OHV While all modern automotive piston engines have the valves arranged over the cylinders, OHV engines were named because earlier engines (like the 1930's and 40's) had valves beside the cylinders, not OVER them - so OHV was a a way to differentiate the OHV valve style from say, the Flathead engines. OHV engines have the camshaft below the cylinder head, and thus use lifters and pushrods to help actuate the valves that are in the cylinder head. Compared to OHC engines, they allow for better packaging, but are less efficient compared to OHC designs due to increased valvetrain mass. To open a valve, the camshaft pushes on a lifter, which pushes a pushrod, which pushes on a rocker am, which opens the valve. OHC engines don't have the weight of the pushrod to overcome. While the weight of a pushrod & lifter is seemingly insignificant, when you consider it can account for more than 15% of the valvetrain mass, and it has to open a valve up to 6000 times a minute (or more), it adds up to measurable difference. It's all about inertia - the less weight it has to move, the less energy is required to open the valve, and thus, there is more energy that can be transferred to the crankshaft - meaning more HP to the wheels.

SOHC (Single overhead camshaft)   Sometimes referred to as just OHC. The camshaft is in the cylinder head, and the camshaft opens the valve either almost directly (with a shim between the cam lobe and valve stem), or by actuating on a rocker arm. In either case, it doesn't have the added mass of a pushrod. Most SOHC designs have just 2-valves/cylinder, but some manufacturers (like Honda) have SOHC engines that use a 4-valve/cylinder arrangement. SOHC engines have a camshaft in each cylinder head - so an SOHC V-8 actually has two camshafts

DOHC (Double overhead camshaft) A DOHC arrangement is normally used to take advantage of a 4-valve/cylinder arrangement. A typical DOHC design has two camshafts in each cylinder head. The benefits are the same as an SOHC engine, with the added breathing of 4valves/cyl (see below)

4-valves/cylinder. It's all about inertia and breathing here. The key to high specific output (meaning HP per liter, or cu. in.) is breathing and low pumping losses. In the '60's and 70's, to get more HP out of a given displacement a manufacturer would install bigger valves. HP comes from cramming more air and fuel into the cylinder, so a bigger "door" (valve) can let in a bigger air/fuel mixture per stroke . The problem with a bigger valve is that a bigger valve weighs more than a smaller one (assuming the same metallurgy), and this means to control the extra weight as it's flung open is the spring that closes the valve has to be stiffer. A stiffer valve spring (multiplied by the number of valves), means more energy is spent overcoming the valve pressure, thus partially offsetting the gains of a bigger valve. Another disadvantage of a big valve is that at lower RPM's, the intake charge has a lower velocity and low RPM torque and driveability. suffer. Enter a mulitivalve design. Two 30mm valves give more breathing area than a single 60mm one, and each valve weighs less than a 60mm one, as well. I can hear you saying "but TWO 30mm valves weigh more than a single 60mm one!". True, but for each cam lobe, there is less mass to overcome, and the smaller valves don't have to open as far nor do the valve springs have to have as much spring rate. A 4-valve design almost gives you something for nothing, as at lower RPM's the intake velocity is good for each port (they're small ports) so you get good low RPM torque and driveability., while at high RPM's there's sufficient flow (there's two ports) to make good HP. In practice, there

In layman's terms, a DOHC 4-valve/cylinder (or even 5v/cyl) increases HP without sacrificing low RPM torque and driveability. It effectively extends the torque curve. Below is a chart of two identical displacement engines, one is SOHC 2v/cyl, and the other is DOHC and 4v/cyl : See the graphs below (click on images to see full-screen view). For those real nit-picky, you can think of this as 4v/cyl vs 2v/ cyl, as I am aware there are 4v/ cyl SOHC engines)

Nissan VG30E  vs VG30DE
2v/cyl vs 4v/cyl - torque	2v/cyl vs 4v/cyl - horsepower

These engines are from the same manufacturer and the same engine family - probably as close as you can get for a direct 2v/cyl vs 4v/cyl comparison. Both are Nissan 3.0L EFI engines. Both engines are relatively stock (only changes are those required to fit them into a 510), and both engines are in the same type of car (early 510). Note that they are essentially identical in torque (one more than 8ft-lbs difference) until 4250rpm, where similarities end. The 4v/cyl torque curve extends another 2000rpm! Since horsepower is torque*rpm/5252, increasing the RPM and maintaining the torque means more HP. Look at the HP chart and you see what I mean. If you race there cars and force both cars to shift at 4250rpm, they will be dead even. This also means in normal driving, in traffic, you will feel no difference.

Those of us who like to rev our engines, however, get to feel the added benefit of breathing, and we get to stay in a given gear longer. So where the 2v/cyl guy here has to shift to 2nd at about 5500rpm, the 4v/cyl car stays in first for another 1250rpm. That means for 1250rpm, the car in first gear is handily out-accelerating the guy who had to shift to 2nd. Doesn't seem like much, but in 2nd and 3rd, you're in those gears longer, which means a longer time for the 4v/cyl guy to stay one gear lower, adding to his lead.

Another advantage of a DOHC design is it makes it easier to add variable valve timing. Not all DOHC designs implement this, as it adds cost, but with variable valve timing, the engine can dynamically adjust the intake and exhaust timing to give an even better spread of low RPM and high RPM power. Honda/Acura, BMW, and Nissan/Infiniti are some of the manufacturers that have variable valve timing on some of their DOHC engines.

The drawback of DOHC? Cost (more parts, more money), packaging (the cylinder head is bigger due to all the cams and valves up there), and maintenance (you have to adjust twice as many valves).