It's *NOT* valve *AREA* which is important ......
[quote=fpv_gtho;245873]Well the main thing i like about DOHC is its ability to have 4 valves rather than 2, afterall you'll cover more area with 4 valves rather than 2 so you should be able to get better breathing out of the engine from that[/quote]
..... it's the circumference of the valve(s).
Air only flows around the edge of an open intake valve.
Exhaust only flows around the edge of an open exhaust valve.
The middle of the port is blocked by the head of the valve. No air or exhaust flows through the valve head.
The area of a valve goes up with the square of the radius. A valve which is twice the radius / diameter has four times the area of the smaller valve. It only has twice the circumference.
Two valves of the same diameter as the original valve also have twice the circumference but only occupy half the area of one valve which is twice the radius / diameter.
You run out of space to put larger valves more quickly than you run out of space to put more valves.
Look a little harder and you will find them ....
[QUOTE=Slicks;254072]I havent heard of any I pushrod engines...[QUOTE]
All of the following which I have driven over the years had in-line engines with pushrods and overhead valves.
1960 Ford Falcon, 144 cubic inch in-line 6
1964 Simca 1000, 57 cubic inch in-line 4
1967 Plymouth Valiant, 170 cubic inch in-line 6 - aka "slant 6"
1972 Ford Pinto, in-line 4 of unknown displacement (~2 liters?)
1972 Volvo 1800E, 2 liter in-line 4
1983 Renault Alliance, 1.4 liter in-line 4