In racing, you want all the power your motor can make to reach the drive wheels. To that end, we commonly use steel motor mounts to replace the factory rubber models on Hobby Stocks or Street Stocks. When the rules for more advanced classes allow it, front and rear motor plates hold the engine rock steady. Any severe driveshaft angles cause a power loss. We can also be sure to dial in the rear pinion angle to keep all of the power coming out of the engine when it's at maximum torque.
There is one more area that can be tweaked to ensure there is no drag on your driveline. Although it involves using a dial indicator, it's not that difficult to accomplish. It might just extend the life of some of your driveline components. If you already dial in your bellhousing, you are probably in the minority of those in racing who take advantage of this simple but effective procedure.
Start with the block plate...
Start with the block plate in place. Add the flywheel, but not the clutch, on a block with a crankshaft.
Friction is a big culprit. It works against you in two ways. The first is friction on the driveline, which robs horsepower. Second, friction due to misalignment can cause premature failure of driveline components such as U-joints, driveshafts, clutches, pressure plates, throwout bearings, rear ends, and-worst of all-your expensive transmissions. The basic idea is to make sure the bellhousing you use is perfectly in the center of the crankshaft coming out of the engine, not the center of the block. The rear end of the crank lines up with the input shaft of the tranny. All of the aforementioned components will work off the proper alignment of that input shaft.
Bad alignment of the bellhousing and its related components comes from a number of sources, the most common being core shift, when the block has flexed permanently for a variety of reasons. Most blocks, iron or aluminum, are cast and contain plenty of voids such as the bores, water passages, and even the overall design of the block. They can shift when torque and/or heat are applied from assembly, rebuilds, or engines running at operating temperature. Casting blocks is not an exact science. Some variations are not only tolerated but expected due to the inconsistencies of flowing a liquid metal into a form or mold and then having it cool under a variety of conditions.
Lower the bellhousing into...
Lower the bellhousing into place.
Another culprit is align-boring of the block. Align-honing will do it but to a lesser degree, as less metal is taken off. When align-machining is done, the centerline of the crank is affected. Even if it is slight, that's just enough to throw off the bellhousing. Also consider any other machine operation that may be done to the block. Whenever any material is taken off the block, the core can shift and throw your alignment off.
Even the bellhousing itself can be off. It is, after all, a machined part susceptible to variations. It may start within tolerance but move when attached to a block slightly out of tolerance. Though most bellhousings will be in tolerance, it's good insurance to find out just how the two match up. The transmission centerline is another source of runout that can kill components. If the input shaft or the bearing retainer on the front of the tranny is off, a misalignment of the block and bellhousing can exaggerate the runout.
In a bad-case scenario, a misaligned tranny will start to wear out the input shaft. The front bearing retainer will also wear and cause bad shifts, even to the point of hanging up a shift. If the clutch is thrown out of alignment from the input shaft running out of round, it cannot engage correctly. Add the breaking of transmission cases and/or mounting ears, and you've got a bigger dent in your racing budget. It's no different than any other assembly in that it will work more effectively when in total harmony.
There's another reason to consider dialing in your bellhousing. Once all the components are bolted together, virtually all misalignment will be undetectable. You won't know you may be slowly destroying your drivetrain and components.