Big Valves and porting?

[nkaminar]

Exhaust scavenging depends more on the exhaust system than the exhaust valve. The exhaust pulse produced when the exhaust valve opens travels down the exhaust pipe at the speed of sound and reverses when it reaches the end to become a negative pulse traveling up the exhaust pipe to help suck out exhaust. The speed of the engine and the length of the exhaust pipe have to be matched to get this advantage. A diffusing cone at the end of the exhaust pipe will extend the rpm range where scavenging has an effect. Having equal length pipes that combine will allow each cylinder to reinforce the scavenging effect. You can see this on older race cars where the exhaust pipes combine and then there is a diffusing cone.

[BRENT in 10-uh-C]

Originally Posted by nkaminar

Exhaust scavenging depends more on the exhaust system than the exhaust valve. The exhaust pulse produced when the exhaust valve opens travels down the exhaust pipe at the speed of sound and reverses when it reaches the end to become a negative pulse traveling up the exhaust pipe to help suck out exhaust. The speed of the engine and the length of the exhaust pipe have to be matched to get this advantage. A diffusing cone at the end of the exhaust pipe will extend the rpm range where scavenging has an effect. Having equal length pipes that combine will allow each cylinder to reinforce the scavenging effect. You can see this on older race cars where the exhaust pipes combine and then there is a diffusing cone.

You are correct ...if you are quoting 20+ year ago technology. That outdated technology mindset also applies to surface finish on port work. Much has changed since then.