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CR250 -01 head/compression/detonation

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Hallo

Just shaved of 2.5mm of my head bike that is a cr250 2001. And this was the resault (picture). Was this a bit to much?? I always use Shells V-power with 2.5% oil. Can not test due to cylinder is going to be replated!. The piston is a Wiseco. And it´s standard dim 66,4mm.

Questions

  • To much shaved of?
  • Need better fuel?
  • Risc for detonation damage?
  • Squish with to much 11.5mm? Should it be smaller like say 9mm???? (68,5-45,5/2=11,5)
  • More questions I forgot?

Here is a pic from my cell.

IMAG0301-edit.jpg

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Honda cr250 -01

BSA B33 -53

Honda Hornet -09

KTM 400 supercomp -97 (sold)

Honda CB500T -75 (sold)

Edited by nmd

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There is a dedtonation risk ,dont know what has been done ie cylinder raised,lowered , porting,machining the combustion chamber deeper to compensate etc but if all is standard the compression may be way too high

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Your squish area is 56% of the bore...that's ok...

Squish clearance of 0,8mm is quite small, but ok if it's real! You must check it with all parts bolted together (you don't know your piston position in TDC and your head seal height). You can use a wire of tin-solder put it through the spark plug bore in the edge of the combustion chamber and slowly turn your engine. Messure the thinnest part of this wire and your have your min. squish clearance. With this you have also checked the next part...is your piston flat or domed... could affect your squish clearance.

The compression factor of your engine can't be calculated from your information so check it!

Should be under 8,5:1 or perhaps 9,0:1 but not more (real compression).

Feel free to ask if something isn't clear

Edited by nine-thirtysix

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The squish clerance was checked befor i disassembled and it was 1,4mm and when i shaved of 2.5mm of the cylinder i made the squich so that it should be 0.8mm.

The piston is flat. The cylinder is 100% stock.

What more info must i have to calculate the compression. I can not measure it right away because it´s haven´t been returned to me yet!

-------------------------------------------------------------------------------------------

Honda cr250 -01

BSA B33 -53

Honda Hornet -09

KTM 400 supercomp -97 (sold)

Honda CB500T -75 (sold)

Edited by nmd

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For caculating only you need the squish angle, dome radius,spark plug volume and the exhaust port height.

You could also gauge the capacity of the head including the spark plug like this. fllung16ai.jpg

Then you still need the rest capacity of the head seal and perhaps the cylinder and still the exhaust port height.

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For caculating only you need the squish angle, dome radius,spark plug volume and the exhaust port height.

You could also gauge the capacity of the head including the spark plug like this. fllung16ai.jpg

Then you still need the rest capacity of the head seal and perhaps the cylinder and still the exhaust port height.

man, you are intimidating this guy with all the info but obviously, .8mm is too small of a squish. normal squish is atleast 1mm to 1.4mm.

OP should have went with pipe change or porting to achieve certain power.

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IMO, you will need better fuel, assuming shell is supplying you with 91 octane (95 RON) since completely stock, the cr250 requires 92 octane (R+M)

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Shell V-Power has here 100 octane...so 0.8mm is ok! You don't want to loose much more unburned fuel!

100 octane in your countries is not 100 octane in the US. yours is a RON rating, ours is a combinatin of RON and MON and then averaged. at 100 RON, its still better than ours, but dont let the numbers throw you.

Edited by mynewcr250

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Don't know much about were differences come from but personaly I would run this clearance easily.

You will hear your piston knocking on the head while braking if it's not enough :bonk: just kidding

Now more important is that you know your compression ratio!

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Can i ask why is your head 68.5mm and your cyl 66.4mm? Im guessing that with a height of only 8.3mm and by looking at the dome shape your comp ratio is going to be very high. Stock head is almost double that height. When you get the cyl back measure the trapped volume and work out the uncorrected comp ratio before you run the engine just to be on the safe side...

Edited by dmcca

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The squish angel i 0 degrees. ... Should i change this to perhaps 0.5 or 1 degrees?????

Dome radius.....will measure

Sparkplug volume and the complete head volume.....will measure

Exhaust port height ... .. will measure when i get my cylinder back.

I will do a compresion test beforer i start it up. What would the absolute maxiumum value be, 260 psi???

The dome radius is 68,5mmm because the oem gasket was that and the head was that before i shaved it off!

In any case it will be pretty interesting how this will turn out. Broken engine or a engine that will be a little more nasty then the stock!

The other performance upgrade on the bike is Boysen RAD valve and a HGS exhaust system!

Edited by nmd

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nice you have a rad valve and HGS. i have HGS and shorty. i do have power reeds on my stock cage. why dont you try get that cylinder to have a boyesen port job like one of my cylinders have?

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No we don't know we write about something we don't know and never have done......joking..

I think nmd would ask if he also don't know it or has a problem with it...But thanks for anticipation.

I haven't red the article but hope it really helps...

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Ok cylinder is back..

Port jobb...would love to have but.....don´t have the money for it....

The squish angel i 0 degrees. ... Should i change this to perhaps 0.5 or 1 degrees?????

Dome cc + gasket = 13.3cc

Exhaust port height ... .. is 39mm from top of cylinder to top of exhaustport

Found a website where they calculate comp ratio and it look I got 11.15 look at picture

compinfo.jpg

Is it to high????

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Your compression ratio is way to high! If you are sure that you have the correct head volume then youe should make the chamber bigger!

You could run between 1:9,0 and 1:9,5...I think stock is 1:8,7 ?

Personally I would leave your sqishband alone and would remove the volume needed in the chamber area around the sparkplug...you can also go a little deeper then the spark plug around the threads (watch toroidal head)!

But this is something where everybody will tell you something different just from everones personal likings and experiences.

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Ok

will do the chamber a bit bigger.....with that "toroidal head" design ...

I 100% sure of the head volume........

will make it a bit bigger so that i run 9.5 in comp

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Hey

Put all the pices together yesterday and went for a test rider and I most say that the torqe is much better no in low and mid.......didn´t want to rev it like hell on the first ride with new cylinder and all.

The head is 14.3cc with gasket and 1mm squich....did that 1mm squich just to be on the safe side!.... It´s not 100% torodial design but i have no knocking sounds what so ever! Execpt the standard 2-stroke sound.

Will take it to the track next weckend and do some more testing it´s just that next weckend......because i have to give the forks some service!

If you want to my jetting is main 390, pilot 27.5, and needle is stock and in stock postion. And I 80m above sea level. Enginemod is HGS exhaust, HGS muffler, Boysen RAD valve and headmod

IMAG0310-edit.jpg

Edited by nmd

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