FrankenRat, Part III

Tired of fooling around with a mild camshaft, our demented Dr. FrankenRat has bolted a serious camshaft into the monster motor in hopes of achieving 500 hp. The doctor works feverishly, at first trying a slightly bigger cam, then letting it all hang out in hopes of big power. As the monster cackles to life, the doctor anxiously anticipates the power run, pacing the floor of Duttweiler’s dyno cell as his reanimated creation surges against the dyno and the power needle swings toward a big horsepower number…

It’s surgery time in the FrankenRat lab. So far, we’ve dabbled with bolt-ons and minor cylinder-head mods in hopes of making better power. But now it’s time to get serious. It’s time to open up this engine and let it breathe with a bigger cam. We consulted the Crane Cams catalog and picked a pair of hydraulic-roller cams that looked as if they could bring something to the horsepower party without sacrificing too much low-speed torque.

With the large-by-huge 318cc rectangle-port iron heads on top of a 454ci engine, inlet velocity through the intake ports is probably not the best, which tends to sacrifice some power. So by keeping the cam specs mild, we figured we might be able to squeak 500 hp out of this big-block since that would be right at our 1.1-hp/ci threshold. Since FrankenRat has always been targeted as a street engine, the plan was to achieve this 500hp power level without resorting to a long-duration cam that would kill torque.

There are several important details pertaining to these late-model Rats that require specific attention. Ignore them and you will break parts right out of the box. The first thing we learned with the GEN V and VI Rat blocks is that there is a limit to the amount of lobe lift the stock lifters can accommodate. The best way to increase lobe lift with any performance cam is to reduce the size of the base circle. This keeps the lobe smaller than the cam-journal diameter. Otherwise, the cam physically won’t fit in the engine. Reducing the base circle also drops the lifters deeper into the lifter bores.

This may not seem important, but it is when you consider that all factory hydraulic-roller lifters are retained with a cast bar that fits over the top third of the lifter body. When a reduced–base-circle cam is used, the lifter can actually drop out of the bottom side of the retainer, causing all kinds of havoc when the lifter turns sideways and gouges the camshaft. We’ve seen it happen!

The simple fix is a set of long-body Crane hydraulic-roller lifters. The longer body’s additional height allows it to remain in contact with the factory retainers when used with an aftermarket camshaft. Unfortunately, these lifters are not only expensive but also heavier. This places an additional load on the valvesprings in an already heavy valvetrain. That’s a lesson we learned last month when the stock valvesprings couldn’t control even the addition of a set of roller rockers. We added a set of Crane roller valvesprings with much more pressure, but this is an ongoing problem with these hydraulic-roller–tappet big-blocks. More spring pressure is not the answer because the additional pressure will actually push oil out of the lifters—pumping them “down” and creating massive clattering from the excessive clearance created when the engine comes down from a high-rpm run. If you recall from last month’s story, we retained the dual Crane valvesprings with a rate of 478 lb/in and a seat pressure of 132 pounds at 1.906-inch installed height.

Despite all this, dyno man Ed Taylor braved the big-block hazards and started with a conservative cam with 226/236 degrees at 0.050, roughly 15 degrees more intake duration compared with the original cam, with a lift of 0.587/0.610 inch for the intake and exhaust. We decided to also include the Crane 1.7:1 roller rockers we had tried last month since we now had better valvesprings. The slightly bigger cam only bumped the torque slightly, with peaks of 529 lb-ft at 3,800 rpm and 480 hp at 5,600 rpm, but we didn’t think the power increase justified the investment. We decided the next-larger cam in the Crane catalog might be a better choice. Once again, Taylor yanked the sutures on FrankenRat and opened him back up.

This time, we went with a more aggressive Crane hydraulic-roller cam with 236/244 degrees at 0.050 and 0.610/0.632-inch lift using the 1.7 roller rockers. We were now tantalizingly close to that 500-peak-horsepower mark. On the first few pulls, Taylor hammered on FrankenRat and saw horsepower numbers of 494, 495, and 498, but 500 eluded him. Finally, after several attempts, Taylor matched a hot oil temperature with a cooled intake manifold, and that combination squeaked FrankenRat up to an even 500 hp at 5,700 rpm with a peak torque of 530 lb-ft at 3,900 rpm. This gave the Rat a fairly wide 1,800-rpm powerband between peak torque and peak horsepower.

The bottom line is that we’ve now made 500 hp with FrankenRat and as much as 530 lb-ft of torque. That’s enough to push a typical 3,600-pound Bow Tie beater into the 11s with decent traction. As odd as it sounds, we think these factory rectangle-port heads may be choking some of FrankenRat’s pure potential as a monster for the street. We have some ideas on different heads, but we’ll save that for next time.

Click Here To View FrankenRat, Part I
Click Here To View FrankenRat, Part II