I have been following the discussions lately regarding the milling of heads
and blocks, as well as some thoughts on valve lift. To that end, I have a
few thoughts to add. I also have a thought or two for Shawn's carb setup.
First, the concept behind, and benefits of squish. Triumph engines have
closed "bathtub" chamber heads. That means the effective diameter od the
combustion chamber is significantly less than the cylinder bore diameter.
This leaves a flat section of the chamber where the head and piston crown
surface are paralell to each other, this section is better known as the
quench area. The 6 cylinder TR engine in stock form allows about .060
quench height (.040 gasket, -.020 deck height). That is, the piston top is
.060 away from the head in this area at TDC. Why is this a bad thing? Two
reasons; inefficient combustion and poor emissions. Inefficient combustion
is our main corcern, and here is what happens. As the piston reaches TDC,
the deep quench height is such that a fair amoun of fuel-air mix is trapped.
Two things happen, the mix detonates due to a locally higher pressure
(diesel style) or it never does burn raising hydrocarbon emissions
substantially. This is one reason why the PI motor was a pinking, stinking
bomb. So, if we reduce this quench height enough, the mixture is actually
evacuated towards the center of the chamber leaving almost no unburned
mixture, and creating a turbulence in the chamber. Such a turbulence aids
in mixture distrbution and flame propegation, aiding complete combustion,
reducing required advance, and allowing for better lean-cruise performance.
What is the magic number? Rule of thumb (for a short stroke V8) is .020,
with .002 added for every 1000 RPM above 5000, but the benefit is lost above
.040. Given a 6000 RPM redline, and a TR's long stroke and whippy crank I
think .030 total is a reasonable target. Think about the old school combo
of a zero decked block and a copper shim head gasket which the likes of
Kastner ran competitively. Current trends in chamber design typically call
for shallow open (bore diameter) chambers, or tight quench heights.
Now second, lets think about TR heads. Examination of the chamber of a US
spec head will show about a .125 vertical step around the chamber between
the angular surface of the chamber roof and the deck surface. Oddly enough,
if this portion of the head is machined away, you will end up with a UK spec
head thickness and about 9.5 compression. Yes Virginia, Triumph just left
the castings a bit thicker for our side of the pond. On a properly finished
head, -.010 deck, plus a .125 head shave should yield a 10:1 motor which
will run on pump premium. What constitutes a properly finished head?
Following a good head skimming by your machine shop, the combustion chambers
will have really sharp edges. Round these, and any other sharp edges off;
as they act like glow plugs causing pre-ignition, while wreaking havok on
airflow. Next look at the bowl areas of the ports. Blend the sharp ridges
under the valve seat flat, but maintain the short-side radius. In the
chambers, smooth the angular ridges emminating down from the chamber roof.
Beyond this, some valve unshrouding, shaping, and flow balancing is in order
if you want to go all out.
Given all the discussion of cams and rockers, bear in mind that stock
installed height (regardless of rate) springs are good for about .430 max.
Port stall probably happens at about the same lift. Beyond that lift is a
good thing, and the greater lift for a given duration loosely equates to
greater torque at the same RPM. Other thoughts, rollers reduce friction and
guide wear but move their mass out to the tip. You do the dynamics here.
For a given base circle, lash should be increased proportionate to the
increase in rocker ratio. Small base circle cams, such as regrinds must run
a larger lash clearance due to their ramp angle. The order of mass
reduction is parts that go back and forth first, then parts that go round
and round, and finally front to rear over the vehicle, maintaining F/R
balance.
Carb mix thoughts, if you are rich under an accelerating load, try a lighter
damper load. If under a steady load, like 3000 RPM and 75% throttle, lower
the needles if you can. If you are as low as you can go while maintaining a
decent idle, and still rich under steady load, lighter springs are worth a
try. Dont cut too much, as it is hard to put it back. Instantaneous leaning
while exiting a corner, or mushy throttle response, try reducing damper
piston free float by placing a shim under the e-clip. Given the higher
density altitude in Ft Collins, and if you are rich everywhere, try lowering
the float levels slightly. Too much, and you will be completely out of the
zone. But if is too rich...
Anyhow, just a few thoughts.
Nate Steiner
Boise, ID
CF140U
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