Author Topic: CR250M inlet port height different, why?  (Read 2777 times)

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Offline John Orchard

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CR250M inlet port height different, why?
« on: September 08, 2023, 05:32:25 PM »
So here is another useless rambling post from me :-)

I've known for many years of the odd inlet port of the Honda CR250M, but I only just now wonder why? I guess I look at employing such trickery on another bike. With the top of the two inlet port windows at different heights, below is a link to the cylinder liner. I guess at a couple of the following reasons ....

* Added crankcase turbulence?

* Flow from each port directed in a different direction?

* With one port starting to close before the other, could it give a mild supercharging effect to the later closing window?

By all reports the CR250M was the fastest 250 in its time, the inlet port being the only thing that seems to stand out from the 250's, makes me wonder, hmmm.

https://www.ebay.com/itm/184383481671?hash=item2aee1ca747%3Ag%3AEcMAAOSwfs9fJG2q&fits=Year%3A1973%7CModel%3ACR250%7CMake%3AHonda

Feel free to offer your own crazy ideas :-)
« Last Edit: September 08, 2023, 06:37:12 PM by John Orchard »
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Offline facthunt

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Re: CR250M inlet port height different, why?
« Reply #1 on: September 09, 2023, 09:10:29 AM »
Lower ring gap

Offline John Orchard

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Re: CR250M inlet port height different, why?
« Reply #2 on: September 09, 2023, 11:23:32 PM »
Lower ring gap


Oh that sounds boring :-(
Johnny O - Tahition_Red factory rider.

Offline PeterC

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Re: CR250M inlet port height different, why?
« Reply #3 on: September 10, 2023, 03:55:15 PM »
I perhaps it was a way of balancing the top end with the bottom end powet. Many years ago sidecar guys ported multi cylinder 2 strokes, to produce more power on the drive side of the crank and lesser porting on middle and lesser again on the ignition side of the crank. supposedly to broarden power and teduce twisting of H1 cranks. was the go at one time, perhaps prior to cr250m!
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Offline pokey

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Re: CR250M inlet port height different, why?
« Reply #4 on: September 10, 2023, 09:59:43 PM »
There was an old mag article that said it was for angles , this was also the time when crank stuffing was promoted. Ask yourself, if it was angles then why didnt all use it the next year and Honda would have kept doing it and none of that happened, perhaps Honda did it just to confuse everyone else.
 Often the simplest answer proves to be the best and such is the case with this "adjustment" for the ring gap and still allow relevant charge amount to be induced within the relevant allocated time.

Plenty of very experienced people have tried even more different methods of improving a 2T engine by modifying the bottom end and they all came to the same conclusion. SFA benefit from anything and nothing that a good pipe couldn't overcome by scavenging. Not corking the flywheels or installing a turbo crank or changing volume as the pipe does the best job of scavenging unless you are using forced induction like snow mobiles and thats a whole new story.

Angles , volume and velocity are all nullified once the transfers are in charge. The inlet is just a method to get the charge into the engine and the bottom end is convenient for it and could even be remote like the first patent on the 2T engine. The bottom end doesn't care about where the charge came from as its a glorified metal tank with an inlet and outlet valve(piston/reed/disk).  Its the transfers the head the pipe and the ignition that give performance, with the CRs its mostly the pipe that did a decent job of scavenging with a decent head design and a well matched gearbox..

Offline sleepy

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Re: CR250M inlet port height different, why?
« Reply #5 on: September 13, 2023, 05:29:25 PM »
I go with the piston ring support with piston at the bottom. Question is how much difference would there be if the high cut side was lowered to match the other side. My answer would be SFA as that part of the port is only uncovered by the piston for a very short time and would have zero effect on flow angle or anything like that. The bottom of the transfer port would be in the same category as the piston edge is masking the port for most of it duration.