Here's a little more info for those that are interested.
When you look at a pushrod motor, it doesn't make sense. You have to understand how the pushrod V8 evolved. The first V8s were designed like the flathead Ford. The valves were mounted parallel to the cylinder and opened upwards into the combustion chamber. The designers placed the camshaft in the valley, because that was the logical location. From there, the camshaft pushed almost directly on the valve, much like an overhead cam engine of today does. After that, Buick designed an engine where the value opened sideways into the comubstion chamber. The valve opening and closing motion was perpendicular to the motion of the piston. In order to accomplish this, they left the camshaft down in the valley, and placed rockers inside the valley to actuate the valves. It was later discovered that in order to get engine efficiency and power up, the compression ratio had to come up. In order to get the combustion chamber volume down, engine manufacturers more the valve to the top of the cylinder, where it has remained to this day. Instead of moving the camshaft to the top of the engine, when Chevy invented the SBC, they took the Buick design with the rocker, and added a pushrod to the valve system to have the valve open vertically into the combustion chamber. That is how the pushrod engine evolved.
Now, a lot of people don't like OHC engines, but I don't think most people understand why. OHC engines do not necessarily have a distinct hp/liter advantage. They do however have a huge advantage in the RPM range. When you have a pushrod engine, there is always a limitation on how fast you can spin it. In order to turn the engine faster, you have to put much stronger valve springs in it. Bigger valve springs, means they wear out faster, and they wear out the valvetrain quicker. This is all comes down to reciprocating mass. A pushrod engine has a huge valvetrain mass (valves, rockers, pushrods, lifters) that is reversing directions with ever motion of the valve. The OHC engine has just as much valvetrain mass, when you consider you have the added belts or chains and multiple camshafts. However, all of this mass is rotating in one direction. The only reciprocating mass in an OHC engine is the valve itself. It doesn't take a lot of spring pressure to reverse so little mass.
Now, most Mustang people don't like OHC engines, because the 4.6 sucks so bad. And admittedly, it sucks. But it's not because it's an OHC engine. The 4.6 sucks, because it's a small bore engine (3.55" to the 5.0s "4.00"). Small bore engines have trouble making power N/A. Simple physics, the force exerted on the piston is the combustion pressure multiplied by the surface area of the piston. A 4.6 engine has only 79% the piston area of a 5.0 engine. So with the same combustion pressure, the 4.6 is only getting 79% of the force down on the piston. In my opinion, Ford should have designed an engine for the Mustang with a bigger bore, and a lower deck height, and obviously a shorter stroke for RPM. If the modular engine had the same 4.00" stroke and 3.00" bore as the pushrod 5.0, you could make it sounds just as nasty, and make even power out of it with less effort. The smaller bore engines do have an advantage when you put the boost to them though. That is why you see these Honda Civics with 2.0 Prelude engines running high 10 second quarter miles in street trim. Those smal bore engines can take a lot more boost, and much higher combustion pressures without detonating.
Kurt
Is this correct?
