Power, Torque and rolling roads

Discussion in 'Engine' started by Mark300SL, Oct 21, 2002.

  1. Mark300SL

    Mark300SL 1962-2010. Gone, but not forgotten.

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    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 :D

    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 :D






    OK Mr Blower - you asked for it :D - 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 :D :D


    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 _:D


    Love the F.O.R.D swear filter :D

    If anyone got this far - class dismissed


    Mark
     
    11 people like this.
  2. Sp!ke

    Sp!ke Administrator Staff Member

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    Excellently written....even a dummy like me managed to take that in. :)

    Now, can you explain the laws of coefficients of friction in the same laymans terms .....especially the bit regarding tyre widths ignoring the rules and providing more grip with a larger contact patch that one really fazes me a little.

    errr...and you never did disclose *exactly* what you do for a living, you certainly got my interest up a bit.
     
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  3. OP
    OP
    Mark300SL

    Mark300SL 1962-2010. Gone, but not forgotten.

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    Cheers Sp!ke

    Thats why I wrote it - I understand that not everyone understands the maths and physics behind engines as well as I do
    To put it into context with something that everyone who was there could understand was what made it pleasurable.

    I was a fully qualified mechanic for about 16 years and I have modded cars since I was 15, - but the lure of money made me move to IT - specifically I am a Wide Area Network engineer. Its not the best paid job in the world, but it pays better than mechanics and you go home clean! - as a bonus you are much more inclined to work on your own car or friends vehicles at the weekend - that is - I'm not sick of cars by the time Saturday comes around!

    But I took to networks very quickly and found myself teaching LAN and WAN data networking (routers) technology for Ericsson!

    The combination of mechanical ability and knowledge plus the intructional skills that were forced upon me in the teaching world - make long technical posts fun!

    Mark
     
  4. GrahamC230K

    GrahamC230K MB Club Veteran

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    Very nice explanation Mark, thanks.

    Just one problem.

    None of that front wheel drive rubbish here please :D

    Only Joking :p
     
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  5. Paul

    Paul Hardcore MB Enthusiast

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    Please Sir, can I go to the toilet? :D
     
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  6. GrahamC230K

    GrahamC230K MB Club Veteran

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    Are we allowed to use calculators? :)
     
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  7. OP
    OP
    Mark300SL

    Mark300SL 1962-2010. Gone, but not forgotten.

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    I've got 2 Honda Crx's, _1 Vx Nova 2.0, a Rover 216 like no others, and a 300SL

    So thats 4 FWD and 1 RWD - So the Mercs outnumbered, if not outclassed :)

    Paul - How many times have I told you not to pee in the Hondas !

    Graham - you need a calculator !


    Mark

    There are 10 types of people who understand Binary - Those who do - and those who dont !
     
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  8. V12

    V12 MB Club Veteran

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    That is THE best explanation ive ever read...much better than when i try to teach it to trainee's...

    Mucho Respecto :)
     
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  9. GrahamC230K

    GrahamC230K MB Club Veteran

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    So at 2500rpm I have 99 BHP!


    207 x 2500 /5252 = 98.5


    Cool.
     
  10. robert.farmer

    robert.farmer Hardcore MB Enthusiast

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    Mark300sl, sorry to butt in but do you know anyone at Ericssons in Burgess Hill, im asking 'cause my stepson works there. Thanks Rob
     
  11. OP
    OP
    Mark300SL

    Mark300SL 1962-2010. Gone, but not forgotten.

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    Sorry no

    I'm an independant contractor - via the company I work for - my Boss was the original ACC router instructor before Ericsson took over, and we just carried on teaching for them.
    Though I know a couple of the Ericsson boys there I deal mainly with Ericsson in Guilford.

    Mark
     
  12. jimmy

    jimmy Hardcore MB Enthusiast

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    So is there a calculation to work out the torque from the Hp? Or is that a really dumbass question.

