Deep in the bowels of his secret laboratory, the wicked Dr. FrankenRat wrenches feverishly on his latest effort in horsepower reincarnation. His work consumes his every waking thoughtinjecting new power into the ageless Rat motor. His lifes work is focused on transforming that 60s heavy-metal monster into a next-millennium motor with greater power than ever before. The doctor is clearly obsessed with Rats and is equally contemptuous of those pesky small-blocks. Ill show those pitiful Mouse motors some real power, he mutters as he dotes over his latest malevolence. Step quietly into the crazed doctors lab, my friends. If we are quiet and abstain from impertinent questions, perhaps we might benefit from his evil genius...
OK, so maybe our dyno guru and engine flogger Ed Taylor isnt the automotive equivalent of the original Dr. Frankenstein, Ken Duttweilers dyno room isnt quite as imposing as the doctors laboratory, and, unfortunately, we couldnt find anybody warped enough to impersonate Igor, the humpbacked assistant. But none of that reality detracts from a clearly powerful story. When it came time to do a story about a big-block crate engine, the obvious engine to choose was the GM Performance Parts 454 H.O. Just last year, we bolted a couple of mild improvements into the GM Performance Parts 502/502 engine and sent it to Minnesota as a Grand Prize giveaway. This time we thought wed look at a more affordable Rat; an engine that the average enthusiast might consider if he were looking for big-inch torque and horsepower. We will treat this Rat motor just like we did our original crate engine buildup of the 350ci Goodwrench engine. We have a multipart series planned for this 454 that should deliver some interesting technical material as well as a few surprises. Well spend this first part getting acquainted with the big lug.
Our FrankenRat 454 is a little like its fictional counterpart. The Gen VI engine uses pieces that have evolved out of the primordial ooze of the original 65 Mk IV 396. However, it is not really a true Gen VI in the same sense as a production Gen VI big-block. Well take a quick look at what differentiates the Mk IV Rat from its current Gen VI cousin. The GM Performance Parts H.O. 454 has its roots in the production Gen VI iron block, but there are several changes. The most obvious is that the 454 H.O. block benefits from retaining the mechanical-fuel-pump boss on the passenger side of the block for the legions of carbureted fans. Other improvements that began with the Gen V in 1991 include a one-piece rear-main seal, provisions for a hydraulic-roller camshaft, a priority main oiling system, and a net-lash rocker system (more on that later). The Gen VI engines received hydraulic-roller cams in 1996.
Another significant difference for this 454 H.O. engine is that it employs a 1053-forged-steel crankshaft instead of the production nodular-iron piece. As a result, the H.O. engine uses a specific externally balanced flexplate that is not the same version thats on the production engine though the damper is the same. There is also a six-bolt aluminum timing-chain cover that is specific to the Gen VI Rat motor. Because of the one-piece rear-main seal, the Gen VI also requires the matching one-piece oil pan.
The heads used on the 454 H.O. are iron rectangle-port heads that are the service-replacement heads for the late-model Gen V and VI 454 engines. The heads feature a 118cc combustion chamber, 2.19-/1.88-inch valves, and a 325cc intake port. This head will not interchange with earlier Mk IV blocks since several coolant passages have been relocated.
Its also not a good idea to try using production Mk IV heads on a Gen V or VI block for the same reason. However, all aftermarket Rat heads will interchange on the Gen V and VI blocks, because those companies have modified the heads to seal the offending coolant passages. The other major cylinder-head change utilizes what GM calls a net-lash rocker system. Because of the complexity of adjusting valves on the independent valvetrain system, GM eliminated the adjustability using a bolt that passes directly through the rocker arm, a ball fulcrum, and a stand. This works fine with the production camshaft but causes geometry problems when it comes time to swap to a performance camshaft. This is one of the items well swap for next months upgrade for FrankenRat.
The rest of the 454 H.O. is fairly straightforward big-block fare, complete with an aluminum dual-plane that looks to be a direct descendant of an earlier factory dual-plane intake with the standard big-block intake bolt pattern. On the exhaust side, the iron heads still accept the standard big-block exhaust flange, so at least there is no drama here.
Once we were familiar with our latest rotund rodent, it was time to bolt him up on Ken Duttweilers dyno and pull the handle to measure FrankenRats mettle. Since big-blocks are often used in trucks, we decided that some enthusiasts may use stock-type exhaust manifolds for the ease of maintenance, so thats how Test 1 was configured. We added a pair of iron Rat exhaust manifolds feeding into restrictive 2¼-inch exhaust pipes plumbed into a pair of generic turbo mufflers. While this exhaust system appears small, we figured it would be representative of a stock-type exhaust system. Up top, we added a Holley PN 0-4779C 750-cfm mechanical-secondary carburetor and a Performance Distributors DUI HEI distributor.
The Rat delivered impressive torque at the very bottom, testing at 2,400 rpm with 415 lb-ft of torque. It grunted up to 450 lb-ft at a >> still-low 3,400 rpm, which makes this Rat a natural for a tow rig or even a fullsize RV. The horsepower was limited by the iron exhaust manifolds but was still capable of 404 at 5,600 rpm. These numbers are lower than GM Performance Parts claims for the engine, but thats because the GMPP baseline test used headers similar to Test 2. Still, this is excellent power for a near-stock 454.
Test 2 reveals what happens when this Rat is aerobicised with a set of headers. As you can see from the dyno chart, headers are probably the single most important component you could bolt onto the big-block to enhance performance. This is especially important since the rectangle-port big-block heads are not known for their excellent exhaust-port flow, so any improvement on the exhaust side will do wonders to unlock power. This boosted torque from 450 lb-ft to a solid 500 lb-ft. Better yet, the entire torque curve improved. Even at 2,400 rpm, the torque was up an amazing 35 lb-ft over the iron manifolds.
Horsepower was also a big gainer with a jump up to 432 at a still-reasonable 5,300 rpm. The low peak number is related to the very-shortduration cam that this engine comes with, and to a valvetrain fuss point that kills power above 5,400 rpm. This is slightly more than the rated power of 425 hp, so its clear that this engine has much more potential, which well bring out by adding a couple of performance accessories.
Since this is such a big engine, we were curious to see if a larger carburetor would help, so the good doctor Taylor rounded up an 830-cfm Holley HP-series carburetor and bolted it on. This accounted for gains of as much as 10 lb-ft of torque and 10 hp virtually all the way through the curve. This is a more expensive carburetor, but it clearly delivers.
We have just barely begun to tweak on our good friend FrankenRat. Next time, well install Cranes slick net-lash conversion kit, which will allow us to run roller rockers with more ratio. So dont let our pal FrankenRat scare you away from Rat motors and mondo torque. We have only just begun the power reincarnation process!