Originally Posted By: A_Harman
No, I don't know what the BSFC is for either engine at any of the hundreds of operating points that are on a modern engine map. We're talking in general trends here.
The total load on the cam is about the same because there are twice as many valves and springs in the DOHC engine as there are in the OHV. The trouble is, a DOHC V8 has 4 times as many cam bearings as an OHV V8, and about 10 feet of cam chain instead of 2. More parts rubbing together, more friction.
Name an engine where the size of the bore doesn't limit the size of the valves. Pushrod or not, a 2-valve wedge combustion chamber does not allow for as much valve area as a 4-valve combustion chamber. The 4.6 Ford modular 2-valve had small valves, and it never made the same specific output numbers in production form as GM's pushrod engines.
You aren't following me
That's exactly my point. That on a given bore size, if you have a 4-valve head, it has the potential for more valve area. So on an engine with a reduced bore size (like the 305 or the Modular) the potential for greater power output is going to be in the hands of the engine that can move the most air... .basic operating principle here. And that engine is going to be the one with the most valve area, all things equal. Which means that if we take a pushrod engine like the 305 and compare it to Coyote, as they have similar bore sizes, well, we both know which engine will make the most power across the board.
With the pushrod engine you are generally (yes, I know there have been exceptions in the past) constrained to 2 valves per cylinder. And the size of those valves is directly related to bore size.
However, if you look at the modular, yes, they had a 2 valve version. But they also had a 4-valve version that made significantly more power, and the latest evolution of that philosophy is Coyote, which still has a small bore (relatively speaking). That is not something you can do (easily... or it would be popular) with a pushrod engine. Limiting bore size for emissions whilst increasing valve area through simply having more of them.
And this is where your argument about GM putting money into combustion chamber tech comes into play. It isn't for power output, but rather for emissions.