Text and Photography by C. Asaravala, J. Mikelonis

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 vaporware.

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 manufacturer’s or retailer’s 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 valve.

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.

Understanding Volume
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 theory.

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.

In This Article:
A look at how the popular Air Flow Research small-block Ford 165cc heads fare against the rest of the aftermarket offerings.

Also See:
Part II - Installation and Dyno Testing AFR 165cc Heads

The 165cc heads feature 1.90"/1.60" intake and exhaust valves, with either a 61cc or 59cc combustion chamber volume.

The 165cc heads were set up with 7/16" studs and guide plates, and dual springs for roller cams up to .550" lift.

AFR's "D" hole exhaust port is a direct carryover from their immense success with Chevy designs. The 165 and 185 heads use a 69cc exhaust runner volume.

We deburred any sharp edges in the combustion chamber to reduce the chance of hot spots, which lead to preignition.

Tech Archives Project Cars Readers Cars Feature Cars