Paging Slipdog , Derek Harris , and engine builder.

Gotta watch the decimal points! You are talking in thousanths like .004" instead of .040" HUGE DIFF lol.

By taking off material from the head and lessening the cc amount, the secondary comp ratio rises. This increases the engine potential of making more power, (as well as heat), effeciency, and you are basically "exciting" the engine into producing more of what is has. Current engine designs made by the mfg are set and tested to make a certain amount of power on a spec'd octane, stated in the manual. If you increase the CR, your octane will need to go up to keep something called detonation at bay.

Deto can easily described in laymans terms as "pipe bang", or motor knock. What is happening there is the uncontrolled combustion of gases at the wrong "time" as the engine fires off and combusts fuel that is underrated for the motor. It is never good to have signs of deto, the top of the piston gets a sandblasted look, pitting occurs, and the engine can be severly damaged due to low octane with a higher spec engine requirement.

So, if you lower your cyl height by changing the base gasket thickenss, you are retarding the port timing and basically "dropping" all of the ports in relation to the stroke. This tricks the engine tune and will increase the effective stroke and yeild less of a "hit" and add torq to the power curve, with a slight loss of top end over-rev.
Keep in mind, the same rules apply, by lowering the cyl, you just took away volume from the head area so the octane must follow to keep deto away.

The squish clearance is a slightly diff animal in that a tighter squish is typically desirebale bc the bikes usually come through with excessive squish. Performance gains can easily be had with a better running engine simply by tightening the squish AND removing cc's from the bowl so your CR stays the same and the octane can stay whatever is spec'd.

Thats why you see some tuners, (yes, shameless self plug), advertise a "pump" gas head mod in addition to the race head mod. What is usually happening there is the cc's are checked before the head mod, (for ex lets use 15cc), and documented. The gasket surface(s) are cut to reduce the squish clearance,(lets say to 1mm) but no material has been taken away, (so now your cc is 14cc), this means your CR just went up, your octane needs to go up. Conversely, if you take the squish down to 1mm and then REMOVE material from the bowl to get back the 15cc, your octane can stay the same.
The benefit mostly with the head mod and cc reduction is effeciency. This is due to the tighter squish, the engine will run smoother, cooler, you will see a mileage increase as well as increased power. I do this mod for a lot of guys who ride single track and need to stretch out the tank, most see in increase, some more than others, but that depends on conditions, elevation, etc.
If you straight up mod the head for more hit, keep in mind that this will slightly stress out the trans, clutch, engine parts, make more heat, and start to move the line away from reliable to less reliable.

The best answer to most of this is to tailor your needs to your budget and stick with it and enjoy your riding. If you can afford to have a full on race motor that requires race gas, frequent rebuilds, etc then go for it. If you just want a little more and keep the relaibility of the motor as best as you can, it is always a better choice to lower the octane requirements and get what you can out of it in the long run.

Jbub

.010" is a noticeable change in port timing.

You can use the base gasket to move squish around only a few thousandths.

Just measure your current base gasket thickness and stay with that for future use. Let your motor builder adjust your head and port timings.

You don't need a VHM head if your having your motor modified. It's just bling

Hey Jessie

That was great info Josh. I'm riding a basically stock YZ, just an exhaust and v force. But I was thinking about getting the head done as I know they're pretty sloppy from the factory. After reading this though, I think I need to find out where to get a thinner base gasket first and set my squish to that, as I tend to like more low to mid and rarely rev a bike out. What do you think? Do you know where to find different base gaskets, I know KTM offers them for their bikes but I've never seen them for the Yamahas.

I have personally assembled JBONE modified engines. His two stroke work is absolute perfection. +1 for Josh.

Hi Jbone

Jeffro, sorry for asking a question when you were waiting for an answer to your own. Interesting info here for sure!

I would just narrow the squish with a slightly thinner gasket.
You might loose a bit of port timing, but the narrower squish and slight compression raise will add across the board.
It's simple, cheap and reversible.

Without dyno runs, you don't really know how much better things are getting. Plenty of after market pipes and silencers actually loose power!

@hellion- Lee, the YZ250 you have could use about .020" off the cyl base, this will retard the port timing a hair and get you some torq. Also, you can use some epoxy in the back rear transfers to have them short circuit across the piston crown. That will add a little bump down low. If only you knew someone who could do this??

@681- thx for the kind words!

Don't really see where there's much for me to add here, carry on!

I used Athena for different thickness base gaskets. They were so cheap that I bought a load so I could experiment.

http://www.athenaparts.com/eng/models/view/4840-section-33/yamaha-yz-25…

Stock will likely be 0.5mm on your YZ. Check your squish and then buy the gasket required to achieve your desired squish measurement.

Eric Gorr did this on cylinders he did for me.

Thank you JBone! I'll give you a call to catch up and schedule in said mods with my next top end job, that sounds exactly like what I'd want.

Every two stroke I own gets squish checked. Cometic is another good outlet for custom base gaskets for bikes. One nice thing about the KTM 65/85 is OEM offers a good selection of gaskets to set deck height and squish. I love threads like this because I always seem to learn something more about setup..........

Cometic will make base gaskets to your needs.