cr250 Squish clearance


3 replies to this topic
  • 4x4r

Posted November 07, 2008 - 07:06 AM

#1

all this talk about squish clearance being set. Stock always having too much clearance. How about throwing in a domed piston to correct the clearance. also how much a dome would be needed to produce the correct clearance and at what compression would that put us? 8:1 is almost extremely low compression. Compression that low is normaly set for Forced induction (turbo/super charged cars) Don't 125c bikes run 14 - 15:1 compression at the nationals? why not do that to a 250cc bike? This is something i'll be working on next -

  • dmcca

Posted November 07, 2008 - 07:58 PM

#2

The 05+ Cr's already have domed pistons. Besides you need to match the piston crown to the head to get the right squish angle and squish clearance. The easiest way to get it spot on is to either mill the top of the cylinder or the head. Either way usually requires reshaping the dome to get compression back to where it should be after removing the excess material.

With compression, there are 2 ways of describing it on a 2 stroke, one is Corrected Compression Ratio (CCR) which is the compression ratio taken from when the piston just covers over the top exhaust port, and Uncorrected Compression Ratio (UCCR) which is taken from when the piston is at BDC. Obviously CCR will always be a smaller number cause its only taking into account a smaller movement of the piston (from the top of the Exh port to TDC) whereas UCCR takes into account the full sweep of the piston from BDC to TDC. Most modern 2 strokes have a CCR of about 8:1, which just happens to usually equate to an UCCR of about 15:1.

This is basically done because a piston on a 2 stroke wont actually start to compress the mixture until the exhaust port is closed, up until this time the incoming mixture can escape through the exhaust port so if we were to only look at UCCR it could be misleading. The whole trick with a 2 stroke is to get the pipe to create the right pressure waves so that this lost mixture is forced back into the cylinder just before the exh port closes. In reality these pressure waves are a form of forced induction... as you mentioned.

There's a really good book by A. Graham Bell called Two Stroke Performance Tuning... Its easy to read and will teach you a huge amount! I've had it a year now and keep going back to it every couple of weeks to re-read another chapter. Its safe to say its my favorite book! What a geek huh? Anyway you sound like you're interesteed in this sort of stuff so do yourself a favor and look into it. Its available on Amazon.

  • 4x4r

Posted November 07, 2008 - 08:48 PM

#3

I am most definitely interested in this stuff ! Have been ever since I was a little kid and use to watch my friends father build national caliber quads. Stock is simply just not in my vocabularly LOL.. Although now i have finally learned useable power is the trick to making a good engine. Any one can make a lot of peak power

  • dmcca

Posted November 07, 2008 - 09:54 PM

#4

I am most definitely interested in this stuff ! Have been ever since I was a little kid and use to watch my friends father build national caliber quads. Stock is simply just not in my vocabularly LOL.. Although now i have finally learned useable power is the trick to making a good engine. Any one can make a lot of peak power


yeah thats spot on about useable power. I made a conscious decision about a year ago to learn as much as i could about 2 strokes cause i was always 'playing' with things but not really understanding what i was doing. Ive still got a huge way to go but i'm loving the process of learning. My next project is going to be a home porting job, aimed at increasing low end power. Ive just bought and old lathe and am looking for any excuse to split my cases any day now... just noticed my gear level is a bit loose, maybe its time to tear it down!

By the way i made a suggestion under the Suggestion Box section of TT a few weeks ago to create a new section relating to Porting and Performance... if you're interested in this stuff go over there and show some support by adding a comment to the post. there are some other good 2 stroke websites around but it would be great to have it all together here on TT





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