Setting the timing in a C900 plus notes about setting up the MSD BTM and a pressure control valve

• Get a timing light. I strongly recommend getting one that has a "dial" and a display (usually digital, can be analog gauge as well) in it. This will make it possible for you to set the advance where ever you want, without the need for extra marks on the flywheel. Using a lamp without the adjustment the BTM measurements would be a major PITA
• Flywheel is located inside the clutch cover, it is visible through a "check hole" on top of it, covered with a plastic lid. What you need to look at in there are two marks: there's a straight line on the static metal piece in there (this is a reference mark) and as you rotate the flywheel you'll see the actual timing marks. Really the only way to do this is engage 5th gear with engine turned off and pushing the car forward. Do this with an assistant - or alone, inch by inch - so that the flywheels ends up in a position where the marks on it are visible through the hole. Takes some patience!
• If you got a timing light with no adjustments/gauge, you need to look at the marks on the flywheel to find the one marked with the timing value you're after. But if you've got an adjustable lamp, just look for the first line (zero-line)
• What you need to do is to mark these two lines (the one that's stationary and the one from the flywheel, either the 0 or the desired timing) with something. I used some silver paint.
• Make sure you don't mix up the timing marks with the two metal "dots" also in there. They're not what youre looking for. Saab has a factory adjustment tool that checks timing using those, but that's a different story
• Now just hook up the timing light to battery positive and negative terminals. The inductive pickup goes to the plug wire of 1st cylinder (make sure you've got it right, opening the spark plug cover to make sure would not be a bad idea)
• It's time to start the engine. It should be warmed up to a normal running temp and idling at about 850rpm (most timing lights with a gauge also show rpms)
• Now take the timing light and press the button on it. The bulb should start flashing. There'll be one flash per every engine rev
• Point the light into the timing marks. The strobe light will have an interesting effect in there: the marks on the rotating flywheel should appear to be freezing. Now what you need to do, is to align the two marks you just painted. The way to do this with a non-adjustable timing lamp is to rotate the distributor (there's just one bolt holding it). Be careful, it is very sensitive! Turn it a bit, and then use the lamp again. If my memory serves me right, turning it clockwise will advance the timing, anti-clockwise will retard. You'll see! This is much easier if you have an adjustable timing lamp. All you need to do is to point the light into the marks and turn the knob on the lamp so that the timing marks will align (in this case we're talking about the mark at 0-degs, remember!). Now you can read the actual timing from the gauge on the lamp
• adjust by turning the distributor if neccessary. If you're getting the btm, I would start at 18 degs of initial timing. Note that this measurement should be done with the vacuum hose going to the distributor disconnected

Hmm, that's about all you need to know about setting the basic timing.

The idea of retard is quite simple: the timing advance has to be lower on boost as it is on the idle. The theory behind this is a bit complicated, but you do not need to know much about it. If you're getting the BTM, it will do the job for you! That's why you need to get rid of the mechanical retard built into the dashpot on the distributor. It comes of by loosening two screws on it. There's an axle connecting it to the internals of the distributor, be gentle when pulling it out! Now the function of the mechanical retard should be very simple as you have the part in your hands: the pressure applied into the dashpot will push out the axle. And vacuum will do just the opposite. You do not want to touch the vacuum operation, only to defeat the retard. The way to do this is to look at the "knob" welded on the base of the dashpot. This is what prevents the axle from extending all the way out as the pressure is applied. What you need to do is to bend that knob so that it will not allow the axle to extend. There is only about 2mm gap in stock form, you need to get rid of it. Just punch the knob with something so that it bends. Don't bend it too much, you do not want to have it pushing the axle into the "vacuum" area. That's all!

The best way to test that you really got rid of the retard is to first check your basic timing (above) and then apply pressure to the dashpot (i used a half-empty tire, a hand pump with a pressure tank would be even better) and check the timing again! No matter how much pressure you have in there, the timing should not have retarded a lot. I have one degree left in my unit and that's fine.

Now comes the btm: when you first install it leave the retard knob at zero. You do not want to run around with excess retard. Setting it properly is a timing light excercise as well! Hook up the light. Connect the pressure tank (or whatever youre using) into the hose going to the BTM box. Now what you really need to have is a pressure gauge somewhere in that hose. This is important because you need to know at which pressure you're at when checking the timing. Start by pumping about 15PSI into the tank. Now with the engine idling, measure the timing (this is really ackward if you don't have a timing light with a gauge). What you want to see is a timing of about 10 degrees at 15PSI. You need to experiment with the BTM pot to archieve this. The potentiometer should need to be turned only a few degrees up from the zero level, so be careful!

As soon as you got it to 10 degs at 15PSI it would be a good idea to measure the values from the whole pressure range. Start by noting the basic timing once again (just to make sure you got it right), this is without any pressure going to the BTM. Should still be 18 (or whatever you decided). Now pump some pressure into the BTM. Measuring the timing for every PSI added would be my recommendation: you can plot a curve which should be linear: this will confirm that you have not made mistakes in the measurements.

The last trick in the field of timing would be the pressure control valve (or bleed valve). This is not absolutely necessary, but if you're running with pressures much beyond 15PSI it is strongly recommended. The idea of the valve is to limit the pressure going to the BTM to a certain level. This way the BTM will retard the timing in a linear fashion until a certain level and then stop retarding, no matter what the actual boost is. This is useful because the ideal retard curve up to about 15PSI goes so that the timing at that point is about 10 degs BTDC. No matter how much you're boosting, you do not want to have much more retard than that. The pressure valve, installed in the hose going to the BTM, should be set so that it peaks the pressure at about 16PSI (my experience: again, you should experiment yourself). This way the BTM will retard the timing all the way to 9 degs BTDC. Now the actual boost in the engine may well be 20PSI but the timing will still be at 9 degs. This has worked well for most people, but again, you need to experiment yourself to find the best setting for your car.