Higher Compression Stock Cylinder Head

[ERBE]

At this time Im not ready to port the cylinder yet, what is a good CC amount for race gas use on a stock port cylinder?? Or am I better off looking for a Cool Dome System than using a stock head?
Yeah lots of bottom end comming !

Adding Paul Turner Midrange Pipe
V Force 2 reeds
36mm A/S carb
Looking into a lighter flywheel if needed
Lighter componets all around

[2muchquad]

Use a thinner head gasket,removing material from the head will DECREASE your compression,not good if your looking for more bottom.I think the 18cc domes on the cool heads require race gas.

[ERBE]

Hummm having the head cut (removing gasket sealing area) and lowering the dome will INCREASE compression, smaller dome = increase compression. (also needing the angle of the squish correct also)
Head and base gasket wise I order custom thickness depending on the squish clearence that is required.


Originally posted by 2muchquad
Use a thinner head gasket,removing material from the head will DECREASE your compression,not good if your looking for more bottom.I think the 18cc domes on the cool heads require race gas.

[Dave83]

Im thinking that .035 squish is what a good local engine builder told me the stock r head/cylinder ran well at.You can get the compression up a bunch with a thinner gasket.I always ran the 2 thin pieces of a stock gasket with good results.

[2muchquad]

Hummm having the head cut (removing gasket sealing area) and lowering the dome will INCREASE compression, smaller dome = increase compression. (also needing the


Yeah,i was thinking he was talking about the dome area,surely shaving the head will increase compression.A good rule ,20/1000s of a inch for play,race,fine with pump,more than 30 and you'll need race gas.

[All250R]

Originally posted by ERBE
At this time Im not ready to port the cylinder yet, what is a good CC amount for race gas use on a stock port cylinder?? Or am I better off looking for a Cool Dome System than using a stock head?
Yeah lots of bottom end comming !

Adding Paul Turner Midrange Pipe
V Force 2 reeds
36mm A/S carb
Looking into a lighter flywheel if needed
Lighter componets all around

Teh stock head is close to worthless as far as it's ability to help the motor extract peak performance. The squish angle is so much steeper than the dome angle that large amounts of cool, never burned fuel get dumped right into the exhuast pipe never used for making power. It was intended to be a very "safe" head design, complimenting the mellow pulses of the stock pipe for riders who might not be able to handle sudden power on an ATV/ATC. You can lug that setup in the same low RPM all day long and it will never detonate or overheat.

To redesign it to a more performance oriented head it needs to be welded and re-cut. If you're planning to run any kind of compression, you should think about a cool head (or something like it) and take advantage of the increased surface area on the dome.

[ERBE]

Very interesting, I was wondering about the limitation of the stockTRX250R head.
Ok then if I went with a cool head what dome CC? for 180lbs
will the dome and squish still need to be modified on the lath?
I ask because the LT500 Cool Head domes are not done correct on the squish angle and need to re angled to prevent detonation, Im not racing so I was thinking the squish clearance would be between .050 and .060 doing the solder test.

[All250R]

Originally posted by ERBE
Very interesting, I was wondering about the limitation of the stockTRX250R head.
Ok then if I went with a cool head what dome CC? for 180lbs
will the dome and squish still need to be modified on the lath?
I ask because the LT500 Cool Head domes are not done correct on the squish angle and need to re angled to prevent detonation, Im not racing so I was thinking the squish clearance would be between .050 and .060 doing the solder test.
Yea, it's all kind of interesting stuff for sure.

You are supposed to know your target compression ratio which is a function of volumes, so basically bore size deck height and dome size. Then you have a more clear idea what cc's are needed to fill the gap. You're not really supposed to consider psi as the sole determination for octane requirement. 2strokes are not valved engines and the compression changes with rpm due to pipe and porting effects.

Ideally, you know or have a a builder that knows how to fine tune the squish too on top of targeting a compression ratio. Most people can just buy a larger than needed dome and be fine. So it's a matter of how much time you want to put into getting a couple extra ponies from the motor. But to sum, you should take some measurements a builder asks for if you want to get the most from your money.

p.s. if your 250-275 cc motor is running under probably 9k, a .040" squish clearance is I think generally considered pretty ideal. 500's need a little more room.

[2muchquad]

One more thing to take note of,actual compression on a 2 stroke doesnt start until the piston clears the top of the exhaust port,just another thing to throw a wrench into the mix.Theres all kinds of hidden hp if your willing to blueprint the motor.How much cash do you have and how much effort are you willin to put in chasing max performance..:(