Turbo Location
"An extreme example is comparing a rear-mount turbo to a conventional underhood turbo. If you look at an overlay of dyno graphs for the two with the exact same turbine size and the same A/R ratio, a conventional underhood turbo system will spool-up over 1,000 rpm earlier on a V-8 engine. Now you can tweak the A/R ratio and use a different size turbine wheel to help the rear-mount turbo make up some of that difference in spool-up time. However, when you go smaller, you limit power potential by increasing backpressure and reducing the energy that goes into the turbo. That's still fine for moderate-horsepower applications, but for high-end street cars whose combinations border on a race setup, the turbo must be positioned close to the exhaust ports."
Turbo sizing
"The first thing to be determined when sizing out a turbo is how much power you want to make. It takes approximately 1.5 cfm of air to support 1 hp, so if you want to make 1,000 hp, you need a turbocharger that flows 1,500 cfm. You then need to determine a target boost level. For a pump gas motor, you want a turbo with a higher volume of air at a lower boost level. There can be multiple turbos that meet those requirements, which is where compressor maps come into play. A more efficient turbo produces less heat as the air is compressed, and a compressor map tells you how efficient a given turbo is at your target cfm and boost level. Generally, 60-percent efficiency is the bare minimum, and anything in the 70- to 80-percent range is optimal. Displacement isn't as critical when choosing the compressor side of a turbo. You pick that based on cfm and how much power you want to make. However, displacement has a big influence on selecting the exhaust turbine."
Camshafts
To promote exhaust scavenging, naturally aspirated and blower cams typically favor the exhaust side on lift and duration and have a fair amount of overlap. The strategy changes a bit with a turbo since it's driven by exhaust gas. "With a turbo cam, you're trying to build up exhaust velocity to spool the turbo, so you limit the overlap," explains Greg. "A hydraulic turbo cam usually has less than 5 degrees of overlap; a solid turbo cam has negative overlap, where the exhaust valve stays closed for so long that when it opens, you have an extreme amount of cylinder pressure exiting the port." Likewise, turbos don't require lots of valve lift, and timing is of much greater concern.
O-Ringing
If your tune and fuel system are on par, forced induction shouldn't be a chore. The cost and hassle of O-ringing is definitely a chore, so when can you get away with skipping the procedure? "For pump-gas applications, O-ringing is a good idea at anything beyond 15 psi of boost," says Greg. However, it shouldn't be used as a crutch to mask other issues. "It's one thing to O-ring because your motor can't properly seal extreme cylinder pressure. It's another thing to do it to prevent head-gasket failure, when the real problem is detonation. All you're doing then is buying some extra time until you experience some serious engine failure."