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The sixth generation BMW 3 Series Sedan F30/F31/F34 and the first first generation 4 Series Coupe F32/F33/F36. Get the latest 3 and 4 series pricing from our ordering and pricing guide sticky thread.

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  #51  
Old 06-30-2013, 02:02 PM
d geek d geek is offline
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Originally Posted by bmw_or_audi View Post
I am not confusing Hp and horsepower, but most people seem to think they are independent things. They are not. If you have one at a given RPM, then you have the other.

Hp = torque * engine rotational speed * crappy unit conversion factor.

In daily easy to moderate driving, yes, you feel the 'd being a little more powerful. But if you really call on the power, the very responsive 8-speed will quickly kick down and put you in the right powerband and the advantage is over.
So do you really think driving a 320i would be comparable to driving a 328d? The 'd has 40% more torque.

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Originally Posted by bmw_or_audi View Post
... I could save a lot more by going for the 320i instead, which is the more comparable car in terms of power.
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  #52  
Old 06-30-2013, 03:16 PM
bmw_or_audi bmw_or_audi is offline
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Originally Posted by d geek View Post
So do you really think driving a 320i would be comparable to driving a 328d? The 'd has 40% more torque.
God no. Absolutely not. First, let's forget about torque, because more torque "down low" also just means more Hp down low. So let's stick with Hp. now let's plot the Hp of both engines on some normalized RPM axis that puts the peak Hps at the same spot. These peaks are about the same. But as you move to lower RPMs, the diesel will have higher peak Hp there. So its curve will be "fatter."

Now if you are driving aggressively, meaning with aggressive gear selection, both engines are producing comparable Hp, and so the diesels low end Hp will not come into play at all.

But if you are driving around down, from stop to stop, then you have no choice but to go through the lower RPM range when you start and you will feel the difference there. You could negate that to some extent if you do high RPM launches, but no one will do that. But once you have gone through most of first gear, the 320 can match the diesel, as long as aggressive gearing is used. But again, I concede that most people rather be more relaxed with throttle and gearing, so the diesel will feel more powerful under those circumstances. But if you really ever need all the power, it's there to be tapped so long as you put the transmission in the right gear.

So how different they will feel will depend on the type of driving you are doing. I am sure you already know everything I said above. But I feel like it is important to elaborate on these details for anyone who is interested at all in the topic.

All else being equal, I would pick the engines in this order: 328, 328d, 320. But all else is not equal, so people will pick according to their priorities: price, mpgs, servicing, etc., all come into play.
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  #53  
Old 07-01-2013, 10:38 AM
fleuger99 fleuger99 is offline
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Originally Posted by bmw_or_audi View Post
God no. Absolutely not. First, let's forget about torque, because more torque "down low" also just means more Hp down low. So let's stick with Hp. now let's plot the Hp of both engines on some normalized RPM axis that puts the peak Hps at the same spot. These peaks are about the same. But as you move to lower RPMs, the diesel will have higher peak Hp there. So its curve will be "fatter."

Now if you are driving aggressively, meaning with aggressive gear selection, both engines are producing comparable Hp, and so the diesels low end Hp will not come into play at all.

But if you are driving around down, from stop to stop, then you have no choice but to go through the lower RPM range when you start and you will feel the difference there. You could negate that to some extent if you do high RPM launches, but no one will do that. But once you have gone through most of first gear, the 320 can match the diesel, as long as aggressive gearing is used. But again, I concede that most people rather be more relaxed with throttle and gearing, so the diesel will feel more powerful under those circumstances. But if you really ever need all the power, it's there to be tapped so long as you put the transmission in the right gear.

So how different they will feel will depend on the type of driving you are doing. I am sure you already know everything I said above. But I feel like it is important to elaborate on these details for anyone who is interested at all in the topic.

