I'm not an expert on engine cycles or camshafts, so bear with me...

In the compression stroke, when a given cylinder is at right at TDC, both valves should be completely closed, right? Now, assuming that we're at TDC in the compression stroke, if the crank were to be rotated in either direction from TDC, should one valve open *immediately* after the crank is off of TDC, or is there some range of the crank rotation where both valves will still remain closed? In other words, are both the exhast and intake valves closed only exactly at the instant when a pistion is at TDC, or are they both also closed a little before and/or a little after TDC?

The reason that I ask is that I'm running a leak-down test on my L98 engine with a (supposedly) stock cam, and I just-so-happen to have the intake off while I'm doing it. One of the things that I'm noticing looking into the lifter valley while the crank is being rotated is that, as I approach the TDC for a given cylinder, a valve for that cylinder is closing. Also, it would seem that if I rotate the crank past TDC, the other valve opens rather quickly after the other one has closed.

Anyway, one of the things that I'm trying to figure out doing this leak-down test is if my cam is degreed-in properly. When I built this engine and put the (stock) timing chain on, I simply used the "dot-to-dot" method; at the time, I knew nothing about "degreeing-in" a cam (the cam didn't even come with a degree-card). So, basically what I'm trying to figure out now is if my cam is, in fact, degreed-in (roughly) as it should be without actually degreeing it in (because I don't have the tools).

If my understanding is correct, then my cam should be properly degreed-in now (which is what I want to make sure of)...

 

Your first statement is correct, during the compression stroke both the intake and exhaust valves will be closed at TDC and quite a few degrees before & after TDC.

Consider that many degrees before TDC the engine has closed the intake valve to build cylinder pressure and several degrees after TDC combustion has occurred and pressure is driving the piston down, so the exhaust valve is kept closed until the piston has travelled roughly halfway down the bore.

What you are likely witnessing when you see the lifters rising and falling close to TDC is the cycle of the camshaft 180 (camshaft) degrees after the compression stroke and this is actually the 'overlap' phase where the exhaust valve is closing as the piston approaches TDC and the intake valve is starting to open at the same time.

Keep in mind that the engine's 4 cycles take 720 crankshaft degrees to complete, and that TDC could be between the compression and combustion stroke, or between the exhaust and intake stroke.

Hope I've interpreted your question properly here??

 

About that stock timing chain...

If you're concerned with degreeing an aftermarket camshaft, steer clear of the OEM timing set and go with a good quality aftermarket set. The OEM timing sets very often retard the camshaft timing a few degrees - in case you haven't heard this one before.

 

TDC occurs TWICE for each cylinder in each engine cycle.

If you follow the sequence of a cylinder's events starting at the firing point, the piston travels downward for ½ of a crank revolution with both valves closed. This is the "power" stroke, during which time the heat energy released during combustion is turned into mechanical energy. Some time shortly before the piston reaches bottom dead center, the exhaust valve begines to open. The crank rotates another 180°, ½ revolution, with it open; this is the "exhaust" stroke. As the piston nears TDC again after one full crank revolution, the exhaust valve begins to close, and the intake valve begins to open. As the piston passes through TDC this time, those 2 things are usually happening at the same time (the valves are "rocking"). The crank continues to turn, having now completed one full revolution; the intake valve is open while the piston once again goes down to the bottom. This is the "intake" stroke. Sometime around when it reaches the bottom, the intake valve closes. The piston begins to move up again, compresing the mixture in the cylinder. This is called the "compression" stroke. As the piston reaches TDC this second time, 2 full crank revolutions later, it fires again.

Dot-to-dot is the "other" instance of #1 TDC besides its firing. It is #1 exhaust stroke ending and intake stroke beginning, and #6 firing. You can't run a leak down test on #1 with the engine at that point, since both valves are open at that time; you need to turn the crank one full revolution from there, then you'll be at #1 firing. Both dots at 12:00 is #1 firing. From there, rotate it exactly 90°, and you'll be at #8 firing; then 90° more, and you'll be at #4 firing; and so on through the firing order. When you have completed all 8 cylinders, the crank will have rotated through 2 complete revolutions, the cam will have gone through exactly one complete revolution, and the dots will both be at 12:00 again.

 

A leak down test will not tell you anything about wether the cam is installed properly.

You really need to stay away from cars... hee hee hee

As the piston nears TDC on the overlap stroke the exhaust valve is closing and the intake valve is just opening.

You can aproximately check the proper positioning of the cam by finding the point of Split Overlap. Or the point where both
lifters are raised equally during the overlap stroke.

 

Here is some very basic 4 cycle engine info:
http://www.howstuffworks.com/engine1.htm

 

Thanks for the very thorough answers. The main thing that I wanted to know for sake of the leak-down test was if achieving cylinder compression (i.e. both valves are completely closed) was a matter of finding a single point on the degree wheel or if a matter of getting within a range of degrees on the wheel. I can see now that there is some overlap that I can work in.