Theres some not so accurate info given out here, first off, you set up the heads to match porting, the higher you make the exh port the more you must machine off the head to regain LOST compression from the bigger exh port. ((Compression doesnt start till the exh port is closed)). The simple check as tomseal6 did is not accurate as to the whole picture, he merely checked cylinder volume by the head volume, well that doesnt tell you what the engine has for a real corrected compression ratio. Yamaha uses the same corrected compression ratio as I do, this is because its the most accurate way. The longer the exh port is open the LESS compression you have, 12.67:1 uncorrected is just fine with porting and using real 93-94 premium octane fuel, theres 100's of these same sled engines on this site alone running that. It matters as to fuel quality, you do need the octane to prevent detonation, if your using 87 octane it will detonate. If your in a area where they have used alot of ethanol to cut the fuel youll also have problems because youll need more of it to get the same octane level. Alcohol needs more volume, (bigger jets) then gasoline does. There were ALOT of problems last year with fuel quality and even bone stock sleds having burn downs.
If your gonna cut the heads on a srx do it the right way and set it up for the fuel quality youll use all the time.
More compression is more power but theres a ceiling, and fuel octane will dictate that. Theres also ways to run more compression and not have large octane levels, this involves changing the squish angle of the heads, so it traps less heat at the edge, this where all the detonation will focus on, the edge of the exh side of the piston and sometimes all around under the squishband to edge of piston.
By adding base gaskets your adding deck clearance and no matter how much you cut the heads you cant remove the deck clearance. This will ruin the way the heads operate, youll now then have a space around the top edge of the piston crown and the closer to the top of the cylinder the ring is, the better the seal. If you add 2 extra base gaskets youve added another .020" deck clearance. Stock a srx has right around .010-.012" deck clearance. So all together youd have .032" deck clearance, meaning at TDC, the piston will now be down inside the bore by this much. The other down side is the piston crown edge will now not be timed with the floor of the exh port.,hampering the return pulse wave from the pipe. The best set up is exactly flush at BDC, and a flat exh floor in the cylinder will yield the best power results.
Also, the 98-99 cylinders are NOT the same, they have differnt port timing, not "just a lip", the exh. port is differnt as are the transfer port timing. I actually prefer to use the 98-99 cylinders when building a mod motor for the better (lower)transfer port timing, the exh port is just changed to the desired timing.
