cam timing
 

This is where Wayne's book leaves a lot to be desired (and I can not understate this!!!!)

Trying to set timing but not getting anywhere (well that is an understatement as a friend is helping but, not sure if I'm getting it).

Any and all help is appreciated!

Big picture:

You need to have the cams with the dot up and the crank at Z1 to start with. You measure the intake valve opening at Z1 on the overlap stroke. The valves are closed at the top of the compression stoke and both slightly open when the crank is turned 360 degrees at the top of the exhaust stroke. The amout the intake valve is open at this point is what you are measureing to check the timing of the cams. You then need to adjust the relationship of the cam to the crank so that the valve is open the correct amount. If you keep this overview in mind then the instructions may make more sense.

-Andy

Mark,
Andy has provided a great overview regarding cam timing.

Let's review exactly what you are trying to accomplish.

You want the intake valve to open as the piston has reached the top and has begun to move towards the crankshaft, some might call it down.
With a Porsche motor it is toward the center of the motor.

Typically the valve begins to open anywhere from TDC to as much as 30 degrees before TDC.

This intake stroke begins at a point called "overlap" or "crossover"

The piston has just reached the top of the cylinder, completing the exhaust stroke. At this time the exhaust valve has just closed and the intake valve has begun to open.

This position is referred to as Z1 TDC overlap or crossover. This is not the same as Z1 TDC compression.

At the Z1 TDC overlap position the intake valve is just beginning to open.

This provides us with a measurement we can read with a dial indicator of just how much the cam has opened.

Typically it is at the beginning, just beginning to run up the lobe of the cam.

Depending upon the specific cam, the number you are looking for can be anywhere from 1.0mm for a cam that has only opened 1.0mm at this point to as much as 4mm or 5mm for a cam that has opened several degrees before Z1 TDC overlap.

Cam specifications are given in crankshaft degrees relating to Z1 TDC overlap.
The trick is the European cams are spec'd at 1.0mm open.
That is the valve has already moved 1.0mm (0.0040").

So a cam that is spec'd at 4 degrees BTDC has really begun to move around 24 to 26 degrees BTDC, but it hasn't reached 1.0mm open until 4 degrees BTDC.

Remember 1.0mm is only 1/25th of an inch.

So you set the dial indicator on top of the flat portion of the spring retainer when the rocker is riding on the back, (base circle), of the cam.

As you rotate the engine (clockwise only) you want the lobe controlling the intake rocker to begin to move the rocker as the motor is reaching Z1 TDC overlap.

At Z1 TDC overlap you want the dial indicator to read 1.3mm, 1.5mm, whatever the spec is for your cam.

If the dial reads low at Z1 TDC overlap you need to advance the cam.

If the dial reads high at Z1 TDC overlap you need to retard the cam.

If you are working on cylinder one, The "dots" or the "numbers" or the keyway will be pointing down on the leftside cam.

When both sides are done, both cams will point down at Z1 TDC overlap for number one cylinder.

Conversely when you are at Z1 TDC overlap for number four, the right side, the "dots" or "numbers" or keyways will both be pointing up.

This is where you want to leave the motor, because this is also Z1 TDC compression for number one. So when you install the distributor, have the rotor pointing at number one, about 3 o-clock.

This is just one way of describing cam timing.

Do a search on "cam timing"

There are many people that have described this procedure.

There may be one that makes it all clear.

Once you have done it, you'll wonder why it seemed so complex. It isn't.
You are just timing the camshaft to the crankshaft.
Have fun,

I've started at the Z1 and 930 cam dot and know that I do one full 360 rotation and there is a point where I need to stop, pull the pin, finish the rotation and put the pin in. This is where I'm not sure of the exact measurement. I used 1.45mm before to top measurement on the dial indicator but I believe that is for an early sc cam. I don't know what I should be using for a later cam or if I'm even thinking clearly.

Hey Doug and Andy, that was great!

Here is a link that helped me:

http://forums.pelicanparts.com/show...ighlight=Timing

That link only went to the main rebuilding page. Would you mind sharing it again or cut and paste?

