Fiat Abarth Cam profiles for 16 valve type 232 Engine

[mickwood]

Guy - I was measuring up an original Abarth 236 profile inlet cam this weekend. so i could publish details. (11.4mm cam lift) I measured cam lift at each 1 degree interval with a dial guage all the way through the cycle. I was just plotting the data to smooth it (lets face it, getting single degrees of cam rotation is not fantastically accurate - could easily be plus minus 0.1 degrees and occasionally 0.2 degrees), but you can pick it up through the integrals). However it has turned up some slightly odd results insofar as it looks very slightly assymetric - Not enough to be deliberate I think, but I see almost no sign of wear on the cam either. Do you have any feedback on the tolerances on the original machining - Difference is not a lot - lift in the middle of ramp up can be delayed by as much as one degree later than hitting the same lift height when the valve is closing.

Any thoughts?

[Guy Croft]

Mick, hi

sorry I missed this. Can you initially email me the plot in Excel format please?

GC

[mickwood]

Guy - sorry was travelling

Data attached

First column is as measured. Measurement used dial guage accurate to 0.01mm - for small openings i noted error to 0.005mm. Measurement on closing was taken rotating cam backwards to ensure there were no problems with cam bucket sticking and giving false readings. Interestingly base circle of cam was not circular at all but had error range of about 0.02mm

Of course I made the beginners error of timing the peak to the measured high point on the cam rather than taking an average based on the shoulders (but see below)

Since there is bound to be measurement error of easily plus/minus 0.1 degree (cam was rotated by hand and rotation was measured against a standard Burton disc guage attached). I then smoothed the profile by looking at the delta of lift per degree. (see charts)

To correct for inaccurate identification of peak i compared profile on ramp up with profile on closing. For a symmetrical cam, this should give a good feel for cam TDC and also of wear. This is where i started to get odd results - best fit for most of the rotation came with peak retarded half a degree - however even then there was quite a bit of asymmetry with lift opening up to a peak of about 0.2mm faster than closing at around 80 camshaft degrees befor Cam tdc and then decaying to zero around 40 degrees before CTDC. If it had been the other way then i might have put it down to wear but this seems odd.

If this makes no sense to you either, then I will go back and run the exercise again

[Guy Croft]

Mick, hi

I took just your lift data and overlaid it on one of my symetric profiles as shown below, but placing your full lift centre more or less over mine and as you can see there is no assymetry on your cam, the data variations you have seen are part measurement, partly a natural feature of a ground item, the more used the cam the more you get. Base circle deviation eg can look alarming on paper but from a reputable cam grinder say like Kent and Piper it is actually technically insignificant because the cam goes thru a major wiping-smoothing phase in the first 10 minutes of running anyway.

Whilst I do of course respect you for having a go, to be frank I caution people not to try and 'map' their cams as it can be, well, really confusing.
When I want something mapped I get Kent to do it on their computerised system. That measures flat-follower lift to a very high level of accuracy and rules out any deviations caused by bucket movement etc. It doesn't cost a lot and the results are beyond dispute.

GC

[mickwood]

Thanks Guy

The purpose of having these mapped was sort of an "insurance" policy in case i ever need to replace them - I dont know anybody who makes replacement cams for thsi engine (the outside cam cover type 16 valve type 232A/B engine used in 124 Abarth).

I will follow your suggestion and talk with Kent about having them maped so they have the data on file if ever i do need to replace

Mick