In reply to a message from spitzstick sent Fri 18 Nov 2011:
Hi Roger
It’s not possible to give a full technical answer to a non-
technical question. You made a vague request for ‘experience’
without any reference to the vehicle or its intended use. How long
is a piece of string? Besides which, who even knows what a D-type
cam is? One thing is for sure, it’s not radical…
The XK engine started out with lift of 0.3125’’ and timing of 15,
57, 57, 15 which is what the timing stayed for the life of the XK
engine, give or take. The lift did increase pretty early to 0.375’’
but that too stayed the de facto lift for every XK engine. The
later parabolic cams changed very marginally but were developed
primarily to boost refinement.
My original D-type workshop manual describes the D-type cam as
having 0.375’’ lift and timing of 30, 60, 60, 30. That’s all it
says. So standard lift and a tiny bit more duration. Just for
comparison, the same manual gives nine headings of data for the oil
pressure relief valve spring…
So we’re not talking wild extremes here, we’re talking basically a
road cam that was a fraction better than the production version
when coupled with race inlet/exhaust systems to make use of the
timing change, with no requirement to idle as smoothly or be as
docile in traffic as a road car, even though the D-type generally
was a complete pussycat compared to contemporary race cars. You did
not say what your car is or where/how you propose to drive it so
this may or may not be relevant.
Remember the objective was endurance above everything, not outright
performance. Is that your aim? If your aim is not to survive 24 hrs
non-stop, why use the settings for those who did want that?
Doubtless by the time fuel injection was tried , followed by the
3.8 engine for the Ecurie Ecosse ‘semi-works’ entry, they
experimented with more lumpy profiles, but that was after the
manual was written and those cams were never fitted to production D-
types. I haven’t seen those cam specs written down anywhere and
would be interested to know what evidence any seller uses to
justify the label ‘D-type’ on any cam different from 3/8’’ and
30/60/60/30.
I haven’t researched it for Jags, but I can’t see any cam grinder
selling a so-called performance cam with identical lift and almost
identical timing to the cam in every production slush-box smog-
regulated sedan. Would you pay serious money for one? So I doubt
you can even buy true D-type cams unless you go out of your way to
commission a set.
Your second question about regrinds versus billets is nothing to do
with D-types and is a matter of personal preference and cost.
There’s no ‘right’ answer, although technically at the ultimate
limit it would be better to have well made billet cams and normal
shims for reasons of reciprocating mass, and possibly eventual
durability depending how much they ground off the base circle. But
the reason people grind the bases circle is primarily to gain lift,
and D-type cams are standard lift. So…
Pete–
1E75339 66 D, 1E33100 66 FHC, 1R7977 69 OTS, 65 Mk2 3.8 MOD
Cambridge, United Kingdom
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