Why Can't I just.....

[flmont]

Well I'am curious as to why I cant just do the valve to piston measument with just the cam installed ???? do I really need to hook up the chain...I should be able to turn the cam and take my readings should'nt I....and save all the rest of it for my cam timing ...????? TIA Frank

[Tim Walsh]

you need to time the engine because the 911 engine is an interferance engine. If you don't time the engine then you are garanteed not to pass the valve/piston clearance test since the valves and pistons occupy the same space, just not at the same time. You only need to have 2 rockers per side installed though.

[dtw]

If you just put the piston at TDC and turn the cam independently of the piston, you will almost certainly hit the piston. The 911 is an interference design. You must actually time the cam to the crank as indicated, because the relationship of the valve positions to the piston positions is dynamic. Do you have the engine rebuild book? If so look in there, Wayne spent a lot of time putting together a graph that expresses the positions of the pistons and valves as waveforms. It should answer your question.

[flmont]

Ahh Ok ,..Yes I have the book ..I thought it was just a static measurement ....Thanks Frank

[911pcars]

Unless I'm missing something here, you can check valve-to-piston clearance without the chains. Here are a couple of methods:

Find absolute TDC, then mark the pulley as such.

Find some lightweight springs to replace the regular valve springs for no. 1 cylinder; just enough to keep the valves closed. Rotate cam to TDC/power stroke (cam lobe at heel); Install rockers for no. 1 cylinder; adjust valve clearance to zero. Position no. 1 piston at TDC.

With a dial indicator at the rocker tip or valve retainer, push down on the rocker to open the valve until it touches the piston. Measure the travel. Repeat on the exhaust valve. Record these as measurements, 1-In, 1-Ex.

With the piston at BDC (out of the way), rotate the cam by hand, measure and record the max. valve lift as 2-In, 2-Ex.

Your intake valve-to-piston clearance is 1-In minus 2-In. Same calc for the exhaust side. Repeat this for no. 4 (other bank). I think that should work.

There are other ways too (modeling clay or soft solder on no. 1 piston, rotate through cycle, measure thickness from valve depression).

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

[Tim Walsh]

Sherwood,
I don't get what you're trying to say since the closest valve/piston clearance isn't at TDC. It's actually somewhere near it but not at it. The piston actually chases the intake valve back "home" (retracted) and then the exhaust valve chases the piston back down cylinder. Because of this and the fact that near TDC the piston moves very slowly it's not AT TDC but somewhere before or after do you have the least piston/valve clearance.

The relationship of the valve to the piston cannot be reasonably determined without timing the engine. Again, the waveform chart in the rebuild book clearly illustrates this relationship and also proves why this is a dynamic test that must be done on a timed engine

[911pcars]

It's difficult to visualize crankshaft rotation numbers (that's what cam timing refers to) to where the piston is in the cylinder, but at TDC, the piston moves relatively slowly compared to what the crankshaft is doing. So, while maximum lift of the valve may occcur within several crankshaft rotation degrees on either side of TDC, the piston is right there, at or very near TDC.

So the question is: If the piston is at maximum extension at the top of the cylinder and the valve is at maximum lift into the chamber, are these parts going to get even closer together elsewhere during the 4-stroke cycle? I don't think so, but I may be wrong.

Sherwood

[Tim Walsh]

hmm.. In your scenario then if your maximum lift of the valve happens when the piston is near the top of it's stroke and they have enough clearance that would imply that the engine isn't an interferance engine. But if it isn't an interferance engine then why do we bend valves and break rockers when we loose timing?

I must be missing something here but I'm not seeing it.

[jgparker]

Sherwood,

From my own measuremets, I can say that the minimum valve clearance is at about 30 degree BTDC overlap for the exhaust, and 30 degrees ATDC overlap for the intake valve. The graph in the rebuild book is great, and shows the spatial relationship very clearly (Figure 8-2 on page 172, and it's in the bonus CD). I'd post it, but that would be a copyright violation.

JP

[911pcars]

Ahh. Okay guys. I just slapped forehead with open palm. Thanks for the clarification and reminder. Sorry if I contributed any confusion.

Short (correct) answer: Install the timing chain(s)

Sherwood