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Exhaust back pressure

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Gonna put a custom exhaust on my a-81 will it need back pressure and can it pretty much be a straight through pipe?

Cheers Matt

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I won't claim to be an expert on this particular small engine type, but I do know my way very well around most other engines. That being said, this would be my engineering take on it, not based in real life experiance, and without being to rocket siency:


Flow and timing optimisation on a 2 stroke system has a profound effect! BUT, this being 4 stroke, the effect is much less profound, and perhaps minute on this engine type I would guesstimate. Yes, if done absolutely correct, you can pull extra out of an engine, doing the fine tuning correctly. BUT, compared to the negatives you can add if you do not know what you are doing, the gains are probably minute.


I will give you these tips for making an exhaust that do not add negative, and will perform better than any kind of restrictive system, albeit not being a system tuned work of art.


-Do not use to small or too big pipe diameter. Follow the size you have in the exhaust port, and do not go bigger either.

-Avoid turbulens in the exhaust gas. Make the pipework flow nicely, without sharp bends and obstacles. The more smooth and laminar you can make the flow the better.

-Make the silensor big enough to flow your volume without creating resistanse, and still be abel to bring the DB's down. Put the silensor as far away from the engine as possible.

- Use heatwrap on the pipe for a stretch after the engine, perhaps all the way to the silenser

-Get the heat load away from the engine area.


Now you have taken care of the exhaust. I would suggest taking a look at the intake too. Engines love cold air that flows freely. Make sure there is no restrictions in that end also, and it will probably reward you more than the exhaust work, and all together, your engine will have a better life............


Now go find some good 5w40 fullsynth oil  too, and it will thank you in the long run.............. that's what I use, and in a little while there will probably be a shitstorm for that comment ;-)


Sorry for the grammar oopsies:-)



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Thank you skipper a very thorough reply which explained it very well. Much appreciated buddy. 🖒

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