All else being equal, I would pick the engines in this order: 328, 328d, 320. But all else is not equal, so people will pick according to their priorities: price, mpgs, servicing, etc., all come into play.
How can you talk about performance and only focus on HP? HP = top end speed where as torque is acceleration. So, with a diesel having its peak torque low down in the RPM range that is what affect acceleration from a traffic light for example.

I had a VW Touareg TDI with the 3.0 litre V6 and off the line it was a beast but once I hit 80Mph it suddenly ran out of steam. It had 406lbs ft torque at only 2K RPM so pulled like a train. But top end HP was only 225Hp at 4K RPM.
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  #54  
Old 07-01-2013, 11:59 AM
Arciga18 Arciga18 is offline
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Originally Posted by bayoucity View Post
You shall know next week.

Sweet. I hope that I'm impressed.
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  #55  
Old 07-16-2013, 10:32 AM
bmw_or_audi bmw_or_audi is offline
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Originally Posted by fleuger99 View Post
How can you talk about performance and only focus on HP? HP = top end speed where as torque is acceleration. So, with a diesel having its peak torque low down in the RPM range that is what affect acceleration from a traffic light for example.
Repeat after me. If you know the max Hp at any given RPM, then you know the max torque at that RPM, and vice versa. Acceleration is a function of engine torque and transmission gearing (= torque at the wheels). It's no different than a bicycle with gears: on a high gear, you need a lot of force but you can pedal slowly; on a low gear, you need little force, but you better be able to pedal fast. Power gives you how the two combine to give you acceleration, or equivalently torque at the wheels. Power = torque x rotational speed.

The only reason that the diesel feels really strong at a traffic light is because car gears go only so low. But once you get going, that factor disappears. Although it is still there is you shift at lower RPMs, like many people do in normal driving. But power is a downshift away when you are moving, whereas that option is indeed not there at the traffic light.
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  #56  
Old 07-16-2013, 12:24 PM
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floydarogers floydarogers is offline
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Originally Posted by bmw_or_audi View Post
...The only reason that the diesel feels really strong at a traffic light is because car gears go only so low. But once you get going, that factor disappears...
There's a chart in the "Advanced Diesel with BluePerformance.pdf" file that's floating around the www that explains this very well (page 26).

Although the M57 makes much more torque than the N52 that is also charted there, notice that the torque curve for the N52 goes all the way to 7K, while the 425 ft-lb of the N57 starts dropping off at 2400 and redline is 4800. Because of the flat N52 torque curve, HP keeps rising to near the 7K redline (where it matches the M57). However, on the M57 the torque drops off before HP peaks - this is because rising RPM is still sufficient to increase HP (TQ * RPM * factor = HP).

Although the diesels have much taller differential gearing, it's not enough to compensate for the rapid drop-off in torque after 2400. The BMW triple-turbo diesel (M50d) uses the additional turbo to supply more air at high RPM to keep the torque curve at peak longer (I believe).
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  #57  
Old 07-16-2013, 02:12 PM
bmw_or_audi bmw_or_audi is offline
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Originally Posted by floydarogers View Post
Although the M57 makes much more torque than the N52 that is also charted there, notice that the torque curve for the N52 goes all the way to 7K, while the 425 ft-lb of the N57 starts dropping off at 2400 and redline is 4800. Because of the flat N52 torque curve, HP keeps rising to near the 7K redline (where it matches the M57). However, on the M57 the torque drops off before HP peaks - this is because rising RPM is still sufficient to increase HP (TQ * RPM * factor = HP).

Although the diesels have much taller differential gearing, it's not enough to compensate for the rapid drop-off in torque after 2400.
That's exactly it. The diesel has to shift up, which means less torque at the wheels.

Just a note though. Hp always peaks at a higher RPM than torque. You can prove this using basic calculus. I actually have it written out somewhere on vwvortex.com. Only some very minor assumptions had to be made, which are valid for every engine power/torque I have seen so far (basically, if you don't have any double peaks in the torque, you are guaranteed this property).
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