This may seem like a chapter worth skipping over on the way to the more exotic heads, but if anything stuck with you from Chapter 3, then you know that flow velocity is pre-determined by port cross-sectional area, lift, and valve size. Given that, a small cross-sectional area intake port head with good intake and exhaust flow is an excellent choice for a mild street engine that is intended for miles of trouble-free performance. The heads detailed in this chapter fall in the under-180-cc intake port volume range, and are especially good for 350-ci engines as well as smaller-displacement motors such as 327 and even 302-ci smallblocks that are not intended for high RPM horsepower.
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In what seems almost like a time forgotten, there was a period in which, besides the iron Chevy Bow Tie, there was little to choose from for a mild small-block other than stock heads. Chevy began building lightweight castings that seemed to crack the moment you unbolted them from an engine and the core market looked in danger of drying up. This was when World Products began casting iron stock replacement (S/R) cylinder heads for both the 305 and 350-ci small-block. The initial run of small-valve S/R heads was so successful that World introduced the S/R Torquer, which was the larger 2.02/1.60-inch valve version of the S/R. As you can see from the flow charts, compared to the Vortec production head, the S/R is down 12 to 20 cfm on the intake side across the board, so performance would be down slightly in comparison.
It’s a similar story on the exhaust side, and this head would certainly benefit from simple modifications to the pocket area, especially on the exhaust side. We get into more details in the porting chapter, but typically these heads are cast and machined using high-volume production techniques, which leave a restricted throat area that can use some tweaking. Merely increasing the throat diameter on the exhaust and doing nothing else significantly improves horsepower above 4,000 rpm while costing no power loss anywhere else. Production and the S/R heads generally have a weak exhaust port as evidenced by the drop in the exhaust-to-intake percentage from 82 to barely 60 percent at 0.600-inch lift. By opening the throat area, this percentage improves significantly.
As an example, we’ve included a second flow sheet on a mildly ported version of the S/R. Granted, this head received more than mere bowl work, but it also shows the gain from a head that is generally considered a mere stock replacement piece. The intake improvements are minimal since most of the work is concentrated on the exhaust side. Note at 0.300-inch lift, the port flow improves by an amazing 40 cfm, which computes out to a 30 percent gain in exhaust flow at that one data point alone. The average E/I improves to just over 70 percent, which means less camshaft exhaust duration is necessary to make power. If we compare the ported version to a stock Vortec, the intake is still down slightly, while the S/R employs a larger intake valve. However, on the exhaust side it’s a completely different story. The ported S/R rocks the Vortec (which admittedly is exhaust limited), revealing a significant potential power gain. The ported S/R is up 30-plus cfm over the stock Vortec, which means that you might be able to get away with a mild dual pattern or even a single pattern cam with exhaust flow numbers of this kind.
While not as good as the Vortec in stock trim, there are times when it’s not financially feasible to use the Vortec, which opens the door for the S/R. It’s also worth noting that the S/R retains its stock bathtub-shaped combustion chamber which is also a bit lazy when it comes to combustion efficiency. This was done, again, as part of the head’s “stock replacement” origins. This head has been around now for two decades and still serves a useful position as an inexpensive improvement over a used production head. The porting information is intended to show that even a relatively obscure, stock replacement head can show promise for a mild street engine if the engine builder is willing to invest a little effort into improving flow. We must also use this occasion to voice the obvious warnings that you cannot just go in and start hogging out exhaust ports in hopes of improving flow. There are important procedures that must be followed if improved flow is to be obtained. But the potential is certainly there if you do a good job of home porting. We go over all that in Chapter 10.
Dart Iron Eagle SS
Dart has long been considered a racing or ultra-high-performance cylinder head company, especially given owner Dick Maskin’s extensive experience in drag racing, especially in NHRA Pro Stock. But what many enthusiasts don’t know is that the company has also been not-so-quietly building a reputation as a full line cylinder head company when it comes to the ubiquitous small-block Chevy. Case in point is the 165-cc Iron Eagle SS head, which is Dart’s entry-level cylinder head running 1.94/1.50-inch valves and a 72-cc combustion chamber. Right off the mark, Dart’s technology advantage is immediately revealed by the combustion chamber. Most of the heads in this category are somewhat dated and rely on the older bathtub-shaped chamber designs of the 1970s. Dart’s Eagle SS head steps up with a much more refined heart-shaped chamber that is not only compact with a restrained surface-to-volume relationship for its 72-cc volume, but also utilizes this shape to help push the combustion process over toward the exhaust valve. This is also a straight plug head and is available from Dart either bare or complete with valves and standard production diameter 1.250-inch valvesprings.
