Stock-Block 535HP Screamer - D.S.S. Engine Build

With the rollers installed, the ARP studs were coated with a slow-drying, nonhardening thread sealer and installed finger tight. Although these studs have an Allen socket in the end, this is not so that you can tighten them down in the block-it's there so you can remove them easily at a later time. To achieve the correct preload, and load the block threads evenly, the studs must not be tightened into the block any more than finger tight. Be aware that if you intend to use a header adapter plate, there can be an interference problem with the regular ARP short outer studs. The fix is to use either shorter studs or go to ARP 12-point bolts.

With the studs installed, the head gaskets-Fel-Pro's latest multi-layer design-were positioned. These need to be laid on the block with the brass button upward, as shown nearby.

At this point, the heads went on. The ones chosen for the job were AFR's 205cc runner, CNC-ported, Outlaw street/strip heads. There has been a lot written about these heads and their near legendary performance. That they produce results is unquestionable, but after spending some time on the flow bench, there are a few facts to offer that may not have seen the light of day before, so check out the sidebar on our AFR heads.

Over the years, MM&FF has written a lot about the AFR small-block Ford heads-all of it pretty much glowing testimonials of their excellent performance capability. Where does this all come from? Well, some testing on a more exotic flow bench might reveal a few facts not mentioned in previous write-ups. First, here are the basic head specs so you know what we used.

The casting used is AFR's basic Ford offering but with CNC -machined ports and chambers. The intake port has 205 cc, which makes it slightly better suited to a 350-inch engine than our 331. The intake valve, at 2.080 inches diameter, is a little larger than is often used with a conventional Ford head configuration.

(Due to an oversight during the build of our 535 horsepower small block in the March 2008 issue (Stock-Block 535HP screamer, page 150), author David Vizard mistakenly reported that the heads used on the engine were the AFR 205 competition heads, when in fact the actual heads used in the testing were a pair of AFR 185 street heads which are actually emissions legal and far less expensive. While 535 HP is still a very respectable number, it's even more impressive coming from a set of street heads that retail for $1450! AFR is confident the much higher flowing, larger runner 205 cc race heads would have actually produced even stronger results in that application probably edging the final numbers a lot closer to 600 HP., Ed.)

On the exhaust side, the 75cc port is fed from a 1.6-inch-diameter valve, and the exit point of the port itself is raised 1/8 inch. This small amount should not affect header fit, but it does give the head designer the opportunity of increasing the short-side turn for better flow. The head casting shows its heritage in that the exhaust port shown here still has provisions for the emissions exhaust passage. This was the port flow tested, and the hole seemed not to affect the flow as the numbers delivered were strong.

The combustion chamber checked out at 58 cc, which with the deck clearance used, the head gasket volume and the volume contained in the D.S.S. pistons' valve cutouts, delivered a CR of 10.4:1 on our 331. About the time this Ford head was in the development stage, AFR's founder, the late Ken Sperling, was heavy into swirl and wet flow testing. This prompts the question as to how much of this head's success is owed to the possible incorporation of such technology.

The included flow chart shows the flow numbers we saw on our freshly calibrated bench. If you want to talk about numbers at 0.700 inch lift as is so often the case, then the intake produced 307 cfm and the exhaust 207. But that's not the real story. What seems to make these heads work is that both the intake and exhaust show excellent low and midrange flow. At low lift, this is very much a function of good valve seat design, and at midrange a combination of seat and port design. Even with less peak flow, a fat flow curve will usually outpower its lesser low-lift flow counterpart. Also, good low- and midrange flow has the ability to produce a power curve that hangs on significantly longer at the top end, thus advantageously delaying the shift point until higher rpm. The fact that the curves have largely leveled off above about 0.600 lift shows that the ports are well matched size-wise to applications where intended valve lift is in the 0.625 to 0.700 range. If a port continues to show increased flow much above the maximum valve likft used, it indicates the port is too big for the application.