Text and Photography by C. Asaravala,
Back in late 1999 when we were putting together our "Head
to Head" article, comparing virtually all of
the small-block Ford (SBF) cylinder heads on
the market, there was one cylinder head that we were not
able to procure. The SBF Air Flow Research (AFR) head,
at the time, was nothing more than a few prototypes on
the trade show circuit. In fact it was our initial discussions
with Rick and Scott Sperling (AFR founders) that prompted
our massive head shootout. The Sperlings were so confident
their heads would substantially out flow and out perform
the competition, that they were jumping at the chance
to prove it in an independent article. Unfortunately production
issues kept delaying the release of the AFR 165 and 185
heads, and we, like many others, wrote the heads off as
Well it's now 2003, and the production problems that delayed
the release of the AFR heads has long been resolved, and
thousands of sets of 165 and 185cc heads have made their
way into the hands of some very happy owners. Remarkably,
we haven't heard a single complaint in how they perform.
In fact the buzz has been so overwhelmingly positive,
we decided it was time to reopen the case and examine
a set for ourselves.
We obtained a set of AFR 165 heads, and had them tested
on a flow bench. We then compared the numbers against
the all the heads we ran back in the original article,
just to see how this head fares with the competition.
However, something we didn't do in the first article,
that we attempt to do here, is explain some of the theory
behind what makes a powerful cylinder head.
Street and Strip
Peruse through any manufacturers or retailers
catalog and you quickly see two general categories of
SBF heads, "street" and "strip". The
terms street and strip are subjective.
What constitutes a strip engine to one, would fall short
as a street engine to another. We simply use these terms
to identify the two major categories of small block Ford
cylinder heads offered in the after-market.
These two categories are determined for the most part
by one thing, the size (volume) of the intake runner
- the tract through the head in which the air and fuel
mixture travels towards the combustion chamber. Heads
with 170cc - 190cc intake runners tend to fall into
the "street" category, while those with 190cc
to over 200cc are considered "strip".
Along with the runner volume comes valve sizes, although
here the distinction is not always consistent. Most
manufacturers use a 1.90", 1.94" or occasionally
2.02" intake valve on their street heads. The strip
heads tend to use a 2.02" to 2.08" intake
The exhaust side of the head tends to stay fairly consistent
regardless of the category or manufacturer of the head.
Most all SBF heads use 1.60" exhaust valves, and
between 60-66cc exhaust runner volumes. Some of the
larger strip heads use as much as a 70cc exhaust valve.
The intake runner volume and valve sizing is perhaps
more critical to the size and rpm range of the engine
than it is the street or strip use of the engine. It's
a fair assumption that the "street" heads
are best suited for 289, 302 and up to the milder 347
stroker engines. The larger "strip" heads
are good candidates for high-revving small blocks, or
347, 351 and 351 based stroker engines. Naturally there
is a lot of overlap here, but experience shows these
rules tend to hold fairly well.
Note that for the sake of simplicity we're not concerning
ourselves with the effects of porting. A professionally
ported street head can often match or exceed the performance
of a strip head. This article covers only "out
of the box" heads.
So far we have talked about runner volume, and we
have generally categorized small-block Ford heads into
two categories, street and strip, based on intake runner
volume. Now we need to go one step further to understand
the meaning of volume.
The volume of a cylinder or tube (i.e. the runner )
is measured using the formula V=L x pi x (r x r). Where
L = length, pi = 3.14, r = radius (half the cross section.)
Keep in mind a runner is not a perfect cylinder due
to it's complex shape, and this formula is just used
In this equation we can see that the volume is affected
by both the length of the runner, and the radius (half
the cross-section). A 170cc volume
across a 2" long runner would have an average cross-section
across the runner, that is twice that of the same volume
in a 4" runner.
For the most part the "street" aftermarket
Ford heads do not vary significantly in runner length.
This is because in order to make a "bolt on"
head the aftermarket has to restrict their designs based
on the stock head and engine architecture. Significantly
increasing the length of a runner requires changing
where the intake and exhaust ports are located. Typically
such modifications are reserved for "strip"
or race heads, where modifications to the intake and
exhaust manifolds is not of concern.
In general, a longer runner and smaller average cross
section will promote better velocity at low rpms, which
translates to more torque, better fuel atomization,
and crisper throttle response- all the ideal characteristics
of a street car. Larger cross-sections, and shorter
runners, yield better top end power. However because
the majority of steet heads on the market only vary
by 20cc or so in intake runner volume, it is hard to
tell which one will perform the best. To do this we
need to look at flow data.