Best Exhaust System for Four-Stroke Engine?

I maybe wrong but I don't think a 4 stroke exhaust needs much tuning...you just want to get the gas out as quick as possible and hopefully quietly.
Better to spend your efforts on getting the gas into the cylinder as efficiently as possible, as (unless you add a blower) there is only atmospheric pressure pushing it in, whereas the exhaust has the piston pushing it out.
A 2 stroke exhaust does need tuning.

Del

ok thank you. but also volumetric combustion gases should be considered right in order to tune the bes effeciency of the exhaust performance, isn't? if it's contribute much to the performance of engine, how can we to analize that parameter by doing calcukation? actually the race is just in range of 200 meter.

mus

There are a lot of variables involved in tuning any exhaust system. the two that are highest on the list are engine speed under your ideal conditions, and exhaust tube length before any restrictions

On engine speed, are you working with a relatively low speed engine or a high rev build?

For low speed it is assumed that you want highest possible torque so you go with a relatively large diameter pipe and minimize the bends before any restrictions, such as a muffler or major bends.

For the high speed engine, a smaller diameter pipe is useful, keeping in mind the exhaust pulses are coming relatively fast, and you want to keep the reflected pulses fairly strong to help evacuate the cylinder, with the muffler at a specific distance from the valve, to time the the reflected pulse with the opening of the valve.

Best places to find the info you will want are speed shops that deal in racing engines.

ok. before this, i had used a pipe with smaller diameter as standard size as manufactured. I step up the cc of engine and increasing area of fuel intake and exhaust port. I used a bigger cuberator than standard one. something was be happen like had choked flow in the pipe. I suspected that the pipe was not support the combustion gases flow. The machine can be assumed be as a fast. What do you think about this case? Actully the machine was set up to drag race about 200meters.

If you are making an exhaust for a four cycle engine a old school rule of thumb is to fasten the straight pipe to the exhaust port. Draw a piece of chalk in a straight line longitudinally along the pipe at a spot you can easily see it. Start the engine an watch the caulk line intently and at the point the chalk burns begin the bend of the pipe.

Does knowing where and how to bend an exhaust pipe have a revelence

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The exhaust system will be tuned to the (? motocycle) engine. To which engine will the exhaust system be attached? Then search for the system designed for that engine.

Dennis Waller