As some of you will know I'm active on 3 different forums - and I'm considered a technical 'expert' on both the Rover and CRX forums
On Sunday we had a CRX-UK GTG/rolling Road day at PTS in Luton.
20 little 1.6's were on form making loads of power but not huge torque
Later on Sun evening a couple of posts turn up asking about how to interpret the results
I took a lot of time to explain the results and after 40 mins of typing I had a masterpiece that I think explains things quite well - here it is for your information (if you need it) and ignore it if you like
OK Mr Blower - you asked for it - but first we need some background information
The first thing to remember is that BHP is not measured on a dyno - only torque, BHP is a factor of torque and engine speed - its just a theoretical calculation
First, power, what do we mean by that?
Well, we usually express power in terms of the amount of time it takes to do a certain amount of work... for example, one horsepower is measured as the amount of work required to lift 33,000 lbs over 1 foot in 1 minute (huh -that's obvious, _). As this is such an obviously Scottish measurement (as James Watt first devised the calculation) , we have a metric equivalent... the metric horse could lift 4,500Kg a metre in a minute... 98.6% of a good British horse. The Europeans decided to call it a PS (Pferde Starke - German for Horse power!)instead of an HP to cover their shame. Also, in the newfangled metric system, 1hp is the equivalent of 746 Watts of electrical power. So, to recap briefly:
1 HP = Roughly the amount of work a horse can do lifting coal up a mineshaft, assuming his heart was really in it = 1.014 PS = 0.746 KW - easy huh _
Now we need a way to measure this output - so we use a defined force or "brake" to see how much energy we need to apply to stop it - Hence "Brake Horsepower" - and is defined as it's maximum performance at a certain rpm.
The other thing we babble on about is Torque... Torque is the amount of force applied to turn something multiplied by the distance from the axis of it's rotation... sounds all weird, until you realise that we use the engine to rotate the front wheel, so torque is something that would be nice to calculate. Something interesting is that 1hp is 550ft/lbs of torque per second.
Now, it's fairly easy to measure torque... this is where the dyno comes in, and we calculate horsepower from an engine's torque output multiplied by the revs...
A dynomometer is just a heavy drum (brake), an accelerometer and a computer... if you know the weight of the drum, and you know how fast it's being accelerated, you can calculate the torque that must be being applied to the drum. What you also want to take into account are the frictional losses on the drum, and the air temperature at the time (which is why you'll see air temperature, pressure and a correction factor calculated by the software... as air temperature goes up, so the effective power output goes down, so the correction factor has to go up to normalise this).
Taking a Dyno Run
( As used today at PTS - differs slightly if they use an inductive loop)
So, to measure torque, we strap the car to the dyno, start it up and run it up in the gears to 3000RPM the dyno operator holds the car at a steady 3000 for 6-7 seconds and the dyno learns the road speed for that car at 3k. ( Some dynos use an inductive loop to accurately measure RPM - but the rev counter method works fine for most modern cars) - now with a given road speed the RPM can be calculated.
... cars tend to do their power runs in 4th gear, as it's the best gear for acceleration at speed and less chance of the wheels slipping, as the calculation errors get smaller the bigger the numbers are. 4th is used because on cars these days 5th gear tend to be a bit of an overdrive.
The throttle is gently floored, and the dyno slowly allows the speed of the engine to increase - this measures the torque of the engine. When the engine reaches maximum RPM, the operator puts the car in neutral, and allows the wheels to decelerate of their own accord - this measures the losses of the transmission,driveshafts,bearings brakes (if they are sticking) and tyres
Now here comes the maths
BHP = Torque (lbft) x RPM
_ _ _ _ -------------------
_ _ _ _ _ _ _ _ 5252
This means that BHP is always equal to torque at 5252rpm _- if its not then there is something wrong!
Now using Pauls results from today we get max power of 169.3BHp @ 7869RPM but 115.0lbft of Torque. The important factor here is that it makes its maximum power at nearly 8000rpm!
Andy Blower made more torque 123.5lb/ft - therefore it must be quite a bit more powerful right? - WRONG ! - he made his max figure at 7309rpm - and as BHP is proportional to speed and his engine was making its torque at a lower RPM value - he gets 169.4 BHP - a whole 0.1bhp more _
Shirish made 129.8lb/ft at 7466rpm - which equates to 170.4BHP
So now we have 115, 123.5 and 129lb/ft of torque - all at different engine speeds - All giving an output of 170BHP give or take a little bit
The moral of this essay is - it is better to make torque at high RPM for a screamer! - and thats why beause Hondas rev so high they produce the power!
