It’s obvious that hot rod parts have evolved tremendously over the past 70 years or so. One of the best examples is the small-block Chevy cylinder head. A scant 30 years ago, the iron “fuelie” casting was the single choice for any small-block, race or street. Today, dozens of specialized heads are available for every type of small-block imaginable.
Recently you may have noticed race-only small-block heads with unusual valve angle designations, such as Chevy’s race-bred, 18-degree head. This refers to the angle of the valve in relation to the deck surface. Standard small-block Chevy heads position the valves at a 23-degree angle to the deck. In 1989, Chevy introduced the 18-degree Bow Tie aluminum head that shifted the valve angle vertically while simultaneously raising the intake port to take advantage of this angle. The advantage of the valve angle change is improved airflow due to placing the valve in a more “upright” position relative to the piston. Raising the port also necessitated a dedicated intake manifold, requiring a sizable cash investment to switch to these heads.
THE TWIST
Trick Flow Specialties is an aftermarket cylinder head manufacturer best known for its excellent small-block Ford aluminum castings. Taking on the small-block Chevy market, it decided to take advantage of this valve angle change. Since interchangeability is so important, the first order of business was to ensure the head was a 100-percent bolt-on. This involved some compromise. First of all, the intake mounting flange would have to be compatible with the standard small-block Chevy intake manifold. Second, the valvetrain would also require significant modifications.
The intake valve was angled even more vertically, to a 13-degree angle. However, this valve angle change also required repositioning the intake rocker arm stud and mandated a new pushrod guideplate. These pieces are included with the Trick Flow Twisted Wedge head while retaining the use of stock-type rocker arms and pushrods. While the intake is dramatically altered, Trick Flow elected to leave the exhaust valve at its production 23-degree angle.
Altering the intake valve angle also changes the valve’s angle to the piston, which means the valve will not match the valve relief pockets machined into most small-block pistons. According to Trick Flow, camshafts with up to 238 degrees of duration at .050 tappet lift with a 106-degree lobe separation angle and .512-inch valve lift will work without requiring machine work to the piston valve reliefs.
THE TEST
Dyno testing is the only real way to evaluate engine components, so we collected all our parts and bopped over to John Baechtel’s Westech Performance Group facility to use his SuperFlow 901 dyno. To evaluate these new Twisted Wedge heads, we decided to baseline the test with a set of stock iron castings and then install the Trick Flow heads for comparison. The test engine was a 355ci small-block with flattop TRW pistons, a mild Crane HMV-266 dual-pattern hydraulic cam and a set of Crane 1.5-ratio aluminum roller rockers. The cam specs out at 210/216 degrees of duration at .050 with .440/.454 lift and a 114-degree lobe separation angle. The iron heads were rebuilt 882 castings equipped with 2.02/1.60-inch valves, guides, springs, screw-in studs and guideplates.
Since aluminum heads can tolerate a slightly higher compression ratio, Trick Flow equips the Twisted Wedge head with a 63cc combustion chamber (the 882 castings typically have 76cc chambers). Using a .041-inch gasket, compression computed out at 9.8:1, while the iron head compression came out at 8.9:1 using a .041-inch head gasket. The Trick Flow compression may seem high, but with aluminum heads this is a safe compression on 92-octane pump gas. In fact, Trick Flow recommends only 33 degrees of total timing, which also reduces the risk of detonation.
For induction we decided on a mild Edelbrock Performer dual-plane intake along with a matching Edelbrock Performer Q-jet carburetor. On the exhaust side, Hooker 15/8-inch primary tube headers vented the spent gas with an open exhaust, 18-inch exhaust collectors and Fel-Pro header gaskets.
As you can see from the power figures, the baseline combination made respectable power with the iron production heads, making 357 lbs-ft of torque at 3500 rpm and 294 hp at 5000 rpm. This is typical of a short-duration-cam small-block with stock iron heads and corresponds nicely with earlier tests on this engine. Now it was time to bolt on the Twisted Wedge heads.
With the engine still warm, we put on our Mechanix Wear work gloves and replaced the iron heads with the Trick Flow castings. With the ARP head bolts torqued, we installed a new set of Bosch Platinum spark plugs and buttoned up the engine with fresh Fel-Pro gaskets. After the engine was refired, we set the ignition timing at the same 36 degrees and ran the first test. Then we pulled the timing back to the recommended 33 degrees of total timing to evaluate that change. We were in the midst of studying jetting changes when our test came to an abrupt halt with the failure of a connecting rod (see “A Death in the Family”).
Despite the engine failure, we recorded a couple of full-power runs with the Trick Flow heads to give us a basis for comparison. Peak torque rpm increased from 3500 to 4000 and boosted torque to 392 lbs-ft (an increase of 35 lbs-ft), while horsepower improved 41 ponies from the baseline 294 to 335 at the same 5000 rpm. In addition, the Trick Flow heads carried the power further up the rpm scale, making 59 more horsepower at 5500 rpm over the stock heads (although the peak was still at 5000).
WHAT WE LEARNED
The flow bench tests revealed that the Twisted Wedge head enjoys a strong exhaust port compared to the intake port flow. This means these heads would probably perform best using a single- pattern camshaft with the same duration and lift on both the intake and exhaust lobes. Our test engine was equipped with a dual-pattern cam with more exhaust lift and duration, which is probably not necessary with these heads. We estimate by merely using a cam having the same duration and lift and by tightening the lobe separation slightly, the Trick Flow heads would probably jump to around 345 to 350 horsepower, for a 50-plus horsepower gain, while also pumping the torque curve.
We also believe that a single-pattern cam with duration of around 215 to 220 degrees at .050-inch tappet lift would make even more power, especially if combined with a high-rise dual-plane like the Edelbrock Performer RPM or Holley Contender intake. This cam choice would result in less idle vacuum and probably lope slightly at idle.
While the Twisted Wedge concept is not a radical improvement over other similar aluminum small-block Chevy cylinder heads, it delivers a comparable power increase over stock heads. One clear advantage is the price. Summit offers these heads (at the time this story was written) at about $200 a pair less than similar competitive aluminum small-block heads. The choice is yours. Wanna get twisted?
