Thread: Exhaust valves
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Old 10-22-2015, 03:01 PM   #12 (permalink)
darcane
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Quote:
Originally Posted by Daschicken View Post
My grandpa's 1999 yamaha r1 had an exhaust butterfly valve, but I doubt it was computer controlled as the thing had carbs. Although it did have a MIL system that had codes for the exhaust valve, so it could be.
This is Yamaha's EXUP system, and the computer module controls when the valve opens and closes. I have the same engine on my bike-engined-car project, and have to defeat the system in order to allow the engine to run properly even though it is carbureted.

It's not entirely accurate to say this system increases backpressure. The header should be tuned for optimal scavenging at whatever RPM peak torque is made at. Restricting the exhaust adjusts the exhaust pulses to better scavenge at low RPMs. This actually reduces backpressure at the exhaust valve when the exhaust valve opens.

For a dedicated economy car, rather than try to replicate this complex exhaust valve system, you'd be better off designing a header that is tuned for running at low RPMs. That would be cheaper and far easier to accomplish, and likely more effective. Although, I doubt you will see measureable gains over the stock exhaust.
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