EFI
EFI offers a level of tuneability that carburetors just can't match. "We have guys with turbos and carburetors that are making good power, but they're more the exception than the rule since EFI makes tuning so much easier," says Greg. Thanks to devices like MAF sensors and fuel pressure regulators, most factory EFI systems adapt easily to forced induction. "Even with more basic tuning methods, like using an FMU-which simply increases fuel pressure under boost-all you have to do is pull out 2-3 degrees of timing and fill up with premium fuel. Then you can run 8 psi of boost without any other tuning whatsoever."
A/R Ratio
So much confusion surrounds the question of what the A/R ratio of a turbo is, and that's because it's difficult to explain. A/R, or area/radius ratio, refers to the area of the turbine housing cross-section divided by the distance from the center of the turbine shaft to the center of that cross-section. Although the turbine housing reduces in diameter as you move toward the turbine shaft, the A/R ratio remains constant for any given spot along the turbine scroll. The rule of thumb is that as the A/R gets bigger, a turbo will make more power, but won't produce boost until higher rpm. "If you could unwrap the turbine housing it would resemble a cone, and the smaller the A/R ratio, the smaller the nozzle gets as you progress toward the turbine," explains Greg. "That increases backpressure and allows less exhaust volume to go through, but it also increases velocity, which reduces turbo lag." Varying A/R ratio is a fine-tuning tool, and the ideal A/R ratio varies from one application to another. "On paper, it might seem like you'd want to make power as low in the powerband as possible, but many road racers don't want a ton of low-end coming out of corners."
Single Or Twin
In theory, twin-turbo kits offer quicker spool-up and greater power potential. How do these theories pan out in the real world? "We do a lot of Outlaw and Pro-style drag cars, and the only reason they ever ran single-turbo setups is that the rules didn't allow twins," says Greg. "On a 360ci motor, two 76mm turbos will spool-up several hundred rpm quicker than a single 106mm turbo. Some racing classes are so competitive that a marginal advantage can make a big difference." On the other hand, those rules don't necessary apply on the street or in mild-race applications. With ball-bearing turbos, the advantage of a twin system in spool-up time isn't all that much, and making boost earlier in the powerband isn't always a good thing. "Even with a single turbo, you can get a car sideways in any gear, and it's easy to make more power than you can use on the street. A twin system will only make that worse."
Controlling Boost
The ability to alter boost-therefore power output-at the touch of a button is a perk turbos offer that no other power adder can touch. It offers the flexibility to dump in some race gas and crank up the boost without ever turning a wrench, but how does it work? "Both manual and electronic boost controllers simply manipulate the boost signal going from the discharge side of the turbo into the top of the wastegate," says Greg. "When you want more boost, the boost controller bleeds off some of the boost signal going into the wastegate, which keeps it shut longer to increase boost." Electronic boost controllers are very advanced these days, enabling users to alter boost in each gear, and shutting off the motor to prevent damage if the turbo overboosts.