This is my second in a series on design features of Mercury Racing’s QC4v engine.
Torque on our new 1350 is Monster! It rises fast, from 700 rpm idle, and is flat at 1,370 lb-ft from 2,500 to 5,250 rpm and generating 1,350 peak horsepower, before tailing off toward a red line of 6,500. On 91 octane pump gas, not race fuel. Whoa!
In other words: Big Fat Monster Torque is more than sufficient to lift the nose of a 48 MTI catamaran, carry it all the way through a hard turn and still plant everybody firmly in their seats for an extended period of acceleration. Just ask my friend, Randy Scism, owner of Marine Technology Inc., about his first test session before the Ft. Lauderdale Boat Show. (Or read, elsewhere on this Blog: MTI Spooled Up!)
Big Fat Monster Torque is born in the inlet system: Air enters the engine via filters (dressed in sensuous carbon fiber) on its way to the compressor section of each turbo. From both turbo compressors, air is driven into a common plenum chamber.
Because the air is now compressed, it’s at higher pressure and temperature than the air outside. Temperature drops as the air passes through an intercooler and exits to the cool side of the plenum. The plenum is modulated to a constant pressure by computer operated valves.
Leaving the plenum, air flows through twin throttle bodies (also computer operated – drive by wire) and into a crossover chamber. The crossover balances and feeds air to each bank of intake manifolds.
Inlet manifolds feed air down eight individually tuned runners, through timed fuel injector sprays (again, computer operated), around inlet valves, and into combustion chambers where it will meet a violent transformation into Big Fat Monster Torque!
Mercury Racing’s pressurized intake system is different. How? Small-increment, boost pressure modulation. We don’t use a simple turbo “waste gate” to dump overpressure. Conventional boost control methods don’t apply. Rather, QC4v retains “pressure on tap” by monitoring internal pressures, temperature, rpm and throttle position as well as outside atmospheric conditions – to control plenum pressure within a narrow range. This assures optimal performance and virtually no turbo response lag. Also, unlike supercharged engines, the pressure map is non-linear: Available pressure isn’t belted directly to the crankshaft, so it isn’t rpm dependent.
From there, the intake of QC4v is somewhat conventional: tuned intake manifolds distribute air evenly to each intakerunner– because each “hole” has to deliver its share of the work.
Properly executed new technology and exceptional design results in engine with breathtaking “feel” and unbelievable punch. This never gets old!