This article is one in a series that have been released in conjunction with Wayne's book, 101 Projects for Your Porsche 911. The book contains 240 pages of full color projects detailing everything from performance mods to changing your brake pads. With more than 650+ full-color glossy photos accompanying extensive step-by-step procedures, this book is required reading in any Porsche 911 owner's collection. See The Official Book Website for more details.
Other than the very expensive factory manuals, there really isn't a very good source of documentation on the process for setting the cam timing on the 911 motor. Without delving into too much detail, this project will show you exactly what is involved with setting the timing, along with a few tips and tricks. This is not meant to be a weekend-type project, but more of a knowledge base, so that you can be up to date when your mechanic talks about setting the cam timing on your newly rebuilt 911 motor.
Obviously, the best time to set the cams is when the motor is being rebuilt. However, the cam timing can be changed and altered with the engine in the car. Quite a few things need to be removed or placed out of the way: engine mounts, A/C compressor, spark plugs, valve covers, some heater hoses, fuel injection components: but it is indeed possible. For the sake of simplicity, we will assume that you are putting the motor together on a bench, but the same procedures apply to the motor when it's in the car.
The first step is to make sure that the valves are adjusted properly. This is a very important step, and if the valve clearances are slightly off, then it will affect the timing setting that you will dial in for the cams. The clearance on both intake and exhaust valves should be 0.10mm (0.004"). Take a look at Pelican Technical Article: Valve Adjustment, Adjusting Your Valves, for more information.
Once you are assured that the valves are set at their proper clearance, you need to attach a dial indicator gauge to the cylinder #1 intake valve in order to read the amount of physical valve lift. Make sure you properly convert all the numbers if you don't use a metric dial gauge. The gauge should be mounted to one of the studs located on the cam towers. There is a specific Porsche tool for this process but if you have a universal dial gauge holder, sometimes you can get away with a hose clamp. Make sure that the gauge tip is perfectly parallel to the valve, and positioned exactly on the edge of the spring retaining collar. Adjust the dial gauge so that the indicator arm is pressed in at least 10mm (about .4"). This will provide enough clearance for the dial indicator arm to travel as the valve moves up and down.
Now, rotate the engine so that the Z1 mark lines up with the case half underneath the fan. If you are adjusting the timing with the distributor installed, make sure that the distributor is pointing at the small notch in its housing, indicating Top Dead Center (TDC) for cylinder #1.
Using the special cam holder tool (Porsche tool P202), rotate the camshafts so that the small punch marks on the cams are pointing upwards. If you are performing this step with the engine in the car, you need to remove the large cam holding nut that constrains the camshafts.
It is very important to note that when the cam nut is removed, and the cam is disconnected from the timing chain, it is possible to have the valves hit the top of the pistons when you are turning either the camshafts or the crankshaft. Turn the crankshaft and the camshafts very slowly, and if the slightest resistance is felt, make sure that you rotate the camshafts slightly in order to relieve the pressure. Moving quickly at this stage in the game can cause great harm to your engine.
If you are assembling the motor from the ground up, then you will need to place the aligning dowel pin into the cam sprocket. In this position, one of the alignment holes of the camshaft will line up with one of the holes in the sprocket. They are slightly offset in order to give a significant degree of adjustment. Using Porsche tool P212, (or the screw tip on an old spark plug) place the pin in the only hole that is lined up with the sprocket. You can look through the camshaft sprocket, and it will be obvious which hole the pin needs to go into. Simply place the pin in there and unscrew the tool: the pin will not fall out.
With the engine at TDC for cylinder #1, and the small punch mark on the cams pointing upwards, the engine now can be timed for firing on cylinder one. Start with the left side of the motor, and use vise-grips to clamp the end of the wheel sprocket lever arm to the edge of the chain housing. This is done to increase the tension on the chain and to get a more accurate reading. On the right side, it is a bit more difficult. It's important to make sure that you keep a lot of tension on the chain at all times during the timing process. If you have a mechanical chain tensioner, you can use it on the right side of the motor during the timing process and swap it out before you seal up the chain housings. The mechanical chain tensioner was used originally on Porsche race cars, and is not spring loaded. You can purchase these tensioners from most parts houses and use the tensioner as a tool for keeping the tension on the chain tight during the timing process.
