Tony:
Cam events are easy to calculate. Just a little algebra.
LS = lobe spacing (cam degrees)
ID = intake duration (crankshaft degrees)
ED = exhaust duration (crankshaft degrees)
ADV = Camshaft advance (cam degrees). + is Advanced, - is
Retarded
Inlet opens: BTDC = ID/2 - LS + ADV
Inlet closes: ABDC = ID/2 + LS - ADV - 180
Exhaust opens: BBDC = ED/2 + LS - ADV - 180
Exhaust closes: ATDC = ED/2 - LS + ADV
It is permissible for a result to be negative, which means that
if the result is normally BBDC, it is ABDC and vice versa.
Here is an example using my TR6 cam with 270i/272e with 109 LS,
+2 advance.
In opens BTDC = 270/2 + 2 - 109 (28BTDC)
In closes ABDC = 270/2 - 2 + 109 - 180 (62ABDC)
Ex closes ATDC = 272/2 - 2 - 109 (25ATDC)
Ex opens BBDC = 272/2 + 2 + 109 - 180 (67BBDC)
Almost all cams will give more power with very little loss of
torque by advancing the timing 3-4 degrees. Likewise with free flowing
exhaust (The stock TR6 muffler does NOT meet this requirement) adding 10
degrees duration on the exhaust will add power and mileage with no loss
of drivability or torque.
It would seem intuitively that closing the intake vale at 62ABDC
would be a disaster, since the piston is well on its way back up the
cylinder, but a couple of things work in our favor. First air has
inertia like all matter. So the air and fuel briefly continue to move
into the cylinder even though the piston is moving back up the bore.
Secondly, the piston is barely moving for the first 20 degrees or so of
crank rotation, so it really does not start to push back on the inlet
charge until much later than you would imagine. Similar effects govern
the other valve events.
With this cam, I have lost the glass smooth idle of the stock
cam but the idle is still very acceptable - no lope but with a deeper,
coarser sound and a slight increase in sound level. As I stated before
the casual observer would not be able to tell that the engine was
modified. Good midrange power, no loss of torque thanks to the raised
compression (9.6:1) and of course that WONDERFUL straight six sound.
Purrs like a kitten driving down the street, pulls away from stop signs
effortlessly. I would not want to add any more timing however. I am
pretty fussy about drivability, and I know if I had gone any further my
satisfaction level would have been less.
Cheers,
Vance
-----Original Message-----
From: Tony Gordon [mailto:triumph@2simpleusa.com]
Sent: November 13, 2007 8:10 PM
To: Navarrette, Vance
Subject: Re: [6pack] cam opinions
Vance,
<snip>
Back to the cam question: If we were to look at a stage 2 asymmetric
pattern, from what I've seen the early PI duration (270-280) would seem
to
be pretty useable, so your suggestion of a 270i 280e would make sense.
Given the geometric advantages of a roller rocker, I'd probably plump
for a
0.29 or 0.31 lift (which gives a valve lift of 0.46 or 0.50) and the
cam is
running in Spit cam bearings ... you list valve timing for your
"preferred"
cam: can you share with me how I can calculate the valve events from
your/my
figures? Sorry to appear dumb (I am with respect to this topic), but I
want
to learn so I can try and spec a cam I think I want, and your thoughts
on an
asymmetric grind seem to make sense, and if I can work with your on
determining the valve event timing, I'd like to try our such a grind.
Cheers,
**************************************
Tony Gordon
72 TR6
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