Let me try that again:

http://forums.pelicanparts.com/showthread.php?s=&threadid=97342&highlight=Cam+Tim ing

It took me to the thread where I imbedded the reference, not the reference place that I took the above from... I'm confused, but scroll down the thread and you will see where I posted the reference, click on that and you will get there.

The number you need to use depends on the cam you are using and the model year of engine. My data from Elgin indicates the following:

930 turbo cams for 1978-1981 .65mm-.80mm,
SC cams had 3 different settings thoughout the years even though the cams are the same. .9mm-.11mm, 1.1mm-1.4mm, or 1.4mm-1.7mm.

Tell us the year motor, type of cam and port size and maybe we can give more guidance.

-Andy

Stock sc, 1982. Although, wayne mentions that advancing the timing can give power increase. He does not indicate how much. Go figure.

Engine is a stock top-end rebuild.

The US spec 82 SC should have 34mm ports. I'd use the more advanced settings with that engine. 1.4mm-1.7mm. Personnally I'd shoot for 1.4-1.5mm and call it good.

-Andy

I think Waynes book does a more than adequate job of explaning this rather simple process.

Cyl 1TDC, Dots up. Measure timing at TDC. Adjust to meet spec. TDC Cyl 4, Measure timing, adjust, done.

Yes that's somewhat oversimplified but don't overcomplicate this simple process and intimidate yourself.

Eagledriver,

Are you saying that I should stop at 1.4 to 1.5 before reaching the top measurement on cam travel? (in otherwords, where I started from and on the second 360 turn of the crank)

I'm not sure I understand what you are asking. I suggested a setting of 1.45mm plus or minus .05mm for your cam timing. Your intake valve should be open this amount when the crank is at TDC on the overlap stroke.

If you are reaching this value with the crank somewhere else then you have a different cam timing.

-Andy

What I'm asking is that I am rotating the crank 360 degrees then turning for the second 360 but stoping the 1.45 short of valve closing on the second go around, right??

Ok reading back through this thread I think I see what you're talking about. First of all there is nothing about valve closing in the procedure. As you approach TDC on the overlap the valve will begin to open. You want it to open 1.45mm as the crank hits TDC. What you can do is stop when the valve opens to 1.45mm and take the pin out then turn the crank to TDC and put the pin back in. Yes this will work in theory. The problem is that the cam will try to turn when you do this because the rocker will be pushing the cam backwards. So you need to do something about that. One thing you could do is just before you loosen the bolt and pull the pin you could back off the rocker adjuster so the rocker is loose and not pushing on the cam. Of course you'll have to re adjust the rocker after putting the bolt back in and rechecking everything.

Hope this helps. I think we might finally be on the same page.

-Andy

I've adjusted the rocker to the stock clearance. So on the second crank rotation, once the valve is closed, look for the indicator to drop 1.45mm? This should occur before Z1. When I was doing it with the help of a friend, we got what we thought was right on the timing when the pully was about 2/3 the way around on the second crank rotation. Were we off or were our thoughts in the right area??

It sounds like your timing is way off (about 120 degrees). This should be very noticable just looking at the dots or flats or whatever you have on the cams to show timing. Note that 120 degrees off on the crank will be 60 degrees off on the cam. So if you turn the crank to TDC you may see the cams at 60 degrees canted one way or the other if my assesment is correct.

Maybe take a break from it and re-read the book and this post as well as other posts to see if you can get the picture.

You are setting the relationship between the cams and the crank. When you have it right the valve will open to 1.45mm when the crank reaches TDC. Note that the #4 intake valve will open to 1.45mm 360 degrees after the #1 intake valve did once you have both cams timed correctly.

Be careful when you are turning the engine. If you have the timing off enough to hit the piston on the valve you could bend the valve.

-Andy

So the valve should be 1.45mm dropped and heading in at Z1?

The engine has been turing over smoothly.

Anyone else??

Thanks so much Andy.

Exactly