Setting the cam timing of an engine using the Lift @ TDC values rather than the peak angle is a much simpler and faster way of correctly timing the camshafts.
With a Single cam engine, timing via peak angle is relatively easy since the relationship between the inlet and exhaust lobes is fixed by the camshaft. However, trying to set the timing on a twin cam or quad cam engine is far more difficult as you have to have a starting point so none of the valves are accidentally bent while turning the engine over.
As we need a setting point for the cams, it makes much more sense to actually time the cams at Top Dead Center and avoid having to move the crankshaft at all.
As we know the full details of the camshaft, its a simple matter to work back from the peak lift position to find the lift when the crank is at TDC.
First we need to find the true top dead center position of the crankshaft. As the piston 'Dwells' at TDC we need to take 2 measurements while the piston is moving and then find the center point of these (See Pics 1-3).
If possible, its much safer to have the camshafts removed to do this!!
If thats not possible pay careful attention to which valves are open and which pistons are coming up the bore!!
Now we have the true TDC position, turn the crank so the pistons are out of the way of any possible valve contact(on a 4 cylinder engine you can turn the crank 90 degrees so the pistons are level half way up the bore)
Next, make sure the vernier pulleys are loose and move freely.
Start with the inlet cam, take the lobe that is just opening the valve (no.1 or 4 on a 4 cyl engine). Rotate the cam so the valve is closed and the follower is on the base circle.
Zero the dial gauge and then turn the cam so the valve is open the correct amount (as indicated on the data sheet), Pic 4
Remember, with mechanical profiles you need to subtract the valve clearance from the lift@tdc figure.
Next move the dial gauge to the exhaust camshaft and repeat the above procedure (pic 5)
Now we have the camshafts set in the correct position we can carefully bring the crankshaft back up to the TDC position we found earlier. Make sure you are bringing the correct cylinder up the bore. On a 4 cylinder engine, the valves on 1 and 4 are safe but cylinder 2 and 3 will have the valves open. If you feel any resistance then stop and find out what the problem is. Its VERY easy to bend valves when turning the crank!!
Fit the cambelt and tension it with the vernier pulleys still loose so the cams and crank stay in position.
Next tighten the pulleys and turn the crank through at least 720 degree's to bring it back to its original position.
Double check the reading on the dial guage (which should still be on the exhaust lobe).
Usually you will have to slightly correct this, make sure you always turn the engine in the direction of normal rotation to keep the tension on the correct side of the cambelt.
Now move the dial gauge back to the inlet side and double check the lift @ TDC figure.
After correction make sure you turn the engine over to double check, its not unusual to have to repeat this a couple of times on some engines but your patience will pay off in having the cam timing absolutely perfect.
Top Tips:
To lock a camshaft in position, place a small piece of paper under one of the cam caps. Dont forget to remove it afterwards!!
To extend a dial guage, remove the tip and use 3mm Aluminuim TIG welding rod as an extension. Grind a point onto the end and it will screw into the end of the guage. These can be easily bent around camshafts, made long enough to reach to the piston crown etc. See pics 6 & 7
Masterclass:
When the lift @ TDC values are very small its easier to measure the lift @ BDC. This is very useful for engines with variable cam timing or very short duration profiles. The measurement can also be used to double check the setting that was made using the lift @ TDC. On a 4 cylinder, it is very straightforward as you can do the lift at TDC on cylinder 1, and the lift at BDC on cylinders 2-3 for intake-exhaust (depending on the firing order) using the same TDC mark on the crank pulley.
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