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I heard this from the guy that builds engines for my boss who happens to be the national amature frestyle jetski Champ, and second in the world and also races rd 400's. This guy really knows his stuff, He IS Team Scream. Now he's a 2 stroke guy, but I listen to what he says just the same.This man know how to make huge horse power, He said unless your on the track dont' get a full system because you actually lose a little bottom end because It has a bigger collector. Your also looking at a huge difference in price. I bought a carbon fiber Yoshimura TRS slip on, and a BMC filter. Now...after I did this my bike ran like a big steaming nugget of poo. So I would not go with the " you don't need to tune with a slip on" theory. So, I bought a PCIII and loaded a slip on, air filter map, adjusted the accel pump......Rocket..The difference is so huge I was shocked. This is not even a custom tune. I will probably do that a little down the road when funding permits. But I'm thinking this..when they assemble your bike at the factory, do you think they custom tune every one...I think not. I'm sure they have a standard this bike, this tune thing going on. So with that in mind a custom tune might not even be necessary.

Where you gain HP is completely in the design of the pipe. There are some where you are correct and others where you are not. With the M4 on my bike, we had no loss at the bottom and a huge gain at the top, with the Akropovich we had the biggest gain in the mid-range, which helped alot coming off the corner.

I don't measure gains by the seat of my pants, we do it on a dyno.

The reason you needed a slight adjustment with the slip-on was because you opened up the intake side with a freer flowing air filter. Had you not done that you would have had minimal effects with the slip-on, other than a mild gain.

A custom tune allows you to get the most out of the dollars you just spent for the performance upgrade. If you've already spent $1200 on a full exhaust and another $225 on a PClll, go the extra mile and do it right with a custom tune. A downloaded map will not give you optimal gains.