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Exhaust port

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i have my jug off of my DT200R for a top end rebuild. I noticed that the exhaust outlet is smaller than the pipe inlet ID. 32mm dia. vs 36mm. The exhaust gasket is also around 36mm id

 

Would you expect any performance increase by matching the outlet to the pipe diameter?

 

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i wouldnt say they should always match. yz250 been using oval style exh hole that doesnt match the pipe. it seems to be working for them

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On a two stroke they should match.

And the pipe would preferrably match the cylinder.

On a 4t you don't want it to match.

Why ?? dont we want it to match on a 4 stroke

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There is no such general rule regarding 4-strokes.  Some engines do benefit from having a step that close to the valve, but many do not, and work better with the step (if any) located farther down stream.  The theory is that the step benefits the "blow down" phase at the beginning of the exhaust cycle, but that doesn't hold up in every case. 

 

Other examples included early big block Chevrolet heads, where the "square" exhaust ports were modified into a "D" shape by raising the port floor.  This was a completely different situation in which the port floor had originally been so low relative to the valve that it resulted in a lot of eddying and turbulence within the port itself.  Flow was actually increased by raising the port floor in spite of the reduced cross section of the port.  Nothing to do with blow down in that case.

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i dont believe there is a general rule saying they should match on a 2t either. i have purposely mismatched a exh port and exh pipe mostly out of curiousity to see the results. ill report back when i have some answers

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actually i didnt intentionally make a mismatch. i intentionaly reduced the size of the exh tunell. similar to what greyracer mentioned . and in doing so, my end result was a mismatch of the exh and pipe. i think the yz250 and some of the later years cr250 cylinders might be based on a similar theory but im not positive

Edited by harryhandshake

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Thanks for the input guys, Still not sure what to do. When I get my cylinder back from the machinist, I'll measure my exhaust port effective area and compare it to the outlet. I would think if the area of the outlet is less than the effective area of the port then it would probably benefit from increasing the outlet. but I don't know

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