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      12-01-2015, 01:04 PM   #1
hector
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Engine selection for a desired output

how does bmw, or any other manufacturer, decide what displacement engine to choose when by, for example, adjusting boost in a turbo app., they can easily arrive at the same total output for 2 engines of different displacements? for example, take the 320i in u.s. spec putting out 180hp/200ft-lbs, output that the 1.5l 3cyl could achieve if it's specific output were the same as the 2.0 4cyl in the 228? there are i'm sure pros and cons to each method, some of which may be marketing related as opposed to technical. specifically, with weight reduction a priority, wouldn't the smaller mill save substantial weight?
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      12-01-2015, 02:24 PM   #2
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Quote:
Originally Posted by hector View Post
how does bmw, or any other manufacturer, decide what displacement engine to choose when by, for example, adjusting boost in a turbo app., they can easily arrive at the same total output for 2 engines of different displacements? for example, take the 320i in u.s. spec putting out 180hp/200ft-lbs, output that the 1.5l 3cyl could achieve if it's specific output were the same as the 2.0 4cyl in the 228? there are i'm sure pros and cons to each method, some of which may be marketing related as opposed to technical. specifically, with weight reduction a priority, wouldn't the smaller mill save substantial weight?
I believe that BMW arrived at the solution of a common cylinder size and altering overall displacement based on the number of cylinders and power output on the state of tune of the engine. This has allowed them to maximize their production efficiencies globally.

I have an i8 and the 228bhp from an FI-3 cylinders is very impressive. I lament the fact that they didn't go with the 4 cyl in the same state of tune which would have made the i8 a real supercar.

Cheers-mk
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      12-01-2015, 02:44 PM   #3
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Quote:
Originally Posted by hector View Post
how does bmw, or any other manufacturer, decide what displacement engine to choose when by, for example, adjusting boost in a turbo app., they can easily arrive at the same total output for 2 engines of different displacements? for example, take the 320i in u.s. spec putting out 180hp/200ft-lbs, output that the 1.5l 3cyl could achieve if it's specific output were the same as the 2.0 4cyl in the 228? there are i'm sure pros and cons to each method, some of which may be marketing related as opposed to technical. specifically, with weight reduction a priority, wouldn't the smaller mill save substantial weight?
It's a matter of finding the right compromise between durability, weight, efficiency, cost, and other parameters. I'd agree with you that building the lightest engine that gets the job done is likely a big priority right now.

Like you, I've wondered why the 320i still uses a 4 cylinder when the 3 cylinder should be sufficient. I'm sure costs must favor it for the time being in this application for some reason, even though it requires more material to manufacture. In time the I3 will likely take over as the standard engine at this power output.
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      12-01-2015, 05:24 PM   #4
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Quote:
Originally Posted by mkoesel View Post
It's a matter of finding the right compromise between durability, weight, efficiency, cost, and other parameters. I'd agree with you that building the lightest engine that gets the job done is likely a big priority right now.

Like you, I've wondered why the 320i still uses a 4 cylinder when the 3 cylinder should be sufficient. I'm sure costs must favor it for the time being in this application for some reason, even though it requires more material to manufacture. In time the I3 will likely take over as the standard engine at this power output.
thx
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      12-02-2015, 02:57 PM   #5
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BMW likes to use engines with cylinders measuring .5 liters of displacement each. That is the largest explanation behind the size of the engines that go in their cars. As for target power output, it's all maximizing cost savings, economy, packaging, and longevity.

Those 100+ hp/L engines look great on paper, and are typically fun to drive, but they pay a price in long-term reliability. The greater stress something is under, the shorter it's lifespan. It's a basic rule of engineering. While a 1.5T I3 and 2.0T I4 may make similar power levels, the I3 is working much harder to make it.
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      12-03-2015, 11:28 PM   #6
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Look at what BMW does with the diesel engines they offer... all of these are basically the same mechanical setup & displacement... just different software tuning... four, four cylinder engines using basically the same mechanical components.

4 cylinder engines:

316d - 85 kW [116 PS]
318d - 110 kW [150 PS]
320d Efficient Dynamics - 120 kW [163 PS]
320d - 140 kW [190 PS]

6 cylinder engines

325d - 160 kW [218 PS]
330d - 190 kW [258 PS]

and...

335d - 230 kW [313 PS]
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      12-04-2015, 02:50 PM   #7
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Dack, the motor in the 325d is actually an I4 now (it was an I6 in the E90 era though). Also, there is the 381PS N57S I6 at the very top of the range.
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      12-04-2015, 03:39 PM   #8
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Quote:
Originally Posted by Dackelone View Post
all of these are basically the same mechanical setup & displacement...
4 cylinder engines:

316d - 85 kW [116 PS]
318d - 110 kW [150 PS]
320d Efficient Dynamics - 120 kW [163 PS]
320d - 140 kW [190 PS]
Usually there are some differences. for instance the 316d/318d have a different turbo than the 320d I think.

But also differences in brakes etc are common. That also adds up to the cost of a car. It's all about cost and filling in market segments.
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      12-15-2015, 10:15 AM   #9
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Might be offtopic but how can they sell a X1 25i as a 28i in North America or a 530d as a 535d? Same goes for the 328d which is a glorified 320d finally back in 2011 the 330i has 272ps and now it's down to 252. I feel like the number doesn't mean much anyway.
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      12-15-2015, 11:31 AM   #10
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Quote:
Originally Posted by BimmerMat135 View Post
Might be offtopic but how can they sell a X1 25i as a 28i in North America or a 530d as a 535d? Same goes for the 328d which is a glorified 320d finally back in 2011 the 330i has 272ps and now it's down to 252. I feel like the number doesn't mean much anyway.
They haven't meant anything for a while now.
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