Turbocharging is one area where size definitely matters. The Turbonetics turbo on the right develops enough boost for about 200 hp; the one on the left is good for 1,800. Like other turbo makers, Turbonetics can recommend-and build-the right turbo system for your application.
After fuel, compression ratio is next on the to-do list, especially when adding a turbo where there wasn't one before. A compression ratio (c/r) of 9.5 or 10:1, typical in a naturally aspirated car, will produce cylinder pressures that are way too high when there's compressed air flowing into the chambers. Instead, a turbo motor wants a c/r in the 8 to 8.5:1 range. Dropping those c/r points can be as easy as installing a thicker head gasket, though if that won't do it you may need new, dished pistons. If you have to replace the head gasket, a metal one will withstand more boost than a composite gasket. If you're replacing pistons, be sure the new ones are forged, not cast.
Most of our experts agreed that, beyond pistons, there's no need to upgrade the rest of the reciprocating assembly if you're running mild (7 to 10 psi) boost levels, though GReddy's Chung and Turbonetics' Tanaka said they've found that Civic rods are more prone to snap than others. "In an Integra, for example, the piston ring lands will give before the rods go," said Chung.
The engine's ignition might need some work. "You'll want to retard the spark at the top end to avoid detonation," said Craig Gibbs, an applications engineer at Garrett who races a turbocharged Volkswagen in his spare time. Added Tanaka, "Some cars have great ignition systems, while others may need an aftermarket box, like an MSD unit, to help with the spark."
Heat is a turbo system's number-one enemy, and you can see why. The exhaust gases that drive the turbine wheel are incredibly hot, so the turbine side of the turbo has to be built from heat-resistant metals, like cast iron (for the housing) and Inconel (for the turbine blades).
Gibbs also noted that, while using the right camshaft in a turbocharged engine is critical to its performance, "there are a lot of opinions out there about camming. It depends on what you want to do." For solid, all-around performance on the street, Gibbs recommends a cam grind that's mild enough to idle well and produce good vacuum, but one that also offers good mid-range power.
Gibbs had some plumbing advice, too. On the compressor side, he noted "you'll want the least amount of restriction and the biggest pipe the packaging will permit" to get all the compressed air efficiently into the cylinders. Use the diameter of the compressor inlet as a guide for pipe diameter.
On the turbine side of the turbocharger, "get the turbine as close to the head as possible to make it easier for the exhaust pulses to get to the turbo," Gibbs said. Also, you want a large-diameter downpipe from the turbine outlet to the catalytic converter to reduce backpressure that might slow the turbine down.
Some think only serious boost levels require an intercooler, but our experts felt that even systems putting out 7 to 10 psi would benefit from the addition. By cooling the air after it leaves the compressor, it becomes denser, so more air can be packed into the cylinders. This is an A'PEXi Hybrid Intercooler, which uses different-shaped fins top to bottom to channel air for maximum cooling.
Turbo Accessories
Top on the list of items you should add to a turbo system is an intercooler, said most of our experts. A few, in fact, considered an intercooler mandatory at 7 to 10 psi. "That much boost will heat the intake charge," explained Gibbs. "You may have problems keeping the intake cool without an intercooler."
A blow-off valve was next on the accessory list, ranking higher than a boost controller. Why? Compressor surge, caused by excess pressure trapped between the compressor wheel and the throttle plates, can flow back to the turbo and "shock" it, said GReddy's Chung, which will impact the turbo's bearings. Blitz's Liao added that even if a factory turbocharged car came with a blow-off valve, it's a good idea to replace it with an aftermarket unit, since the OE valve won't handle as much pressure as an aftermarket valve will.
On the exhaust side of the turbo, wastegates fall into two types: internal (sometimes called integral) and external. Which you use will depend on several factors. Generally the external 'gates can handle greater pressures, as they're physically larger than the internal units. But if underhood space is tight, an internal wastegate may be the better choice. Another wastegate tip from Gibbs: An adjustable wastegate, whether it's internal or external, will allow you to tailor pressure relief-and therefore boost levels-by changing the rate of the valve's spring.