Vintage Dirt Bike Q & A

Since the MR175 has an unbridged exhaust port, adding exhaust eyebrows like the CR125 required drilling pilot holes from outside the exhaust port into the cylinder and shaping them into ports. This leaves a small bridge on each one for ring support.

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Wow, those auxiliary ports must have been hard to drill, especially on the left side of the port! They came out great, though. Your exhaust setup is now similar to the Yamaha TZ250G, just without the power valves. It should be real easy on the rings, too.

I was going to try this along with the boost port, but chickened out and just widened the existing ports. My (auto) machinist did the exhaust port and passage, I'm real happy with the work he did:



The exhaust port is now 68% of the bore width at the widest point, which is a little less than the max recommended by the Bell book (the TZ's main center port is 73% of bore width). The passage was opened up from the port as straight as possible to a 39mm exit I.D. to match a '76-'78 CR125 manifold, requiring a lot of metal removal. I use a hi-temp O-ring between the inner manifold stub's beveled edge and the cylinder recess, with no leakage so far.

Ray

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'74 CR125M (175cc), '75 MR175, '82 RM250Z, '08 YZ250F, '14 Zero FX electric, '14 Zero MX electric, '18 Alta MXR electric

The widest part of your port is half way open. The tunnels or eyebrows are open right away and send a stronger signal to the pipe and it's supposed to make it hit harder on the pipe on the top end.

It's actually widest at about 1/3 open (the top is 45mm wide, bottom is 44mm, top edge & corners are quite rounded to help ring life), but you're right - all else being equal, your arrangement will have faster/better blowdown at high rpm and a more powerful initial pressure pulse. I'm using a very unusual pipe that should work pretty well with the single port, but only until 8,250 rpm or so when power falls off drastically.

I tried modeling a single 10mm tunnel port on the right side, but it didn't add as much flow as widening the main port so I went that route. Two ports like your setup would have been a different story.

Ray

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'74 CR125M (175cc), '75 MR175, '82 RM250Z, '08 YZ250F, '14 Zero FX electric, '14 Zero MX electric, '18 Alta MXR electric