Is this new diagram for the book clear? (2nd Version of Image)

[911pcars]

I guess my observation was about an hour late; somebody else got there earlier.

I wrote:
"It might suffice to say (or add a caption to the chart) that the piston is closest to the valves during the overlap period (end of exhaust, beginning of intake) and piston is at TDC."

and Wayne responded:
" ..... that's not correct! As the graph shows, at TDC, the valves may not be closest to the pistons. It's on the upward stroke that they are closest (of course, it depends upon the camshaft). That's the point I'm trying to make here - that the TDC point is *NOT* where you have to worry about interference! "

Wayne,
I'm trying to understand what you're saying. If the piston is indeed on it's upward stroke (not near TDC), then there is no way BOTH valves will be closest to the piston. The only time BOTH valves are open is during the overlap period as the ex. valve is closing and the intake valve is beginning to open.

If you're referring to one valve. becoming closer, i.e. the exhaust valve, yes, as the chart points out, the exh. valve-to-piston clearance narrows, but is especially critical as the piston approaches TDC. Likewise, during the intake stroke, the in. valve begins to open while the piston is moving up the cylinder.

The further away from TDC, the greater the valve-to-piston clearance you will have, and conversely the valve-to-piston clearance decreases the closer the piston is to TDC. With difference camshaft profiles this could be different, but in general terms that's what happens. A cutaway of a Porsche cylinder going through a 4-stroke cycle would tell us exactly what's going on but I suspect it's like most other 4-stroke cycle engines. We might be splitting hairs with the number of crank degrees vs actual piston location in the cylinder. The crank rotates quite a bit while the piston is around TDC.

Sherwood

[David Ceruti]

Some comments (its early in the AM so forgive me if these are a little incoherent)

The comments on scale (to show that the piston travelled farther than the valves) made sense to me when I read it but now on version 2 there is too much dead space at the bottom of the graph

The clearance graphic doesn't make that much sense to me. It isn't clear from the graph why you have put it where you have. Maybe you could add a third line to show piston to valve clearance as it varies with the degrees of rotation

You could also add small diagrams showing the piston and valve in a cylinder at critical points - this may be overkill for your target audience though

And my two cents worth about the valves on top or bottom discussion - they should be on the side. Isn't the book about Porsches - horizontal engines - LOL

[Zeke]

Originally posted by Wayne at Pelican Parts


Odd, because I think of it backwards from how you guys think of it. When you're assembling the engine, the pistons are at the top, and the valves are at the bottom. If you stand next to the engine, and look down at it, the pistons are located above the valves...

-Wayne

O, and it's upside down.

Works either way. What's hard to visulize is the piston moving sideways along the bottom (rotation) axis on the graph. I can't do graphics on the 'puter like you guys, but could you use a bar graph with the valve graphs extending down from the top axis on the chart? In three colors it would show the overlap and the potential interference as the bars came to almost meet in the center. Rotation still on horizontal axis and travel on vertical axis. I know this sounds like the same thing as what you already have, but I'm seeing the horizontal axis extended and the individual bars topped with lots of little pistons and valves showing their relationship at each piont much like individual frames from a movie. Wish I could show you.

[A Quiet Boom]

Second version is much better but, the degree markings could be confusing to those less knowledgeable. The current diagram has crankshaft degrees at both the bottom and top, I think it would be better to place camshaft degrees along the top near the valves to give folks a better understanding of cam timing. This would also help folks understand how retarding or advancing a cam could cause piston to valve clearance issues. As an aside this graph could also be used to describe how valve float from an over rev condition can cause the valves to hit the pistons.

[ChrisBennet]

I think you're d**m lucky to get any graph at all sonny. In my day...
-Chris

[Wayne 962]

The suggestions about camshaft degrees at the top is a good one. Also good may be a third line showing piston to valve clearance.

Sherwood, the point where the piston is closest to one of the valves is not at TDC. It's actually before TDC, because the valve is still open quite a bit. Contrary to popular belief, when you over-rev and your valves hit the pistons, it's not happening at TDC. At TDC, there is quite a bit of clearance, as the valves are almost completely closed. It's when the piston is coming up towards the valves, and they are still open where the clearance problem is. I have a whole section in the book dedicated to checking this...

-Wayne

[michel richard]

This suggestion will overlap some of the others made, but perhaps having 2 of the clearance arrows: 1) clearance is smallest here 2) clearance is greatest here, or 2) This is clearance at TDC, etc . . .
Look forward to buying the book.
Michel Richard

[island911]

if you want show over rev, when the valves float, do something like. . .

. . .of course this is *if* the valves go thru the pistons

[old_skul]

Good idea, Island.

Wayne, I'd include all four strokes of the engine.

[arcsine]

Wayne,
How about something like this?

[Wayne 962]

That's not bad Gary, but it portrays the wrong message. The distance at TDC is *not* what you need to worry about...

Hmm, let me think about this some more...

It's not an easy thing to convey...

-Wayne

[island911]

I really like Garys choice of colors for the valve paths, and the addition of closed valves at the end points. . . .I don't think he was too focused on the accuracy of the sinewaves.

[arcsine]

OK then, to de-emphasize TDC clearance which is not the issue, would you want to empahsize max intake lift clearance or some generic timing as shown here?

[David Ceruti]

Small point - you may want to caption it MINIMUM valve clearance