Tech What exactly does 198bhp mean?

[bronze]

Hello All,

I have 57 plate Type R GT which is unmodified.

Honda quote a power output of 198bhp, but it seems that a number of contributors on here, and on the baseball cap wearing forum who have had their vehicles on a dyno, report power output somewhat in excess of that figure, as much as 10-12 bhp.

Obviously given manufacturing differences there will be variations, but my question is:

What does the 198bhp figure actually mean?

Is it a guaranteed minimum? or If we took a random sample of 100 engines would the overall average power output be at least 198bhp and if so does that mean that some engines actually leave the factory with less than the quoted 198bhp?

[Pottsy]

It means.... very little really.

It should be a minimum, measured at the flywheel, with certain ancillaries (needed to run the engine) connected.

To measure this, you need to remove the engine.

The rolling road measures power at the wheels (which is considerably less, because power is lost in the gearbox, diff, CV joints, bearings and tyres) and then a computer makes a guess at the actual power by looking at the resistance on the over-run and applying a total-guess-fudge factor.

Dyno runs are good for comparing similar cars, and for comparing before-and-after when modifying.

HTH

[illegalhunter]

I think they just put it there to try to convince you of the power output.My cooper s turbo was supposed to be 175bhp, when it was dynoed i got 192bp.The mini was un modded.

[allan40alt]

BHP is the actual 'pulling power' of an engine.

See here:

Mechanical horsepower
See History of the term "horsepower" The term "horsepower" was coined by the engineer James Watt (1736 to 1819) in 1782 while working in the performance of steam engines. This occurred while using a mine pony to lift coal out of a coal mine. He conceived the idea of defining the power exerted by these animals to accomplish this work. He found that, on the average, a mine pony could pull (lift by means of a pulley) 22,000 foot-pounds per minute. Rather than call this "pony" power, he increased these test results by 50 percent, and called it horsepower i.e. 33,000 foot-pounds of work per minute.
Assuming the third CGPM (1901, CR 70) definition of standard gravity, g, and the international avoirdupois pound (195, one mechanical horsepower is:
1 hp≡ 33,000 ft·lbf/minby definition= 550 ft·lbf/ssince1 min = 60 s= 550 × 0.3048 × 0.45359237 m·kgf/s since1 ft= 0.3048 m and= 76.0402249068 kgf·m/s1 lb= 0.45359237 kg= 76.0402249068 × 9.80665 kg·m²/s³g= 9.80665 m/s²= 745.69987158227022 Wsince1 W≡ 1 J/s = 1 N·m/s = 1 (kg·m/s²)·(m/s)Or given that 1 hp = 550 ft·lbf/s, 1 ft = 0.3048 m, 1 lb = 4.448 N, 1 J = 1 N-m, 1 W = 1 J/s: 1 hp = 746 W
cross multiply and cancel out:
550 ft-lb/s0.3048 m4.448 N1 J1 W1 hp1 ft1 lb1 N-m1 J/s
= 745.66272 W or 746 W

So I reckon your engine could pull the equivalent of 198 horses!

PS: I was once told that a high BHP usually means less gear changing as you get a much wider 'pulling' range and a car can pull better in a higher gear at a lower road speed, than a lower BHP engine where it would be necessary to change down and increase revs.

[NEEMSTAR]

i have read that aswell - where on the same dyno, same day the EP3's were getting stock 197, whilst the FN2 was getting 210+.

[coolice]

Hey,

I was always under the impression that a higher BHP figure meant a higher top speed, but a higher torque figure meant less gear changing as there was more power to pull through the gears.
Of course it's all relative to gearing etc. which alters from car to car.

Torque is what propels you off the line and helps you acelerate, the higher this figure the better hence why diesels are not slow in acceleration.

BHP is good for speed, but having BHP & no torque will limit performance.

198 BHP is the equivelant of having the power of 198 horses under your bonnet.

There has been articles I've read in the past which cover the differences between BHP & Torque, I will have to see if I come across one.

This is an interesting read : Puma Race Engines Technical Guide - Power and Torque - how they are related and how to calculate them

It seems that this sort of question is open to lots of answers

Ian

[gjackson]

The manufacturers figure is on a test bed (out of the car) in a controlled environment. This can't be achieved on a rolling road.

You can make a rolling road show anything you want.

As said the only use of a rolling road figure is if using the same car same day as a before and after snapshot when making mods. Even then environmental factors will have changed.

Other than that rolling roads are just for willy waving.

[mike20]

I guess we need to assume that all manufacturers include the same 'fiddle factor' when deriving their published figures for power, torque, fuel consumption etc.
If this is true, then I assume that you can use these figures for comparing one car to another.
There's no way I can manage anything like the 53.5 mpg stated by Honda, so does that mean the power output figure is just as dubious?

I know these figure have to be checked by the various motor certification agencies, but I imagine that some manufacturer's are better at getting their cars through than others.

[gjackson]

Originally Posted by mike20

I guess we need to assume that all manufacturers include the same 'fiddle factor' when deriving their published figures for power, torque, fuel consumption etc.
If this is true, then I assume that you can use these figures for comparing one car to another.
There's no way I can manage anything like the 53.5 mpg stated by Honda, so does that mean the power output figure is just as dubious?

I know these figure have to be checked by the various motor certification agencies, but I imagine that some manufacturer's are better at getting their cars through than others.

Exactly, manufacturers use defined controlled conditions. One rolling road can use completely different parameters in different conditions and therefore comparisons are limited.