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slip-on exhaust

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I put a full exhaust system from an 2006 yz450f on my 2009 yz450f due to stock is very restrictive and hurts power. Now the 06 exhaust slip-on is basically a big hole if you look in it with packing around the big hole. I am sure guys with an 06 yz450 buy aftermarket slip-ons. So my question is how does an aftermarket exhaust slip-on improve on an exhaust like that. A hole is a hole.

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When is a hole not a hole, would be the question. Your analysis is too simplistic, and it's also based on an assumption that isn't necessarily operative.

Properly done, a tuned exhaust system for a four-stroke that uses a "glass-pack" type of muffler should be designed so that the tuned portion "ends" at the point where the mid pipe meets the muffler. Any improvement made by a slip-on muffler in most such cases has to be accomplished by improving on the design and/or length of the mid pipe. Elements such as changes in diameter, where they are placed, and whether they are tapered or stepped figure into a complex picture.

The flaw in your premise is the assumption that the stock system will be improved by a slip-on muffler. Some are, but a good many are not, most YZF's other than the '08-'09, and the '10's included. Usually, the most that is accomplished on a YZF with an OEM slip-on is loosing some weight and gaining some bling, frequently along with more noise, and an occasional change in power one way or other. Yours is different because the OEM can has two perforated baffles the gases are forced to flow through, which is an old and discredited design. I was frankly astounded that any such thing was built, but there you are.

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Like Gray said, there's more to it than the hole. The diameter, the length, and the midpipe characteristics all come into play. Aftermarket slip-ons have all sorts of different configurations. Take the FMF factory 4.1 for that bike for example. The midpipe gradually increases in diameter up to the muffler entrance, then the core of the muffler is actually tapered, then it tapers back down near the entrance. The overall length is shorter than the stock 06 silencer. Obviously these two silencers will give different characteristics.

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