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Home » Archives » August 2012 » Warning: Science Ahead #1:Engine Performance

[Previous entry: "This Post has Nothing to do with Cars. Well - OK - It Does have Something to do with Cars."]
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Warning: Science Ahead #1:Engine Performance


Science Ahead

Science Ahead will be a regular column here at the CarCynic. To find more, use the search box on the left or hit here for a canned search for other Science Ahead articles on CarCynic.com.

Here's a quick test: Which of the Following 2 vehicles has the higher-performance engine?

Truckette and Corvette (44k image)

The Citroen Truckette, of course. Surprised? If you disagree, you need a quick lesson in Engine Performance.

First of all note that I did not ask which vehicle has the more powerful engine, or which has the bigger engine. I also did not ask which has the more efficient engine (Would you have picked the Truckette for that one?). Those are all different questions.

Also note that I asked which has the higher performance engine -- Not which is the higher performance vehicle. Again, different questions, with different answers.

Performance is not size, or power, or even efficiency.

Let's tackle "efficiency" first because it's easy to show the difference between performance and efficiency. Adding a battery pack and motor/generator to a car -- as in a Toyota Prius or Honda Insight for example -- can increase it's fuel efficiency, but few would argue that either of those cars have high-performance engines.

Here's another way to look at it: Race car mechanics want every bit of performance they can get from a racing engine, but unless the rules of the race strictly limit available fuel, they are not too concerned about efficiency.

Performance, to state it simply, is having something do what we wish it to do, while not doing what we do not want it to do.

In a car, we want the engine to move the car without adversely affecting handling, breaking, etc.

If you want your car to go faster, the first thing you may think of is installing a larger engine, and this may work -- up to a point.

Top Gear fan submission picture
A bigger engine does not mean higher performance.
Photo from Top Gear fan submissions - Original rights owner unknown.


Increasing engine size does (should) increase power, but it also increases weight. Engine weight decreases (other things being equal) handling, breaking, etc., and weight (by itself) actually decreases acceleration.

The true performance of an engine is the ratio of Break Horse Power (bhp) to weight. However, auto makers do not usually tell us the weight of the engine... but they do tell us the displacement, and since displacement is the primary factor in determining engine size and thus weight, the ratio of HP output to displacement is the best indicator of engine performance.

This is why Turbocharging is an excellent way to boost engine performance. The turbocharger, and associated plumbing, adds little weight, but can increase HP significantly. -- And before you supercharger and nitrous people soil yourselves in anger -- Yes, there are other ways to boost performance too.

So how about the 2 vehicles shown above?

The Citroen Truckette shown above (1976 AK 250) has a 435 CC 2 cylinder engine rated at a not exactly awe-inspiring 26 bhp. 435 CC, for us Americans is right at 26 cu. in., so that's almost exactly 1 HP per cu in.

For a conventionally aspirated automotive engine, 1 HP per cu in. is actually pretty good, at least for the 1970's.

The 1976 Corvette has a 350 cu in Chevy small block, and while some Corvettes did break the 1 HP per cu in. mark, (and Chevrolet made quite a big deal of it at the time) the engine in a Corvette like the one pictured produced 250 bhp. (at best)... that's only 0.71 HP per cu in. -- significantly less than the Truckette.

But it gets worse. I'm using numbers for break horse power, not HP at the drive wheels. Both vehicles have gearboxes, and they both loose some power (as heat) there, but the Corvette also has water and and power steering pumps, which the Truckette does not, so the available power to displacement ratio is worse for both cars, but the difference favors the Citro├źn even more.

I'd also like to mention that this is not the same as volumetric efficiency, although volumetric efficiency is one of the biggest factors in engine performance. It's easy to understand the difference if you take the case of an engine running with an incorrect mixture ratio. It may have great volumetric efficiency at moving a nearly incombustible mixture through the engine, but it's power output is not going to be very good at all. Everything has to be right for good engine performance, and that is why HP to displacement ratio is am important parameter.

So when car shopping, or when discussing automotive performance don't just look at displacement, or just at HP, look at the HP to displacement ratio -- That's performance.