When we last visited the evil Dr. FrankenRat, he had convinced us to bolt a blower on our Scoggin-Dickey-supplied 454 H.O. engine. This simple step (OK, it was expensive too!) was worth 825 hp and 751 lb-ft of torque at 4,900 rpm. Rather than getting greedy by trying to better those numbers, we decided to tame our savage Rat and return to the land of the normally aspirated with a set of oval-port heads.
Some may question the need to try a set of oval-port heads. Our reasoning starts with the concept that Rat motors are finicky engines. Everybody thinks that these big engines need a monster head to feed those large-by-huge cylinders. That may be true for 1,000hp Pro Stock engines, but for a street-destined 454, we think there may be some torque to be had by employing the services of an oval-port head.
Our idea was to make more torque, even if we had to sacrifice a little peak horsepower to get there. Why do this? Because despite the fact that everyone talks about horsepower, torque should get equal billing. Combine strong torque with good horsepower and you have an outstanding street combination that's fun to drive and delivers impressive numbers at the dragstrip. This particular combination is intended as a mild pump-gas big-block with a goal of big torque and good throttle response. This could be used in a mild Chevelle or Camaro street combo, a pickup for towing (although we'd suggest one step smaller on the cam), or even in a ski boat where low-end torque is king.
In Part IV of our FrankenRat series, we tried a set of World Products Merlin rectangle-port heads packin' a 310cc intake port, 2.300/1.88-inch valves, and a 119cc combustion chamber. As you can see by the power curve we included (Test 1), with our Crane cam's large 236 >> degrees at 0.050-inch tappet lift, the rec-port Rat made an honest 512 hp at 5,700 rpm and 518 lb-ft of torque at 3,900 using an Edelbrock Performer RPM Air Gap manifold. Rather than search for a set of production iron oval-port big-block heads, we went with a set of World Products Merlin oval-port heads. These heads feature an identical 119cc chamber but with big 2.30/1.88-inch valves and--most importantly--a smaller 269cc intake-port volume. Compared to the rectangle-port heads, this is a reduction of 41 cc's, which theory says should be worth low-speed torque and throttle response--all good things for a torquey street Rat. The comparison would also be more equal since the compression ratio would remain the same at 8.75:1. In fact, this is one place where we could probably pick up perhaps 2 percent more power by increasing the static compression three-fourths of a ratio up to 9.5:1 with this size camshaft and still be able to run on 91-octane pump gas.
The rectangle-port heads enjoyed the advantage of a 30-degree back-cut on the intake valves before we bolted them on the engine--something we didn't do with the oval-port heads.
To maintain a level playing field, we decided to install the same Crane valvesprings that we used on the rectangle-port heads since we had run into so many problems with valve float on the previous test with the rectangle-port heads. We also tried 1.8:1 roller rockers at one point but suffered the same float difficulties that plagued us in the previous test.
For our first foray into testing the oval-port heads, we retained the larger of the two Crane hydraulic-roller cams from the rectangle-port head test in the Jan. '03 issue. This turned out to be disappointing, since FrankenRat let us down by >> underachieving on both the torque and horsepower compared to the rectangle-port power curve. We chalked this up to a "too-big" camshaft and installed the milder camshaft to complement the torque-enhancing oval intake ports. The smaller cam offered 10 degrees shorter duration while still delivering valve-lift numbers close to 0.600-inch lift in order to give the oval ports a chance to work.
Bolting this package together resulted in a torque gain right off idle with torque numbers closer to what we were anticipating. As you can see from the dyno curve, FrankenRat liked the shorter duration, closing the intake valve sooner and allowing the engine to make more cylinder pressure at lower engine speeds to enhance the torque. This combination cranked out a dazzling 448 lb-ft at 2,400 rpm with torque peaking at 523 lb-ft at 3,700 rpm. When you bolt on smaller intake ports and a shorter cam, you must expect to lose horsepower at the top--that's the inevitable compromise. FrankenRat actually lost more than we expected, peaking at 459 hp at 5,550 rpm. This is a difference of as much as 62 hp at 5,700 rpm from the rectangle-port/big-cam combo that made 512 hp at 5,700 with the Performer RPM dual-plane. The single-plane World manifold made even more with 523 hp but was down on torque. Again, the engine struggled to overcome its valve float difficulties that certainly didn't help horsepower production. This is due in part to the heavy hydraulic-roller tappets and the large 2.300-inch valves.
But don't be too disappointed. We plugged this conservative oval-port/mild-cam power curve into our now-familiar 3,600-pound Chevelle using Racing Systems Analysis' Quarter Pro computer program. The simulation uses a 3.55 rear gear, TH400 trans, a 2,600-stall converter, and 26x10 sticky street tires. We also simulate at 100 feet of elevation with a 72-degree F day with good air, so this is like the perfect pass that the engine and car could muster. Despite the mild curve, the simulation says the car has the potential to produce an 11.68 pass at 117.60 mph, which is plenty stout. Even if the oval-port combo only runs 12-flat, that's still impressive! With the same gearing and converter, the rectangle-port combination was worth 11.31 at 122.2 mph, so you can see that the big heads and that extra 50 hp is certainly worth roughly 0.40 second and 5 mph.
We were about to conclude our fat Rat testing when we learned a couple tricks that might increase the power on FrankenRat. If these ideas work, we'll be back with a full report.