Delta Force
Banned
I think once you go to multi-valve the pushrod becomes less economical in terms of construction. The beauty of your average pushrod engine is a minimum of expensive, machined parts. The cam is about all that needs machining, lifters and pushrods are mass produced and rockers are even stamped out of steel sheet. That all works because the geometry is simple and standard.
Once you start getting fancy geometry for rockers and multiple valves then it becomes easier just to machine another cam and minimise the rest of the valetrain by putting the cam up on top of the head.
As for blowers, the biggest problem for roots type blowers is heating of the intake charge. If intercoolers became more common then roots type blowers could be seen on a lot more production cars.
Most multi-valve engines tend to be dual camshaft, at least for engines with four or more valves. Have there been any dual camshaft pushrod engines? Could the cams/valves be doubled and separated into intake and exhaust systems without having to change everything else?