This is the first in a series about design features of Mercury Racing’s 1350 horsepower, quad cam, four valve (QC4v) engine. Throughout the series, I will strive to present unavoidably technical content in non-engineering language.
Bigger Purpose: The only reason to have a high performance marine engine in the first place is to produce thrust from a prop so as to push a boat to: a) go someplace or b) outrun somebody – and come back. Props, drives, transmissions, engine innards and controls all have to do their part, but I skip now to the weakest link in the current high-performance chain: the valve train. (See “Why Rev Limits are…Limits” on this blog.)
I will explain some engine basics, so that one can appreciate the elegance of our QC4v creation: A gasoline engine executes a series of controlled explosions in confined spaces. Its job is converting these recurring forces of combustion into rotary motion thereby creating torque (force about an axis) and power (rate of performing work – or play). A valve train’s job is to get air in and exhaust out while helping create that confined space for these explosions – by closing against their seats in the cylinder head. The bigger the “bombs” (and the more efficient the conversion-to-rotation bits are), the more horsepower comes out the spinning end.
Four valves = bigger bombs: 1) Two inlet valves per cylinder increases the area available to get fresh air in. 2) Turbo compressors on each cylinder bank increase the inlet pressure to get more fresh air in through that valve area even faster. 3) Inlet air intercoolers (between the turbo compressors and inlet valves) get denser fresh air in. More air (faster and denser) means bigger bombs. 4) Two inlet valves and chamber design also enables better fuel/air mixture motion. Better motion (and centralized spark plug location) makes for quicker and more thorough combustion. All that makes bigger bombs with lower tail pipe emissions and less fuel consumed. 5) Two exhaust valves increases the area available to get combustion by-products out. Quicker evacuation of spent, bomb leftovers means there is more time and volume available to refill the confined spaces with new bomb makings. Bigger, denser, combustible cylinder refills mean bigger bombs.
Thirty two valves need four cams: Pushrod engines, like Mercury Racing’s 700SCi, use a single cam in the valley between cylinder banks to poke pushrods, through passages in the block, up into the cylinder heads. That’s a cost effective solution for a two valve head. However, there is not enough space for all the pushrod passages and cam lobes needed to actuate 32 valves. Instead, we use two cams in each cylinder bank: a quad cam, four valve configuration. While there are more moving parts, those parts are stressed less. Also with this design, there is less mass and lower valve train inertia: Revs come easier and rev limits can be higher – 6,500 rpm on our 1350.
Valves like finger followers: Finger followers are individual levers acting between each cam lobe and its corresponding valve. Their effect is the difference between a soothing massage (good) and karate blows (not good) to the valve stem. That “purposeful gentility” delivers valve actuation that is smooth, precise, high velocity, long lasting and low maintenance. Direct acting fingers have low inertia and are extremely stiff compared to the “pogo-stick” cam-on-stem. It’s, “Oooh, rev me some more, you smoothie!” It’s not, “Whack, whack, bang. You’re done.”
Four cams like three chains: We use three chains – one chain from the crank to a cam drive sprocket, then two chains from there to drive the twin cams in each bank. This reduces cam speeds to half engine speed (required, if it’s going to function) while keeping sprockets small for narrower valve angle (enabling better combustion). It also gives durable and maintenance free cam synchronization.
There you have it. That’s the simple logic behind our QC4v valve train revolution. We know the engine likes it. When the engine is delighted, an owner soon will be.