For years, carmakers have been in the habit of putting some of the technical innovations they’re proud of on the back of their cars via badges. This practice probably peaked in the ‘80s, but it’s still around, to some degree. Today I want to look at a badge that was once quite common to see on car buttocks: a series of letters that read DOHC.
I know for a great percentage of our readership, this is like asking what your own nostrils are for, but let’s not forget that the world isn’t just populated by car nerds like us. So, for all those people, here’s what DOHC means, and it’s a pretty technical thing to wear on a car badge, if you think about it: Dual OverHead Camshafts.
Okay, so to understand why dual overhead camshafts are worthy of being immortalized in chrome-coated plastic on the back of your car for the world to see, we need to break down just what this all means.
Those letters are standing for two modifiers and one (plural) noun, so let’s start with the noun that’s getting modified: camshafts.
An engine’s camshaft looks like this:
The job of a camshaft is to make the valves on an engine’s cylinder head open and close at the right times. Each cylinder has valves for letting fuel/air mixtures in, and letting spent waste gases out. The timing of when these valves (which resemble golf tees sitting in holes) open and close is very important, and that timing is contained in the camshaft, which is, essentially, a physical, rotating record of motion.
Seen in cross-section, each of those lumps on the camshaft, called lobes, is designed to push on another part to cause the valve to open or close. When the pointy part of the lobe swings around, it opens the valve, and when the lobe swings past, the valve gets closed by a spring.
The shape of the lobe is a record of the linear motion of the action, wrapped around itself into a loop, if that makes sense. This illustration may help:
Get the idea? Good. There’s, of course, much more deep one could dive into pretty much any part of this, but that should be good enough for what we’re doing here.
Now that we know we have a camshaft to control valves, I should admit to you that you can’t always move the valves directly off the camshaft as you see in that animation. That’s where we get into what overhead cams (OHC) are about.
On engines that do not use overhead camshafts, the camshaft is close to the crankshaft, in the lower part of the engine, often right inside the crankcase, driven right off the main crankshaft, often through a timing chain. These sorts of engines are called pushrod engines, because they transmit the motion of the camshaft to the valves via a rod that pushes, which we call a pushrod, because of it’s inventor, Nathaniel T. Pushrod.
No, I’m kidding. You know why it’s called that.
While engines with overhead camshafts have been around since at least 1903, it wasn’t all that common until the 1960s, at least for volume-production cars.
Cars with pushrod engines do have some advantages over overhead-cam engines: They tend to be cheaper to produce, and they can be designed to be shorter packages, since the camshaft is contained within the engine block (or between the cylinder banks in V-type engines), yielding a lower center of gravity.
They also have some significant disadvantages: Pushrods can get long and somewhat substantial, so they have a fair amount of mass, which means that they carry a lot of inertia, which means that at very high revolutions, it’s harder to get them to quickly stop and change direction, which can lead to a condition known as valve float.
In the case of valve float, the engine is going so fast that the inertia of those pushrods, the lifters that sit between the pushrods and the cam, as well as the rocker arms (which turn the pushrod’s motion into valve lift) can cause the valves to remain open long after the nose (the “pointy part”) of the cam lobe has swung around. This can reduce engine power and efficiency.
So, if we want higher-revving engines, we need to get rid of all that inertia that’s being stored in those pushrods and get the camshaft way closer to the valves. That means sticking it on top of the engine, or if you were living in the oil pan, that cam would be overhead.
So, a big advantage of having the camshaft overhead is that you eliminate the pushrods and all their inertia, and can make an engine that can rev much higher. That’s great!
Sure, there’s drawbacks, like now your engine will be taller to accommodate all that extra hardware on top (or wider, if it’s a horizontal engine), and it’s all more complex (with a giant timing chain or belt or gears), expensive, and, arguably, delicate, at least compared to most pushrod engines.
Once you move that camshaft up top, though, the mind starts to race: Is there more that can be done? We know that the bigger the valve area you have, the better the engine can breathe in (fuel and air) and out (exhaust gases), which helps the engine make more power and be more efficient.
A regular pushrod engine can really only actuate two valves per cylinder (well, Cummins sort of cheats with a Y-shaped rocker arm for their three-valve-per-cylinder pushrod B-Series engines) and there’s limits to how big you can make those two valves—so what if you had more valves per cylinder, for more valve area?
That would be better, and if you have two (or, you know, dual) overhead camshafts, you can do that rather easily. I mean, technically with one overhead camshaft, you can have three, but for four (or more) valves, you really want two.
Here’s a little explainer of why four valves are better than two:
So, if you have a pair of camshafts on top of your engine, one for the intake valves, and one for the exhaust, you can use those camshafts to actuate those valves directly (well, essentially; there will be a shim in between), with no pushrods or rocker arms, and you can have twice as many valves per cylinder.
Sure, there’s some added complexity with a whole second camshaft, but, in some ways, it’s a pretty small leap from one overhead cam to two, and you get the huge advantage of easily doubling your valves. Plus, with that extra cam, you’re now able to independently change intake and exhaust valve timing using a Variable Valve Timing system to optimize power and efficiency. So why not?
All of this multi-valve technology started trickling into major manufacturer lineups in the ‘70s and rushing into the mainstream in the 1980s and into the ‘90s. As it did, carmakers rushed to advertise their developments in making more power from smaller engines. That explains the DOHC badges, along with a host of other Twin Cam advertisements when they could manage.
So, back to our original question: What’s such a big deal about DOHC that carmakers stick the letters on badges and slap them on your car?
The big deal is twofold: first, the overhead-ness of the camshafts mean that the engine can potentially rev higher thanks to less valvetrain inertia, and the dual-ness means that each cylinder can easily have more than two valves (yielding better intake and exhaust flow) and the intake and exhaust valves can be controlled independently using variable valve timing. These things combined can make a dual overhead cam engine more attractive than a pushrod or even a single overhead camshaft design.
So, that’s worthy of telling everyone behind you about, I think.