What affect does rpm have on bhp?

  • Thread starter luchre
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luchre

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luchre
Apologies if this is a stupid question but though I love racing cars round in GT I know very little about them.

Thing is wanted to race the various cars from the Indy 500 enduro using their actual bhp rather than the GT game figures to see if I could beat the AI. Was OK with the R10 as it's game figures seem to match up with reality - 650 bhp.

Went for the BMW v12 LMR next.

In game machine has 827 bhp, trawling the net and it's actual power is 580 bhp. However in game rpm is 4400, real life 6500. As I'm not good enough to beat the AI with a 580 bhp 4400 rpm engine ( tried and failed) was wondering if the 580 bhp could be increased to compensate for the lack of in game rpm. Is that viable or is that a daft idea that totally misunderstands the relationship between bhp and rpm. If it is viable can anyone give me a clue as to a math formula that would allow for an accurate bhp increase as opposed to me just sticking in a random number.

If anyone could give any help it would be much appreciated.
 
Torque (in lbft) @ rpm x rpm/5252 = Power (in hp) @ rpm
 
Ok thankyou for that, didn't realise needed torque as well - which just shows my ignorance. Think understand formula presented but 'ahem' once I've got some numbers put together might pass them by you if that's ok.
 
Torque is a turning force expressed in pound-equivalent force feet (lbft) or kilogram-equivalent force metre (kgm) - or the force equivalent to a given mass under normal gravitational acceleration acting on a lever of given length.

Power is simply the amount of torque per second. One horsepower is the amount required to lift a 550lb mass through 1 vertical foot against gravity in 1 second - 550lbft/s.

Given that torque is a force acting on a crankshaft, you can find out the power by calculating the time that force is acting - and since you have a time function in the form of revolutions per minute, it's easy to do.

So if a car produces 120lbft at 2000rpm, it makes (120 x 2000/5252) hp at 2000rpm - 45hp. If it produces 120lbft at 6000rpm, it makes (120 x 6000/5252) hp at 6000rpm - 137hp. At 5252rpm, torque in lbft is equal to power in hp (and torque in kgm is equal to power in PS).
 
Right you explain that very well. But could you explain to me why two sets of numbers are given in the stats. Found two sites giving me engine info; conceptcars.com and ultimatecarpage.com. Both agree the V12s' hp is 580 @ 6500rpm. Ultimate expresses torque as 678nm/500lbft, concept gives 500lbft @ 4500rpm.

(500x4500/5252) = 428hp. Made me think 'Ah' so (500x6500/5252) must equal the 580 number but it doesn't it equals 619.

So why are there two rpm figures given? Have noticed it many times playing gt but never given it any thought. What's the relationship?
 
luchre
Right you explain that very well. But could you explain to me why two sets of numbers are given in the stats. Found two sites giving me engine info; conceptcars.com and ultimatecarpage.com. Both agree the V12s' hp is 580 @ 6500rpm. Ultimate expresses torque as 678nm/500lbft, concept gives 500lbft @ 4500rpm.

(500x4500/5252) = 428hp. Made me think 'Ah' so (500x6500/5252) must equal the 580 number but it doesn't it equals 619.

So why are there two rpm figures given? Have noticed it many times playing gt but never given it any thought. What's the relationship?
Click to expand...

Peak torque and peak power won't be at the same point of the rev range, so the peak power value and location is given along with peak torque value and location.

For your example, peak power of 580hp @ 6500rpm would result in 469lbft @ 6500rpm (580 x 5252/6500) - less than the peak torque. Peak torque 500lbft @ 4500rpm would result in 428hp @ 4500rpm (500 x 4500/5252) - less than the peak power.

Remember, the formula is:

(Torque [in lbft] @ rpm) x That rpm/5252 = (Power [in hp] @ that rpm)

And the reverse is:

(Power [in lbft] @ rpm) x 5252/That rpm = (Torque [in hp] @ that rpm)
 
Right think I see but now getting late for work so have to go - many thanks for info, would like to come back tomorrow if that ok.

cheers luchre
 
Nice Famine.....I was just going to post the equation but your explanation is great. I never really went into depth with it like that and think my tired brain might have just felt a spark....thanks for that.
 

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