"DOHC" has become a term commonly found in advertisements and reviews when referring to vehicle powertrains, usually referring to the powertrain's technological finesse.

But what is "DOHC", anyway? And why does it matter?

First off, DOHC stands for Dual-OverHead Camshaft, meaning that each bank of cylinders has two camshafts controlling the valves. For an inline engine (virtually all 4-cylinders), which has one bank of cylinders lined up, this means 2 camshafts total. For a V-style engine (V6, V8, V10) this means 4 total camshafts, as each head gets their own double camshafts. By having two camshafts per head, each camshaft is dedicated only to the intake valves or the exhaust valves, not both, and because of this, they can be located directly above the valve.

By having the camshaft directly above the valve, it eliminates the need for rocker arms that are required in non-DOHC designs, and thus has fewer moving parts. Because of this, DOHC engines generally can rev higher than other engines as they have less valvetrain inertia, although the actual difference is not tremendously important in non-racing engines (more on this later). DOHC engines lead to easier implementation of multivalve engines, as they don't require rocker arms and other supporting hardware for each valve, thus allowing for easier and more efficient breathing of the engine.

The bad thing about DOHC engines is that they are very large and bulky given their displacement and they are expensive. Having 4 camshafts adds a lot of space in the heads and makes the engine take up much more room than other engines. For example, the Chevy Corvette motor takes up less space than a Cadillac Northstar engine, even though the Corvette's engine displaces six liters and the Caddy only displaces 4.6. Having additional camshafts also makes the engine much pricier and requires the use of a long timing chain or belt.

SOHC (single overhead camshaft) engines are similar to DOHC engines except they have one camshaft per head (so one camshaft total for an inline engine, two total for a V-style engine). This splits the benefits and problems inherent with DOHC designs - they require less space because they have half as many camshafts, but have additional mechanical complexity because it is impossible to locate one camshaft directly above both intake and exhaust valves, thus requiring hardware to translate the camshaft's movement into moving each valve. This additional machinery adds valvetrain inertia. SOHC engines also require a long timing chain or belt.

OHV (overhead valve or pushrod) engines are on the other end of the spectrum from DOHC in that they are the simplest, lightest, and least expensive design used in current vehicles. In these engines, there is always only one camshaft, regardless of configuration. Virtually all modern OHV engines are V-style (with the exception being the Cummins Turbodiesel, which is an inline-six). The camshaft is located directly in the middle of the engine, in the block, in between both banks of cylinders, and actuates the valves with long metal rods called pushrods. This leads to multiple effects:
- OHV engines have simple engine heads, as they do not contain camshafts and only have the rocker arms;
- OHV engine heads are much smaller than other designs, leading to increased space efficiency;
- OHV engines generally cannot rev as high as other designs because the pushrod adds valvetrain inertia;
- OHV engines have timing chains that are generally permanent.

It is much easier to implement cylinder shutoff technology (such as GM's Active Fuel Management, or Chrysler's MDS) as well as variable valve timing in OHV engines. As an example, GM added variable valve timing to their 3.5 liter V6 with little more than a camshaft phasor, which is a simple and inexpensive device, and yielded over a 10% increase in horsepower with no difference in torque or fuel economy. A caveat is that variable valve timing can never be as sophisticated with only a single camshaft because the intake and exhaust timing cannot be changed independantly of each other.

One common issue that is often brought up about OHV engines is that they are unable to rev freely; this isn't true, case in point being GM's 3900 V6, 6.0 "LS2" V8, and 7.0 "LS7" V8. The first two redline at 6000 RPM, which is above what most people are unwilling to drive at; the LS7 redlines at 7000 RPM which is as high as most DOHC designs.

So which is the best? Of course that depends. I personally like the OHV engines that GM produces as they are powerful, efficient, and compact. Honda currently is the main user of SOHC engines and they make great powerplants. Most other producers use DOHC designs and these are good motors too. But here are a few vehicles, and while there are tons of other factors such as transmission options, this can give you an idea:

OHV
GM 6.0 liter V8 "LS2"         400 hp/400 ft-lb   18/28 mpg
GM 7.0 liter V8 "LS7"         505 hp/470 ft-lb   16/26 mpg
GM 3500 V6 "LZ4"            224 hp/220 ft-lb   22/32 mpg
DCX 5.7 liter V8 "Hemi"     340 hp/390 ft-lb    17/25 mpg
Ford 3.0 V6 "Vulcan"         154 hp/180 ft-lb    18/27 mpg

SOHC
Honda 3.0 V6                  244 hp/211 ft-lb      21/30 mpg

DOHC
Toyota 3.0 V6               190 hp/205 ft-lb        20/29 mpg
Toyota 3.5 V6               267 hp/244 ft-lb        21/31 mpg
GM 3.6 V6 "HF"           255 hp/252 ft-lb        20/28 mpg

So, basically, what you might notice is that 1) a V8 engine with 400 horsepower gets better mileage than a V6 with 154 hp and that 2) the particular engine matters much more than how the valves are actuated. Look at the huge difference between the Ford OHV V6 and GM's OHV V6, the GM powerplant is vastly superior in every way. Or even look at Toyota's DOHC engines, the 3 liter unit is vastly inferior to the 3.5 they offer.

So ignore the silly, childish comments in the media about "antiqued pushrod powerplants" and how outdated they are - they are plenty competitive. Base any decision you make on the performance of the powerplant, not how the valves are moved.