Forum » How to Degree a Cam » Cam timing?

Cam timing?

How to Degree a Cam

Discussion and questions related to the course How to Degree a Cam

= Resolved threads

Page 1

What I dont understand is when you adjusted the intake cam timing 5 degrees it misaligned the "factory" timing Mark's on the 2 cam gears. Then when u put the plado on the piston and did timing u aligned the "factory" timing Mark's on the cams. Wouldnt that be put the timing off?

Since I didn't watch the video, and I am having some trouble understanding your question, a couple of points.

It sounds like Andre(?) used what is known as a vernier, or adjustable timing sprocket (AKS gear). This is made of two main parts - the first, inner, part is bolted to the camshaft - the second, outer, is driven by the cam' belt, or chain. The advantage is that while a standard, one-piece, sprocket has a fixed relationship (position between the sprocket and camshaft) the adjustable type allows them to be moved relative to each other.

What I suspect has happened is that Andre has altered the adjustment by 5 degrees (actually 2.5 degrees as all camshaft figures are relative to the crankshaft, not camshaft - it can be confusing at first ;-( ), so when he set the sprocket to the correct timing mark the camshaft was actually 5 degrees different from where it would normally be with a one piece sprocket.

Then, when using the playdough/plasticine, he was checking the actual timing to be used. However, there may be another explanation, he may have adjusted the camshaft timing by 5 degrees to check that, if he later wanted to try changing it from the specified timing, he would still have clearance.

There are a couple of main reasons for this being done - sometimes the machining of the parts are all slightly off, although each is within tolances, and they can all add up to quite a big difference (usually called stacking, or accumulating, tolerances). For example, if the tolerance is +/- 0.5 degrees, then there could be +.5 for the crankshaft AND the crank gear woodruff slots - worse, the camshaft and the camshaft sprocket wooldruff slots would be effectively double, because it rotates at half crankshaft speed (think about it) and if the head and block were skimmed, that could be another 0.5. That would be 0.5+0.5+2(0.5+0.5)+0.5 for a total of +3.5 (crankshaft) degrees - this can make a big difference with the engine's performance. Using the adjustable sprocket allows this to be corrected. As an aside, you may like to check out "datum", and the importance of measuring from a single, fixed point or value.

Second common reason is that some engine combinations may perform better with the camshaft(s) a little advance, or retarded, from the specified timing. As an aside, that last was a subtle trick used back in the day to give 'standard'spec' engines a little more top end - in spec' class racing a little bit here, and a little bit there, can add up to a nice advantage over the opposition.