Abgeschickt von Guido am 28 April, 2004 um 16:36:29
Antwort auf: Nockenwellendaten gesucht !!! von Mrobster am 27 April, 2004 um 18:11:37:
CAM TIMING - HOW TO DIAL-IN CAMS
YOU HAVE TO DIAL-IN TC CAMS OFF THE BLOCK. IF YOU DON’T BELIEVE ME, TRY FOR YOURSELF!
Tools required: Mag base dti (dial gauge), 6“ protractor, sticky stuff to fix protractor to camwheel, pointers, dummy cam bucket, and a metal plate to fix to the cambox or vice to hold the whole thing in and to which a dti can be fixed. In the RH shot, I am assessing the approx pos of full lift (FL) on a 2 liter inlet cam.
After setting the dti to zero on the approx FL pos, swing the cam say, 1 mm clockwise and set the protractor to zero. This will put the cam-bucket contact on the flank of the cam and well away from the dwell region on the cam nose. It’s no good guessing at the position of full-lift on a cam with 7 cam degrees (14 crank degrees) of dwell at full lift, so we’ll use the dti and protractor to determine the geometric centre of two equal and opposite flank positions. Clear? I though so...
... So having set the protractor to zero, I now turn it counter-clockwise 1 mm from appr FL and noted the reading (above center), 36 deg. Because it’s a symmetrical cam the FL position - or to be more correct, the centre of the FL dwell position - must be at 18 deg (above right).
Whilst the position of the dwell phase in the engine cycle is not hypercritical, any inaccuracy in setting the lobe centreline will upset the commencement and end of the whole valve event. This does matter. If you are prepared to hunt for good cams, set them up right or don’t bother at all...
The FL position is very important, as it tells us - for example - that the inlet cam should be FL 110 crank deg after TDC, and by deduction, because the cams rotate at half-crank speed, the inlet cam should be at 110/2 = 55 cam deg before cam-full-lift at TDC.
In the same example, the exhaust cam would be set at 55 cam deg after-cam-full-lift at TDC.
The cams rotate clockwise viewed from the camwheel end. Remember that the only relative cam position we can ascertain readily is that of TDC on the overlap phase at the end of the exhaust stroke - ie: both valves open, the ex closing and the inlet opening up. This is the position the cams should be set at when we put the head on the block...you can easily recheck the FL positions once the head and cambelt are fitted, and make adjustments (provided you are confident of your piston-to-valve clearances), by using adjustable camwheels.
As you can see above I have marked the FL position on the end of the cam and the cam box with two pin-punched ground marks (above left). I like to infill the marks with white paint. The protractor is then zeroed again, and as this is the inlet cam I turn the inlet camwheel counter-clockwise (viewed from the camwheel end) - or backwards compared with engine direction of rotation - by half the number of crank degrees at which I want FL in the inlet cycle ie: 110/2=55 cam deg. As you can see I have marked the TDC position of the inlet cam with a pin punch mark and red paint (above right).
In summary, in this 110 deg example, the inlet will be 55 cam degrees before cam-full-lift at TDC
giving cam FL in crank degrees at 110 deg after TDC (ATDC). The inlet is opening up at TDC on the overlap phase.
With the exhaust cam - for say 110 deg timing - I will turn it 55 cam deg clockwise after cam full lift,
giving ex FL in crank degrees at 110 deg before TDC (BTDC). The ex valve is shutting down at TDC on the overlap phase.
Now. You can work out almost everything you need to know about a cam by measurement, lift at tdc, full lift, duration, start and end of valve event, but you have to know one single important piece of information. The optimum position of cam full lift (FL). You either know this for a fact or you are guessing. I have done my fair share of guessing.
Optimum FL can only be determined by patient dyno testing and track test confirmation with an experienced and consistent driver.
Of course for everything I say there will be a bloody fool who will declare that all TC cams work best at 110 or any other crank degrees FL. He is of course bluffing and has never dynoed and actually swung the cam timing to-and- fro to assess the impact of the changes on the torque curve on any particular engine.
FL timing is NOT just about the theoretical relationship between piston speed and valve lift position, it is to do with inlet tract shape and length and exhaust layout, and importantly, if you’re doing this seriously - where the driver wants the torque.
If you want some generalisations here are a few: Supercharged engines like early inlet opening and turbo ex cams like late ex opening. Full race engines like the ex cam opened as early as possible. Race engines like lots of overlap for top end and low-down-torque-engines don’t.
Beyond that I’m not prepared to comment, except perhaps to say that I have experimented with numerous TC cams of varying origin, and of course my own profiles and have determined that optimum in/ex FL can be anywhere between 99 deg and 112 after/before TDC.
There is no guarantee that even a reputable cam grinder’s recommendations we’ll be prefect for your engine or that inlet and exhaust timing figures should be the same...
99 - 113 deg is a huge range, and if you guess at it - don’t expect your engine to be fully optimised.