Well guys I've finally done something I've been meaning to do for years. I
have made a contraption that traces the lobe profile on a camshaft, onto
graph paper, with the degrees rotation on the horizontal axis, and the lift
on the verticle. It is pretty dodgy, (i.e. not at all precision engineering)
but it has certainly revealed some intersting things regards our cam specs
provided by the men at Rootes.
The reason for this (some might say madness) is I have been running what
according to the rootes spec sheet is really a full race cam in my race car.
The cam comes from a Hillman Hunter/Sunbeam rapier fastback of 1974 and is
specced by rootes as 38-72 72-38, i.e. has a duration of 290 degrees. It was
the wildest standard cam I could find, hence my fitting it. But it
originally comes from a single carb or dual carb, family road car. And
indeed in my race car, the engine pulls very cleanly from 800 revs and
really has no "coming on cam" feel to it. Something has definitely been
amiss. I have been considering fitting an isky SB-3 cam which is specced at
33-73 73-33 giving 286 degrees duration, which is a semi race cam... but
according to the specs from rootes, my cam should actually be wilder!
After reading various books I discovered the whole "cams should be measured
at .050" camlift scenario" whereas engine manufacturers such as rootes give
timings that are are from when the valve first leaves it's seat till when it
returns. Thus, if the clearance is set to 14 thou, and the rocker ratio is
1.4, this equates to only about .010" camlift. So I have been wondering what
the rootes cams would measure as when done "correctly", i.e. to .050" spec.
Well, now I know!.
I have put a few cams I have through this test, and there is a reasonable
amount of error, due to the basic construction technique of this device..
but nevertheless it gives a reasonable approximation of the true picture
Cam 1 Sunbeam Alpine Series 4a/ Humber sceptre mark 1
Rootes say 19-57 61-15, duration 258 degrees, overlap = 34 degrees
The first thing is that the graph shows an incredibly slow initial opening
period, which must be the quietening ramp to ensure minimal tappet noise.
The .010" point is part way along this ramp, and as such the valve is
initially opened very slowly. When measured at the .050" camlift points the
duration is really only about 200 to 210 degrees, and the overlap between
the exhaust and inlet valves is non existent, and may actually be negative
by 10 degrees or so. So this is really a very mild cam. The camlift is about
0.260" giving about 0.36" valvelift
Cam 2 Sunbeam rapier H120, Hillman hunter GLS (twin weber std, 110hp)
Rootes say 58-66 84-40, duration 304 degrees, overlap 98 degrees
Whoa! the engine shouldn't even run below 3000 revs with a cam like this,
and checking it out as for the previous cam, rootes radical timing specs can
be revised as such. Duration approx 240 degrees, but it does actually have
some genuine overlap of about 40 degrees. It's initial quieting ramps are
much smaller, maybe to keep the valve on the seat as long as possible to get
rid of heat. So this is definitely a bit more than a standard road cam, but
certainly not the race cam it presents itself as.
As such, I have not measured my cam in the race engine, because of just
that, it is still in the race engine, but from the pattern that has emerged,
I would think it probably has a true overlap of no more than 220 degrees,
and as such the iski cam should indeed wake my engine up somewhat.
If anyone has some comments on this, please let me know. I have found it a
very interesting exercise, and am interested in any feedback, or previous
findings by other people that relate to this.
Greg
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