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How do i calculate my 4 stroke exhaust size & legnth?

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I have just got my son an old 1985 xr 250.

These bikes has been known to punch out big h/p with twin carbs and exhaust. RFVC (30hp std).

I know the less exhaust pipe curves the better and where it does curve try and make them long and flowing.

In order to obtain maximum results matching the carbs, cam and capacity how would i work out my pipe size and length?

Thanks Matt.


We got 14hp out of a 7hp xr 100 so i hope for at least 45 from the 250.

Giddee up 👍

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There are several different theories, most of them fairly similar in what they result in. Punch in "exhaust tuning length", and you see what I mean. There is quite a lot to consider in the question. Cam timing, engine size, bore/stroke, rpm, etc.

Here are some examples:





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Ok from what i can read thus far , if my header is 1 8th of an inch larger than standard then the power band moves up some 500rpm and will move down if inside diameter is reduced in the reverse, I understand that point however i dont understand why the large collection box on the dr 350 instead of a straight through pipe ? is this purely to reduce noise or dose it have some beneficial reason ???


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The important dimension for tuning is the distance from the exhaust valve to the pipe end.

A couple of thoughts on the muffler shown in the previous post; the pipe terminate into a large volume that provide good tuning resonance. The large volume dampens some of the sonic energy. The last part of the muffler provides more sound reduction.

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The entire DRZ exhaust is primarily concerned with noise reduction. It's not tuned (or tunable, really) in any sense. It simply attempts to reduce noise as much as possible with a minimum of restriction. None of the sonic/pressure tuning info will really apply to it.

sorry hope you dont mind 500cc just we are at the same point of experiment at the same time 👍

my situation is that I had bought a 1991 clapped out dr350s bought as a non runner for tinkering purposes.. ( it had a chipped top ring and goosed carb) , Instead of buying rings and a s/h carb then selling it for some profit i decided to dig a big HUGE hole so that i have to keep her for ever :jawdrop: so in the last year ive ported the head and fitted new valves, fitted a 441 cc jug with je piston also higher comp , beefed up the clutch with my own personal mod, gold did zvm 525 chain and sprockets 15x44 , fitted a 39mm fcr mx

and dialed it in to a point where i now want to bin the original exhaust as it weighs an absolute ton and i believe it is restrictive on many areas of the experiment.

why not purchase a good aftermarcket system ? i hear🤣

1) they are as rare as rocking horse sheet in the uk and 2) I prefer the learning curve associated with doing it yer self

Part 1 of my plan is new muffler that is light weight and non restrictive

Heres one i made earlier lol


this muffler is titanium from a gsxr 600 has been sitting in my garage for 7years so good doner , cut it down too 32mm length must weigh approx 2-3 kg , I also have a crf 250r complete system to use if deemed better ????

my thoughts are 🙂 and i would love to hear some educated feedback on this

a) use the gsxr stubby and have a custom connection made to fit the original header ? or :cheers: have a new custom header made to fit the diameter of the crf muffler connection and then having the gsxr set up to fit this header ? effectively allowing me to have the choice between the cfr muffler or the custom gsxr muffler ? I also note that the crf header is some 2mm wider inside diameter tubing than that of the dr 350s original

Having read the links that Grayracer has kindly provided i realise that the system length is important and thanks to Chuck too

so should i keep original diameter or go wider and build 2 systems instead of one

Im hoping to be able to run a bigger main jet little bit more up top as bike runs outta steam in 6th gear at 8000 rpm instead of 9000,but dont want to lose the torque i love thanks guys :applause:

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