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re ALFA twin cam - the cam is located above the valve & bucket follower (longer distance between valve seat and cam centerline = greater total expansion) The cam is nested lower in the 911 design by virtue of its rockers which decreases the effective length of valve seat to cam centerline.
That would not explain it. The Jaguar twin cam used 0.004" and 0.006" for twenty years and it is a very similar design (Morin bucket pat. 1916) to the ALFA. The Lotus twin cam used similar 0.006-0.008" clearances. The Jaguar clearances were increased to 0.012" for a complex parabolic cam design with very different ramps to produce the exact lift and duration but much quieter operation. The ALFA has a similar construction, a wider VIA, does not have parabolic cams, but uses a 0.020" lash spec. Now that I think about it, the sodum cooled exhaust valve cannot be the reason, because the intake spec is just as wide.
I think in the case of Jaguar, competing with the hydraulic lifter cam noise level in a luxury car had more to do with the tight clearances than any technical reasons . But it is a useful example because the MFG was able to reduce noise at much higher clearances by redesigning the cam profile. The ALFA specs are closer to a racing twin cam hemi, 0.016-0.020", and it may just be that the market for sports cars with tuned exhaust notes can tolerate more tappet noise.
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Paul
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