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strangewhispering

How dumb is it to run a small exhaust on a big cylinder?

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I tried to build a sleeper motor with a Mugello 200cc barrel on a big block case with one of those Series 1/2 big bore clubman exhausts that SCK sells now.  It ran pretty good but the CHT was kinda high with the carb jetted well for performance.  I removed the silencer (the final tailpipe that comes out of the box) and the temp went down a bit and the power improved a bit which made me suspect that the exhaust might be a little choked by it's small box capacity but I don't know anything about gases and exhaust science stuff, this is why I'm asking).  Then I ditched the exhaust altogether and switched to a Tino Satchi series 3 big bore clubman and the CHT went down so low that I thought it was a misread, like not ever getting into the yellow, to the point that I could even retard my timing because it runs so cool.  So, the question is:  Is there a limit to how small you can go on an exhaust in relation to the displacement of the cylinder?  If that's true then why do stock V3$p@ P200s or Rallys run fine with stock exhausts that have boxes that look as small as the little Series 1/2 clubman I was trying?  Aren't there people who run clubmans on TS1's?  Wouldn't that cause the same problem?  For reference the overall setup is this: Varitronic ignition with the timing set to 21 degrees at 2000rpm, PHBL 25mm carb breathing through a GP scoop and drilled stock airbox with a CLW high flow filter, Mugello 200cc top end on a Jet 200 case, 125-130lbs of compression.  Once I swapped to the series 3 clubman pipe and re-jetted to suit the bike ran really well and cool and I completed a 1000 mile road rally on it so it all worked out.  Thanks y'all, Hi Chris!

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The smaller pipe is too restricted and the burned charge isn't getting out fast enough, so heat builds up quick. 

If I were building an engine like that with a small pipe and a Mugello, I would drop the ports, but still have a huge exhaust port. The problem with the Mugello is tht you'd need to remove material from bottom of cylinder to bring them all down.

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2 hours ago, strangewhispering said:

I tried to build a sleeper motor with a Mugello 200cc barrel on a big block case with one of those Series 1/2 big bore clubman exhausts that SCK sells now.  It ran pretty good but the CHT was kinda high with the carb jetted well for performance.  I removed the silencer (the final tailpipe that comes out of the box) and the temp went down a bit and the power improved a bit which made me suspect that the exhaust might be a little choked by it's small box capacity but I don't know anything about gases and exhaust science stuff, this is why I'm asking).  Then I ditched the exhaust altogether and switched to a Tino Satchi series 3 big bore clubman and the CHT went down so low that I thought it was a misread, like not ever getting into the yellow, to the point that I could even retard my timing because it runs so cool.  So, the question is:  Is there a limit to how small you can go on an exhaust in relation to the displacement of the cylinder?  If that's true then why do stock V3$p@ P200s or Rallys run fine with stock exhausts that have boxes that look as small as the little Series 1/2 clubman I was trying?  Aren't there people who run clubmans on TS1's?  Wouldn't that cause the same problem?  For reference the overall setup is this: Varitronic ignition with the timing set to 21 degrees at 2000rpm, PHBL 25mm carb breathing through a GP scoop and drilled stock airbox with a CLW high flow filter, Mugello 200cc top end on a Jet 200 case, 125-130lbs of compression.  Once I swapped to the series 3 clubman pipe and re-jetted to suit the bike ran really well and cool and I completed a 1000 mile road rally on it so it all worked out.  Thanks y'all, Hi Chris!

This is a fun topic to dive in to. Not many people get in to it.

The CHT temps are not the end all to everything. There is an inverse relation ship between CHT and EGT (exhaust gas temp, which to me is much more important for tuning) . As CHT goes up the EGT will go down and vice versa, between a certain range. 

 

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And you would want to ADVANCE your timing if it were running that cool. You wrote "retard my timing because it runs so cool."

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