How much horse power is gained with aftermarket exhaust on quads?

I've been looking at some exhaust for my 1989 TRX250x fourtrax and was wondering which slip-on exhaust will give me the most torque and horsepower. The websites all pretty much have the same description so I really have no idea which to buy. Will there be a noticeable difference in power or is it not even worth buying. I've mostly been looking to get a Big Gun EVO R (https://www.biggunexhaust.com/product/evo-r-slip-on-honda-trx-250x-87-92/) or the FMF Power Core 4 (http://www.fmfracing.com/Products/ATV/129)

Added (1). I should probably note that the engine is on a full rebuild (full bottom- tranny and a full top end)

In the Motorycle News Magazine they made a series of tests with after market exhaust pipes and the conclusion is simples: new noise, same power.

If you really want to get more power from the same engine the best way to do it is a full replacement of the exhaust pipe, dimensioned carefully to provide a new range of power.

Bear in mind that the power (performance) of an engine is one of the three factors affected by tunning the engine.

Power VS fuel econony VS durability

If you increase one of these factors, one or two of the other factors will be decreased.

Thus, if you increase the power most probably there will be a decrease fuel economy (the quad will consume more fuel for the same distance traveled) or a decrease in durability (reliability) … Or both.

Remember that we're talking about a 28 year old engine that surely has had its part of abuse. Reliability may be an issue.

Nothing more than 5% on a pristine engine, for a nearly 30 year old expect less.

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