|
Oversquare means that the bore (piston diameter essentially, but not precisely) is larger than the stroke (the amount the piston rises and falls). This means that the displacement is attained with slower piston speeds, allowing the engine to rev high.
The BMW M3 engine is a classic example, as are most Honda V-TEC engines.
Also, on those rev machines, the engine components tend to be of light weight. Years ago I built a small block Chev 355ci engine that could rev to nearly 10,000 rpm, and do 8000 realiably. Thanks to aluminum pistons and special rods, the moving mass was greatly reduced. The crankshaft was forged steel and carefully ground for the engine.
The problem with oversquare engines is the sheer size of the piston. The flame has a great area to cover and the spark plug can be quite a distance from the edge of the piston. Also, highly oversquare engines, as they are designed for high rpm applications, tend to have small combustion chambers (high compression ratios) and that means detonation problems. You need the gasoline to ignite at a higher compression point, and that is why premium gas is needed. It burns with less volatility than the regular gas. Also, thos esmall combustion chambers often mean reduced valve lift for the amount of fuel required, and even piston cut-outs might not be enough. The valve surface gets bigger,and this means a fuel "dump" can occur. That is why high rpm engines tend to have two intake valves, as it increases the total valve area without reducing the swirl effect of a smaller valve. Brilliant.
You can influence the character of an engine with intake and exhaust tuning as well. Long intake runners allow the air to slow, which is desirable for low revving engines. MB makes a neat variable intake runner that allows long paths during low rpm for greater torque and the path to shorten during high rpms for greater efficiency at high rpm. BMW makes their own version too. An open exhaust allows the cylinder to empty faster, but this in NOT always better. Most folks don't understand about cylinder head flow and the timing of exhaust valves. You need the exhaust system to allow the cyclinder to empty at the CORRECT rate, and with most street engines, some backpressure is required. You don't want the exhaust to empty too fast if there is cam overlap, but not too slow either. Now, that M3 engine could probably run with an open exhaust, but OE systems have become so good, there would be little improvement with an open exhaust, and might get you a noise ticket.
Engineers spend lots of time trying to get a certain feel to a driveline. That includes the engine, the flywheel, the transmission ratios and the final drive ratio. Mercedes' cars have typically been tuned for decent low rpm performance, but excel in midrange and low-high (like 5,000-6,000) performance. Take the old M103 W124 300E. The engine made 177hp, and on paper should be a stoplight killer for it's day. But, tuning made it a different car. Once rolling, it could take on most comers, and even outrun the 560SEL. Years back the car mags grinned when the 300E could outrun a Porsche 944.
Some cars are tiring to drive thanks to that tuning. I spent some wheel time in the E46 BMW M3. It's a hoot (except for quality problems) but might get tiring on long trips and daily commutes. The engine has no flywheel effect and needs lots of concentration when crawling in traffic. The engine makes HUGE power, but not much down low, and this means rowing the shifter constantly. However, in the right conditions, it's the king-of-the-hill. However, guys like me would probably take the 330i for real world living. Keep a fast motorcycle in the garage for those moments when you feel like tearing up the road.
|