The P head exhaust port has a lot of sharp edges and can really benefit from just a little grinding.
A word of caution though: just behind the valve guide and in front of the thermactor divot is a water jacket, so be careful in this area.
Starting with the outboard side of the throat, as with the intake, the throat
undercuts the valve seat defined by a sharp edge.
Smoothing this wavy ridge on the inboard wall of the
Forming a generous radius on the outboard ridge really
helped .4", .3" picked up some
flow too. But .5" lost a bit of flow.
pocket helped .5" recover a small amount of flow.
The ugly sharp ridge on the short side radius is just
begging for some blending and the numbers show it across
the valve lift range.
The back wall of the bowl, while brandishing a sharp edge
where the throat undercuts the seat, showed little
improvement when blended.
The short turn radius, outboard wall and back wall blended
Applying a small radius to the top cut of the seat is a
slight detriment to low lift numbers. I saw the same
results with the intake but thought it was an error on
my part. Now I see the same results on the exhaust. I
thought this might improve the low lift numbers but
instead the opposite happened.
While the picture below shows the sharp
edge of the top cut around the seats, the picture above
shows the small radius that I applied
Trimming the guide on the outboard side netted some
noteworthy results from .3" and higher.
While the higher valve lifts took advantage of my trimming
the outboard side of the guide boss, a little trimming
on the inboard side of the guide helped out the low lift
Cutting the chamber wall back to the cylinder gasket
line, to unshroud the valve, provided no flow improvements.
Looking at the roof of the exhaust port exit, you
can see the bulging area that was provided for the
Removing all that thermactor plumbing provided good
results for the midrange lifts.
A threaded wand shows how the exhaust valves
wide seat surface and sharp edge at the
bottom help direct the air flow straight down
the exhaust port. The sharp edge causes the air
to separate from the valve, so that it won't
have a tendency to follow the valve surface
and flow across the port. I will not undercut
the exhaust valves.
I am using Ford Racing Products GT40P shorty headers.
They have 1 5/8" primary tubes. The supplied gasket
is just high and low enough, and a little wide on the sides.
The exits are measured for width.
The header tubes are also measured for width. The actual
inside of the tubes are measured, not the flange. The
header tubes and exhaust exit are nearly the same width
I will not increase the width of the exhaust exits on
The supplied gasket shows how the headers will line up
with the port exit. While the port floor lines up well
with the header, there is ample room at the top.
While exploring the exit with a threaded wand, I happened
to discover that the port exit did not like having the clear
plastic velocity tube moved to close to the roof of the port.
If I moved the tube any closer than about 1/8" to the top of
the port, the pressure sensor on the 'Flowbee' would rise,
indicating a reduction in flow. I will keep this in mind when
doing anything to the port roof.
While exploring the port exit with a velocity probe, I
discovered something that I really wasn't expecting. While
the upper left side of the exit has a very strong
velocity, the lower right hand corner has a very low
velocity. While this may not seem so unusual, what I
really didn't expect was to find that the lower right
corner was actually producing a slight vacuum at .1"
and .2" lift! When I applied a threaded wand to this
area, sure enough, the air was moving into the
lower right corner of the port exit. I tested a stock,
untouched, port and found the same results.
I experimented with clay, building up walls and corners,
trying to see if anything would relieve this occurrence.
I found that filling the lower right corner helped a
little by eliminating the vacuum sooner in the valve
lift. This indicated that no material should be removed
from the right side of the port.
This velocity map shows the negative flow areas in red.
The velocity map doesn't have enough resolution to show
that the upper left hand corner of the port exit always
had the highest velocity of any other area, and the lower
right corner always had the weakest velocity. To properly
illustrate this would require many more grids and shades
of gray. So any cross section enlargement should only take
place on the left side of the port
Raising the right side of the port exit roof about .04", and opening
the lower right corner, began to produce only very small
improvements in flow, and as the velocity map shows, velocity
began to deteriorate in some areas. I am going to consider
this configuration as optimal for my application
GT40P Intake Port
More good stuff!