post: #1 » Download Post**

Had a chance to compare the barrells of an MC18R2 and an MC21 (not sure if its an SP or not) for anyone interested. With a quick look they seem very similar.

Same overall height ie 86.3mm

Exhaust port seems the same size and in the same position within the bore ie approx 28.2mm from the E/P roof to the top edge.

Its the transfers that seem higher in the MC21. Seems to be about 1.5mm by my rough estimate which sounds a lot and may be my crap measuring.

Just using my fingers and not interested in the outright measurements just the differences so its rough.

**Thu Jan 28, 2010 12:12 pm » Post: #2 » Download Post**

Standard MC18R5K map overlaid on a standard MC21 map.

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**Thu Jan 28, 2010 12:34 pm » Post: #3 » Download Post**

MC18 barrels are better than MC21 (port wise).
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**Thu Jan 28, 2010 5:46 pm » Post: #4 » Download Post**

Will my bike go faster if i put mc 18 barrels on Very Happy

**Thu Jan 28, 2010 11:10 pm » Post: #5 » Download Post**

why not try cr250 barrels?

**Fri Jan 29, 2010 12:47 am » Post: #6 » Download Post**

Have you seen how big CR250 barrels actually are?!
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**Fri Jan 29, 2010 1:48 pm » Post: #7 » Download Post**

nope. are they big?? so a pair of cr500 barrels would need new crank cases making??

**Fri Jan 29, 2010 2:16 pm » Post: #8 » Download Post**

wouldn't a CR250 barrel mean you end up with a 500 (250x2)? a CR500 barrel would provide you a 1000 (500x2).

I woudl guess your thinking about a CR125 barrel and not a CR250 barrel.

either way, my guess is the port map woudl be much different due to the different power characteristics needed for the different applications.

**Sat Jan 30, 2010 3:01 am » Post: #9 » Download Post**

yeah the off roaders are tuned for a better spread of power and more accessible torque. Aren't CR500s supposed to make about 56 horsepower at peak, according to Honda's specs?
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**Sat Jan 30, 2010 6:46 pm » Post: #10 » Download Post**

hey lets put cr 500 barrels on and make a 1000cc 2 stoke lol sorry guys but this is getting to far fetched
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**Sat Jan 30, 2010 7:41 pm » Post: #11 » Download Post**

How can you tell the difference between an R2J and an R5K cylinder?

**Sat Jan 30, 2010 8:35 pm » Post: #12 » Download Post**

malc wrote:hey lets put cr 500 barrels on and make a 1000cc 2 stoke lol sorry guys but this is getting to far fetched

have a look on the PB forum, someone is building one.

**Sun Jan 31, 2010 9:34 am » Post: #13 » Download Post**

StephenRC45 wrote:MC18 barrels are better than MC21 (port wise).

So those tiny changes in shape make for much better flow?
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**Sun Jan 31, 2010 4:53 pm » Post: #14 » Download Post**

It's more timing rather than flow. But as stock MC18 have the most "competitive" porting.
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**Mon Feb 01, 2010 5:54 am » Post: #15 » Download Post**

The power valves look to be a completly different shape too.
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