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Discussion Starter #1
hi this is my first message thankyou for having me.i race a trx250r at grasstrack events i was using totaly standard barrel that ran going okay i have now fitted a barrel thats been ported and head milled it went okay but lacked abit of power out of corners.my psi is 150/155 with 1.5mm squish with spacer plate set up,i wanted to get my compression up im sure the exhaust port has been taken up a bit much and this is lowering my psi and also my head has had material taken out of it,so i bought a 1987cr 250 head to try out as blank to work with also i read on here that the cr head has better cooling.my original head is near 21cc volume the cr head is 16,5cc,so with cr head my compression was 240psi with 1mm squish and 190 psi with 2mm squish.i have done a uccr following carlos method and it works out to13;1using cr head and 12.26:1 with my milled head, is this the same compression that manufacturer quotes as being 7.7:1 for 1989 trx 250r doesnt seem right to me so would appreciate some help with understanding the difference. is a 2mm squish ok to run with or should i get cr head milled so i can run with a 1.5mm squish and abit less compression thanks paul.
 

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The CR head does not have better cooling, bigger water jackets don't mean better cooling, they can actually slow down flow and increase temperatures. The CR head was designed for the water jackets in the CR cylinder. Honda designed the ATC/trx250R head to match the ATC/TRX85-89 cylinders....a lot of miss-information on the internet.

If your exhaust port is cut higher than it should be, not much will help put Torque in the Low to Mid range, that is what you need getting into and out of corners, not high exhaust port timing.

13/1 or 12.26/1 (not the same as CR 7.7:1) is good for 110 Race Fuel (actually 110 race Fuel is 107 Octane not 110)...should end up around 190-200 PSI.

At 1.5MM tto 2.5MM squish clearance you are at what Honda set on the stock cylinders from 85-89...2MM squish is high, however; it is "OK" to run, rather a 'Safe' area to be in.

As an FYI; on VP110....250R's seem to be the happiest at 1.5MM squish, 18-19CC domes. It is hard for me to tell what you have, how much your Head was shaved or cylinder 'decked', however; follow UCCR closely and you will have a good, strong running 250R.

Carlos
 

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Discussion Starter #3
thanks for your reply carlos.so is uccr generally just used as a guidance to what octane to use!so do you recommend i ditch the cr head, the plan was to get some power back and the original head with it has had material taken out of the dome so isnt helping my compression,just want to get some use out of this ported barrel i do have another standard head dont know what the volume of that is! yes the exhaust port has been taken up and wider quite a lot and was okay at high rpm. i did lower the barrel .5mm to help as i also read that would help on here! i run it on av gas which i think is about 106 octane i will check with the airfield i get it from, is their any disadvantage to haveing a larger squish 2mm compared to 1.5mm for example other then lowering compression! alot of questions i know but i hate not understanding and relying on miss information and wasting time and money for no gainso its good to hear from someone with reccomendation.many thanks paul
 

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UCCR is a great tool to determine Octane, Dome CC's & Design, Performance, ect...

Nothing wrong with the CR250R head if setup correctly, however; the TRX250R Head is a better choice IMO as explained above. You can also get the TRX250R head Dome re-shaped/cut to improve Performance along with adding o-ring to eliminate the gasket all together....works great; increases compression, minimizes leaks, better performance. Initially a bit more expensive than just an OEM gasket, mostly due to all the machining, o-ring glands, o-ring sets...afterwards the cost of o-rings is a fraction of the cost of an OEM head gasket...just an option to consider.

The larger Squish is safer,allows lower Octane, less compression, more reliable, lower or tighter squish increases compression, Torque goes up from Low to Mid range...top end remains basically the same.

Carlos
 

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Discussion Starter #5
thanks for clarifying a few points for me think ill stick my trx head its around 22cc at 1.5mm squish uccr of12.71 its just my gauge psi is only 150 due to the big exhaust port i think,dont want to run race gas so not alot i can do i dont think!one thing i have noticed is my trx head the squish doesnt start tapering till its about 1mm inside of bore which is 67.5mm do i need to get the squish recut to the bore circumferance thanks paul
 

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Discussion Starter #6
hi sorry carlos should posted this reply to my stock head cc post.thanks for your help wish their was a company that specilizes in trxs in the uk parts are hard to come by here and importing from the usa is expensive due to import tax and then handling charges, second hand parts are virtually none exist thanks paul.
 

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hi sorry carlos should posted this reply to my stock head cc post.thanks for your help wish their was a company that specilizes in trxs in the uk parts are hard to come by here and importing from the usa is expensive due to import tax and then handling charges, second hand parts are virtually none exist thanks paul.
Most welcome; just ask the questions needed Paul via .NET or call/text BDT Motorsports...if we can, we will help :)

Carlos
 

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Discussion Starter #9
UCCR is a great tool to determine Octane, Dome CC's & Design, Performance, ect...

Nothing wrong with the CR250R head if setup correctly, however; the TRX250R Head is a better choice IMO as explained above. You can also get the TRX250R head Dome re-shaped/cut to improve Performance along with adding o-ring to eliminate the gasket all together....works great; increases compression, minimizes leaks, better performance. Initially a bit more expensive than just an OEM gasket, mostly due to all the machining, o-ring glands, o-ring sets...afterwards the cost of o-rings is a fraction of the cost of an OEM head gasket...just an option to consider.

The larger Squish is safer,allows lower Octane, less compression, more reliable, lower or tighter squish increases compression, Torque goes up from Low to Mid range...top end remains basically the same.

Carlos
hi carlos should my squish start angling up from the edge of the bore as mine starts about 1mm inside the bore so itsso the head is flat into the bore for 1mm does this matter thanks paul
 
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