Gaining more power from a proven combination can be tricky. It's easy to do too much, forgetting that "bigger is better" isn't always true.In this case, we were looking to extract more power-and faster e.t.'s-from the 11:1, 358ci solid flat-tappet small-block in Matt Daeseleer's '70 Chevelle. The car had a best run of 12.68 at 110 mph going into the project, so it was no slouch, and we certainly didn't want to mess up a good thing. Fortunately for all involved, we didn't. Moving more air through this motor with a set of big, 235cc runner Motown 23-degree cylinder heads, teamed with a bump in cam specs via a set of Scorpion 1.6:1 rocker arms, proved to be exactly a formula our test subject liked-especially at the track.
Daeseleer's A-body is set up for heavy breathing. It was fitted with a set of World Products Sportsman II iron heads-still certainly a good head, and these had been massively ported. The carb is a 750 Mighty Demon, the intake a Super Victor with a 2-inch spacer, and the Hooker headers have big 1 3/4-inch primaries. The Ultradyne solid cam spec'd out, with 1.5:1 rocker in place, at 0.588/0.583-inch lift on the intake/exhaust, 276 degrees of duration on both sides, and an aggressive 106-degree lobe separation angle. In other words, this is a strip/street combo rather than vice versa.
We did what you should do: called up World, described our combo, and asked for advice. We were a bit surprised at the recommendation they gave us, which turned out to be a set of Motown 23-degree aluminum lungs with 235cc runners. Those seemed awfully big for a "mere" 358. On the other hand, we were looking to improve an already aggressive combo, so we needed to move more air; that we were starting with a set of heavily modified heads in a car that sees most of its duty at the track led the World team to this conclusion, so we went forward.
After making baseline pulls on the Mustang chassis dyno at the Source Interlink Tech Center, it was out with the old and in with the new. Installing the new Motown 23-degree heads was relatively straightforward, but there are a couple wrinkles to keep in mind. One is that these heads require offset rocker arms on the intake side due to the longer length intake valves and big ports. Since the 1.5:1 rockers Daeseleer had on his car weren't going to work, we called up Scorpion for a set of its 0.150-inch-offset Race Series rockers. While we were at it, we decided to amplify our cam specs by stepping up to a 1.6:1 ratio.
For this project, circumstances dictated that we order our Motown 23-degreees in bare form-World also sent us the appropriate springs for the solid flat-tappet cam in our subject 358, along with 7/16-inch studs and guideplates. We completed the package with Manley valves, oil seals, spring locators, retainers, and locks, all installed by Alan Bessant at the conveniently close BEP Performance Center. Manley also provided us with the correct pushrods, after we installed the heads and used a checker tool to determine the length needed. It was 7.900 inches, 0.100 inch longer than where we started.
Other than that, the only snag in the fabric was that these heads are wider than the Sportsman II's, so installing the driver-side headers around the steering shaft was an adventure. It required some finagling, as in removing the shaft and loosening up the headers, but with patience it can be done. With 13/4-inch headers like Daeseleer runs, we also had to disconnect the neutral lock linkage that runs from the steering column to the frame then over to the transmission. We'll seek a solution to that later and check to make sure we're not in gear before turning the key for now
Which led us back to the dyno for our "after" runs. Long story short, we gained power-16 hp at peak, along with a nominal 4 lb-ft of torque. The important thing here is that while we did lose some horsepower and torque between 3,900 and 4,300 rpm, the new combo leaped back ahead at that point and stayed there. At the top end, peak power happened 500 rpm later, at 6,000 rpm, and stayed strong post-peak, still making 320 hp at 6,200 rpm. At the track, this translated to the car's typical 1.80 60-foot times, while the stronger top end lopped three-tenths off the Chevelle's e.t, bringing it in at 12.35 at 111 mph. It's a nice gain, with power happening where this car wants to run at the track. In this case, bigger is better.
Quick Notes
What We Did
Swapped out the ported Sportsman II heads in a '70 Chevelle in favor of a set of 235cc Motown 23-degree aluminum lungs and Scorpion 1.6 rocker arms.
Bottom Line
We picked up more horsepower and torque and ran a three-tenths-better e.t.
Cost (Approx)
$2,550
 Our first step as we began...  Our first step as we began our runs was to do a little carb tuning to make sure we were making maximum power in baseline form. To facilitate this, car owner Daeseleer had an O2 sensor bung welded into his exhaust system. We ended up with an 80/91 jet combo and a 2.5 power valve in the Mighty Demon 750 carb. |  Daeseleer then dove in and...  Daeseleer then dove in and went to work as only a hands-on car owner can, quickly stripping away the Chevelle's intake system and preparing to remove his tried-and-true, mondo-ported Sportsman II iron heads. |  The semi heart-shaped combustion...  The semi heart-shaped combustion chambers feature a 2.08/1.60-inch valve combination with some bowl blending work done-the 64cc volume maintained our test subject's 11:1 compression ratio. |