Backpressure
Ideally, backpressure between the cylinder head and turbocharger should be limited to a 2:1 ratio. In other words, if you have 10 pounds of boost in the intake manifold, exhaust backpressure should not exceed 20 psi. Too much backpressure will pop gaskets and destroy pistons. "The three methods used to control backpressure are varying the A/R ratio, exhaust turbine size, and wastegate size," says Greg. "For example, if you bought junkyard turbos that don't have the ideal A/R ratio for your application and have too much backpressure, you can use a bigger wastegate to bleed off excess exhaust pressure. It's obviously not the best way to make power, but it can be done." Even at a 2:1 ratio, the additional backpressure a turbo creates compared to a naturally aspirated motor has a marginal impact on power output.
Compression Ratio
For maximum power on pump gas, it's better to run lower compression and more boost than it is to run higher compression and less boost. "Many late models come from the factory with between 10- and 10.5:1 compression; limiting boost to 7-8 psi before detonation becomes an issue," explains Greg. "If you lower the compression to 8.5:1, you'll be able to safely run 15 psi and make gobs more power than at 7 psi." Lowering the compression too much can adversely affect off-boost drivability and turbo lag, but it's much less pronounced in a V-8 than in a small-displacement four-cylinder. "As far as compression is concerned, the sweet spot is between 8.5- and 9.5:1." That said, the proper way of lowering compression is with a dished piston, but alternative methods work nearly as well. "Quench is extremely important in a naturally aspirated motor, but we've never had a problem with detonation when using a thicker-than-stock head gasket to lower compression. There have been instances where we've added 0.040-0.060 inch in head-gasket thickness, and we've never had issues with uneven burning, detonation, or extreme EGTs."
Clocking
Both the exhaust and compressor housings of a turbo are attached to a central bearing housing. "Clocking" a turbo just refers to the orientation of the housings in relation to one another, which can make fitting a turbo into tight spaces much easier. "To clock a turbo, all you do is loosen up the bolts or V-band clamp on the exhaust housing and rotate it," explains Greg. "There really are no clearances that you can mess up." However, there are some pitfalls to avoid. The oil inlet and outlets should be as close to 12 o'clock and 6 o'clock as possible. "You can be a little off and still be OK, but if you stray too far, you can have problems with the oil system and its seals." Perhaps the most important thing to remember is properly torquing the bolts back down on the exhaust side. "The torque spec for those 51/416-inch bolts is usually 15 lb-ft, but a lot of guys will overtighten them, and have them pop off once the turbo heats up and expands."
Bearings
As with an engine, friction is the enemy inside a turbo. Ball bearings are a recent innovation that decreases spool-up time. Additionally, ball-bearing turbos are much more durable. "One of the most commonly failing parts in a turbo is the thrust bearing, which is located right behind the compressor wheel," says Greg. "The thrust bearing wears out from the resulting surge of not having an adequate blow-off valve, and ball bearings can take 50 times the amount of load as a standard bearing." However, ball-bearing turbos usually cost $600 more than a conventional turbo, and may not be worth the cost if turbo lag isn't a problem and you have a quality blow-off valve. Heat is another factor detrimental to bearing life, and it's long been argued whether or not a turbo motor needs to sit at idle after being driven hard. "If you're just going to the grocery store, it's not necessary. However, if you hit full-boost the turbo can spin at 70,000-100,000 rpm, so it will keep spinning even after the motor is shut off if you don't give it a chance to spool down." Synthetic oils and the advent of water-cooled bearings go a long way toward preventing oil cooking, but it's not a bad idea to take extra precautions. "If you take care of a turbo properly, it should be good for 100,000 miles."
Wastegates
Since the turbine and compressor wheels of a turbo aren't physically driven by the engine, exhaust must be bled off before reaching the turbine to limit boost. That responsibility falls on the wastegate. There are two types; most factory turbos use internal wastegates, but beyond 450 hp, external wastegates are mandatory. "Since the flapper valve in an internal wastegate's diameter is only about 71/48-1 inch, the exhaust volume that can pass through it is fairly small," says Greg. "Once you exceed 450 hp, you can run into backpressure or boost creep issues." While internals are integrated into the exhaust housing, externals are positioned between the exhaust primaries and the exhaust housing of a turbo. In addition to supporting higher horsepower levels, an advantage of externals is that their diaphragms can handle extreme temperatures better and are, consequently, more reliable. So any turbo system that makes big power will boast an external wastegate.