Ken's corner #2: Pushrod vs. OHC

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Ken's corner #2: Pushrod vs. OHC

Postby ken.lagarec on Sun Sep 22, 2002 10:58 pm

Welcome back everybody, hope you enjoyed my first (long winded) post on spark plugs. Today's topic is for Johnny and everybody else interested in engine design: what's the difference between a pushrod engine and an overhead cam engine.

The answer is quite simple: they are simply 2 different designs of driving (opening and closing) the intake and exhaust valves.

To complete the answer, I'll explain a little more about engines first. The engine (strictly speaking, not including the intake and exhaust manifolds, etc..) is composed of a block and a head. In the block, there are the cylinders where the pistons go up and down pushing on the crankshaft. The head is composed of the intake and exhaust ports which direct the air/fuel mixture into the cylinders on the intake (or induction) stroke and let the exhaust gases out on the exhaust stroke (on 4 stroke engines, there's induction, compression, power and exhaust strokes). As the pistons move up and down during those strokes, the intake and exhaust valves open up of block off the intake or exhaust ports respectively. The valve opening and closing is determined by one or more cams. Where the cam is located and how it actuates the valves depends on the design.

1. Pushrod engine. This is an old and proven technology in which the cam is located inside the block, next to the crankshaft. As it rotates, the lobes push on a rod (hence pushrod :!: ) that actuates the valves through a rocker. An illustration of this is seen at http://www.keveney.com/otto.html: Image
Although the pushrod design is mostly associated with American cars now, it has been used by most manufacturers at one point or another.

2. Overhead cam (OHC) engine. This is a more modern design in which, as its name implies, the cams are located above the valve ports, inside the head. In this case, the cam lobe can push directly on the valve stem (or through a cylindrical tappet) or it can actuate the valve using a rocker as in a pushrod engine, but without the rod. A single overhead cam (SOHC) has only one cam per bank of cylinders and has lobes for both the intake and exhaust valves. It pretty much always actuates the valves using rockers. A dual overhead cam (DOHC) has a separate cam for the intake and exhaust valves and can push directly on the valve (as in the KA24DE) or using a rocker (as in most Honda DOHC heads or the SR20DE I think).

Image

>>EDIT<<
Here is a scanned image from the FSM of a 1995 240SX showing the 2 cams that act directly on the valves (no rocker involved)

Image. Bigger image here: http://pages.infinit.net/klagarec/240sx/240sxhead.gif
>> end EDIT <<

Advantages/Disadvantages:
1. Weight: pushrod+, ohc-. Although there is the added weight of a pushrod, you can build a V engine with a single cam at the base of the V as opposed to 2 (SOHC) or 4 (DOHC). The pushrods are generally lighter than cams. Also, a pushrod engine has a lower center of gravity than an OHC because of the location of the cams (except for boxer engines - yay Porsche :D ).

2. Compactness: pushrod+, ohc-. Same problem. Cams on top make very talll engines. The BMW 2002 engine is so tall it has to be tilted to fit the engine bay. This also makes boxer (horizontal) engines very wide.

3. Efficiency: ohc+/-, pushrod-/+. Because of the additional inertia of the pushrod, the valves on a pushrod engine start to float (not follow the cam lobe) at much lower RPM than on an OHC engine. It's also possible to build lighter cams in an OHC because they have less weight trying to flex them than with a pushrod. However, in a pushrod engine, the cam gear can be directly rotated by the crankshaft, leading to crisp cam rotation. On an OHC engine, the gears are connected by a belt or chain that has some flex, that can lead to timing inconsistencies at high RPM.

4. Reliability. I'm pretty sure the OHC design has a longer lifetime because of the lesser number and weight of components but this is just my guess.

5. Tuning. dohc+/ohc-/pushrod-. You can more easily change the valve timing by separately adjusting the gear for the intake or exhaust, either by changing the gear altogether or by altering it through a variable valve timing system. I (stress I) have yet to see a pushrod engine with variable valve timing or lift although it is technically feasible. Also, changing a cam in an OHC engine is much easier because the block doesn't have to be pulled.

So which is best? F1 uses DOHC so it's got to be better. Yet somehow, in 1994, it was a Mercedes-Benz built pushrod engine that wone the Indy 500...

P.S. I'll try to get a few images and diagrams scanned at work this week to illustrate things better.

Ken[/url]
Last edited by ken.lagarec on Mon Sep 23, 2002 8:52 pm, edited 2 times in total.
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Postby Nismo on Mon Sep 23, 2002 7:34 pm

cool animated gif... thanks for the good read.
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Postby Black Madgic on Fri Sep 27, 2002 4:00 am

Ken>

The OHV engines tend to have a longer working life. Especially in trucks. One generalization with OHV engines is they are higher torque. The KA24DE and KA24E are exceptions to that rule though. They OHC engines tend to rev easier and much higher, but have less torque so they are found much more in higher performance cars. But at the same time the Corvette still uses pushrods and 2 valves per cylinder....and look at the power they are getting out of that engine!!

I've found the OHC engines generaly need a rebuilt much sooner than OHV engines. I think that this has a lot to do with where the power is in the RPM band as well as the fact most OHC engines operate at higher RPM's most of the time.
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Postby ken.lagarec on Tue Nov 05, 2002 5:20 am

Good one. This I can answer off the top of my head as I've looked into it quite a bit. I'll write something up soon.

Ken
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