28i vs E89 3.0 coupe bhp/torque didn't make sense?

R

RX-78

Active member
 Liverpool
Forgive me before I even start. I'm a simpleton and I don't pretend to understand all of the calculations used in working out torque but I just can't get my head around this.

E86 (sorry, originally wrongly written as e89) 3.0si coupe 265bhp 1385kg max torque 315nm at 4000rpm
28i 245bhp 1475kg max torque 350nm at 4800rpm

How can a car with lower max bhp with heavier weight have better torque? I know it's at higher rpm but surely that makes no difference when calculating max torque at any point?

P.s. I've read 28i has solid break at front where as 3.0 coupe has vented break both front and back. Did this make any difference?
 
Firstly, are you comparing your E86 to the E89? Just you say E89 3.0si Coupe which does not exsist and the figure for the E89 30i are differnt to what you have quoted.

As for the turbo, yes that makes an impact but the 18i, 20i and 28i all use the same engine and are all turbo charged with a twin scroll set up. The 18i has 156bhp, the 20i has 184bhp and the 28i 245bhp. That's quite a range for one engine.

A larger engine will usually have more bhp than a smaller engine abd BMW could go on forever getting more power out of each engine by using different turbos etc but they have built the 4 cylinder cars for efficent and EU regulations.
 
Yes, that's a typo I meant e86 (my current car, which is why I was comparing the two). I'll amend the to post y2k Reese the confusion.

I know same engine can be mapped to give different output so different bhp can appear on "same" engine (although there was a debate about if the components where different, not just the mapping).

What I don't get is the torque.
Torque = force x distance
Given the car is similar in size, I assumed distance remained the same so the force must be the variant here
Force = mass x acceleration
Or rather acceleration= force/mass

As their acceleration is roughly same (looking at 0-60 figure. Obviously acceleration curve may be little different) and 28i is heavier then e86, force must also increase in order to sustain the same acceleration. But the bhp is less on 28i, nevermind higher.

Have I got my physics wrong or is bhp not force? It's been a long time since I did physics so I may be very wrong here.
 
Assuming the 3.0si produces it's peek torque, 315Nm, at 4800rpm it would be making 212bhp, where as the 28i will be making 236bhp at the same rpm. However we know that the 3.0si is making less torque at 4800rpm so power is lower as well. So while the 3.0si is making more power it's closer to peek power for less time. This is important because it's not the actual peek value that's important, it's the average power during the acceleration pull that's the important thing.

To the 0-60 run it's self - A 2 gear pull and using a wide rpm range in both gears. In this case the 28i has a massive advantage due to its higher torque level across a wide plateau in the low through to mid-rpm registers. This launches the car down the road in 1st gear so hard that the 3.0si is unable to pull back the 1st gear deficit. I'll try to rememberer and make a graph demonstrating this when I get off holiday.
 
Thanks tech, I kinda get it. I think :oops:

Because the acceleration (0-60mph) is an average of acceleration that took place, acceleration at the point of max torque may not be the same in both car? And therefore my calculation is fundamentally flawed?

Z4M - both cars 0-60 are same @5.7secs. One of the reason why I went for 28i as I didn't want to drop the performance but didn't need a better straight line performance either (though I would have happily accepted a better performance if money allowed :D )
 
RX-78 said:
Thanks tech, I kinda get it. I think :oops:

Because the acceleration (0-60mph) is an average of acceleration that took place, acceleration at the point of max torque may not be the same in both car? And therefore my calculation is fundamentally flawed?

Z4M - both cars 0-60 are same @5.7secs. One of the reason why I went for 28i as I didn't want to drop the performance but didn't need a better straight line performance either (though I would have happily accepted a better performance if money allowed :D )
Click to expand...

Well I think that an 28i manual will be simular performance to an 35i Manual - and would welcome an opportunity at an meet of doing an 30-70 test? :fuelfire:
 
RX-78 said:
Forgive me before I even start. I'm a simpleton and I don't pretend to understand all of the calculations used in working out torque but I just can't get my head around this.

E86 (sorry, originally wrongly written as e89) 3.0si coupe 265bhp 1385kg max torque 315nm at 4000rpm
28i 245bhp 1475kg max torque 350nm at 4800rpm

How can a car with lower max bhp with heavier weight have better torque? I know it's at higher rpm but surely that makes no difference when calculating max torque at any point?

P.s. I've read 28i has solid break at front where as 3.0 coupe has vented break both front and back. Did this make any difference?
Click to expand...

The engines torque output has nothing to do with the cars weight, HP is calculated by a formula using torque x revs
That's how a car with a higher torque output can have a lower HP figure (it revs lower)
Regards
 
techathy said:
Assuming the 3.0si produces it's peek torque, 315Nm, at 4800rpm it would be making 212bhp, where as the 28i will be making 236bhp at the same rpm. However we know that the 3.0si is making less torque at 4800rpm so power is lower as well. So while the 3.0si is making more power it's closer to peek power for less time. This is important because it's not the actual peek value that's important, it's the average power during the acceleration pull that's the important thing.

