FrankenRat, Part II

The evil Dr. FrankenRat is less than satisfied. His monster is alive, but not yet powerful enough. So far, his tests have been superficial with minor surgery aimed at better breathing. Our dour doctor is impatient to achieve his goal of over 500 hp.

Last time we introduced you to Project FrankenRat, the GM Performance Parts 454 H.O. engine that we intend to flog on for several more installments. In our first episode, Dr. Ed Taylor performed a couple of bolt-on power parts procedures that netted us 505 lb-ft of torque at 3,600 rpm and 441 hp at a mere 5,400 rpm. The engine is equipped with a mild hydraulic-roller camshaft, which is the main reason peak power arrives at a relatively low 5,400 rpm. Torque is still impressive at 505 lb-ft, especially when the engine also makes a healthy 450 lb-ft at 2,400 rpm. That would flatten your eyeballs, assuming you could hook it to the ground.

But now it’s time to step up again. The easy thing to do would be to bolt in a big cam and see what kind of power it makes, but many enthusiasts prefer to do less dramatic changes that don’t involve going that deep into the engine, so we decided to try some simpler tasks first. Last month FrankenRat ended up with an 830-cfm Holley carburetor, a GM Performance Parts dual-plane intake, and a set of Hedman 1¾-inch headers, all running through a set of Borla’s new Pro-XS stainless steel 2½-inch mufflers.

The first thing we decided to do was bolt on an Edelbrock Performer RPM Air Gap intake. After the simple swap, FrankenRat responded well to the changes, adding torque throughout the entire power curve—as much as 20 lb-ft in a couple of places and never less than an additional 15 lb-ft between 2,600 and 4,700 rpm. That’s a strong increase for a simple swap. This encouraged us to try something else.

Since our FrankenRat 454 H.O. is a Gen VI engine, it also utilizes a net-lash rocker system that does not allow the valvetrain to be adjusted. The system differs from the typical small- or big-block adjustable rocker with a bolt that runs down through the rocker and tightens against the head. Crane offers a net-lash conversion kit that uses an ARP stud that bolts into the rocker bolt hole in the head, allowing the use of either stock stamped rockers or Crane’s aluminum roller rockers. We decided to try this system on the FrankenRat.

The reality of testing engines is that not everything we test is successful. This is one of those cases. The Crane system bolted right on with no problem, using a set of Crane Gold 1.7:1 roller rockers to replace the stock stamped-steel pieces. However, once Taylor spun the engine through its powerband, he didn’t have to even look at the report on the computer screen to know the engine didn’t like this setup because he could hear the engine run into valve float at the end of the test. The numbers merely reflected what he heard.

This has nothing to do with the quality of the Crane net-lash conversion parts. The problem lies with the big-block’s inherent valvetrain weight. Adding a heavier rocker with a more accurate ratio that accelerates the valve quicker contributed to a valve-float problem that the stock valvetrain just barely avoids. While the engine lost 18 hp at 5,500 rpm compared to the previous test, what was interesting is that FrankenRat lost power throughout the entire pull, averaging a loss of 5.3 hp for every point in the entire rpm band!

While we call this valve float, the reality of the situation is actually a loss of control of the valves as they approach the seat. The heavier intake valve tends to bounce off its seat on the closing side, allowing cylinder pressure to escape. This is only a minor problem at lower engine speeds, but you can see that it becomes nasty at 5,500 rpm. Taylor immediately went back to the stock rocker system, and the engine came right back to its previous power level.

The simple fix would have been to merely add more valvespring to the stock heads. But we decided to go ahead and yank the heads and do several modifications all at once. Taylor removed the iron Gen VI heads and had Duttweiler Performance open up the intake valve seat from its stock 2.18-inch diameter to 2.25 inches using Manley’s Street-Flo stainless steel valves. We also swapped in a set of Manley exhaust valves but kept the stock 1.88-inch diameter. The larger valve seat also necessitated an abbreviated pocket-port session, but Taylor only spent about two hours total on the heads just removing any sharp edges under the valve seat.

Finally, Taylor also had Duttweiler machine out the valvespring pocket area to use a larger Crane dual spring with a damper and then added a set of 1.8:1 roller rockers to the package to increase the valve lift slightly over the stock 1.7:1 ratio. These 1.8 rollers are a bit more expensive than the 1.7:1 rollers, so you might want to consider sticking with the 1.7 ratio, especially if you plan on changing cams anyway. We also had to change valve covers for more clearance when we added the roller rockers.

With FrankenRat all buttoned back up using new Fel-Pro head and intake gaskets, Taylor again bolted the Rat to Duttweiler’s dyno and ran a series of timing and jetting exercises that culminated in a max of 528 lb-ft of torque at 3,700 rpm and peak of 465 hp at 5,200 rpm. This is a total improvement over last month’s story of as much as 27 hp and 31 lb-ft of torque, but we expected to see larger gains based on the changes we made. Realistically, the real limitation here is still the rather short-duration (211 degrees at 0.050) >> camshaft. To really take advantage of the rectangle port heads and the work put into them, this monster Rat needs a longer-duration camshaft of at least 230 degrees at 0.050 with more lift to get the maximum power out of the current combination. We’ll work on that for next month.

In retrospect, we probably should have bolted the camshaft in first along with a set of better valvesprings and the net-lash conversion. That would have gotten us further along. But this test is still valid since it illustrates what works and what doesn’t on this Rat. Even if you leave the valvetrain stock and add just headers, intake, and a good-sized carburetor (which are all easy bolt-on parts), this Rat will easily make almost 520 lb-ft of torque at 3,500 rpm and a solid 450 hp. For many Rat aficionados, that’s plenty of power. The problem is, we want to make more. So watch for the next episode where we bolt in a Crane hydraulic roller cam and make even more power!

Click Here To View FrankenRat, Part I

Click Here To View FrankenRat, Part III