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Measuring Up

A cylinder's swept volume is determined by its bore, or diameter, and the stroke of its piston. Typically, we give these dimensions in both English and SI units, in. and mm, millimeters. Displacement of an engine is this swept volume times the number of cylinders. D = [∏ (B/2)2 x S] x C, where D is displacement, B is bore, S is stroke and C is cylinder count. We cite displacement in both cu. in. and cc, cubic centimeters; also, recall that 1000 cc is 1 liter.

An engine's compression ratio is the quotient of its cylinders' largest volume (when a piston reaches its lowest point, bottom dead center) divided by its smallest volume (when the piston is at its highest point, top dead center).

Compression ratios of gasoline engines typically vary from 9.0:1 (for those engines benefiting from forced induction) to figures approaching 12.0:1. Diesels have compression ratios ranging from 14.0:1 to as high as 25.0:1.

An engine's crankshaft translates the reciprocating linear motion of its pistons into a twisting action, a torque, on its output shaft. The English unit of measure of this torque is the lb.-ft. (Imagine grasping one end of a 1-ft. ruler with a 1-lb. weight at its other end.)

Torque is measured on a brake dynamometer (essentially a device "braking" the rotation). It's recorded at successively higher engine speeds so that a peak figure and rpm are determined. This appears in our Data Panel as Torque reported in lb.-ft. @ rpm.

Horsepower is torque per unit time. In automotive terms, derived from readings on a brake dyno, it's cited as brake horsepower, bhp. In English units, the relationship between torque and horsepower is given by horsepower = (torque x rpm)/5252. In our Data Panel, peak Horsepower(SAE) is reported in bhp @ rpm.

A word about SAE: Different test conditions are used to evaluate torque and horsepower, among them standards promulgated by SAE (what used to be called the Society of Automotive Engineers) and in Europe by DIN (Deutsches Institut für Normung, the German Institute for Standards). As engines react differently to changing test conditions, there is no precise formula relating bhp SAE to bhp DIN.

The basic unit of electric power is the watt. One horsepower, abbreviated hp in electric nomenclature, is 746 watts, equivalently, 0.746 kW.

In our Data Panel, we cite specific power, bhp/liter, as a general indication of an engine's technical sophistication. Normally aspirated engines produce 65–110 bhp/liter. (For example, the Pontiac G8 GXP's 415 bhp and 6162-cc V-8 calculate out to 415/6.162 = 67.3 bhp/liter.) Engines with forced induction, turboed or supercharged, exceed this, often gloriously. At 530 bhp and 3.6 liters, the Porsche 911 GT2 is our current record holder in this regard, at 147.2 bhp/liter.

We cite an engine's Redline/limiter for a couple of reasons. First, by policy we avoid routinely exceeding an automaker's redline in our performance testing. (We figure it's red for a reason.) Also, these days, engines often have an rpm limiter, an electronic safeguard protecting against a missed shift or downright abuse.

Fuels and Their Byproducts

Our last two items in the Engine category of a Data Panel are Fuel injection and Recommended fuel. Each could be a tech feature of its own.

The simplest fuel injection these days is port injection, delivering the metered dose to the intake port. Most do this sequentially, i.e., cylinder by cylinder, under electronic control.

The most advanced gasoline systems today are direct injection, delivering their fuel directly into the combustion chambers. A benefit of this is enhanced control of where, when and how the fuel is metered. There's also a cooling effect, which allows higher compression ratios.

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