Does anyone know a formula or methodology for translating “motor power” between cc and horsepower??
This is the one formula which I have never been able to locate . . . . . .
I’m no mechanic or physicist, but I don’t believe its as simple as that – I think there are many factors governing horsepower other than a straightfoward relationship between engine capacity and horsepower.
[quote=“Hartzell”]Does anyone know a formula or methodology for translating “motor power” between cc and horsepower??
This is the one formula which I have never been able to locate . . . . . .[/quote]
CC is cubic centimeters. That would then be the actual size of the motor’s piston chambers. So a 2L has two litres of combustion space. MAking it more powerful than a 1300 say.
But, as Sandman says it’s not as easy as that. Weight ratios, rev capacity etc all make a difference.
There is a horsepower/ metric conversion however.
Watts to Horsepower Watts x 0.00134
Hosepower to Watts Horsepower x 746
Kilowatts to Horsepower Kilowatts x 1.34
Horsepower to Kilowatts Horsepower x 0.746
Typical brake specific output can be as little as 50bhp per litre of capacity from an older design and up to around 100bhp/l for modern high-performance engines, both normally aspirated and turbocharged. Thus you see about 120bhp from a typical modern 1.6l engine in a passenger car.
The horsepower number doesn’t directly relate to the engine displacement (cc). It’s all dependent on the engine design.
For example, in the late '80s, the Chevy 5700cc V8 in many American sedans put out around 190 horsepower (hp), which is about 33.3 horsepower per liter.
The late '80s Ford Mustang GT has a 5000cc V8 with 225hp for 45hp per liter.
The 2000cc I4 found in the Acura RSX-S puts out 200hp, which is 100 hp per liter.
For more enthusiast-oriented (read: expensive) cars, you get into more exotic technologies:
Honda S2000, 2000cc I4, 240hp, 120hp per liter
BMW M3, 3200cc I6, 333hp, 104hp per liter
Horsepower numbers don’t tell the whole story though. Those high horsepower small displacement engines need to be revved up to high RPMs in order to generate that kind of power. They generally are lacking in low end torque, which is a function of how many cc the engine has. The Chevy and Ford V8s in the first examples by virtue of it’s displacement has a lot of low end torque, which is what gives you that neck snapping acceleration from a standing start. But, they run out of steam on the top end. That’s where the high-revving modern 4-valve per cylinder engines shine, for example on a racetrack. That’s not to say that the large displacement engines haven’t improved over the years, but they still generally post lower hp per liter numbers. (Chevrolet Corvette Z06, 5700cc V8, 405hp, 71 hp per liter).
Then, there’s turbocharging and supercharging, which boost horsepower per liter numbers even higher:
Nissan Skyline GT-R, 2600cc I6, 320hp (estimated), 123hp per liter
Mercedes Benz C32, 3200cc V6, 349hp, 109hp per liter
Subaru Impreza WRX STI, 2500cc H4, 300hp, 120hp per liter
Mitsubishi Lancer Evolution VIII, 2000cc I4, 271hp, 135.5hp per liter
Anyway, to simplify things, I’d say for the average modern engine that you might find in a new Taiwanese built car, 70hp per liter is a good rule of thumb to follow. But by no means is it an absolute rule.
Ben, that’s a really interesting post. Thanks.
One thing I would like to clarify is the distinction between torque and horsepower. It’s clear that you know about this already but it bears drawing out a little more. I used to think, based on what I heard people saying, that ‘torque’ referred to low-end pulling power only. Then I read up on it and found that in fact torque is a factor which is involved in calculating power at any engine speed.
Horsepower is achieved by combining torque and revolutions per minute. Engines with comparatively small displacements but high power achieve that by being able to rev. fast. Yet although their peak torque is perhaps less than that produced by a larger capacity engine, it’s significant nonetheless. If an engine were to be capable of revving quickly but had extremely low torque, the end horsepower figure would also be low. Conversely, something like a waterwheel produces enormous torque (imagine trying to stop one rotating) but at a very slow speed. Were it geared up to produce a few thousand revs., the torque at that point – and hence the power – would be low.
You talked about low end torque producing ‘neck-snapping acceleration from a standing start’. Yet with enough clutch slip, surely a car with peak torque and power at higher revs. would accelerate just as fast or faster? Still, it doesn’t always feel good to be revving the nuts off an engine. Perhaps a vehicle with peak torque and power at lower revs. is more relaxing and forgiving to drive. On another thread, Sandman talked about his old 500cc single-cylinder Yamaha motorbike not having much top end, but being able to drive virtually up a cliff. That sounds like a nice bike to ride; a good wide band of usable power.
One area where small capacity, high power engines really have an edge IMO is when used in conjunction with continuously variable transmissions as found in a few scooters and a few cars.
That is true, for a manual transmission car with enough clutch slip. That’s murder on the clutch though if you do it often. I didn’t mention it in the last post, but perhaps a better indicator is not the standing start, but the rolling start from 5 mph. That will give a better idea of the performance advantage of the large displacement engine over the smaller displacement high-revving engine in more useable day to day conditions, where generally, engines stay in the 1000-3000RPM range. Smaller displacement engines sometimes utilize turbochargers to generate more low end torque to simulate the torque of a larger displacement engine, but then the power delivery is not as linear.
Anyone who’s ever driven a Honda S2000 knows about the Dr. Jekyll/Mr. Hyde personality of the low displacement/high revving engine. At low RPMs, it feels like a normal 2000cc economy car when tooling around town. Rev it up past 5000 RPM, and the engine screams like a racecar as you are pushed back in your seat. Honda realized this, and that’s why they upped the displacement to 2200cc for 2004, so it would be more usable in day to day driving.
IMO, I think the best of both worlds would be a medium displacement (3000-3500cc) 6 cylinder engine with multivalve technology and high RPM capability. That way you have a decent amount of low end torque in the low RPM ranges, but your engine doesn’t have as much rotating mass as a large displacement (4500cc+) engine, so it can still rev up to high RPMs and get high horsepower numbers. A lot of automakers now have 250+ hp 6-cylinder multivalve engines that don’t feel underpowered around town at low RPMs.
As I recall
14 cc’s is equal to 1 HP as a general rule for a standard engine
Turbo adds about 20%
Catalytic converter losses you 5%
Also depends on the fuel you use
But then you have Twin Cams, more valves in an engine, cars tuned to the max etc that throw that 16cc rule out the window