    It's just that Sym, Graham and myself have all done power runs and the graphs only show our peak power (Hp) output and losses but no torque figures at all.:confused:

    I asked the guy who operates the rolling road if we could have a torque reading, he just said no and wouldn't explain why.

    Somebody who is good at maths should be able to work the calculation backwards.

    From the graphs we can work out the road speed at which maximum power was achieved, we should be able to work out from the gearing of the car at what rpm the engine was turning at this point so we should be able to do it. Right?
     
  13. GrahamC230K

    GrahamC230K MB Club Veteran

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    I thought that earlier but could not figure out. Although I trust the power figures as your (Jimmy's) improvement with b4 and after runs, plus Syms are all fairly consistent.

    Apart from they didn't use fans or strap the cars down (their risk & we were only on the rollers for like 3 minutes each), it is the the lack of torque measurement that makes me question the whole thing.

    We need our resident expert .......Mark where are you?

    LOL. I think Sym has PM'd him already with a link to my graph, so hopefully all will be explained - either that or we will be marched off to another RR to try again!
     
  14. GrahamC230K

    GrahamC230K MB Club Veteran

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    OK, no laughing here is the perhaps basic and unconventional power graph I (we) got:

    [​IMG]

    130KW @ the wheels
    30KW transmission losses

    =

    160KW total engine power.


    Using W x 0.00136 = BHP

    176.8 BHP @ the wheels
    40.8 BHP transmission losses

    = 217.6 r/u 218 BHP total engine power.

    Now the graph shows my peak 218BHP @ just over 180 KM/h, which is about 110mph ish. Now peak power is suppossed to be at 5300 rpm, so that would correspond with 4th gear - 5300 rpm, as I can just squeeze 120mph before the rev limiter kicks in.


    From there on I need help :confused:
     
    Last edited: Jul 26, 2003
  15. jimmy

    jimmy Hardcore MB Enthusiast

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    I mentioned that to them, in all their years doing this they have only ever had one car 'jump' out of the rollers and that stopped dead as they have no momentum when stationary. They say some front wheel cars have to be strapped as they get torque steer and wander from side to side.

    They do use fans when the cars are repeatedly run for long times, I saw them tuning a Jag E-type whilst it was running flat out.

    The garage, Hi-tech, are highly regarded as one of the best garages in the area. They sort out problems for many of the main dealers and always have exotic cars there.

    I shall quiz them about the torque measurements on Monday, if they can do it then perhaps this could be a possible venue for a RR GTG?
     
  16. GrahamC230K

    GrahamC230K MB Club Veteran

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    Just trying to look at it objectivley! - and I'm sure others will ask why we have no torque figures. Yours must be monsterous James! Need to be sure of this stuff before I can brag!

    An explanation from them may be the easiest way. I guessed that they did not give us the full strap down and fan job as it was not required for the quick run.

    I think a RR GTG is a good idea.

    I'm sure they would use straps and a fan just for you Vinay.
     
  17. altreed

    altreed Hardcore MB Enthusiast

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    How can you work out the transmission losses on an Auto? You can't put it in neutral at road speed. I believe that is a no-no.

    Also how can you hold the gear and 'floor it' to get the full power. As soon as I 'boot-it' my car kicks down, unless I am at 3500+rpm

    I would like a b4 and after RR when I get my car chipped. Is there anywhere that can do that service, and how much is it likely to cost??

    p.s I worked out what RR is, but what is GTG?
     
  18. sym

    sym Hardcore MB Enthusiast

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    I don't think the idea is to floor it - but to hold it in the gear that will give the most power (you can do this with my auto box), and to gradually build up to maximum revs - peak power output should be somewhere in that range.

    It is possible to slip the car into netural while driving, but don't know if it has exactly the same effect as a manual - so perhaps that's why the losses generally appear to be assumed for an auto - rather than calculated.
     
  19. GrahamC230K

    GrahamC230K MB Club Veteran

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    Get-ToGether


    A car meet basically.
     
  20. scotth_uk

    scotth_uk Hardcore MB Enthusiast

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