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Shard Parts

I'm all for shared parts and, certainly, in my repair shop shared parts really helps me out, because, if I put a part into inventory and it could conceivably fit more than one car or more than one engine, that really helps me out. If everything was unique, boy, you'd have to have three times the inventory that you have today to repair cars.

Here's probably the classic example of all-time of an engine that shares a lot of parts with another engine. This is a 4.3 GM V6 engine and this engine shares a lot of its valve train components with a 5-litre and a 5.7-litre V8 engine. Matter of fact, when they made this 4.3, it was essentially a 5.7 V8 with two cylinders lopped off the block and shortened up to a V6. The same lifters, push-rods, rocker arms, valve springs, valves, same front timing cover, timing chain and sprockets, crank-seal, were all the same on the V6 as they were on the V8. When we talk about engine layout, the push-rods are one area and the cam layout of the engine is one key difference in the engines in many of today's vehicles.

So let's have a look at this push-rod V6 and show you the key differences between this and an overhead cam engine. Our cutaway engine is a push-rod engine and, when we look down inside the heart of this engine, we can see the valve train layout and this is where the key difference is between a push-rod and an overhead cam engine. On this push-rod engine, when I turn the crank, you can see the camshaft turning down in there and you can see one lobe of the cam coming by and actuating this valve lifter. Here's the push-rods in the engine and this is how we get the name, push-rod engine. The push rods actuate the valves via these rocker arms, and, as their name implies, the rocker arms rock back and forth to actuate the valves. This makes for a rather compact engine design and a rather compact cylinder head because the camshaft is down in the block and not up in the head.

Over here, we've got a cylinder head off a single overhead cam engine. This happens to be a 2.2 litre Chrysler four-cylinder and, as the name implies, single overhead cam, you can see a single overhead camshaft right here.

There's the cam lobes that are operating the valves via these rocker arms. The lobe comes around, presses the rocker arm down, presses the valve spring and valve down to open or actuate the valve. On the single overhead cam engine, all the valves are in line, as you can see. And when I tilt the cylinder head up, we can see in the combustion chambers, this is a two-valve-per-cylinder design, we can see an intake valve that's open right here. The cam lobe was pressing this valve open.If we go to the next combustion chamber, we can see an exhaust valve that's off its seat right here.

I'll lay this head back down and we'll go to the next one. This is a double overhead cam head off a Ford Mystique or Contour and, as the name implies here, the camshafts are dual. We've got one cam here operating all our exhaust valves and one cam operating our intake valves. Again, camshafts are in the head. Makes for a little bit bulkier cylinder head.

When I tilt this one up, we can see that it's a four-valve-per-cylinder design. There's our two exhaust valves and two intake valves per combustion chamber. That hole is for the spark plug right there. Just to summarize, the push-rod engine is great for a big displacement low RPM engine, very low maintenance, so it works well in full-size cars, pickups and vans where the RPM is typically low. The overhead cam engines are good for small engines that have to turn high RPM to make their power. The double overhead cam, even higher RPM.

When you talk about racing, for example, NASCAR Winston Cup cars are all push-rod V8s. They're limited to about 9,000 RPM, even when using some rather expensive light-weight valve-train components 9.000 RPM is roughly their limit. When you talk about Indy cars, they're all double overhead cam engines and they turn 13,000 or 14,000 RPM. Formula One car is 16,000 or 17,000 RPM and some of those double overhead cam engines. They're talking about next year having engines as high as 20,000 RPM. It's hard to believe.

But, in any case, the push-rod engine, because of all that reciprocating mass in the valve train can't turn really high RPM. It's got to be bigger displacement to make its power. The double overhead cam engines, smaller displacement, higher RPM, different type of a power bend. I'd like to thank the guys at Oakville Automotive, Bob and Frank, for loaning me these two cylinder heads today.

'Til next week, I'm Bill Gardener for Motoring 2002.


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