I realise I've forgotten an important part of the calculation _
Where did the 5252 figure come from ?
As discussed what we actually measure is torque, expressed in ft/lb, and then we calculate actual horsepower by converting the twisting force of torque into the work units of horsepower.
Visualize a one pound weight, one foot from a fulcrum on an "invisible weightless" bar. If we rotate that weight for one full revolution against a one pound resistance, we have moved it a total of 6.2832 feet (pi times a two foot circle), and, we have done 6.2832 foot pounds of work.
OK. Remember Watt? He said that 33,000 foot pounds of work per minute was equivalent to one horsepower. If we divide the 6.2832 foot pounds of work we've done per revolution of the weight into 33,000 foot pounds, we come up with the fact that we have to rotate that weight at the rate of 5,252 revolutions per minute in order to do 33,000 foot pounds per minute of work, and thus do work at the equivalent rate of one horsepower.
Therefore, the following formula applies for calculating horsepower from a torque measurement:
Horsepower = ( Torque * RPM ) / 5252
Thats where the 5252 comes from
Torque is only half the story. While torque is the force created, it doesn't account for the importance of revs.
Imagine trying to remove a wheel nut from a car with a standard wheelbrace and all the torque you could produce can't loosen the "Kwik fit special" airgunned super tightened wheel bolts. You apply lots of force, i.e. torque, but you still can't generate any rpm. Therefore nothing is accomplished, no power generated despite your cursing and kicking. So - Without rpm, torque is useless!
Two engines may make 125 foot/pounds of torque, but if one is a FixOrRepairDaily turning at 5,000 rpm and another is a Honda turning at 10,000 rpm, the Honda is doing more work than the FixOrRepairDaily, therefore generating more power.
HP = Torque x RPM / 5252
so the FixOrRepairDaily makes 125*5000 / 5252 = 119HP,
but the Honda makes 125*10000 / 5252 = 238HP - must have been an S2000 _
Love the F.O.R.D swear filter
If anyone got this far - class dismissed
Mark
On Sunday we had a CRX-UK GTG/rolling Road day at PTS in Luton.
20 little 1.6's were on form making loads of power but not huge torque
Later on Sun evening a couple of posts turn up asking about how to interpret the results
I took a lot of time to explain the results and after 40 mins of typing I had a masterpiece that I think explains things quite well - here it is for your information (if you need it) and ignore it if you like
OK Mr Blower - you asked for it
- but first we need some background information The first thing to remember is that BHP is not measured on a dyno - only torque, BHP is a factor of torque and engine speed - its just a theoretical calculation
First, power, what do we mean by that?
Well, we usually express power in terms of the amount of time it takes to do a certain amount of work... for example, one horsepower is measured as the amount of work required to lift 33,000 lbs over 1 foot in 1 minute (huh -that's obvious, _). As this is such an obviously Scottish measurement (as James Watt first devised the calculation) , we have a metric equivalent... the metric horse could lift 4,500Kg a metre in a minute... 98.6% of a good British horse. The Europeans decided to call it a PS (Pferde Starke - German for Horse power!)instead of an HP to cover their shame. Also, in the newfangled metric system, 1hp is the equivalent of 746 Watts of electrical power. So, to recap briefly:
1 HP = Roughly the amount of work a horse can do lifting coal up a mineshaft, assuming his heart was really in it = 1.014 PS = 0.746 KW - easy huh _
Now we need a way to measure this output - so we use a defined force or "brake" to see how much energy we need to apply to stop it - Hence "Brake Horsepower" - and is defined as it's maximum performance at a certain rpm.
The other thing we babble on about is Torque... Torque is the amount of force applied to turn something multiplied by the distance from the axis of it's rotation... sounds all weird, until you realise that we use the engine to rotate the front wheel, so torque is something that would be nice to calculate. Something interesting is that 1hp is 550ft/lbs of torque per second.
Now, it's fairly easy to measure torque... this is where the dyno comes in, and we calculate horsepower from an engine's torque output multiplied by the revs...
