High Compression Ratio Pistons

[mangel83]

We're using a 2003 Suzuki GSX-R600 engine, it comes standard with a 12.2:1 CR, we're thinking to take CR up to 13.5:1 or so with aftermarket pistons. But we're unsure if the engine will knock using 100 octane gas, we've been told that without the restriction the engine would definitively knock, butn since we're running restricted nobody knows for sure, any teams have experience with this? Any help would be greatly appreciated. Thanks

[mangel83]

We're using a 2003 Suzuki GSX-R600 engine, it comes standard with a 12.2:1 CR, we're thinking to take CR up to 13.5:1 or so with aftermarket pistons. But we're unsure if the engine will knock using 100 octane gas, we've been told that without the restriction the engine would definitively knock, butn since we're running restricted nobody knows for sure, any teams have experience with this? Any help would be greatly appreciated. Thanks

[Perry Harrington]

JE sells pistons that should fill the bill. Your real CR is only partly affected by the static CR. This is called dynamic compression ratio. The reality is that with a higher static CR, you have to run a more aggressive cam to keep the BMEP from getting out of hand. You can tune the engine to run on 91 octane if you're willing to degree the cams enough. Really, don't go with a higher CR unless you're going to run more radical cams. Those cams are only a benefit if you have an unrestricted air supply. On a restricted engine, you're better off with a stock CR and degreeing the cams to give you more BMEP, pushing the peak power RPM down in the RPM range.

I'd speculate that you'd want your peak power to be between 7-10k rpm. This would give you a pretty flat and broad torque curve.

As for the details, you want to play with intake centerline and lobe separation angle. The ICA has more to do with the peak RPM point, earlier is lower, later is higher. LSA has more to do with how much power you make at any given RPM. The wider the LSA, the broader the torque curve, but the less peak power. The narrower the LSA, the narrower the torque curve and higher the peak power.

Now these are only adjustments within a given cam selection. Those adjustments only affect relative values, not absolute. The absolute values are determined by the basic grind of lift and duration.

The other thing you shouldn't overlook is valve spring pressure. The more lift you run or the more aggressive the ramp, the higher the spring pressure has to be to control the valve.

Since most bike engines are shim under bucket, and are mechanical profiles, you really only have a couple of variables.

Higher initial spring rate will improve (allow you to use) low end power and idle. The open spring rate determines how high of an RPM you can run before floating valves.

It can be worth a lot to properly tune your spring rates, don't just assume that the stock spring is good enough.

--Perry