Mark:
You need to answer some specific questions in order to select a
cam. Any cam is a series of compromises in terms of fuel economy, idle
characteristics, power, smoothness, cost, etc.
One person's street cam is another person's race cam. I have had
people tell me that a 285 degree cam is a "great" street cam. Depending
on the rest of the motor setup and your tastes, it can be a burping,
surging, soggy nightmare, or it can be a "hot" streetable cam that has
acceptable hits on economy and drivability. Incidentally, the factory
offered a 285 degree cam in the PI models, but backed away from it
because the car was deemed to be too 'beastly'.
So, some questions you need to ask:
How important is cold start drivability? Are you willing to warm
up the motor before attempting to drive down the street?
How important are the idle characteristics? Do you want a glass
smooth idle, a bit of vibration, a lumpy 'bad boy' idle, or???
Fuel economy. Does it matter or not? Do you want near stock
mileage, or is a significant hit on mileage acceptable?
Emissions. Do you need to worry about them?
Fuel mixture. Are you prepared to swap out the needles in the
carbs, and perhaps spend some time trying different needles to find the
correct fuel mixture?
Cost. How much are you prepared to spend matching the rest of
the engine to the cam characteristics? Failure to do so will make the
car more difficult to drive and live with, and will yield disappointing
results in terms of power gains.
Assume for a moment that you want the following:
Glass smooth idle
Minimal impact on economy
Good cold start drivability
Minimal change in emissions
Minimal additional modifications (i.e. a low total cost of the
engine work)
A noticeable improvement in power.
Then the 256 cam used by the GT6 and the late TR6s is a good
choice. If you have an earlier TR6, then this is a 16 degree increase in
duration which is good for around 10 HP or so with no other changes (New
lifters are required, possibly a new set of stock springs, but that is
about it). If you have a later TR6, then you are stuck as this is the
stock cam, and you need to look for HP somewhere else besides the cam
(compression, etc).
Now assume that you are willing to give up some cold start
drivability, with a modest increase in idle roughness, some fuel
economy, etc. Then a cam up to about 270-275 degrees duration is the
limit, depending on the lobe center spacing. Assuming 108 to 110 degree
lobe centers, you will have driving characteristics that give up a small
amount of livability, but not much. You will need new lifters, higher
performance valve springs, and you will definitely need to reneedle the
carbs. For the later TR6s (CF series) you will probably also want to
raise the compression to avoid any loss of low end torque. If you raise
the compression past about 9:1, you will have a tendency for the engine
to run on, unless you have an anti run on valve in place and
functioning. This will add some cost. The 270 cam is good for about 10
HP beyond the 256 degree cam, more if you raise the compression.
My rule of thumb - 270 degrees (perhaps a teensy bit more
depending on other variables) is the limit for a street driven car with
little or no compromise to drivability. If you want more cam than that,
you are into significant idle, cold start, and economy issues for a
daily driver, and you will need to spend more time sorting out the
ignition timing, fuel mixture, etc. There are better ways to get power
than stacking on lots of timing. Head porting, compression, and exhaust
work are excellent ways to increase power without the penalties of a
long period cam. They add complexity and cost to the engine work
however, so nothing is free.
As a side note, the 285 degree PI cam also had much tighter lobe
centers than the US cams. 105 or 106 degrees if memory serves. Besides
making the idle rougher and reducing economy, tighter lobe centers pull
the HP peak down into a more usable RPM range and actually increase the
total power output. So don't ignore lobe center spacing when talking
about "a good street grind".
Lastly, rather than talk about "A good street cam" talk about
the characteristics and costs of a cam. I think you will find that one
man's treasure is another man's trash. Then you can choose the "correct"
cam.
Cheers,
Vance
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