Graeme Oxley writes:
> I was wondering if anyone could help me with the choice of cam for the mini
> engine I'm building. The specification is;-
>
> 1275 bored to 1312 (+.040)
> gas flowed and polished head, planed to give 10/1 CR
> Standard Mk3 Cooper S valves with 140 lb. valve springs
> Twin 1 1/2 inch S.U's
> LCB exhaust into 2 inch pipe with straight through silencer
>
> I'm builiding the engine to use in hill-climbing/sprinting. I also need to
> use the car on the road occasionally, so a full-race cam is out. I've heard
> that the 731 cam is a good compromise, and that it works from 1500 rpm.
> Haas anyone had experience with this cam? Bearing in mind that the car will
> only occasionally be used for fast road work, is it possible to get a cam
> "hotter" than the 731, but still usable on the road?
According to the David Vizard book, and an engine I've got with a
731 in it, I would avoid the 731 at all costs. It only works on
a small engine, say 850 or 998, anything larger and you lose much
more than you gain. The only reason that it is so popular is that
it is an official factory cam. In addition it only revs out to about
6500, which is too low. The engine I have is an 1100 bored out to
1150, with a Cooper head etc. It gives a reasonable amount of torque,
but the 731 doesn't seem to give more power until the revs go over
4500, so it's obviously wasting something.
What you need with an A-series engine is to have the exhaust duration
at least the same as the intake, which is where the 731 falls down, as
it has a longer intake than exhaust. The 643 or possibly the 649, would
be the choice of the factory cams.
In any case, for sprinting/hillclimbing you will want a hotter cam
than the 731, especially if it is to be used only occasionally on the
road. My Clubman GT has a 643 cam in it, which was the original
Formula Junior cam. It pulls (in a 1293) from about 25-2800, and
keeps going to about 7200. However, that cam is much older than
the engine configuration you're using, and to take most advantage
of the head etc. you'd probably be better off going to a Kent Megadyne
286, or Piper Magnum 2 285/2. These give much the same sort of top end, with
a little more polished bottom end. Not to mention more lift, which
most (all) factory cams lack.
Any hotter than 285 deg and you won't really be able to drive it on
the road, unless you have a low ratio diff. As it is, a 3.4 diff will
have difficulty in sprinting, as it takes too long to pull the car
up to the top of the powerband. You might consider a 3.765.
(I was talking to a guy who races in the FIA Historic championship,
and he said that when they were allowed to use radials, they would
pull 8000 rpm in top at Monza, using the 3.4 diff (128mph!), but now
they had to use Dunlop CR65s they had to go down to a 3.765 diff).
Monza is not a typical racetrack, as it is mainly straights with a
few chicanes. Hillclimbing would probably be better off with a 3.7 or
even a 3.9 diff.
regards,
thos cohen |Softway Pty Ltd
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