Cylinder-head design can be a tough code to decipher. Port volumes, runner lengths, chamber and bowl designs, valve sizes and locations—change one and you are going to affect something else. On a Chevy 23-degree head it gets even tougher because it’s difficult to keep the combustion chamber walls from shrouding the intake valves. The big red Bow Tie offers more efficient head designs these days (the 18-degree and SB2, for example), but that doesn’t do much good when most sanctioning bodies controlling racetracks across America require Chevy drivers to stick to the tried-and-true 23.

For years, World Products in Sterling Heights, Mich., has offered racing heads for nearly every application you can imagine. When Chevy heads became scarce in the early ’90s, many tracks—especially dirt tracks—simply spec’d in World Products’ Sportsman II heads (back then they were more commonly known as the Dart II). It is by far World Products’ best-selling product. In fact, owner Bill Mitchell estimates that more than a quarter million of the Sportsman II heads have been sold in the 15 years they have been available.

Since then, though, World Products has developed another 23-degree Chevy head, the Motown 220, which is designed for small-block engines 383 cubic inches and larger. For that purpose, the Motown 220 is designed with larger intake runners than the Sportsman II (200 cc vs. 220 cc) and the ability to accept larger intake valves without additional porting (2.055 inches vs. 2.080). But the combustion chambers are also improved as well as the exhaust ports, which should make the Motown tempting even to those racing smaller Chevys.

To find out more about these heads, we took them both to Ken Troutman at KT Engine Development in Concord, N.C. With the help of Troutman and head porting specialist “Big” Al Taylor, we put both heads through the paces on the flow bench, testing several variables, including valve sizes and seat patterns. To provide a baseline, we also tested a Chevy Bow Tie head. This isn’t entirely fair because the Bow Tie head is designed mainly for use in NASCAR’s Late Model Stock division—where it’s the only legal Chevy head—and not for no-holds-barred power production. Our intention isn’t to make the Chevy head look bad. It does its intended job and does it well. It’s just to give a frame of reference against a familiar cylinder head design.

To find out how they worked, we checked the volumes of both the Sportsman II and Motown 220, then began experimenting with different valve seat sizes and designs before moving over to the flow bench for some extensive R&D. Nothing like ruining two perfectly good heads with a different intake seat in every combustion chamber. Oh, the things we’ll do to quench our thirst for knowledge!

Along the way, we also decided that this was a good time to try out Manley’s exclusive “Pro Flow” valves against the standard design. The Pro Flow intake and exhaust valves feature a neck just above the underside of the valve that is smaller than the rest of the valve stem. This design is supposed to improve low-lift valve flow by reducing the restriction created by the valve stem. We also thought that the Pro Flow valves would improve performance by reducing weight. On average, the intake valves lost around three grams by going to the Pro Flow. But it was the 1.600 exhaust valves that surprised us, losing a whooping 10 grams from the standard-stemmed Race Masters to the more curvy Pro Flow units (see the “Weighty Matters” sidebar for more information). The smaller-stemmed valves did show an improvement in flow in most cases. Troutman, who has already used the valves in his racing engines, felt the improvement in flow was enough to translate to performance on the track.

Bow Tie Vs. Sportsman II (w/Race Master Valves)

The first test was to compare the Bow Tie head to the Sportsman II with 2.020 intake and 1.600 exhaust valves (Chart A). All flow-bench tests were done at 28 inches of water at consistent barometric pressure. You will notice that the charts are a little more detailed at low and high lift, which are generally considered most critical. Valve lift is measured in thousandths of an inch. The units in the charts are airflow in cfm (cubic feet per minute).

For this test we used “standard” valve seat angles: 30, 45 and 70 degrees. The Bow Tie head held its own on the intake side until around 0.150 lift where the Sportsman II began to blast by. The Bow Tie maxed out at 0.500 and regressed from that point on. Of course, that’s an acknowledged trait of the Bow Tie head, and it’s mostly because of its small intake port volume at just 182 cc. It can be improved with a good valve job but not fixed without some serious porting work. On the other hand, the Sportsman II just kept on going. The more lift the valves were allowed, the more it flowed. The exhaust flow between the two heads is a lot closer.

