Jim:
Oh, goody! I LOVE the can of worms discussions! Threats
exchanged, hate mail made public, half baked claims, MASS HYSTERIA. The
kind of situation in which I can always contribute by stirring the pot.
=:-)
I am going to kick this off by saying "NONE of the above". They
are all decent cams, but the BEST? No one is making a state of the art
cam for TR6s. Research stopped on this motor many years ago, and most
(all?) cam designs were frozen in a time when emissions were dreaded and
performance was the last thing on anyone's mind.
The 'best' cam depends of course on which set of parameters you
wish to optimize. Power, emissions, fuel economy, smoothness,
durability, etc. Most people respond to this by saying "I want all of
the above". Not gonna happen unless you convert to variable valve
timing.
Allow me to do my "Karnack the Magnificent" act. Ahem. Please
hand me my turban... And the answer is...
For a STREET motor, with power being desired with little to no
impact on drivability and economy, a good cam (according to current
engineering practices) will:
1. Have no more than 270 degrees advertised INTAKE duration.
2. Have approximately 10 degrees more advertised duration on the
exhaust than the intake.
3. Will have a difference of no more than 70 degrees between
ADVERTISED duration and duration at 0.050" lobe lift.
4. Will have lobe centers no tighter than about 108 degrees.
5. Will have the intake valve centerline advanced by 3-4
degrees. An offset cam keyway or vernier timing set can be used in place
of this requirement.
6. Have the maximum lift permitted by the valve train (normally
determined by the size of the valve lifter).
So, "Karnack" sez:
260i/270e with 108 lobe centers. Valve timing is 26/54i, 66/23e.
Duration would be 200i/210e@0.050" and lobe lift would be 0.260". This
would be a very civil cam with decent power, but no one makes it.
If you are willing to spread the lobe centers a couple degrees
then you could go to 270i/280e which would give you a bit wider power
band without affecting idle too much. This would also reduce torque a
bit but would still be very solid. If you use high ratio rockers then
expect the motor to be a bit less docile, as if you had gone up 5-10
degrees in cam timing.
I should also mention that this sort of cam (a so called "dual
pattern" cam) requires low back pressure in your exhaust system, an area
in which the stock TR6 muffler fails miserably. This may be why there is
a dearth of dual pattern cams for the TR6.
Barring a custom cam, the absolute limit for a single pattern
street cam is 270 degrees duration if you want a decent idle and good
torque, with lobe centers at 110 degrees. As the lobe centers get
tighter manifold vacuum and idle degrade, emissions and power climb
while mileage falls. So if the lobe centers are tighter than 110
degrees, plan on reducing the permissible duration below 270.
Cheers,
Vance
-----Original Message-----
From: owner-6pack@Autox.Team.Net [mailto:owner-6pack@Autox.Team.Net] On
Behalf Of Jim Jones
Sent: November 13, 2007 10:23 AM
To: 6pack@autox.team.net
Subject: [6pack] cam opinions
<snip>
My question is what cams would be preferred for a mild upgrade with
emphasis getting more power and torque at low and mid range RPMs? I'm
considering these cams:
S2 cam
TSI 275 cam
BP Northwest TH5, BP270, or BP 150 HP cams
Goodparts GP2 cam
<snip>
Jim
|