Piston to Valve Clearance Explained (Excel Graph)
 

Here's a graph that I generated in Excel:

http://forums.pelicanparts.com/uploa...e-engine28.gif

I modeled it after the 911SC camshaft, and then 'removed' a large chunk of material from the heads in order to simulate a very tight clearance.

The upper graph shows piston travel in blue, exhaust valve travel in pink, and intake travel in yellow. The point where the pistons and the valves are closest is where the distance between the two curves is the smallest. Note that this does not occur at 360 (TDC), but actually at a point a bit after.

The graph below shows the difference between the piston and the exhaust valve (blue), and the intake valve (pink).

As you can see from these two graphs, the piston comes very close to the valve at about 390 degrees. This is why you must check the piston to valve clearances across a wide range of your camshaft motion. Otherwise you may have an interference in the engine.

When you don't time your engine correctly, or you don't keep tension on the chain during a chain tensioner upgrade, your chain can slip, causing the top graph to move to either the left or the right. If it moves to the right, then you will have impact.

On the lower graph, what we care about is the minimum of these two graphs combined. At about 360, we see a dramatic change as the intake valve gets closer to the piston (the exhaust valve is moving away).

Any questions? I think I will add the lower part to my previous diagram.

-Wayne

Wayne,

This is a great way to look at it. Are you going to "parameterize" it and make it available in the tech sessions, if not, you

Re: Piston to Valve Clearance Explained (Excel Graph)
 

I think a simple "allowable angle number" would be more useful to the rebuilder, for this end.
I would say its a given that people can understand that if it's off enough, things *will* hit.
The answer they need is "how far" (each direction.)

The graph modified "to simulate a very tight clearance" will just cause paranoia.

Well, if you're running a high-lift camshaft with something like 'E' pistons (S-cam with E pistons), you will have extremely close tolerances like the ones shown here...

-Wayne

Wayne,
Island 911 may be right.

Putting together a technical book always requires a reassessement of this question:

What needs to be known?
What's nice to know?

It doesn't matter if it's an engine rebuilding book, a cookbook or a computer software manual. The reader doesn't know what he needs to know. You must assume the reader will not know this and thus present information according to those two questions above.

What category does this chart fit into the two questions?

Keep up the good work,
Sherwood

So is it true that the 2.4T is the only engine that is non-interference?

Given that the SC cams have a range of 6&deg of possible adjustment as used from the '76 Carrera 3.0 to the later SCs would an envelope be possible? Perhaps dashed lines of the same color for max advance and max retard.

Darned good question Jeff! My answer is that I don't know. But I'd sure like to hear from those who do.

A son of a friend of mine "borrowed" dad's '72 911T. While "borrowing" it, the right bank cam chain broke. He called me late at night asking me if he had done any damage to it by driving it home.

I found out this way that it was the only "free spin" 911 motor built.

Good luck,
David Duffield

In looking at the 1st graph, you will see what happens when the cam chain slips. The pink and yellow curves will slip to the right with the exhaust line intersecting the blue piston line. Remember when the chain slips, the cam retards.

Good luck,
David Duffield

These graphs are very informative.

http://forums.pelicanparts.com/uploads/bent_valve.jpg

But until you hold one of *these* in your trembling hand, you won't really understand what happens when Mr. Piston meets Mr. Valve.

:eek:

Now wait a minute.....you'd think the broken chain would *not* gracefully fall into the crankcase. But rather, trash the whole timing end of the poor motor.
Drive home? I think not!

:confused:

With all due respects to the intent of the chart, but I think Doug's bent valve pic communicates and illustrates the point very well.

Sherwood Lee
http://members.rennlist.org/911pcars

soooo...

are all 2.4 motors non-interference?

what about alphabet soup 9.9:1 JE S pistons with an E cam in a T case that fantasizes being an RS?

The bent valve needs a shattered cast rocker arm pic to go along with it ...

Alternative horror pics would be a pre-68 forged rocker arm -- unbroken ... along with broken valve and piston with hole punched in the crown, chewed-up cylinder head, and bent rod pics ... to illustrate the reason for using cast rockers!

So is it true to say that on the '72 911T I just purchased which was sold to me with "Engine Idler Problems" would not have had a valve to piston collision even if the tensioner had failed???

Sorry this is OT, but you can see why it is very interesting to me.

If so, then I'll likely have more money to spend at Pelican Parts:D on upgrades after some work to get things running and idler support arms/spacers replaced (pressure fed upgrade done by PO)

I would check out cranes web sight.
They have a very good procedure for checking piston to valve clearance.
http://64.90.9.168/cranecams/pdf/803.pdf
This is a simple process.
Chris

Wow, this stuff is definitely 'NEED TO KNOW' when rebuilding an engine. There is so much information, and this graph explains why you need "Section 8: Checking valve to piston clearance" on just about every single engine you put together!

-Wayne