explaining the concept of pushrod engine today.....

[lemming]

.....to someone.

i would not have been able to do this well about 5 years ago, but i whipped out a piece of paper and drew it.

"cam in block" helped to explain the concept.

it's funny how almost anyone can understand the OHC concept but the pushrod concept if much more difficult to grasp intuitively. anyone else ever have to explain this?

[rumatt]

Care to try again? I'm listening. :eeps:

[blee]

So the crankshaft drives the pistons up and down in their cylinders, as you know. In a pushrod engine, the crank also drives a single central camshaft.

Think of your typical V-8. The cylinders are angled, making the "V." The camshaft is typically placed right inside the V. That's nice and efficient from a packaging standpoint, but there's an issue. The valves are located in the heads, which sit on top of the cylinders. So they're sitting on the "legs" of the V, while the cam is sitting in between them. How does the camshaft get the valves to move?

The lobes of the camshaft are in contact with pushrods. Pushrods are literally metal rods. When the cam turns, the pushrods move up and down. The top end of the rods are in contact with rockers, which are just levers. So when the pushrod moves up, the other end of the rocker moves down. Now the valves are connected to the rockers. See how it works?

here's a great little cartoon of a pushrod engine:

http://auto.howstuffworks.com/camshaft5.htm

[rumatt]

Great explaination and great site.

Now the question: Why is pushrod exciting? Looks like pretty outdated, and limiting technology. (looks like the kind of hack I would come up with, actually. :eeps: )

[FC]

Great explaination and great site.

Now the question: Why is pushrod exciting? Looks like pretty outdated, and limiting technology. (looks like the kind of hack I would come up with, actually. :eeps: )

It is great packaging. You no longer need all this stuff atop the "legs" of the V. Instead, you take up the "void space" in between the legs. It takes a lot of weight of the top of the engine, lowering the cg.

[JST]

Great explaination and great site.

Now the question: Why is pushrod exciting? Looks like pretty outdated, and limiting technology. (looks like the kind of hack I would come up with, actually. :eeps: )

It is great packaging. You no longer need all this stuff atop the "legs" of the V. Instead, you take up the "void space" in between the legs. It takes a lot of weight of the top of the engine, lowering the cg.

Why hasn't anyone brought back the L-head?

:)

[Nick M3]

Great explaination and great site.

Now the question: Why is pushrod exciting? Looks like pretty outdated, and limiting technology. (looks like the kind of hack I would come up with, actually. :eeps: )

It is great packaging. You no longer need all this stuff atop the "legs" of the V. Instead, you take up the "void space" in between the legs. It takes a lot of weight of the top of the engine, lowering the cg.

Because a 7.4L pushrod V8 is smaller externally than that 3.0L I6 you've got in your 330i.

[clyde]

anyone else here actually owned a car with an L head besides me?

[Nick M3]

How about WTF is an L head? :p

[clyde]

Great explaination and great site.

Now the question: Why is pushrod exciting? Looks like pretty outdated, and limiting technology. (looks like the kind of hack I would come up with, actually. :eeps: )

It is great packaging. You no longer need all this stuff atop the "legs" of the V. Instead, you take up the "void space" in between the legs. It takes a lot of weight of the top of the engine, lowering the cg.

Because a 7.4L pushrod V8 is smaller externally than that 3.0L I6 you've got in your 330i.

The advantages are that you can build more power from less weight with less complexity (as odd as that might sound to you at the moment) while putting it in a smaller box (external dimensions).

Also, the more "ooh and ahh" the technology might have (multiple overhead cams, Vanos, VTEC, Valvetronic, blah, blah, blah) the less benefit they provide in boosting output as displacement grows. There's a strong application of diminishing returns in play here. They will still boost output in larger displacement engines, but they do so at a much higher cost and with greater weight and complexity than just upping displacement a little more to achieve the same output.

[John V]

L-head = flathead. The valves are in the block.

JV

[blee]

Flatheads are cool, but I've never owned one. If I ever get around to buying that '32 Ford roadster, I'd be very tempted to stay traditional and put in a hopped-up nailhead.

[Pinecone]

The BIG problem with L or flat heads is the inability to get high compression ratios.

[FC]

The BIG problem with L or flat heads is the inability to get high compression ratios.

I was actually wondering that, and also how much of a pain a valve job would be.

[lemming]

american le mans series is good evidence that pushrods are still viable for not only top speed but also endurance racing (as the car also race @ LeMans).

it will be curious, however, to see if how competitive the C6Rs will be at LeMans compared to the new Maseratis and AstonMartins on/at LaSarthe where top speed comes into play for hours at a time.