1990s hpp cr125 exhaust porting difference

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2/17/2020 12:22 AM

I have way too many but 2 of them have this ground out spot on the exhaust exit. Rt hand side sitting in the seat. This one is a pro circuit ported east coast supercross bike. I did not know if it was flow evening grind job or not. Or just a year over year thing.

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2/17/2020 2:10 AM

Odd it’s not both sides. Roofs had some grinding work. Curious as well

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2/17/2020 4:42 PM

I would have expected PC to match the exhaust flange and exhaust port. That's like porting 101.

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2/17/2020 10:28 PM

450exc115 wrote:

I would have expected PC to match the exhaust flange and exhaust port. That's like porting 101.

No not in two strokes actually

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2/17/2020 11:40 PM

That is stock casting, don't spend to much time on it. Makes no difference

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2/18/2020 8:26 AM

Would you mind sharing the port specs?

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2/18/2020 8:34 AM

450exc115 wrote:

I would have expected PC to match the exhaust flange and exhaust port. That's like porting 101.

Chris99Flannery wrote:

No not in two strokes actually

You'll have to expand on that please?

Abrupt transitions on airflow are never good. Rule of thumb to keep separation from occuring at the walls (boundary layer) is 7 degrees or less. There is some wiggle room around that depending on how pressure and temperature are changing but that rule works in most cases. A drop is better than a lip in transitions from part to part but if you can make it smooth it should be even better. Anytime I do a crank I always gasket match parts and clean up casting marks, the motor always seems to run better and jet cleaner after that.

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2/18/2020 3:15 PM
Edited Date/Time: 2/18/2020 3:18 PM

Honda have used that lip on there RS road engines and even Kit cylinders. I’ve read a test that that removed it an lost measureable hp on the dyno. It’s when the reverse pipe wave hits it is has a positive effect.

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