That's the theory, but the stock AFR's of the RZR are too rich to begin with, so after adding the pipe, we still pulled fuel out of the curves to maximize power.

I found the same with a Suzuki EFI with a new pipe. Initial setup on the Dyno was fine, added the pipe and the ratio was too rich. Talked to a guy at BUB that was running the Dyno and he said it's pretty common

What is really weired is that all ATVs and all UTVs must meet federal emissions standards like lawn equipment before they can be sold now so if they are to rich they could not pass :?: , That is really weired :?: , That would mean that the stock system would be less restrictive then the aftermarket system, When you move more air through the engine you need to compensate with more fuel, That means the engine would have leaned out to do that, In every race car i have dealt with when we take the exhaust off the header collectors we had to go up in jetting to compensate, That is the reason when you go up in altitude the older carburated cars start to run way to rich because of the lack of air, as for the Polaris and your Suzuki It may be because the the ECU and the T-map sensor gets confused and starts adding to much fuel to the fuel map because of the aftermarket system, That would be the only answer to this question, After all They do not have an O2 sensor seeing what is coming out of the engine to compensate, The ecu's aren't smart enough :idea:, Any how this thread was not meant for auto shop 101 and I will stop babbling on this .