Three simple steps to big flow gains
There’s a saying that when it comes to
engine building, "the power is in the ports.” Airflow is the key to
turning up the power output of any engine, and with choices in performance
cylinder heads, there is plenty of flow to be had. However, when money is tight,
budget constraints often dictate cylinder head selection. We see plenty of
Mopar street machines running around. Sure, a swap to aftermarket heads might
be in the cards for later when the funds are available, but is that a reason to
give up substantial horsepower right now? Port work can improve the situation
substantially, but grinding on that old iron has its practical limits.
Professional porting cost bucks, and unless the engine is being built for a
class that demands stock iron, it’s generally impractical. That said, is it
worth pulling out the tools and giving the heads a tittle rework? If you can
handle a grinder, the answer is almost always “yes.”
Professional
porting cost bucks, and unless the engine is being built for a class that
demands stock iron, it’s generally impractical.
Even if you are paying for professional
porting services, it might be worthwhile to consider the options. The key is to
keep the scope of the work at a level that makes sense in terms of time, money,
and effort. We have seen guys go nuts on factory iron, and produce impressive
results, but that is not what we were after here. Our goal is finding the
maximum flow gains for the minimal amount of work. To get there, we looked at
practical steps that produce a big improvement with the minimum level of
modification.
A good example is the set of OE Mopar
small-block iron castings presented here. These heads are a part of a low-buck,
mild 360 street rebuild project, and were fitted with OE 1.88/1.60-inch valves.
A valve size upgrade for OEM small-valve heads is usually worthy of
consideration, especially if new valves are going to be a part of the
reconditioning process anyway. In the case of the Mopar small-block, an
increase in intake-valve diameter to the early 340 engine’s 2.02-inch
specification is a low-cost and common upgrade.
In
the case of the Mopar small-block, an increase in intake-valve diameter to the
early 340 engine’s 2.02-inch specification is a low-cost and common upgrade.
The Rework
The relative benefit of installing the
larger valves will vary with how well the seat work is done on the head
casting. We have seen installations where the valve seat was simply cut with a
larger 45-degree seat angle, and sized with a 60-degree bottom cut. This kind
of installation will hardly change the port throat diameter, and yield little
if anything in increased flow, except at very low lifts. On the other hand, a
professionally cut seat, sized to the valve diameter, including a throat cut at
a minimum of 75 degrees will dramatically open the port’s bowl. The point is
that the results from installing a larger valve can vary substantially
depending upon how the work is performed. For our heads, the valves were
installed with a competition seat job, featuring a five-angle (30-45-60-75-82-degree)
cut that makes the most of the larger valve’s diameter. After machining, the
large increase in the port bowl’s throat diameter was readily apparent by the
ledge left at the point where the newly machined area met the as-cast portion
of the bowl.
After the seat work, the large protruding
ledge left by the seat machining must be blended into the bowl. The stepped
mismatch left at this point will represent a massive flow disruption, negating
any potential benefit of the machining operation. To illustrate this effect, we
flow tested the cylinder heads before and after machining. As seen in the
accompanying flow chart, the high-lift flow in this form was clearly impeded.
Since the porting carbide cutters need to be pulled out and put to work to
blend the seat cuts, we wondered just how much additional flow can be gained by
a few very basic porting steps.
Since
the porting carbide cutters need to be pulled out and put to work to blend the
seat cuts, we wondered just how much additional flow can be gained by a few
very basic porting steps.
