One of the more difficult technical aspects of diesel performance is choosing a turbocharger. If you put enough fuel through a diesel engine, it's going to run up against a point where the stock turbocharger becomes very inefficient. The stock turbo can even overspeed and the compressor wheel can explode, sending metal into your intercooler and engine. Also, having a turbocharger that is mismatched to your engine can cost you horsepower, and we all like horsepower.
So what's the right turbo to choose? Well it depends on many different factors. For racing engines the answer is simple: as big a turbo as you can spool, or that the rules will allow. For a street truck, it is a little more complicated. Drivability has to be a concern. It's no good to sit there waiting for your turbocharger to spool while your buddy passes you, so on a street truck it's better to be a little smaller than too large.
Taking Stock
First off, you have to decide if you need a new turbocharger. Cummins and Power Stroke engines can support 350 to 400 hp to the wheels with a stock turbocharger. Duramax engines can hit a hot and smoky 500 hp on a big tune with a stock turbo. So if you're an intake and exhaust type of guy, you probably don't need a new turbocharger. If you're shooting for 500 to 800 hp (or if you tow), then it's time to start shopping. In the case of an overfueled stock engine (lots of black smoke), buying a new turbocharger will add horsepower, lower exhaust gas temperatures, and be capable of supplying higher boost pressures without risk of failure.
Almost all the companies we...
Almost all the companies we spoke with indicated consumers usually want to buy a turbocharger that is way too large for their truck. Do you really need a turbo that's larger than your head on a street truck? The answer is no, you don't. Custom turbo setups like the one on Meacham Evins' drag truck run about $5,000 to $10,000 and feature turbochargers larger than 100 mm in inducer diameter, but they have no place on a diesel that makes less than 1,500 hp or spins less than 5,000 rpm.
Weights And Measures
When we're talking turbocharger sizes, the most commonly referred to measurements are the compressor wheel's inducer diameter, turbine wheel diameter, and turbine-side housing ratio. All of these numbers affect how the turbo will behave. Stock turbochargers usually have large exhaust housings, large-diameter turbine wheels, and smaller compressor-side inducers. For instance, an HX35 turbo from a 5.9L Cummins measures somewhere around 54/69/14, which means it has a 54mm-inducer compressor wheel, a 69mm-inducer turbine wheel, and a 14cm2 exhaust housing. The size of the exhaust housing determines how quickly the turbocharger will spool up, but having too small of a housing can create high drive pressures (think of it as boost on the exhaust side) which is hard on engines, turbochargers, and exhaust hardware. Non-wastegated turbochargers often have larger exhaust housings to ensure the turbo's drive pressure remains in an acceptable range, even at maximum engine speed and fueling. A popular aftermarket turbo might be in the 64/71/14 range if it's not wastegated, while a quicker-spooling 64/65/14 (notice the small turbine diameter) will need to be wastegated, or high drive pressures, high EGTs, and possible turbo overspeeding will be the result.
It's The Engine, Not The Turbo
When it comes to turbo sizing, the engine design dictates the type of turbo to use. For instance, larger turbochargers in the 66mm to 71mm compressor wheel inducer range might not even start to spool until the engine reaches 2,200 to 2,500 rpm. If you're putting this turbo on a stock engine that makes power at 2,800 rpm, you might be in trouble. After all, a few hundred rpm does not make for a very usable powerband. If you have an upgraded camshaft, ported head, and enough fuel to turn your engine 4,500 rpm, then having your turbo spool above 2,500 rpm might not be a big deal. An engine that is spinning faster will also require more air, so a high-rpm engine will also need a larger turbocharger for that reason.