A dynomometer is just a heavy drum (brake), an accelerometer and a computer... if you know the weight of the drum, and you know how fast it's being accelerated, you can calculate the torque that must be being applied to the drum. What you also want to take into account are the frictional losses on the drum, and the air temperature at the time (which is why you'll see air temperature, pressure and a correction factor calculated by the software... as air temperature goes up, so the effective power output goes down, so the correction factor has to go up to normalise this).
Taking a Dyno Run
( As used today at PTS - differs slightly if they use an inductive loop)
So, to measure torque, we strap the car to the dyno, start it up and run it up in the gears to 3000RPM the dyno operator holds the car at a steady 3000 for 6-7 seconds and the dyno learns the road speed for that car at 3k. ( Some dynos use an inductive loop to accurately measure RPM - but the rev counter method works fine for most modern cars) - now with a given road speed the RPM can be calculated.
... cars tend to do their power runs in 4th gear, as it's the best gear for acceleration at speed and less chance of the wheels slipping, as the calculation errors get smaller the bigger the numbers are. 4th is used because on cars these days 5th gear tend to be a bit of an overdrive.
The throttle is gently floored, and the dyno slowly allows the speed of the engine to increase - this measures the torque of the engine. When the engine reaches maximum RPM, the operator puts the car in neutral, and allows the wheels to decelerate of their own accord - this measures the losses of the transmission,driveshafts,bearings brakes (if they are sticking) and tyres
Now here comes the maths
BHP = Torque (lbft) x RPM
_ _ _ _ -------------------
_ _ _ _ _ _ _ _ 5252
This means that BHP is always equal to torque at 5252rpm _- if its not then there is something wrong!
Now using Pauls results from today we get max power of 169.3BHp @ 7869RPM but 115.0lbft of Torque. The important factor here is that it makes its maximum power at nearly 8000rpm!
Andy Blower made more torque 123.5lb/ft - therefore it must be quite a bit more powerful right? - WRONG ! - he made his max figure at 7309rpm - and as BHP is proportional to speed and his engine was making its torque at a lower RPM value - he gets 169.4 BHP - a whole 0.1bhp more _
Shirish made 129.8lb/ft at 7466rpm - which equates to 170.4BHP
So now we have 115, 123.5 and 129lb/ft of torque - all at different engine speeds - All giving an output of 170BHP give or take a little bit
The moral of this essay is - it is better to make torque at high RPM for a screamer! - and thats why beause Hondas rev so high they produce the power!
I realise I've forgotten an important part of the calculation _
Where did the 5252 figure come from ?
As discussed what we actually measure is torque, expressed in ft/lb, and then we calculate actual horsepower by converting the twisting force of torque into the work units of horsepower.
Visualize a one pound weight, one foot from a fulcrum on an "invisible weightless" bar. If we rotate that weight for one full revolution against a one pound resistance, we have moved it a total of 6.2832 feet (pi times a two foot circle), and, we have done 6.2832 foot pounds of work.
OK. Remember Watt? He said that 33,000 foot pounds of work per minute was equivalent to one horsepower. If we divide the 6.2832 foot pounds of work we've done per revolution of the weight into 33,000 foot pounds, we come up with the fact that we have to rotate that weight at the rate of 5,252 revolutions per minute in order to do 33,000 foot pounds per minute of work, and thus do work at the equivalent rate of one horsepower.
Therefore, the following formula applies for calculating horsepower from a torque measurement:
Horsepower = ( Torque * RPM ) / 5252
Thats where the 5252 comes from
Torque is only half the story. While torque is the force created, it doesn't account for the importance of revs.
Imagine trying to remove a wheel nut from a car with a standard wheelbrace and all the torque you could produce can't loosen the "Kwik fit special" airgunned super tightened wheel bolts. You apply lots of force, i.e. torque, but you still can't generate any rpm. Therefore nothing is accomplished, no power generated despite your cursing and kicking. So - Without rpm, torque is useless!
Two engines may make 125 foot/pounds of torque, but if one is a FixOrRepairDaily turning at 5,000 rpm and another is a Honda turning at 10,000 rpm, the Honda is doing more work than the FixOrRepairDaily, therefore generating more power.
HP = Torque x RPM / 5252
so the FixOrRepairDaily makes 125*5000 / 5252 = 119HP,
but the Honda makes 125*10000 / 5252 = 238HP - must have been an S2000 _
Love the F.O.R.D swear filter
If anyone got this far - class dismissed
Mark
