Metric Mechanic - The Engine Block - Technical Details

Engine Displacement more than anything else dictates the torque output of an engine. Increase compression and crankshaft stroke as well as reducing the reciprocating mass factor into increasing acceleration.

At Metric Mechanic, giving you a larger displacement engine is a standard practice. These larger engines would not be possible without a lightweight reciprocating mass (piston and rod assembly). All our engines are "over bored" to use a very lightweight but strong forged Alusil (high silicon content aluminum alloy) piston with a vented slipper skirt and lightweight wrist pins.

Engines that are stroked use lightweight "I" beam rods, except 3.5 liter engines. Compression ratios of 10.0:1 or greater are standard on all our Performance engines - thanks to Surface Turbulence.

Also, the oil pump is "blue printed" and has a modified pressure relief valve system. A cornering baffle is welded into the oil pan on all 4 Cylinder Sport Engines.

Bottom End Design Parameters
Generally speaking, to make a large engine smooth; counter balance shafts need to be added to the engine or else the reciprocating mass (piston & rod weight) needs to be reduced by the same percentage that engine displacement is increased. For example, enlarging from a 2.0 liter to a 2.5 liter, a 25% increase in displacement would require reducing the reciprocating mass by about the same amount. In our 2500 engines, the reciprocating mass has been reduced by 28%.

Since the piston is further from the center line of the crankshaft than the rod, its weight becomes even more important when reducing the reciprocating mass. Taking 1 gram off the piston is like taking 2 grams off the rod. Also, in a properly designed engine, you’ll find that the rod needs to be about 10 - 15% heavier than the piston to produce a balance of strength between the two.

Obviously, these design goals cannot be met by using heavy factory cast pistons and rods. This is why we have gone to a lightweight forged Alusil (high silicon content aluminum alloy) piston and longer lightweight rods. When stock rods are used for strokes much over 86 mm, the rod angle gets fairly steep - another argument for longer rods.

Using these design parameters and parts has allowed us to build smooth large powerful engines that go way beyond the displacement limits of other BMW tuners.