The 165-cc intake port is the smallest of this category of heads and therefore enjoys a decent velocity rating compared to the other heads in this category. If you look at the intake and exhaust flow numbers, the head is on a par with a production head. This may not seem like much of an improvement over the older iron heads like a 441 or 882 head, but the chamber offers potential that can only be measured on the engine rather than on a flow bench. Clearly, this head would also respond similarly to the World Products S/R head if lightly ported, especially on the exhaust side. The complete head also comes with guideplates, screw-in studs, and quality valveguide seals. It uses a tapered seat, 5/8-inch hex, and peanut-style spark plug.
This is the first aluminum head in this small-port aftermarket category. Edelbrock’s intake-port philosophy emphasizes port velocity for its mild street heads, and the Performer is the entry-level head in this category with a 170-cc intake port with a rather small 1.80-sq.in. minimum port cross-section. This limits the head’s max valve lift flow capacity, but does enhance velocity. The Performer’s 0.100 through 0.400-inch lift flow numbers are decent although not as strong as the GM iron Vortec head, which has a similar port volume with a slightly smaller cross-section.
One of the strengths of the Performer is the sheer number of variants that Edelbrock offers. In addition to the Performer, there’s also the 50-state legal center bolt head, which offers EGR passages to make these heads emissions compatible. The heads come with 2.02/1.60- inch valves with a tight chamber design, straight (non-angled) spark plugs, and 3/8-inch screw-in rocker studs along with guide plates that require hardened pushrods. Edelbrock sells the Performer either bare or complete with springs, retainers, and keepers. One point worth noting is that if stock-stamped steel rockers will be used with the Performer or Performer RPM heads, 0.100- inch longer pushrods are required in order to generate the proper rocker geometry. Edelbrock also recommends limiting valve lift to under 0.450 inch when using these heads on engines with smaller bores than 4.00 inches. This includes 283, 305, and 307-ci engines.
The Performer is ideal for the mild street engine applications where the engine builder wants to step up to a set of aluminum heads over the stock iron castings. These heads can be ported while the exhaust-to-intake relationship (E/I) is decent with averages throughout the lift curve around the mid-70 percentages. Part of the reason for the exhaust performance is that the intake port is somewhat limited. This is one case where the head might actually respond to increasing the crosssectional area slightly to improve upper valve lift flow since the head crosses over in terms of port restriction at just under 0.300-inch valve lift.
Edelbrock E-Tec 170
Edelbrock capitalized on the Vortec iron head phenomenon by introducing an aluminum version of the GM head. This resulted in two different intake port heads, the E-Tec 170 and its larger E-Tec 200-cc cousin. We deal with the smaller of the two heads in this chapter. The 170-cc E-Tec features the same port configuration, 4-bolt Vortec intake flange, and 64-cc chamber volume so that the heads are easily interchanged. The valve sizes vary slightly, with the intake the same as the GM iron at 1.94 inches, but Edelbrock snuck in a slightly larger 1.55- inch exhaust valve.
By comparing flow figures, you can see that the E-Tec intake port is actually not quite as good as its GM iron progenitor. Perhaps we expect too much from Edelbrock, but if the company is going to come out with an aluminum version, we expected it to be far better than the stock iron version. Edelbrock claims that its engineering effort was placed on the exhaust side, which makes sense since the stock Vortec is clearly exhaust limited. But you must look closely to see the flow improvements on the exhaust side. First, Edelbrock raised the exhaust port exit by 0.200 inch to help the port flow and create a more generous short-side radius. Our flow numbers indicate that at 0.400 inch of valve lift and above, the ETec is significantly better than the Vortec. For example, at 0.600 inch, the E-Tec is almost 30 cfm up over the stock iron Vortec, which is a 17 percent flow improvement. But this begs the question of how often are you going to run a 0.600-inch lift cam with a relatively small 170-cc intake port head? At the lower exhaust valve lifts, the E-Tec is virtually even with the Vortec, so it offers little improvement.
On the intake side, it doesn’t take long to realize that the E-Tec does not perform as well as the stock iron Vortec, especially in the mid-lift flow range. At the critical 0.400 inch of valve lift on the intake side, the E-Tec actually flows roughly 10 cfm less than the Vortec. This lack of performance on the intake side, combined with an exhaust port that only flows better at valve lifts that will rarely be attained, underscores why in a dyno test comparison with a relatively mild camshaft there is little E-Tec power improvement (with the 170-cc head) over the stock iron Vortec. Here’s a case where your money might be better spent elsewhere unless having an aluminum head is worth the price of admission. We deal with the larger 200-cc E-Tec head in a subsequent chapter.
Edelbrock Performer RPM
If Edelbrock whiffed on the E-Tec, its earlier entry with a conservative port size, high-velocity, aluminum street head it certainly smacked a solid hit with the Performer RPM head. One reason for splitting up the heads in this book by chapters as well as port area (and volume) is because it’s important to emphasize that heads of like cross-sectional area should be compared on this level playing field, not against much larger heads that can easily out-flow the smaller castings.
