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F30 / F31 / F32 / F33 (2012 - current)
The sixth generation 3 series, chassis code F30. 2013 model year 328i and 335i sedans now in production. Read the F30 frequently asked question thread for all your basic question and dive into all the details in the ultimate F30 information thread.

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  #1  
Old 11-16-2013, 01:08 PM
GG Hyundai GG Hyundai is offline
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Why does 320i and 328i have nearly the same MPG?

Down 60 HP, you'd think it would be a lot more efficient?

Last edited by GG Hyundai; 11-16-2013 at 01:20 PM.
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  #2  
Old 11-16-2013, 01:16 PM
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with less powerful engines, it may take a bit more throttle for the same speed.
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  #3  
Old 11-16-2013, 03:35 PM
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Re: Why does 320i and 328i have nearly the same MPG?

Reduction in boost does not necessarily mean increase in mpg. Just like many people who get a tune report better mpg.

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  #4  
Old 11-16-2013, 04:13 PM
thaibruin thaibruin is online now
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well, the 320i doesn't exist for fuel economy purposes, it's just there to have a lower cost of entry.
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Old 11-16-2013, 04:14 PM
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Re: Why does 320i and 328i have nearly the same MPG?

MPG is optimal fuel consumption. The key ? is how much more fuel can 328i consume under full throttle. Probably more due to the bigger boost hence more horse power.
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Old 11-16-2013, 11:18 PM
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With modern engines, the specific efficiency (% efficiency at a HP output) tends to be very similar, modulo momentum and weight-related losses in larger engines vs smaller engines.

The 320i's engine is not really any smaller physically than the 328i's, and the gentle acceleration used for the majority of the EPA test cycles means that you're not gonna be asking for the extra 60HP from the 328i all that often.
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Old 11-17-2013, 07:11 AM
Alpine300ZHP Alpine300ZHP is offline
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Why does 320i and 328i have nearly the same MPG?

Quote:
Originally Posted by jdong View Post
With modern engines, the specific efficiency (% efficiency at a HP output) tends to be very similar, modulo momentum and weight-related losses in larger engines vs smaller engines.

The 320i's engine is not really any smaller physically than the 328i's, and the gentle acceleration used for the majority of the EPA test cycles means that you're not gonna be asking for the extra 60HP from the 328i all that often.
This. Many forget that fuel economy is largely affected by displacement. This is why turbocharging a smaller displacement engine yields big power and better fuel economy. The 320 engine is, I believe, the same displacement as the 328 and the power reduction is a function of less boost. Same thing with the N63 v8 in the 550 and the S63 v8 that resides in the m5. Fuel economy of the m5 is within a couple mpg of the 550 despite the extra 150 hp. Why? Same displacement in both motors.


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Old 11-17-2013, 07:34 AM
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Moving the same car through the same cycle with the same basic engine. Some modest gains to be expected from slightly lower weight.
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Old 11-17-2013, 08:39 AM
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Re: Why does 320i and 328i have nearly the same MPG?

Don't believe there is a weight difference. Can someone confirm?

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Old 11-17-2013, 09:08 AM
gkr778 gkr778 is online now
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Quote:
Originally Posted by greywolf328i View Post
Don't believe there is a weight difference. Can someone confirm?
According to bmwusa.com, unladen weight figures for base models are:

3250 / 3295 lbs [MT/AT] for 320i;
3360 / 3410 lbs [MT/AT] for 328i

The delta may be due partially to additional standard equipment in the 328i (power seats, etc.)
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Old 11-17-2013, 09:18 AM
thaibruin thaibruin is online now
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Re: Why does 320i and 328i have nearly the same MPG?

Quote:
Originally Posted by gkr778 View Post
According to bmwusa.com, unladen weight figures for base models are:

3250 / 3295 lbs [MT/AT] for 320i;
3360 / 3410 lbs [MT/AT] for 328i

The delta may be due partially to additional standard equipment in the 328i (power seats, etc.)
Though the engines are basically the same, I think there's a different radiator or something

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Old 11-18-2013, 03:40 PM
Gambla Gambla is offline
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With the same engine (2.0L turbo charged N20) I don't understand why there is a 60hp difference. Curious, where does that difference come from? Surely not just the radiator...
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Old 11-18-2013, 03:45 PM
SNye45 SNye45 is offline
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Quote:
Originally Posted by Gambla View Post
With the same engine (2.0L turbo charged N20) I don't understand why there is a 60hp difference. Curious, where does that difference come from? Surely not just the radiator...
The 320 has a de-tuned variant of the same N20/N26 engine found in the 328
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  #14  
Old 11-18-2013, 03:51 PM
jdong jdong is offline
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Quote:
Originally Posted by Gambla View Post
With the same engine (2.0L turbo charged N20) I don't understand why there is a 60hp difference. Curious, where does that difference come from? Surely not just the radiator...
It's software, capitalism at work. BMW knows customers are willing to pay more for more power, and it doesn't matter if BMW merely caps the power output of the engine via software. They know that they can sell certain people a 320i who would otherwise have not considered a BMW due to cost. Better to make some money than no money.
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Old 11-18-2013, 04:05 PM
Jamesonsviggen Jamesonsviggen is offline
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Originally Posted by jdong View Post
It's software, capitalism at work. BMW knows customers are willing to pay more for more power, and it doesn't matter if BMW merely caps the power output of the engine via software. They know that they can sell certain people a 320i who would otherwise have not considered a BMW due to cost. Better to make some money than no money.
It's more than software.

It has different pistons with 11:1 compression vs 10:1 for the 328. That has it also running less boost.
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  #16  
Old 11-18-2013, 04:30 PM
Gambla Gambla is offline
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Quote:
Originally Posted by Jamesonsviggen View Post
It's more than software.

It has different pistons with 11:1 compression vs 10:1 for the 328. That has it also running less boost.
Ah I see. Thx
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  #17  
Old 11-18-2013, 07:54 PM
jdong jdong is offline
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Quote:
Originally Posted by Jamesonsviggen View Post
It's more than software.

It has different pistons with 11:1 compression vs 10:1 for the 328. That has it also running less boost.
I don't think the compression difference can account for all of the 60HP loss. I'd be willing to bet the majority of this was achieved by changing the boost mapping in the DME which in turn may have let them reduce costs a bit on the pistons.
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Old 11-19-2013, 07:08 AM
Jamesonsviggen Jamesonsviggen is offline
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Originally Posted by jdong View Post
I don't think the compression difference can account for all of the 60HP loss. I'd be willing to bet the majority of this was achieved by changing the boost mapping in the DME which in turn may have let them reduce costs a bit on the pistons.
You are not understanding. I never said it was the pistons that accounted for the power deficit.

The difference in pistons means they are less boost friendly, so the ECU tune is matched accordingly to produce LESS boost.

So if you take a 320 and just turn up the wick to produce the same boost as the 328, it is not a safe plan of attack. BMS has a specific map just for the 320 for this reason.
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