Three simple steps to big flow gains
The porting work considered here was just
basic. We were looking for the biggest gains for the least time-consuming level
of rework. Rather than go for the classic sandpapered shine, we sought to just
leave the modifications in a smooth carbide-cut finish, and quantify the
results of this level of modification with additional flow testing. The first
step was to simply blend the machining cuts into the port bowl, a level of work
usually referred to as a "bowl blend." While in the vicinity, a mild
reworking of the port short turn was also performed, and the head went right
from the porting stand to the flow bench, producing the results seen in the
third columns of the flow charts. As can be seen, this level of modification
really unlocked a healthy amount of flow, with strong gains everywhere on the
flow curve. Testing showed an increase of 35 cfm at peak intake flow, with the
exhaust showing a 22 cfm gain.
We
were looking for the biggest gains for the least time-consuming level of
rework.
At this point we could say that the
performance character of the heads was radically changed, with flow numbers
that would produce a very respectable street performance engine. In keeping
with our objectives of maximum improvement for a minimal level of rework, we
looked to one more popular modification on the intake side of the port: the
port match.
The intake port is opened at the manifold
side to match the desired gasket size, in this case a standard Fel-Pro 2013
gasket for the small-block Mopar. Potential flow gains from gasket matching
will depend upon the architecture of the cylinder heads, and in the case of
these particular castings, the narrow and restricted pushrod pinch area is a
definite source of restriction. Again, leaving the work in a smooth
carbide-finished state, the port entrance was gasket matched, working into the
runner to enlarge the pushrod restriction to the practical safe maximum allowed
by the existing iron. Back on the flow bench, the port match/push rod pinch
work showed a gain of 14 cfm, leaving no doubt that this area represented a
restriction.
The
intake port is opened at the manifold side to match the desired gasket size, in
this case a standard Fel-Pro 2013 gasket for the small-block Mopar.
So what did we get for our modest cylinder
head modification efforts? Looking at the flow curve, the lower lift and
midrange numbers really equal some of the better aftermarket cylinder heads in
terms of flow. The competition-style valve job is responsible for that result.
Since a valve job is going to be a part of any cylinder head rebuild it is
worth the effort to find a shop that will do a performance cut. Looking at the
peak numbers, the heads deliver flow on par with a good set of entry-level
aftermarket heads, and actually exceed the flow of some of the offerings on the
market. With an improvement of 50 cfm from just three steps, we consider this
to be a very practical level of modification in terms of cost/time versus the
improvements. That’s enough extra flow to support up to 100 extra horsepower in
an all-out race-style engine, and 50 or more in a typical street piece.
So
what did we get for our modest cylinder head modification efforts? Looking at
the flow curve, the lower lift and midrange numbers really equal some of the
better aftermarket cylinder heads in terms of flow.
