125 rm many MANY mods, how much increase in hp?

[Rice#67]

My YZ250 is 195psi and it runs on 91 pump gas. I wouldn't go too much over that... as there is some variance in the compression guages. There is another guy on here that had 210psi running on 94 octane but it pinged when putting an aftermarket pipe on.

Oh... and if a 250 doesn't work for you... my 125 has 60 hours on the topend and still has 165psi. Never checked it when it was brand new but it has always ran on 91 pump gas.

 

[wileyE]

There can be huge difference in gauges and how the test is done. Taller exhaust port heights can trap less compression and show lower on a gauge, but actual calculated compression could be higher. Don't rely on the gauge unless your apples to apples with the same motor and gauge and just using it to compare with previous readings. Also consider when the bike is on the pipe the scavenge can trap more than the motor size, you could be compressing 135cc in a 125cc cylinder, compression is higher, though the gauge would never show it. Sorry to be a butt, but would hate to see someone wack away at the head trying to get up to some arbitrary number. CC the assembly and calculate the compression ratio, then modify to what your intended use.

 

[Pete Payne]

shoot for the 150psi to 160psi range . Set the squish clearance to 1mm . Squish band angle 2 degree's greater than the piston dome angle (in the squish area ) and the squish band width should be about 6mm wide on one side

 

[125rmcrazy]

thanks pete! i can set up all those measurements easily.

 

[125rmcrazy]

thats an interesting concept though wiley. heres another couple questions? 1 .. would the psi reading be higher when the eng is at operating temp rather then cold? and 2 , i was also told that on a new engine before the ring seats in properly i could get a lower reading then when the ring seats in the bore fully? opinions?

 

[wileyE]

I usually find they read lower on a warm engine by about 5 psi, maybe even 10. Some of this is the sealing from the oil, cold oil film is thick and seals better. Another factor is the ring design, 1 ring vs. 2 rings, dykes ring vs, semi-trapezoid. One seals more off the downward motion of the piston, the other requires combustion to seal. So an identical build motor with equal compression RATIO may show different on the gauge at kick rpms. You wouldn't want to cut a head the based on that difference.

Differences between new , broke in , or worn out could be noted. Depends on how well the new fitment was made and parts used. On a two ring piston new vs break in you might not see much if any change. Big thing is to be consistent on how you make the test, hot, cold, race oil vs. thin injector oil, how many times you kick, whether the gauge trap valve is close to the gauge or close to the plug hole etc etc. That's why I caution comparing actual psi with other riders for any build specs.

 

[125rmcrazy]

i see. well the guage i have the trap valve is closest to the guage. the head i've got has been resurfaced a few times for trueness and i cut the combustion chamber in (same angles and size) to achieve the same squish clearance, because it had gotten too tight after all the surface cuts. im thinking on ebay i can get heads for this bike a dime a dozen, so maybe buy a head that hasnt been touched.. and then set that one up from scratch? thoughts?

 

[wileyE]

The volume of air in the hose will make the reading lower than actual on small engines. No biggie though if your just comparing and ballparking for reference. I have 3 gauges and they all read a little different.

All things being equal the compression should have went up with the overbore and (slightly) lowered exhaust port. How does it run?

 

[whenfoxforks-ruled]

Crazy, buy a new head and send it to Eric.

 

[125rmcrazy]

oh it runs like a top. lots of power well as much as i can expect from a 125.. when it comes on pipe it picks the front wheel up in 1-2-3-4.

whenfoxforks... i may just leave it alone and see how it goes when the spring comes. if i feel i can get more out of it, i'll get another head cheap and set it up myself, i had no reference to use for this one as it was already worked on before by someone else. plus it was damaged. thanks guys.

 

[125rmcrazy]

one last thing.. i cut the squish band diameter to equal the piston size, maybe 1 or 2 thounsandths larger. i didnt like the fact that the piston was larger then the head and i figured i could get detonation in that small area around the edge of the piston. that would probably lower the overall compression a bit too

 

[2strokerfun]

its been resleeved with a cast liner, why do you think that would make a difference to power compared to a chromed cyl?

Cast v. nikasil-coated aluminum isn't a staight question of power. Initially after startup, there would be little if any difference. After running for a while, however, the coated aluminum cylinder should be more efficient in shedding heat and should maintain its power slightly better at operating temps than cast liner. It's probably a difference I would not even notice at my skill level.

 

[125rmcrazy]

yeah that makes sense. but to me a cast liner is more cost-effective because if i lose a piston i can bore the cyl myself rather then have it sent away to be chromed.

 

[whenfoxforks-ruled]

As long as they make the correct material rings for the liner material.

 

[125rmcrazy]

im going to pull the head off again and set the dimensions to what pete told me. setting the squish to 1mm includes the thickness of the gasket right? im thinking it has to because theres alot less than 1mm on just the head now.

 

[whenfoxforks-ruled]

Have you ever measured, cc'ed a head? You will be effectively decreasing the amount of cc's, if you only do the squish.

 

[125rmcrazy]

yes. i've got the tools for that. now, does anyone know what the stock volume is for the head on my bike?