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Project FrankenRat, Part I

Power-Building GM Performance Parts’ 454 H.O.

Jeff Smith Nov 1, 2002

Step By Step

This is the way the 454 H.O. engine comes from GM Performance Parts.

Make sure to check that all the coolant plugs are in place once you have the engine in the car. Always pressure-lube the engine before firing it for the first time.

The first thing we did after painting the engine Plastikote Orange was bolt on a Holley 750 mechanical-secondary carb. The double-pumper (PN 0-4779C) is a perennial performance favorite.

For ignition, we added a Performance Distributors HEI sparker and a set of plug wires to complete the ignition system. Just remember to ensure a full 12 volts to the distributor when it’s in the car.

The Gen VI block offers several revisions to the basic Mk IV design, but the most significant difference may be the net-lash system. Note the bolt that runs directly through the rocker that does not allow changes to lash. When you add a bigger cam, you’ll also need to convert the net-lash design to an adjustable rocker. We’ll show you how to do that next time.

For the first dyno-test, we bolted on a set of iron exhaust manifolds using restrictive 2-1/4-inch pipes and a pair of turbo mufflers. Power was acceptable, but what this Rat really needed was a set of headers.

The next test added a set of Hedman 1-3/4-inch headers for a Chevelle that brought FrankenRat to life.

We included a pair of Borla’s new Pro XS street 2-1/2-inch stainless steel mufflers with the header test. The good news about these babies is that they are priced competitively yet flow like a Borla race muffler.

Under the intake manifold on the FrankenRat 454 H.O. you’ll find a slick oil splash shield that keeps hot oil off the bottom of the intake manifold to reduce inlet-air temperatures in the intake manifold.

The Gen VI block uses different-shaped coolant passages in the deck surface that prevent the use of production Mk IV heads. However, most aftermarket Rat heads are now designed to accommodate both Mk IV and Gen VI configurations.

We made best power by bolting on this 830-cfm Holley HP-series carburetor in place of the 750-cfm carb. With a few minor exceptions, the engine made more power everywhere with the larger carb.

To ensure that everything stayed lubricated, we used Valvoline’s Durablend 10W40.

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 thought—injecting new power into the ageless Rat motor. His life’s 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. “I’ll show those pitiful Mouse motors some real power,” he mutters as he dotes over his latest malevolence. Step quietly into the crazed doctor’s 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 isn’t the automotive equivalent of the original Dr. Frankenstein, Ken Duttweiler’s dyno room isn’t quite as imposing as the doctor’s laboratory, and, unfortunately, we couldn’t 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 we’d 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. We’ll spend this first part getting acquainted with the big lug.

Up Close

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. We’ll 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 that’s 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.

It’s 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 we’ll swap for next month’s 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.

Testing Time

Once we were familiar with our latest rotund rodent, it was time to bolt him up on Ken Duttweiler’s dyno and pull the handle to measure FrankenRat’s 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 that’s 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 that’s 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-short–duration 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 it’s clear that this engine has much more potential, which we’ll 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, we’ll install Crane’s slick net-lash conversion kit, which will allow us to run roller rockers with more ratio. So don’t let our pal FrankenRat scare you away from Rat motors and mondo torque. We have only just begun the power reincarnation process!


Scoggin-Dickey Parts Center
Lubbock, TX 79424
Performance Distributors
Memphis, TN 38132
Hedman Hedders
Whittier, CA 90602
Holley Performance Products
Bowling Green, KY 42101
Racing Systems Analysis
Phoenix, AZ 85022-5736
Friendly Chevrolet
Fridley, MN 55432
Sallee Chevrolet
Milton-Freewater, OR 97862
Valvoline Co.
Lexington, KY 40512



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