To the 0-60 run it's self - A 2 gear pull and using a wide rpm range in both gears. In this case the 28i has a massive advantage due to its higher torque level across a wide plateau in the low through to mid-rpm registers. This launches the car down the road in 1st gear so hard that the 3.0si is unable to pull back the 1st gear deficit. I'll try to rememberer and make a graph demonstrating this when I get off holiday.
Click to expand...

Excellent post and a big +1 to you... Its amazing how many people do not grasp this (usually Honda VTEC owners :rofl: ).

Ive always said that PEAK figures from Tq/Hp mean little - its the AREA under the graph that counts!
 
Twin Turbo said:
RX-78 said:
Thanks tech, I kinda get it. I think :oops:

Because the acceleration (0-60mph) is an average of acceleration that took place, acceleration at the point of max torque may not be the same in both car? And therefore my calculation is fundamentally flawed?

Z4M - both cars 0-60 are same @5.7secs. One of the reason why I went for 28i as I didn't want to drop the performance but didn't need a better straight line performance either (though I would have happily accepted a better performance if money allowed :D )
Click to expand...

Well I think that an 28i manual will be simular performance to an 35i Manual - and would welcome an opportunity at an meet of doing an 30-70 test? :fuelfire:
Click to expand...
I was responding to Z4M-2006, not suggesting e86 z4 ///M has same performance........
 
Smartbear said:
The engines torque output has nothing to do with the cars weight, HP is calculated by a formula using torque x revs
That's how a car with a higher torque output can have a lower HP figure (it revs lower)
Regards
Click to expand...

That's correct the calculation is HP = (RPM x Torque)/5252

Units for torque are lb.ft and at the RPM

and...
Torque gives you acceleration
Power gives you speed
 
Power = work/time
Torque = force x distance
Work = force x displacement or mass x acceleration x diplacement or torque x displacement/distance
Speed = distance/time (or distance = speed x time)

Therefore
power = mass x acceleration x displacement/time or
Power = torque x displacement/time x distance or
Power = torque x displacement/time x time x speed

Didn't that mean:
increasing power should increase torque
Increasing torque should increase speed

I'm getting even me confused now :oops: may be I just need to stop thinking
 
techathy said:
Assuming the 3.0si produces it's peek torque, 315Nm, at 4800rpm it would be making 212bhp, where as the 28i will be making 236bhp at the same rpm. However we know that the 3.0si is making less torque at 4800rpm so power is lower as well. So while the 3.0si is making more power it's closer to peek power for less time. This is important because it's not the actual peek value that's important, it's the average power during the acceleration pull that's the important thing.

To the 0-60 run it's self - A 2 gear pull and using a wide rpm range in both gears. In this case the 28i has a massive advantage due to its higher torque level across a wide plateau in the low through to mid-rpm registers. This launches the car down the road in 1st gear so hard that the 3.0si is unable to pull back the 1st gear deficit. I'll try to remember and make a graph demonstrating this when I get off holiday.
Click to expand...

That's to simplistic and using power to describe acceleration is inaccurate, acceleration of an object is governed by it's mass and the amount of force applied to it, in other words the amount of torque at the driven wheel gives you the force. and then, well there are just so many factors that can shape the acceleration curve of a car, gear ratios, number of gears, spacing between gear ratio's, final drive, driven wheel diameter, tractability, engine torque curve, maximum rpm and etcetera
 
Nope, increasing hp can just rely on increased rpm without increasing torque-look at sars post as he had the correct formula, yours is to confusing :wink:
Regards
 
Smartbear said:
Nope, increasing hp can just rely on increased rpm without increasing torque-look at sars post as he had the correct formula, yours is to confusing :wink:
Regards
Click to expand...

She....I know, how can a woman possibly know all this!! :?
 
lux said:
Smartbear said:
Nope, increasing hp can just rely on increased rpm without increasing torque-look at sars post as he had the correct formula, yours is to confusing :wink:
Regards
Click to expand...

She....I know, how can a woman possibly know all this!! :?
Click to expand...

Oops, sorry sars :oops:
Regards
 
Smartbear said:
lux said:
Smartbear said:
Nope, increasing hp can just rely on increased rpm without increasing torque-look at sars post as he had the correct formula, yours is to confusing :wink:
Regards
Click to expand...

She....I know, how can a woman possibly know all this!! :?
Click to expand...

Oops, sorry sars :oops:
Regards
Click to expand...

:rofl:

An easy explanation with pictures is available for us gents
http://www.epi-eng.com/piston_engine_technology/power_and_torque.htm
 
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