Hi Guys

Just pondering if there is a general relationship between exhauste valve and exhaust port size (where it meets the exhauste pipe).

Ta

Newt

There's a formula for valve size to port size out there somewhere but the one thing I do know is that the exhaust port must always be a few mm smaller in diameter than the headers primary length diameter to stop the exhaust reflection wave entering the combustion chamber. This wave travel back along the wall of the exhaust pipe and if allowed to enter the cylinder head will cause issue with the scavenging effect present during the cam overlap period and interfer with the expulsion of exhaust gasses.

Thanks for the info.

The reason I am pondering is that I have some 044 heads with 35.5mm exhauste valves and the exhauste port where the header attaches is 32mm.

Out of curiosity I compared that to a 1200 head and that seems to also have a 32mm exhauste port (and with a 32mm exhauste valve).

The engine Im looking at building is a 1955cc.

Newt

From what I've read about gas flowing there is a fine line between gas speed and gas flow. This means that if you open a port up then it will flow more gas but if you open it up too much then you'll lose gas speed and that in turn will reduce the filling and emptying efficiency of the engines cylinders. This makes the engine less powerful and torquey especially low down in the rev range. Then at higher revs the engine will come on song due to the piston speed being able to suck efficiently thru the enlarged port.

Thanks - Makes sense.

Moving on to the header, I understand that for my build, I should look at 1.5 inch pipes (rather than 1 3/8). Is that out side diameter or inside? If inside, then that is around 38mm.

My question then relates the step off from say 32mm port to a 38mm pipe. I understand that it is good to allow the gas to expand once out of the port, but is a 6mm diference somthing I whould be pondering - or is it aceptable?

Ta

Generally exhaust tubing is given as the external diameter but exhaust tips are given as internal so for you the 1.5inch headers are 38mm external diameter with a nominal wall thickness of about 1.5mm so that'll give ya 35mm internal diameter.
Hence if the head port at the mounting flange is 32mm then you have a 1.5mm radial lip to stop the reflection wave. I'm no head guru but maybe you could open the port by 1mm but really you'd be better off just cleaning the casting flash from the inside of both the inlet and exhaust ports, to smooth the airflow, and port match the inlet manifolds to the inlet ports. You can easily add a good few BHP for free with a basic clean up of the ports.
I use a dremel flap wheel in a drill as the drill has enough torque to turn the tight fitting flap wheel. Don't go mad. Use your finger to feel how things are going and don't stop in one spot for long or you'll enlarge in a single place.

Thanks - I learn a bit every day.

Always happy to lend my experience wherever I can