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.
One of the most important and basic tasks to perform on your 911 is the setting of the ignition timing. The timing setting is what determines when the spark is going to be fired for each particular spark plug. If the timing is a bit off (retarded or advanced), then your car may not perform at its maximum efficiency and power. It's very important to check the timing periodically to make sure that it's within the correct range for your car.
There are two basic adjustments that need to be made in order to set the timing on early 911s (1965-77). The rotation of the distributor needs to be determined, and the gap between the points needs to be set. On the later cars (1978-83), Porsche used an electronic breaker-less ignition system that eliminated the need for the points. On the earlier cars, the breaker points are controlled by a cam that is located on the main shaft of the distributor. As the distributor rotates, the cam pushes the points open. At this point the coil releases its stored energy and sends a charge to the spark plug determined by the position of the rotor inside the distributor cap. In 1984, the factory employed the use of the Bosch Motronic engine management system, which incorporates the fuel injection and ignition systems together. The timing and dwell of the Motronic system is controlled by its central computer, and cannot be adjusted.
By far, the most important adjustment is the setting of the position of the distributor for adjustment of the timing. Make sure that your points gap is properly set (see below) prior to adjusting the timing of the distributor. This measurement is performed using a timing light with the engine running and warmed up. The timing light is attached to the spark plug line for cylinder number 1, and gets its power from the terminals in the fuse box panel on the left side of the engine compartment. Connect the ground connection of the timing light to the fan housing, or the braided shielding of the ignition wires. The timing light uses induction to sense the voltage running through the spark plug wires. Make sure that you attach the sensor around the wire where it is not protected with the stainless steel sheath, otherwise you might have difficulty obtaining an accurate reading.
Strap the sensor around the spark plug wire for cylinder number one, which is located on the left side of the car, towards the rear of the car. Make sure that the car is warmed up before you take the timing measurement. Also make sure that your idle speed is set to the correct range. For details on setting the idle speed for your 911, make sure that you look at Projects 29-31, which are specific to each different type of fuel injection system. To properly set the timing, you may have to remove the vacuum hoses from the distributor. Check the Porsche Technical Specifications Book for the exact details on what you need to do to properly set the timing. Sometimes, this information is written on stickers attached to either the engine shelf, or the inside of the rear decklid.
With the car running and the idle speed set, aim the timing light at the crankshaft pulley located directly underneath the fan. If all is working properly, the timing light should strobe on and off, and you should be able to see the timing mark on the pulley. Different year 911s had different timing points, but it should be marked with a number "5" or similar marking. This mark indicates the timing mark for five degrees before top dead center, or after top dead center, depending upon the year and engine. If you have a more advanced adjustable timing light, you can set the light to time the car to just about any setting simply by dialing in the value on the light.
The timing setting is changed by rotating the distributor slightly. Take a 13mm wrench, and loosen up the nut that holds the distributor secure to the engine case. While the engine is running and the timing light is shining on the pulley, gently rotate the distributor until the timing mark on the pulley lines up with the notch in the fan housing. It may take a few tries to get it right, and keep in mind that small movements in the rotation of the distributor are all that are needed. Once you have the correct setting, then tighten down the nut on the distributor. Be very careful not to place any tools or fingers near the fan when the engine is running, and watch the cord of the timing light as well.
On the pre-1977 911s, you must set the dwell angle for the points as well as the timing position of the distributor. The dwell angle is the number of degrees of rotation that the points will stay closed when the distributor is rotating. The angle is adjusted by moving and rotating the position of the points with respect to the to the cam. The farther the points are moved away from the cam, the smaller the dwell angle.
The difficult part of this equation is that changing the dwell angle changes the timing of the motor. Therefore, it's wise to set the distributor to the correct location, then adjust the dwell angle, and then go back and adjust the distributor again. You measure the dwell angle using a dwell meter that is hooked up to the points and to ground. The meter will accurately tell you the angle of dwell based on the frequency and length of the points opening and closing. The dwell angle is changed by moving the points closer or away from the distributor cam. They are attached to the distributor using a small screw that must be loosened in order to move them. With the engine off, unclip the distributor cap, loosen the screw, and move the points away from the cam to decrease the angle, or move it closer to the cam to increase the angle. Since there are so many different years and engines used with the 911, check the Technical Specifications book available from Porsche for the exact dwell angle, and also the exact setting for the ignition timing. In general, the 1965-68 911s should be set with an initial gap of 0.016" and the 1969-77 911s should have an initial gap of 0.012".
You can measure the dwell angle on 1965-68 911s by hooking your meter up to the positive and negative terminals of the coil. On 1969-1977 911s, you will need to use a different technique since the capacitive discharge system won't give you an accurate signal. Don't hook up the dwell meter to the coil, or you may damage your capacitive discharge system and your meter. Instead, hook the signal wire to the dwell meter directly to the points wire that is coming off of the distributor. In some cases, it may be easiest to hook the dwell meter to a long screwdriver and place the tip of the screwdriver on the contacts where the wire is plugged into the distributor. You should be able to get an accurate reading from the distributor using this method.
Keep swapping between setting the dwell and the timing until you have the two measurements correct. Keep in mind that if you removed vacuum advance or retard hoses from the distributor, then you might have to reset the idle for your car once you reconnect them.
It's also important to follow the proper vacuum hose settings when timing the car. On some models with vacuum advance or retard, you may need to disconnect or plug the hoses in order to get an accurate reading. Again, check the Technical Specifications book for the exact timing procedure.
It's also important to check the total timing advance that occurs at 6000 rpm. Check it with the engine cold, and then again with the engine warm. Refer again to the Porsche Technical Specifications book for your year car as to which vacuum hoses you need to disconnect from the distributor. Checking the total advance at 6000 rpm may seem scary and very loud, but it's necessary to make sure that the ignition system is advancing properly. You want to make sure that your total advance at 6000 rpm does not exceed the value specified in the Technical Specifications book. If it does, then you may have detonation or pinging at higher RPMs. If the total advance is beyond the specification, rotate the distributor until it comes back into spec. As a result, your idle timing will change. If you find that you cannot adjust both the idle and total advance values to the proper specifications at the same time, then you might have some internal components of your distributor that are worn, or some vacuum hoses that are clogged or improperly installed.
The most essential tool for setting the timing is a timing light. This device attaches to one of your ignition wires, and then flashes a light whenever the engine fires the spark plug that the sensor is attached to. Using this strobe effect, the timing can be set on the engine.
This photo of the engine compartment shows how to hook up the timing light. Attach the positive clip to one of the fuses on the left side of the engine (yellow arrow). The black ground for the light can be attached to any metal point on the chassis, but the most convenient spot is usually clipped to the ground for the spark plug wires (green arrow). The white arrow shows where the inductive sensor is placed around the spark plug wire. Make sure that you don't attach it around the shielded section of the wires, as this will result in a weak signal to the lamp.
At the specified RPM, aim the light at the bottom crankshaft pulley. With this engine, a 911SC 3.0L, the timing should be set so that the second mark appears lined up with the notch in the bottom of the fan housing when the timing light flashes. This mark, labeled with the number five and shown by the arrow, indicates 5 degrees before Top Dead Center (TDC).
The inside view of the Bosch distributor shows the points and the gap that needs to be set prior to setting the timing. The arrow points to the gap that should be initially set using a feeler gauge. This is the same Bosch distributor that is the upgrade for the Marelli distributor for the 1969-71 911s.