This is the 5th in a series on design features of Mercury Racing’s Quad Cam, four valve sterndrive engine.
As I mentioned in The Valve Train That Could, valves exist to get air in and exhaust out. Well, the exhaust isn’t finished just because it’s past the intake valves. It’s got work to do: It’s time to “Peg the fun-o-meter” for some lucky boater! Exhaust heat remains from combustion. All turbocharged engines use that “waste” energy to spin a turbine, compressing incoming air to higher density. QC4v does that – and more.
It’s less commonly known (except by header designers and a few other social deviants), the exhaust flow also has pressure waves racing down and back up the exhaust system. When an exhaust valve opens to expel spent combustion gases, the rapid pressure rise sends a pulse down the pipe at the speed of sound.
In a 2-stroke engine with tuned exhaust, that pulse reflects back up the pipe. It’s on a mission. Pulse arrival back at the port is timed (by pipe length) to increase pressure at the exhaust port and slow or reverse fresh, combustible mix overflow – just before the piston slides back up to close the exhaust port.
On a naturally aspirated 4-stroke engine (one with no turbo, blower or screw charger) with headers, the reflected pulses are timed to decrease pressure at the port when the exhaust valve opens thereby increasing scavenging rate of explosion leftovers. The exhaust valve closes to stop the fresh mix before it overflows. (The pressure pulses are not needed to do that job, as they do in a 2-stroke.)
However on our turbocharged 4-stroke (and here’s where our 2-stoke experience inspires), the pulses are used to accelerate the spooling of the turbos. We separate the exhaust ports in each manifold and plumb them in sequence into isolated runners – all the way to the turbine blades of two, twin port turbos. The timing of the isolated pressure pulses spins up the turbines faster and keeps them spinning longer. That pulsing is a key element of Mercury Racing’s pressurized induction system. Quick spooling compressors — together with short, precise plenum pressure modulation by the computer — virtually eliminates low-rpm torque deficit and turbo lag (which is commonly associated with less technically advanced turbocharged engines).
Without question, the manifold and turbo housing design is more complicated. And it is different on port and starboard banks because of firing order and pulse timing intervals. However, once the design was sorted out, the advantage of quick spooling turbos is “in the pipes” ever after. You and your guests get to savor the experience of Big Fat Monster Torque. Your victims? Not so much.