Sportsman Ii (w/Pro Flow Valves)

Next, we threw in a pair of the Pro Flow intake and exhaust valves in the Sportsman II head to test them against the performance we had just documented from the straight-stemmed Race Master units (Chart B). On the intake side, the advantage of the Pro Flow valves didn’t kick in until 0.400 lift. On the exhaust side, the results were really quite marginal—and actually lagged behind the standard valves in the low-lift numbers. Still, both Troutman and Taylor agreed they would prefer the Pro Flow units on the exhaust side because of the significantly reduced mass that would need to be slung around.

Sportsman Ii (w/KT Valve Job)

Before moving on to the Motown, we did one last test with the Sportsman II to try to get a little more flow out of it (Chart C). To improve the low-lift characteristics, we took the head back to the Serdi seat machine and put in one of KT Engines’ special valve-job recipes. The face of the seat is still at a 45-degree angle, but the throat is cut at 72 degrees to open it up just a touch more, and the top angle is 28 degrees with an eighth-inch radius. Obviously, a radius blend isn’t legal everywhere, but the idea is to guide the incoming air/fuel charge into the combustion chamber at the critical low-lift stage. What we got was a mild improvement at both low and high lift, but a bit of a lag in the mid-range. Troutman felt the improvement was consistent with other heads that had been cut with the same design.

Motown 220

Finally, it was time to get the Motown 220 into the act. Without additional porting, the Motown can accommodate either 2.055 or 2.080 intake valves, while the Sportsman II will accept 2.020 or 2.055. To allow us to make a decent comparison, we cut one combustion chamber in each head to accept 2.055 intakes then tried them out on the flow bench (Chart D). As expected, the smaller port helped the bottom-end flow on the Sportsman II, but by 0.500 lift it had flatlined. The Motown really began to wake up at 0.300 lift and blasted past the Sportsman II, bettering the smaller head’s intake by 40 cfm at 0.700 lift. If you are in a class that limits lift to half an inch, the Sportsman II is definitely a consideration, otherwise the Motown is probably the way to go.

On the exhaust side, though, it’s a completely different story. The exhaust port is one area where the Motown 220 is definitely head-and-shoulders above the other heads (see Chart D). With both the Bow Tie and the Sportsman II, no combination ever produced more than 140.1 cfm of flow—the Sportsman II with a Pro Flow 1.600 exhaust valve at 700 lift. But the Motown, equipped with a standard Race Master 1.600 exhaust, was breathing nearly 140 cfm at just 0.300 lift and went on to a whopping 193.4 at 0.700! For comparison sake, we also fitted the Motown 220 with a Pro Flow exhaust, but the performance on the flow bench was no better.

Motown 220 (2.055 Vs. 2.080 Intake)

The last test was to compare the Motown’s two available valve sizes, the 2.055 and 2.080 (Chart E). The exhaust flow values are the same, since the exhaust valve was unchained. On the intake side, the 2.080 valve showed some improvement, but not as much as the increased size would have in an unrestricted environment.

“The flow improvement is there, but it isn’t a great amount,” Taylor says of the 2.080 valve. “Given the increased weight of the bigger valve (116.5 grams for the 2.055 vs. 124.8 for the 2.080, both Race Master stainless), I don’t believe it’s a good tradeoff. Given the combustion chamber, I think there’s a lot of shrouding with the bigger valve. The 2.055 seems adequate for this head, and I think it’s a plus when you factor in the weight. The valvetrain is very sensitive when it comes to weight in the valves, and weight costs you control. You can have a camshaft with very specific qualities matched up to the right springs and rocker arms, but if your valves are out of control, all those specs just went out the window.

Building the best valvetrain—and engine—is all about making the best compromises.”

Potential

Overall, both Troutman and Taylor pointed to the Motown 220 as having the best power potential. “Using it on a 350-size engine wouldn’t scare me,” Troutman says. “Bone stock on the same engine, the Sportsman II and maybe even the Bow Tie will probably produce more torque on the really low rpm because of the smaller port sizes. But at the higher rpm normally seen on a racetrack, that evens out a lot. Plus, there’s a lot you can do with the engine. I’d probably go with a smaller duration camshaft—stack as much lift on it as you can but keep the duration down—and change the lobe separation. You can also tailor the valve job and rod lengths to get the piston speed where you want it, and I don’t think you’d be missing a thing in the torque category.”

SOURCE
KT Engine Development
www.ktenginedev.com
World Products
Ronkonkoma
NY
6-31/-981-1918
www.worldcastings.com
Manely Performance Products
Lakewood
NJ  08701