The ignition system and an aftermarket ECM tune allows tuners to be able to set the ignition timing for increased power and to avoid detona-tion. "Ignition tuning is critical for boosted applications in order to prevent detonation," Squires says. "On newer stock engines with compression ratios in the 10:1 range, stock timing can be in the 26- to 30-degree range at wide-open throttle. When we tune this type of engine for the turbocharger, we typically have to drop the timing under boost peak by 10-15 degrees to prevent detonation on pump gas."
By retarding the timing, the engine is far less likely to run into detonation, which could lead to serious engine damage in a matter of moments. The overall timing figure itself is not only based on compression ratio and the boost level, but also off camshaft and cylinder head choices as well. "The boost amount, cam, and heads all affect the overall timing number," Head says.
The internals of the engine must also be able to handle the increased cylinder pressure accompanying turbocharging. Included in this area is piston choice, compression ratio, and the type of cylinder-head retention hardware. "Internally, you want to run forged pistons with a moderate compression ratio, though you do want to run as much compression as you safely can," Head says. "Higher compression with less turbo lag means a snappier system, though with pump gas in a street application, you are limited in the amount of compression you can run."
While a stock component-equipped 4.6 or 5.0 can hold up to about 500 hp at the wheels (many combinations have made more than that), for ultimate reliability, an aftermarket block and rotating assembly are a must. Keep in mind that a 9:1 static compression ratio will, under boost, rise to a level that could possibly induce detona-tion. While on paper a lower static compression ratio, like 8:1 or 8.5:1, might seem weak, when under boost, that compression ratio, known as the dynamic compression ratio, will rise right to where it's needed to be at to make the best power.
Running a lower compression ratio helps to keep cylinder pressure down, which assists in avoiding blown head gaskets and detonation. Also, the forged pistons, rods, and crank are more suitable to withstand the increased pressure. "In the engine, you would need forged pistons, rods, and maybe a crank," Napier says. Page agrees with him on this subject. "As with any high horsepower combination, the internal parts of the motor must be up to the task," he says. "Turbo motors generally will need a slightly lower compression ratio, thicker ring lands, and larger ring end gaps." The thicker ring lands on the pistons will allow the piston rings to stay in place under the increased heat and pressure, and will prevent the ring lands themselves from cracking and/or failing.
"Keeping the pressure in the cylinders is another important thing to consider, and a set of head studs and good gaskets will go a long way there," Squires says. "Heads and cams will increase the amount of boost that gets into the cylinders, which will allow you to run less manifold boost to make the same power."
Camshaft And Valvetrain
With camshaft profiles made to take full advantage of the juice or increased airflow offered by a supercharger, we wondered if there was a dedicated cam profile specifically for turbos. The answer we got was quite surprising.
Consensus was that a stock or mild cam would be best suited in a turbocharged applica-tion. "Blowers like more air out, while turbos like more air in, so you want to have a good amount of intake lift and duration, but not as much as a blower or nitrous cam," Campbell says."
Horne agrees with running a small cam, though he differs a bit on the subject from Campbell. "You want a cam with very small duration but with high lift," he says. "Also, you want very little overlap. You want to look for something around 225 degrees of duration. We've seen a lot of people do well with an F-303 cam."
While a custom-ground cam is the way to go to suit the profile to the power level you want to make with your engine, there seems to be a specific set of guidelines as to what works and what doesn't.