I have no experience with the 4 valve Porsche heads but that trend started way back with the 2 valve hemi style Porsche heads.ptuomov wrote: ↑Fri Nov 22, 2019 9:54 am Interesting.
If you go to 1980's and 1990's four valve heads from Germany or Japan, I think you're going to find very low port velocities with this formula.
Take for example 1987 Porsche 928 S4. The port minimum cross-sectional area is about 2.7 square inches. The engine displacement is about 302 sqin with eight cylinders, so 37.75 CID per cylinder. The peak power from factory is at 6000 rpm, making 316 hp. Unless I made a mistake, the formula says 6000*(302/8)/(2.7*360) = 233 ft/sec.
The head itself flows decently considering the port minimum area. It flows 290 CFM with 2.7 sqin MCSA at 28" of water. It's just very big and very slow compared to these numbers listed on the sheet.
Cams are also very short duration, in case that's relevant.
At first glance they look like a well designed classic hemi head and one might ask what can be wrong with that. the answer as you may guess is - 'plenty'!
Try this for a quick list cons:-
*Valve angle relative to the cylinder axis is too wide.
*Ports are too big which leads to low velocity and consequently port energy.
*Ports are poorly designed in terms of shape.
*There is no swirl which may be why the heads have two spark plugs.
*The combustion chamber is so far off what is required that they can almost be termed as a disaster volume.
My thoughts here are that with enough welding rod these heads can be considerably improved.
DV