Reset the indicator of the dial gauge to zero, and rotate the crankshaft 360 degrees, until the Z1 mark appears again. Make sure that you rotate the other camshaft to relieve any conflict with the pistons if you sense any resistance at all while turning. At this point, the dial indicator gauge should read a few millimeters less than the point that you zeroed. It is this distance, or valve lift for cylinder #1, that needs to be set to a specific amount when the camshaft is at TDC for cylinder #4 (where it should be now). On the 911SC type 930/10 motor, this value of intake valve stroke in overlapping TDC with 0.1mm valve clearance must be set between 0.9mm and 1.1mm. To find out this value for your particular motor and cams, check the Porsche Factory Specifications Book for your year car.
In order to change the value that the camshaft is currently set at, simply remove the small alignment pin, and rotate the camshaft until the dial gauge reads the specified amount. Then replace the alignment pin, attach the large spring washer, and tighten down the large cam nut, but not to the final torque specification. Turn the crankshaft 720 degrees, and check the measurement. It should be the same value that you just set. If it's not, remove the alignment pin and repeat the procedure again.
At this point, the crankshaft should be at TDC for cylinder #4. This is 360 degrees off of the point where you originally set the dial gauge to read zero. At this point, remove the dial gauge, and place it on cylinder #4, and repeat the entire timing process for cylinder #4. The gauge should be set to zero before you make any additional turns of the crankshaft (should be 360 degrees off from TDC on cylinder #1).
It's a wise idea to double-check the measurements prior tightening the cam nut to its final torque. Also make sure that your dial gauge is a metric gauge, and if it isn't, don't forget to convert your units over before you're finished.
On the 911SCs, some people advocate advancing the timing slightly, however, dyno tests have failed to reveal any significant increase in power from doing so. I recommend setting your timing values to the factory settings for your camshafts, and looking for horsepower increases elsewhere.
The dial indicator gauge can be installed using the special Porsche tool holder (sometimes called a Z-block), or if you have a flexible holder, you might get away with clamping it to one of the studs on the cam tower. Make sure that the gauge is parallel to the valve, and not cocked at an angle. Setting up the gauge at an angle will affect your final timing setting.
Set the dial indicator tip to sit on top of the valve spring retaining collar. The indicator needs to measure exact valve lift, so make sure that it doesn't slip, and is firmly mounted. Also make sure that it has enough play to measure the entire lift of the valve.
When placing the small alignment pin into the camshaft sprocket, it will line up with only one hole of the sprocket mounting flange. The two flanges are angularly offset in order to allow a slight degree of adjustment when setting the timing. Place the alignment pin in the hole and then unscrew the tool: the pin shouldn't fall out.
When all the timing is set, and you are convinced that it's correct, tighten the nut to its final torque using a torque wrench, the 46mm crows foot wrench, and Porsche tool P202. This is a task that cannot be done without the use of these two tools, as the final torque for the cam nut is extremely high (150 Nm for the early 911SCs).
This diagram clearly shows the relation of valve lift to crankshaft rotation. The 911 crankshaft turns exactly 2 full rotations for every one rotation of the camshafts. The process of setting the timing involves the synchronization of the rotation of the camshafts to the rotation of the main crankshaft. At the point dubbed Top Dead Center Overlap, the crankshaft has completed its first rotation (pushing exhaust gasses out of the cylinder), and is now beginning to open the intake valve. At this point, both the intake and exhaust valves are slightly open. The exact distance that they are open is what is measured when setting the timing. When the engine reaches the specified value for "Intake Valve Lift at Overlap TDC with 0.1 mm (0.004") valve clearance," the crankshaft should be set at the Z1 mark on the pulley, or exactly 360 degrees off from TDC for cylinder #1. Advancing or retarding the cam timing setting will result in the exhaust and intake profiles being moved either to the left or the right on the diagram. It is important to note that this is very different from the ignition timing, which controls when the spark is fired with respect to the crankshaft. This diagram is borrowed from Bruce Anderson's book, "The 911 Performance Handbook," which should be a staple in any 911 owner's library. The book contains a wealth of information on the 911, its history of evolution, and performance modifications that you may wish to perform on your 911. For more information see http://www.911handbook.com.