BHP, PS, KW, CV or what ??

[Marius]

I wonder about some power quotations given in some catalogues. In GB all power-figures are being given in BHP (brake-horse-power). I wonder if this is the same figure as given in france (CV = chevaux) or germany (PS = pferdestärken = horsepower). Is there a difference between these figures ? Should the BHP-figure been multiplied by, let´s say, 0.85 to have the equal in PS ?

 

Reason for all this is that it seems impossible to me to get a 235BHP out of a Vauxhall 16V just by fitting some new pistons, camshafts and valve springs. I just can´t quite believe what figures some companys quote for their engines with very little work done to them. Has anyone had further experiences about power-figures which were promised but not achivable ??

 

[Matt Prior]

Hi Marius,

 

CV, or PS, is metric horsepower, and damnadably close to imperial horsepower, or bhp, as we say over here.

 

Up to 100bhp or so, there ain't much difference, but when it gets to the likes of Audi's RS4 with 380 PS, the actual bhp figure is *only* 375.

 

The difference is this:

bhp = PS (or CV) / 1.014

PS( or CV) = bhp * 1.014

 

Incidentally:

kW = bhp / 0.746

 

 

[Peter Carmichael]

Notwithstanding that I have never known an engine tuner underestimate the power of one of their conversions. Engine tuners tend to use BSPP (Brake Shetland Pony Power).

 

Bolt on upgrades are always restricted to the initial design compromises of the base head casting, which is where having a Cosworth designed Vauxhall head counts for lots of ready ponies. Anything else requires hand finishing (or spark erosion) of the ports. Dave Andrews is very good at pointing out that the detail of the throats and valve seating, especially the short turn into the combustion chamber make big differences to low lift flow. Good cylinder filling is very sensitive to this low lift flow and good power follows.

 

I am sceptical of the power claims for the bolt on Vauxhall conversions, not least because Caterham made exaggerated 218 and 235 BHP claims for their engines. Anybody ever seen dyno curves for a JPE?

[CHRIS CLARK]

Peter

 

I've only seen one 'JPE' lately & that wasn't on a dyno; only Caterhams showroom!!! Pity.

[Pierre Gillet]

Yes,and the power of the 1.6 K SS engine has been oscillating between 138 and 132 PS depending where you read it. Which is the right figure?

Pierre

[old captain slow]

I prefer 140 - at the wheels per a rolling road test that I can't track down now but at a web site mentioned in this chat room some weeks ago. Who's going to prove it anyway when the driver is at least half the problem (in my case anyway. Cr*p driving plus overweight = apparent loss of power)

[JKJ]

I think alot of standard Caterhams have less power than quoted. I have a std 1700 Super sprint and it feels alot slower than my everyday car, a Mk 6 Escort 2000. Even today a BMW 238i annihalated me up Dumail Raise in Cumbria, I really do need a Superlight.

[Peter Carmichael]

Please remember that:

 

"at the wheels" + "rolling road" = "meaningless figure"

 

unless you hold all other variables constant between comparisons, which nobody ever does.

 

"140 at the wheels" from a 1.6 Supersport did not happen. Some punters will believe anything, but physics tends to be amazingly consistent and unswaying in the face of personal belief.

 

(Remembers news story from earlier this week where science teachers were being given special training in overturning 'belief systems' rather than just presenting evidence. Apparently schoolchildren are prone to contorting evidence to support whatever they already believe - a bit like adults, really). smile.gif

[Marius]

Thanks fo all your hints. I was just wondering if I did pay far too much money for my 16V. Sometimes the 7 can be a barrel without a ground-floor....

 

[Tony Martyr]

Power is a value derived from values of speed and torque and a constant. It is the value of the constant that determines the different units of power. One day we may all use kilowatts and the sums will be easy but my guess is that the units of bull**** will be bhp for some time to come.

Speed is easy to measure but torque is not; which is why we don't all have dashboard gauges showing individual half-shaft torque.

Even when is is measured the accuracy of the measurement can vary greatly through all of the classic causes: zero offset, non-liniarity of signal, temperature effects, system hysterisis etc etc. When it is measured by rolling raods a whole extra sources of error come into play as the friction and interia effects of the machine need to be compensated for.

Power figures in catalogues are not to be believed.

I was present at an American engine tuners test cell when he was taking maximum power figures. His technique was to warm up the engine, bring it down to 40% speed (Position on throttle/speed mode on dyno) with WOT then give the dyno a step demand for the maximum power speed. The dyno immediately backed off load to allow the engine to free accelerate to the demand speed and then it banged on maximum braking force when it overshot the set point. This transitional torque required to force the engine revs back to set point was recorded as the maximum engine output and power calculated accordingly. In fact it was up to 20% over the constant max power figure (depending on system inertia etc) - but it was what went into the catalogue.

I think a good accelerometer in the car must be the best objective way of checking the effect and exploitation of power.

[Peter Carmichael]

Tony,

 

I am horrified at your story from that American engine tuner, but not surprised. I wonder why they even pretend to have a process for deriving such a fictitious figure.

 

My first rule as an engine tuning customer is: If the b*ggers won't let you watch what they are up to then run (don't walk) away.

 

(But you have given me an idea about how to get 280bhp from my k-series...)