358 Small Block Cylinder Head & Rocker Arm Upgrade - A Different World
When It Comes to Heads for This Hot 358, Bigger Is Better
From the May, 2009 issue of Chevy High Performance
By John Nelson
Photography by John Nelson
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.
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.
We picked up more horsepower and torque and ran a three-tenths-better e.t.
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.
|WHAT'S IT FLOW? |
|WORLD MOTOWN 23 235CC HEADS |
|LIFT ||0.200 ||0.300 ||0.400 ||0.500 ||0.600 ||0.700 |
|INTAKE ||128 ||188 ||235 ||270 ||281 ||285 |
|EXHAUST ||140 ||165 ||183 ||193 ||200 || |
For our head upgrade, World...
For our head upgrade, World Products recommended a set of its Motown 23-degree aluminum heads with a 235cc runner. These as-cast heads are all about providing lots of airflow for big-cubic-inch small-blocks, so they were certainly up for the job in our aggressive 358 combo.
Everybody gets kudos when...
Everybody gets kudos when a project goes well, and we have to give a little extra on this one to Manley. World sent us the heads, guideplates, and the proper springs for our solid cam so we could make our dyno day. Manley provided the rest on short notice: +0.100-inch Race Master valves, seals, locators, retainers, and keepers.
We decided to make life easy...
We decided to make life easy on ourselves and hauled the heads and all the parts over to BEP Performance in Orange, California, where owner Allan Bessant quickly completed the cylinder head assembly.
Bessant also showed us a neat...
Bessant also showed us a neat trick for installing valve seals if you don't want to spring for a special tool. Just push it on with a retainer, and you're good to go. It may seem obvious, but it was news to us.
The valves in these heads...
The valves in these heads are 0.100 inch longer than stock, so we measured for proper pushrod length with a checking tool. The length came out to 7.900 inches; we picked up a set of Manley's one-piece 4130 chrome-moly rods to do the trick.
World sent out the 3/8-inch...
World sent out the 3/8-inch studs and guideplates we needed-don't forget to use thread sealer on the studs, as they do touch the water jackets.
|DYNO DETAILS |
|Carburetor ||Mighty Demon 750, 80/91 jets, 2.5 power valve, 2-inch open spacer and nitrous plate |
|Intake manifold ||Edelbrock Super Victor |
|Ignition ||MSD 6A |
|Timing ||36 degrees |
|Exhaust ||Hooker Super Comp 1 3/4-inch long-tubes with 3-inch X-pipe and Flowmaster mufflers |
|Fuel ||91/110 octane unleaded mix |
Here's something to remember...
Here's something to remember when setting up these heads-the guideplates can be installed either side up. Depending on which cylinder you're on, one side or the other will provide proper pushrod alignment.
We keep mentioning that the...
We keep mentioning that the World Sportsman II heads this Chevelle was wearing were ported-side by side, you can see why we needed to step up to 235cc heads to get more air in and out of this 358ci.
The 235cc heads require use...
The 235cc heads require use of an offset rocker arm on the intake side. We contacted Scorpion for a set of its Race Series rockers and upped the ratio to 1.6:1 while we were at it. You get eight regular rollers and eight offset pieces in the set.
On the motor, the need for...
On the motor, the need for the offset rocker arms was made readily apparent. The extra 0.150 inch provides the clearance for the big intake ports and the longer valves these heads are fitted with to accommodate them.
Lash on the solid flat-tappet...
Lash on the solid flat-tappet Ultradyne cam was set at 0.16-inch intake and 0.19-inch exhaust. With the 1.6:1 rockers in place, cam specs came in at 0.627/0.622-inch lift intake/exhaust, with 276 degrees of duration on both sides. Lobe separation angle is 106 degrees.
All buttoned up and ready...
All buttoned up and ready for its retest, our big-headed small-block put on 16 more horsepower at peak-which happened 500 rpm later-while still averaging 4 hp more throughout the powerband. It paid off at the track, where Daeseleer piloted his enhanced Chevelle to a quarter run of 12.35 at 111 mph, three-tenths faster than before. You gotta like that!