Re: Big Valves and porting?
 

Thank you. Gaining efficiency is like cooking, and everyone has their own recipe. The closest engine to the Model-A that has seen extensive R&D done is the 5 horse Briggs & Stratton engine used in both drag racing and tractor pulling. You might also follow that ‘circle’ to learn some tricks.

Re: Big Valves and porting?
 

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.

Re: Big Valves and porting?
 

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 That outdated technology mindset also applies to surface finish on port work.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.

Re: Big Valves and porting?
 

What Bob has pictured is a ported AND RELIEVED flathead V8. In them relieving does reduce the compression ratio, relieving is not needed in our A's because in the V8, the valves are not parallel to the cylinders.

Re: Big Valves and porting?
 

I think what Jim meant to say is on the model A. block the valves are not sunken like the V8 flathead is the V8 flathead requires relieving for improved air flow the model A. does not as the valve sit on top of the deck