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Turbo Engine: Turbo Small Block Build

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Nothing like the guttural roar of a turbo engine making 950 lb-ft at 4,000 rpm. It's like the last few seconds of a pool being drained or the chug of a tornado as it slurps up a lake. That kind of power tends to add stress to engine parts, and we've rolled the bones many times with the gods of physics, hoping they wouldn't fill the oil pan with flaming bits of crankshaft. To keep the noise and power and avoid the dry-cleaning bill, we've decided to assemble a long-block turbo engine that can take the load that a 1,000-plus horsepower pounding delivers with the Comp Turbo 67s on our Wrenchrat twin-turbo kit.

The Hammer was the last mule we built nearly five years ago. It was assembled by JMS Racing Engines in El Monte, California, with a Lunati rotator and Wiseco Pro Tru pistons. In a testament to JMS and the bullet-proof short-block, that turbo engine has seen the dyno whip more than 500 times without even a scuffed bearing. Why build another? Compression ratio. The Hammer has between 10.5-11.0:1 compression ratio depending on the combustion chamber and head gasket choices, so adding a lot of boost begs for race gas, taking the fun out of street thrashing.

The Animal will be stronger yet, with a compression ratio in the 8.5:1 area so we can try to control detonation. To add a little science to the mix, we'll explain why we chose each part of this turbo small block and what disaster we were trying to avoid. Since Ted Toki broke in the twin turbos on his '55 Chevy, he volunteered to assemble the new turbo engine at Westside Performance in Los Angeles, California.

Turbo Small Block: The Rotator
Our first plan was to build a 434-inch turbo small block, thinking bigger is always better. Except this time, that is. As the stroke gets longer, the piston gets shorter, and on a boosted engine you run out of dish room to get the compression down. For example, if you have a 9.020 deck and a 6-inch rod with a 4-inch stroke, you end up with a compression distance (CD) of only 1.025 inches from the wristpin location to the crown. Not only is that a bit too thin to hit with the boost hammer, the maximum dish is only 19 cc, since the small end of the rod is so close to the underside of the piston.

To get the 8.5:1 compression ratio we needed to pack in copious boost; we needed around 30 cc of dish, making the big-stroke/big-rod combo a no-go. We also looked at the 5.85-inch rod and 1.170-inch piston option. JE Pistons, for example, has a shelf piston for this combination called the Extreme Duty with a 23cc dish, but it still wasn't enough. Also, we didn't like the 5.70-inch rod and 1.320 CD piston for this application because of the piston's extra weight. Rod failures typically occur just after the piston reaches TDC on the exhaust stroke and the crankshaft is trying to pull it in the opposite direction. A big 4.155 bore combined with a tall piston creates a heavy slug on the end of the rod at 7,000 rpm. In this case, less is more, so we looked for something better.

We spoke to Sean Crawford at JE Pistons about our turbo small block to get his take on the situation. "I'd sacrifice the extra inches all day long to get a workable combination," Crawford says. "We should use thick lands, good rings, and don't compromise the piston [height] as much to make it work." The combo he recommended has a 3.875 stroke with a 5.85-inch rod for 420 inches. This piston has a 31cc inverted dome and a 1.227-inch CD. It represents a balance between the light-piston/long-rod and heavy-piston/short-rod combination and puts the piston 0.0055 below the deck on a 9.020 block. Using the Brodix 233 CNC heads with a 68cc chamber, it gave us 8.54:1 compression.

With the piston and rod combo handled, we spoke to Tom Molnar from K1 Technologies about a good crankshaft. Crankshaft quality is about strength and design. He told us that it is not the amount of horsepower you are making that fails a crank but rather the load and fatigue caused by the number of cycles at that load. "Look at it like bending a straightened out paper clip," says Molnar. "If you make small bends, it will last a lot longer than if you are making big bends. A higher load is a bigger bend failing to clip at the joint."

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