It depends on the sled, and the pipes. A well tuned from the factory sled probably wont see much HP gain unless you go from a single to twin pipes for example, another thing is people use a track dyno and then they dont clutch the sled right after a pipe change and they see a drop in track HP because of clutch efficiency. I think until recently at least Ski-Doo had the best factory pipes, only a few of the bigger aftermarket companies make Doo pipes, and if they do they are always setup to make power at a higher RPM because the factory pipes are about as good as they can get at stock RPM levels, but the others seem to be closing the gap in pipe quality, perhaps now even besting Doo (the F7 is my example) so this is not an easy question to answer by any means.
I remember when Dynotech did a 485 Phazer pipe test. All the aftermarket pipes made more HP than the OEM pipe. There also wasn't a big RPM change. They guessed that Yamaha was conservative in the design. After my PSI pipe fiasco on my 85 MX, it appeared the only way to improve on the factory Rotax pipe was to go to twins.
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