Edelbrock chose to build a decent, standard small-block intake bolt pattern head that emphasizes flow velocity for street-driven engines that could benefit from significant torque gains. Again, it is this velocity that creates the mid-range torque, especially in relatively small-displacement engines like a 327 or a 350-ci small-block. The comparison against the Vortec street standard works well in this case since it can outflow that production head at the higher valve lifts mainly based on a slightly larger cross-sectional area. This is despite the fact that the Performer is an older design that predates the Vortec. This makes the RPM more appealing to an engine builder looking for a bit more horsepower from an affordable aftermarket aluminum head. In Chapter 13 we detail a 355-ci small-block combination that includes these cylinder heads that made a solid 438 ft-lbs of torque at 4,000 rpm while making 424 hp at 6,000 rpm. With a power band between peak torque and peak horsepower of an impressive 2,000 rpm, this is a great engine for use in an automatic where the engine speed drops more between gears, demanding a wider power band. The Performer RPM heads are a bit of an underdog in the current world of high-flow heads, but it should not be overlooked when it comes to price versus performance. This head outperforms most of the heads in this chapter and yet still qualifies as easily affordable.
Racing Head Service Vortec Iron
The most recent player in the replacement Vortec head genre comes from the revitalized Racing Head Service (RHS)—now the brand name for what once was the Pro Action cylinder head company out of New Zealand. The owners of the COMP Cams family of companies recently purchased the offshore cylinder head concern, and the Vortec replacement head is the first of a new line of cylinder heads from the reorganized business.
The original Vortec set the bar rather high for mild street cylinder heads, and by looking at both the intake and exhaust numbers, the new RHS head achieves an interesting level that might make it a player in the mild street head category. The RHS head manages to achieve equality with the original Vortec on the intake from the low lift through the mid-lift numbers up to 0.400 inch. Then at the half-inch valve lift position, the RHS Vortec jumps up to 258 cfm, a solid 20 cfm better than the stock Vortec. Then the RHS head drops off back to stock Vortec numbers at 0.600 inch of valve lift.
The RHS head uses a larger 2.02 inches of valve size to achieve these numbers, which instantly means that with equal flow numbers in the mid-lift areas that its velocity past the valve will be slower (but the pressure will be higher) compared to the stock 1.94-inch Vortec head. Plus, the RHS Vortec offers a significantly larger 1.89 sq.in. of cross-sectional area compared to the GM Vortec with its smaller 1.74 sq.in. This bump in area along with the larger intake valve is the main reason for the increase in the high-lift flow.
On the exhaust side of things, the RHS head offers dramatically better numbers that promise to make this head worth some power. The stock GM Vortec is most definitely exhaust port limited, and it appears this is where RHS spent its development time. Again, it all starts with not only a larger exhaust valve at 1.60 inches, but also a slightly larger exhaust port cross-sectional area matched with a slightly taller exhaust port exit height compared to the stock Vortec. While this points to improvements in flow at higher lift, the RHS head offers a dramatic increase in low-lift flow right off the mark at 0.100-inch valve lift. Compared to the GM Vortec, the RHS head pumps the flow up with a solid 67 cfm compared to the GM head’s 51 cfm. That alone is a 16-cfm improvement, which represents a hefty 31-percent increase.
We simulated these two heads against each other on the ProRacing Sim DeskTop Dyno program, and with a mild 268H COMP Cams flat tappet hydraulic cam, the stock Vortec heads came up with 367 hp at 5,500 rpm with a peak torque of 412 ft-lbs at 4,000. By simply replacing the GM Vortec with the RHS version, the simulation reported an increase to 382 hp at the same peak RPM along with 416 ft-lbs of torque. That’s 16 more hp and 5 ft-lbs of increased torque based on the improved airflow. Averages were less dramatic, with horsepower gaining roughly 3.5 and torque 3 ft-lbs. This reinforces the concept that these new RHS heads are worth a slight power increase. Granted, this is a simulation, but we’ve had amazing success with this simple computer program in predicting naturally aspirated small-block engine packages.
The heads discussed in this chapter may seem tame, but they represent some of the most popular in terms of units sold within the smallblock market. This is because they are intended to fit the area between daily driver and mid-point performance applications that are used far more than the popular press likes to admit. In the performance magazines, it’s all about the horsepower race, so it’s rare to read about mild performance heads because most of the editors gravitate toward the castings that can generate big flow and equally big horsepower numbers. While those are enticing and attract readers, the truth is that most of those readers are more interested in a set of heads that they can both afford and match to the rest of the engine combination they currently point in the direction of the local cruise spot on Saturday night. The beauty of the small-block Chevy is that there are so many potential buyers that the market can afford the broad selection offering that no other engine can attain.
Written by Chris Petris and Posted with Permission of CarTechBooks