Compression questions

[gentner8]

My stock head is milled down .015, and I was debating using a thinner CR head gasket with it.  The cylinder is stock with a fresh 68mm bore.  Would I be pushing my luck with this combo running VP 105 octane T2?
Can anyone recommend a specific gasket to use in this size? 
Thanks!
Jay

[Jerry Hall]

What type of riding are you doing?  What is your piston to head clearance with .015" removed from the head and the head gasket you are currently using?

What are you trying to accomplish by raising the compression?  I know that you are expecting more power, but adding compression without making other modifications will just move the torque and power peaks to a lower RPM.  You may be able to get the same power characteristic by running your current head and head gasket and changing pipes with out having to spend a lot of money for high octane fuel.

[gentner8]

Im an east coast woods rider.  There is a "stage 2" port and polish on the cylinder, but I don't know the specific details of the port job.  There is a fmf fatty and power core silencer, rad valve, 38mm carb, and the airbox lid is off.  My goal is to make the bottom and mid be real snappy and responsive, but I still want a decent top end when I want to cut loose in a straight line.  But most riding will be 1st through 3rd gear riding, in short bursts before another turn.  I was hoping the bump in compression would let me lug the motor a bit more, but I don't want to add flywheel weight.  As for the fuel cost, yeah, VP is expensive, but the gas quality sucks out of the pump, and it's not worth the aggrivation to me.  I'd rather just purchase a few five's over the course of the year.

[broken1]

On 2 of my R's I have stock 85-86 heads & ATC head gaskets which puts the cranking compression around 200-205psi. I have an oem 86 head with .013" removed & when I tried that one with the thicker TRX head gasket I was over 220psi. I like them in the 200-210 range & also run VP 110 but think I'm going to start mixing VP & non-eth 50/50. I ride woods sometimes here on the West coast & with a 12T front sprocket my R feels pretty darn snappy in the woods & it pulls good from right off the bottom & I have similar mods to yours except no porting & I have an FMF Don Emler H-13 gold series MX pipe.

[jcs003]

was anything done to the combustion chamber?  do you have any piston details?

[gentner8]

Wiseco prolite 526M06600.  I'll be putting it back together this coming weekend.  I'll mount the jug, and do some measurements, solder etc.  I have a stock head, and the milled head.  But it sounds like the milled head and atc gasket will be too much?  I half thought it would put me in the 200ish area.  I'll update when I get some time to get out there. 

[jcs003]

anyone running ethanol in their machines these days?  that would make it much easier to meet your RON/MON (octane) requirements.  I know it gives my car an additional 100 or so hp over 93.

[gentner8]

Ethanol in what ratio?  It would seem it could be a jetting nightmare if the fuel isnt always fresh?  I would think it would have a faster evaportation rate than the race gas.

[croat1]

Ethanol?   I thought it’s best to stay away from it?

Race gas, AV or Methanol is what I hear everyone is using.  I don’t remember the last time I ran pump gas for one of mine (maybe 10 years ago).

Joe

[Jerry Hall]

anyone running ethanol in their machines these days?  that would make it much easier to meet your RON/MON (octane) requirements.  I know it gives my car an additional 100 or so hp over 93.

Are you talking about running straight ethanol  or running ethanol laced gasoline?

[Jerry Hall]

Im an east coast woods rider.  There is a "stage 2" port and polish on the cylinder, but I don't know the specific details of the port job.  There is a fmf fatty and power core silencer, rad valve, 38mm carb, and the airbox lid is off.  My goal is to make the bottom and mid be real snappy and responsive, but I still want a decent top end when I want to cut loose in a straight line. But most riding will be 1st through 3rd gear riding, in short bursts before another turn. I was hoping the bump in compression would let me lug the motor a bit more, but I don't want to add flywheel weight.  As for the fuel cost, yeah, VP is expensive, but the gas quality sucks out of the pump, and it's not worth the aggrivation to me.  I'd rather just purchase a few five's over the course of the year.

 
I would try as many different pipe that you can beg, borrow or steel from you friends to find one that gives you the power characteristic that suits your needs.  I have not had favorable results with the FMF fatty pipes on modified 250 R engines.

Your engine does not know or care about what gear your are running.  The shape of your power curve is controlled by the interaction of all of the components that make up your basic engine design.  ( pipe, ports, carb size, reeds, intake system and ignition timing curve, etc). 

A power curve that is narrow and has a high peak power (Salt flat racing engine) is only good for very high speeds if coupled with the right gearing and or tire circumference. 

A power curve that produces maximum acceleration (drag racing engine) will have the highest average power in the RPM range that is defined by the RPM drop after each shift.  Your transmission ratios define the RPM drop between gears. 

A power curve that works best for most outdoor racers is one that usually has the torque peak and the power peak about 500 to 1000 RPM apart on a 250cc single cylinder.  The RPM spread between the torque and power peaks is usually what determines whether the engine is used by a Pro level rider or a Novice.  Pro level riders make less mistakes in the corners so the width of the usable power range for some Pros may be similar to that of a drag engine.

From a engine designers perspective it is easier to build an engine for drag racing than it is for Motocross or Stadium type racing

[gentner8]

While I would love to borrow others pipes, I don't know anyone who has another one.  They are a little more scarce on the east coast, unless you are up in the coal hills.  I had ordered an LED setup, but he told me it would be 12 weeks, so Ill order one at the end of the season.  Unless an ESR comes down the line, but the website doesn't change much on stock.