Hey fellas-

I was reading in an old ATV magazine about cheap hopups for a 400ex. One of the reccomendations was to use a higher compression piston. Does this mean the engine needs to be bored out? Or will a higher compression piston fit the stock moter? Also they said to get a more aggressive grind... what does that mean?

Thanks

 

"High compression" pistons typically have more of a "pop-up" dome to them so that the trapped volume at top-dead-center is lessor. That raises the compression ratio. "High compression" pistons are usually available in most standard and "big-bore" sizes. Measure the bore size, measure the piston, bore or hone if/as needed.

A more "aggressive" grind refers to a more aggressive profile of the camshaft. Camshaft lobes are surface ground into the thier respective shapes, that's where the slang "grind" comes from. The profile of the lobes dictates how long and how much the valve is open(ed)-->Lift(how much) and duration(how long).

The lift is the amount of lift the lobe opens the valve, and the duration refers to how long the valve is held open for given crank degrees of revolution. The amount of lift is often dictated by the physical constaints of the rocker(s) and valve springs.

The duration is somewhat limited by how much air the engine can breathe-->the carb, header, head shape/design. Too much duration can cause rideability problems at low rpm's. The velocity of the intake and exhaust become too low. It's a fine balance to find, but works when done right.

Most cams require a level of modification to go along with them. Some cams will work with all stock components giving modest gains. Where some radical grinds require huge carbs, header, and massive porting to the head to work well, but returning big gains...

It's how fast do you want to spend $$$???

 

Yes, you can get a higher compression piston that will fit into the cylinder without boring, although you might have to get the cylinder machined for the proper piston clearances. FYI...I used to ride with a guy who dropped in a stock bore JE 10.8:1 high compression piston, and while his machine was quicker than mine when it was stock, it wasn't by a whole lot. Basically, his machine had a little more power, but it revved a lot quicker than mine.

If you're gonna tear into the motor though, in my opinion you might as well bore it out to a 416. I just had mine bored out about 1 1/2 months ago, and while it was apart also had a XR400 cam and 39mm FCR carb installed. I like the stock XR400 cam because it helps develop more power in the mid and upper ranges, but it's not so aggressive that I lose bottom end power. For me this cam was a trade-off because I need the bottom end power for trail riding, but also need high revving power when at the dunes. The FCR carb, in my opinion again, is a must if you go with a larger bore and different cam. Anyway, after my mods I raced a few machines at the Pismo dunes this past weekend...when dragging I can pull stock Banshee's and am about dead even with stock Raptors. I might be able to do a little better once I get a different exhaust system...right now I'm just running a modified stock silencer.

 

I have a similar setup (416), and can pull my friends DS650's, QR250's, QR500's, and 430EX's.

BTW-
To answer the original questions:
- You can get a high compression piston for a 400EX that is stock bore or bored over to 406cc, 416cc, 426cc, 430cc, 440cc, 465cc and who knows how many others. Each generate a bit more power over the next, BUT the lower bores rev MUCH faster/higher. (hence why I ALWAYS beat my friend's similarly setup 430EX...although, that's also due to my hotter grind cam than his.)
- The "grind" refers to camshaft grind. In order to gain power from low thru mid and right up thru high, you need a high-comp piston (stock or overbore) *AND* a hotter "grind" camshaft. The 2 compliment each other perfectly, and result in big time hp and torque gains.