OHV vs OHC

Today I thought I would write about OHV vs OHC.  While it should probably be pushrods vs OHC instead, I often hear reference to OHV instead so I thought OHV vs OHC was a better fit.

Most muscle cars use a camshaft in the block and push rods to actuate the valves.  The exceptions would be the Mustang from 1996 on with the 4.6 SOCH motor and the DOHC 5.0.  Also if you subscribe to my theory that a four door can be a muscle car, the Mercury Marauder.  There is also the Ford 427 Cammer, but it was never in a production car.

Both overhead cam and having the cam in the block date back to the early days of internal combustion engines, despite the fact that OHC is perceived as being more modern.

There are advantages to both designs.  The main advantage to the cam in block design is that it is more compact, especially on a V engine.  This is due to the fact that there are less parts on the head of the engine so it is narrower.

The advantages of OHC is that the parts of the valve train are lighter, and the parts are shorter so there is less flexing.  The makes the valve train more stable at higher RPM.

So in the choice between OHV vs OHC, the answer to which one is best would be it depends.  Since most muscle cars came with push rod engines, that choice is easy.  From a technical stand point, OHC has the advantage as long as there is enough room.

The advantages of OHC come from the fact that there is a shorter path from the cam(s) to the valves.  With a push rod engine, there are lifters that ride on the camshaft that get pushed up by the cam.  From there the motion is transferred to the push rods which then move the rockers that move the valves.  That is a lot of parts to move.  That adds more places for things to flex, specially the push rods.  There is also a lot of mass involved for the moving parts.  All that mass needs to be pushed back down by the valve spring once the cam is no longer pushing on the lifter.  With the higher spring pressures needed to push all that back down increases the friction.

With an OHC engine, the cam is already on the head by the valves.  So all that is needed to operate the valves would be be either a small adjuster between the valve and cam, or a rocker assembly.  Due to parts expanding as they heat up and wear issues, it wouldn’t work well or for long to have the cam push directly on the valve itself.

That is definitely less parts to move, and with the less mass, there is less spring pressure needed by the valve spring so there is less friction.  There is also less flexing.  Parts flexing in the valve train would cause the valves to be open for less time and not open as far since part of the motion of the cam is used by parts flexing.

One thing to keep in mind is that with newer push rod engines, the heads are designed so that less lift is needed from the camshaft so there is less flexing to deal with.  Plus with computer modeling manufacturers can design the parts and angles to minimize flexing.

For a muscle car, there are advantages to both, but since most muscle cars were manufactured with push rod engines, there isn’t much debate.

When a manufacturer needs to choose OHV vs OHC, they have to look at the size of the engine, the manufacturing costs and marketing perception.

Many consumers view push rod engines as old tech and therefor not as good as OHC.  While OHC does have benefits, with newer designs the differences in performance are small, small enough that most people wouldn’t notice.

What that means is that if you decide to put a 32V 4.6 or 5.0 in your muscle car or not you will still have a muscle car and the gains for going either way depend on what you want.

The advantages of swapping an OHC engine into your muscle car are the same as swapping any modern engine.  Better gas mileage and driveability.  And as long as you pick an engine with some aftermarket support, there is no loss of performance either.

 


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