I have a 2003 baja that is stock with the exception of a cdi and air filter and i want to step it up just a bit so i would like some input on what is a good 11:1 stock bore piston with a good reliability rercord.I have asked before but i will ask again if anyone has had a pipe for the ds tested that came in at a reasonable decibel and if so what were the results.I am looking at having a 11:1 piston installed along with adding a pipe,clutch springs,ported intake and with my big gun cdi and K&N air filter i should see a fairly significant performance gain?I realize that this will not be the best drag racer but it should make a better all around dune quad with these modifications,any guesses on horsepower increase with these parts?

 

I think if you did the a 11:1 piston with an exhaust, and ported intake it would be a very noticeable over what your are running now. While you have the engine apart cams might be a good thing to think about doing too. Of course this means more $$$, and with new cams you may need stiffer valve springs also.

For exhaust, I would just do a slip on. Get something that you can add a quiet core since you are worried about sound levels. HMF and the Yoshi would be my top two suggestions. Both will get you better flow over the stock muffler and have a spark arrestor and available sound baffles. The cool part is you can easily remove the sound baffles or the spark arrestor for places that don't require them.

 

stock compression is close to 10:1. 11:1 isn't going to do much for you.

 

11.5:1 is as high you can go on this motor on pump gas, I run 11.5:1 and 92 octane pump gas, have been for years very reliably. step up to 12.5:1 and higher and you get all the hassles that go with it, like fuel availability. I know lots of people that have to pack 30 gallons of high octane fuel on the trailer for every ride trip because of their motor setups, and I just pack an empty 5 gallon can and fill it when I arrive at destination. Most of these guys don't go any faster than me either.

But if I was opening up my motor, I would do all or nothing. The piston alone isn't much, but when combined with cams and proper valve springs the combination is real nice. Swapping cams is easy any time, but you really otta install dual rate valve springs with them, and this requires head removal, and the since the head is already off for piston replacement that's the best time to do it. Then you don't have to drop another $50 on a head gasket next time down the road.