I've been doing some measurements on the cam I've bought from Ian, using a slightly improved measuring setup. A word to the wise - that scanned degree wheel in my previous post has some distortion, so I was getting a bias between the cylinder 1 readings and those for cylinder 2. Instead I found a nice accurate clipart of a simple 360 degree protractor, and scaled it up to 275mm diameter so that it fits on the flywheel. I now have not only a more accurate gauge, but also a much more precise gauge - each degree mark is almost 2.5mm wide and I can easily read to half a degree.
The purpose of these measurements is to make sure I have the new cam timed properly. I initially set it up so that the mid point of the overlap was precisely on TDC (and BDC for cylinder 2), but after consulting with the manufacturer I now have it so that this midpoint is 2 crankshaft degrees (=1 camshaft degree) BTDC. The original VW cam had the overlap centred over ~4-5 degrees BTDC, and the lobe separation was slightly greater. The difference between 2 degrees BTDC and TDC is tiny, and probably won't be noticeable, but I was advised that this is the side to err on.
So here's a couple of comparisons between the new cam and the original - cylinder 1 first, then cylinder 2.
Observations?
- The specs are pretty much spot on compared to what Ian posted earlier.
- By advancing the cam by 2 degrees compared to TDC, the downward profile of the inlet lobes is virtually identical to the original VW cam
- The maximum lift is greater on all four lobes than the VW cam, especially on the exhaust lobes. There's variation between the VW lobes, probably due to wear, but none whatsoever on the new cam.
I'm no expert on these things, but I'll have a go at some broad brush predictions of what all this means. Please correct me and/or add your own thoughts.
1) The inlet lobes have the valves opening sooner and further than the VW cam, but the difference is not as dramatic as it is for the exhaust valves. Inlet duration at 1mm lift is actually quite similar to the VW cam, but the 'area under the curve' is a bit bigger. So it'll be able to pass a little more charge into the cylinder in the same time -> higher velocity? Thinking back to
some earlier speculative posts about whether a bigger Pierburg carb would benefit this cam, I don't know if the standard one can provide higher velocity (there was a suggestion it was already quite high), but I suspect there's little to be gained from a bigger carb venturi.
2) The exhaust lobes have longer duration and much higher lift than the original VW cam. I'm not really sure what this means, but I'd guess that it means it can expel the exhaust gasses better, and in combination with my Speedshop exhaust I'm optimistic that this will be an improvement. But what do I know.
