rear whell horsepower
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Guest
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Er, surely the point about BHP is that it's the horsepower as presented to the brakes (ie, the wheels). You don't measure BHP at the flywheel - it's just plain old HP there.
Ergo I would expect my 211BHP S2 to deliver 211 HP to the wheels (when new anyway), and presumably more than this at the flywheel - where the actual figure is interesting but pretty irrelevant [8|][8|][8|]
Ergo I would expect my 211BHP S2 to deliver 211 HP to the wheels (when new anyway), and presumably more than this at the flywheel - where the actual figure is interesting but pretty irrelevant [8|][8|][8|]
mik_ok
New member
I'm afraid you're mistaken mikey.
Horsepower is a work rate. You measure it by applying a braking force ~ hence "Brake Horse Power". This is whether you use a std wheel dyno, or an engine dyno (with the engine on the bench).
At your local dyno, what you actually measure is torque generated at the wheels.
You calculate BHP given that :-
BHP=(Torque/5252)*RPM
{assuming your torque is measured in lbs.ft}
You then measure driveline resistance and estimate the actual figure that the engine must be producing to provide x at the wheels.
All manufacturers rate their engines ~ what arrives at the wheels is less.
Horsepower is a work rate. You measure it by applying a braking force ~ hence "Brake Horse Power". This is whether you use a std wheel dyno, or an engine dyno (with the engine on the bench).
At your local dyno, what you actually measure is torque generated at the wheels.
You calculate BHP given that :-
BHP=(Torque/5252)*RPM
{assuming your torque is measured in lbs.ft}
You then measure driveline resistance and estimate the actual figure that the engine must be producing to provide x at the wheels.
All manufacturers rate their engines ~ what arrives at the wheels is less.
sawood12
New member
Actually the term Brake Horse Power is from rigs used that applied a brake directly to the crank shaft and measured how much braking force it takes to stop the engine which accurately relates to the power of the engine. When manufacturers quote BHP figures for cars they mean the power the engine is generating i.e. 211bhp in the S2's case. The transmission soaks up quite alot of the power though the inertia of the rotating masses and friction so your wheels are seeing a much lower level of power. Dyno's measure power at the wheels and have to mathematicaly approximate the power produced at the flywheel i.e. crank which is the raw power developed by the engine. The only way to accurately determine your engine's BHP is to remove it from the engine and plonk it into a rig that measures the power directly from the crank via a brake.
This is why comparisons of power output between different dyno's is largely irrelevant as there are different assumptions and mathematical algorithms used to calculate your flywheel power. When using a dyno to quantify the effect of mods you should always take it back to the same dyno every time. The actual figures might only be an approximation and not 100% accurate but the relative increases in power should be.
You think this is complicated - you should try aero engine test beds on for size. The physics and mathematics involved is a science in itself!
This is why comparisons of power output between different dyno's is largely irrelevant as there are different assumptions and mathematical algorithms used to calculate your flywheel power. When using a dyno to quantify the effect of mods you should always take it back to the same dyno every time. The actual figures might only be an approximation and not 100% accurate but the relative increases in power should be.
You think this is complicated - you should try aero engine test beds on for size. The physics and mathematics involved is a science in itself!
Guest
New member
At least I was watching the telly [
]
(edit - The 'IT crowd' and 'My Name is Earl' Fen)
I bow to your expertise guys. But Horsepower is a measure of power, as is Brake horsepower. So why the need for the distinction. And ultimately, isn't power as measured at the flywheel a totally pointless figure - what we are interested in is the power at the wheels.
Must confess that it's a long long time since I last talked technical engineering stuff, so I'm probably wrong.
](edit - The 'IT crowd' and 'My Name is Earl' Fen)
I bow to your expertise guys. But Horsepower is a measure of power, as is Brake horsepower. So why the need for the distinction. And ultimately, isn't power as measured at the flywheel a totally pointless figure - what we are interested in is the power at the wheels.
Must confess that it's a long long time since I last talked technical engineering stuff, so I'm probably wrong.
mik_ok
New member
Mikey - there is no difference between BHP and HP ~ the former just became the common term due to the measurement method.
We are of course only really interested at the figre that arrives at the drive wheels, as it provides the motive force. It's the torque arriving at the drive wheels that we are interested in - power figures merely indicate the advantage you can take of gearing to maximise this figure. If you are now wondering what the hell I'm on about, and have 5mins - READ THIS. []
Car manufacturers use engine dynos when developing their engines though, and everybody therefore rates the output of the engine. Which makes some sense - othewise the same engine used in different cars would have differing power ratings. Also the driveline losses change slightly depending on the gear you are in......
We are of course only really interested at the figre that arrives at the drive wheels, as it provides the motive force. It's the torque arriving at the drive wheels that we are interested in - power figures merely indicate the advantage you can take of gearing to maximise this figure. If you are now wondering what the hell I'm on about, and have 5mins - READ THIS. [
] Car manufacturers use engine dynos when developing their engines though, and everybody therefore rates the output of the engine. Which makes some sense - othewise the same engine used in different cars would have differing power ratings. Also the driveline losses change slightly depending on the gear you are in......
sawood12
New member
Also for marketting purposes it sounds far more impressive to say the car develops 211hp instead of 180hp (or whatever the rwhp is for an S2). Engine hp is relevant however because generally gearboxes and transmissions are mature products and are as developed as far as they are ever going to be so the relative efficiencies from car to car will be very similar. And don't forget if you've a 4wd car the wheel horse power will be even less as what is left after the transmission has taken it's share is shared across two axels and not one.
Guest
New member
Interesting stuff.
But I have become distracted from this by the fact that the front of my engine is all wet []
Almost definitely coolant leaking out somewhere. B******ks !
I don't think it's anything relevant, but there appears to be a place to put a pipe on the (now wet) plastic cover (front of engine) which goes over the cam belt and there isn't a pipe on it. Not an obvious place for coolant to go into though.....
But I have become distracted from this by the fact that the front of my engine is all wet [
]Almost definitely coolant leaking out somewhere. B******ks !
I don't think it's anything relevant, but there appears to be a place to put a pipe on the (now wet) plastic cover (front of engine) which goes over the cam belt and there isn't a pipe on it. Not an obvious place for coolant to go into though.....
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