426 pistion replacement question?

[Ztard]

Ok the time has come for a new piston and rings for the mighty 426. Wiseco offers 2 options one is standard 12:5:1 compression ratio, and the other is High compression 13:5:1 ratio.

Is there any reason to not get the higher compression option? Will I need to rejet?, run race gas only?

Pros?

Cons?

[classic92]

you will have to run race gas with the High compression 13:5:1 no matter what. or you well blow your motor i would just go with the standard 12:5:1 compression ratio.

[grayracer513]

you will have to run race gas with the High compression 13:5:1 no matter what. or you well blow your motor i would just go with the standard 12:5:1 compression ratio.

False.

Most people find pump premium adequate for 13.5:1 in the YZF. Exceptions would be at low elevations in hot weather, where you may find that 91 octane's not quite enough to suppress detonation (pinging). If you have pump fuel rated higher than 92, you can use that, or use race gas during the summer.

You may need to rejet in any case, as one of the things higher compression does is to increase the strength of the intake vacuum signal at low speed. That can actually cause the pilot to be too rich. Main jetting should be reviewed also.

The bulk of the power boost you will notice from higher compression is going to come at lower and middle rpm.

One thing you should be aware of is that many of the forged aftermarket pistons available are considerably noisier than the OEM unit. Wiseco is one of the noisier ones. Also, I don't advise using a piston with a single compression ring in a bike not primarily used as a racer only. Single ring setups don't last as long as two rings do.

If it was me, and my bike was mostly for recreation, and I was more concerned with longevity than absolute power, I'd stick with stock stuff.

[Ztard]

As I was hoping Gray...Thanks. I just priced the OEM route, I'm amazed at how close the pricing is. I figured it would be much more? I'll order up from the TT store tonight.

I hope to report out in a few weeks that the new piston and rings helped...if not cure my excessive crankcase pressure issue as well.

[pscook]

One thing you should be aware of is that many of the forged aftermarket pistons available are considerably noisier than the OEM unit. Wiseco is one of the noisier ones.

You got that right. But why?

[grayracer513]

Piston slap or rattle occurs when the piston rocks or shifts in the bore excessively as it goes over top. I don't know the specifics in the case of the Wiseco, but there are 4 main things that contribute to piston noise:

1. Fit: Too loose and it will rattle. Sometimes, especially in the past, the specific alloy used in some forged pistons had to be a higher expansion alloy in order to be worked by the manufacturing methods employed, and that would mean an even looser fit when cold, and more noise with it.
   
2. Shape: Piston skirt shapes are complex. In simplified terms, the skirt is oval shaped viewed from the top, narrower at the pin bores, and barrel shaped when viewed from the sides. This is done to reduce drag in the bore, but the "contact points" must be precisely located and properly shaped to reduce the rocking/shifting effects noted above.
   
3. Rings: The rings help stabilize the piston and serve to damp the shift at TDC. Break a ring on a slipper style piston like a YZF, and you'll hear about it. Twin ring pistons are generally quieter than single compression ring models.
   
4. Pin offset: Offsetting the wrist pin is done to change the rod angle on one stroke or other for a variety of reasons. Isn't done very often. If it's excessive, it will contribute to rattling.
   

When you have a piston making noise, one or more of these attributes is usually the cause.