0.5L of displacement per cylinder

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dparm

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Something that I've noticed about BMW: many of their motors have 0.5L of displacement per cylinder:

I-4s are 2.0L
I-6s are 3.0L
V-8s are 4.0L
V-10s are 5.0L
V-12s are 6.0L

There are a few exceptions, such as the current turbo V-8 that's 4.4L, but in general they adhere to the sizing I described above. Their upcoming I-3 is 1.5L.

Is there some benefit to that "magical ratio"? Or is this done for ease of manufacturing/engineering, i.e. parts sharing.
 
Nothing "magical" about it. It's pretty common though. For years the Toyota Camry V6 was listed as 3.0L.

However, BMW used to have the 2.5L inline-6 as their most common engine.

I take the train to work some days. The latest GE Genesis locomotive has a 175L V16. Over 10L displacement per cylinder.
 
Audi has used this ratio a bit too:

I-4s are 2.0L (GTI, A4, TT, etc)
V-6s are 3.0L (S4, A6, Q7, etc)
V-8s are 4.0L (S6, S8)
V-10s are 5.0L (R8, Gallardo)
W-12 is 6.0L (A8L, Phaeton)
 
Originally Posted By: dparm
I-4s are 2.0L

Not sure there's anything magical. By the way, in Europe, BMW offers I4 engines that have 1.6 and 1.4 liter displacement as well.
 
Originally Posted By: Quattro Pete
Originally Posted By: dparm
I-4s are 2.0L

Not sure there's anything magical. By the way, in Europe, BMW offers I4 engines that have 1.6 and 1.4 liter displacement as well.


Right, but they have to in order to avoid certain taxes on larger displacement engines.

I was just wondering if there was some benefit to that cylinder count to displacement ratio.
 
Originally Posted By: 95busa
Toyota was running the same pistons in many applications for a while werent they?


Possibly. This is what I was getting at -- if many motors use the same bore, you could use the same piston across multiple applications.
 
I have heard that half-liter cylinders usually work the best for most applications.

Smaller cylinders have better volumetric efficiency but a higher ratio of friction to displacement. The opposite is true of larger cylinders. Apparently, for most engines (depending on design, materials, application, etc.), the right balance just happens to fall around 0.5L.
 
I have heard of other engines which share heads with different size bores, but bore center spacing the same. I can't think of them off the top of my head.
 
It's a mechanical/structural reason. The loads at higher engine speeds starts to get too high due to the increase in mass wen cylinder size increase beyond 500 cc. Well, 500 cc is a normal limit when you look at ordinary passenger cars.
Sport cars needs to rev higher and have therefore also usually smaller cylinders. (and vise versa for trucks)
 
Except for the M60 @ .375.

Originally Posted By: dparm
Something that I've noticed about BMW: many of their motors have 0.5L of displacement per cylinder:

I-4s are 2.0L
I-6s are 3.0L
V-8s are 4.0L
V-10s are 5.0L
V-12s are 6.0L

There are a few exceptions, such as the current turbo V-8 that's 4.4L, but in general they adhere to the sizing I described above. Their upcoming I-3 is 1.5L.

Is there some benefit to that "magical ratio"? Or is this done for ease of manufacturing/engineering, i.e. parts sharing.
 
Originally Posted By: dparm
Something that I've noticed about BMW: many of their motors have 0.5L of displacement per cylinder:

I-4s are 2.0L
I-6s are 3.0L
V-8s are 4.0L
V-10s are 5.0L
V-12s are 6.0L

There are a few exceptions, such as the current turbo V-8 that's 4.4L, but in general they adhere to the sizing I described above. Their upcoming I-3 is 1.5L.

Is there some benefit to that "magical ratio"? Or is this done for ease of manufacturing/engineering, i.e. parts sharing.


Don't forget the older 1.8 and 2.3 inline 4, 2.5, 2.7, 2.8 and 3.5 inline sixes, 4.4, 4.6 and 4.8 v8, and 5.0 v12 (these are just off the top of my head).

This said, currently most manufacturers do seem to like their displacement to be somewhere around 1/2 liter per cylinder.
 
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Yes, I know that there are plenty of engines that did not use that formula. I just found it to be an interesting pattern that emerged. The 3.0L I-6 is one of their best-regarded motors and has won numerous awards, as did the 4.0L V-8.
 
There surely is an optimal L/D ratio for the cylinder, though that may change according to what is being optimized (e.g. torque, high RPM breathing, thermal losses through the walls, etc).
 
Originally Posted By: JHZR2
There surely is an optimal L/D ratio for the cylinder, though that may change according to what is being optimized (e.g. torque, high RPM breathing, thermal losses through the walls, etc).


I think longer strokes side load the piston more.
 
500cc per cylinder is an engineering "rule of thumb", when you start out designing stuff.

500cc/cylinder was typically in the 80s the point at which a 4 cylinder needed balance shafts, and the point at which the designer then went to a small 6.

6s and 8s (and 12s) weren't that worried about it, as they have better inherent balance.

In a pure engineering sense, where a car would have been designed from scratch, the bhp/torque would be chosen for the performance/weight, the displacement worked out via typical BMEP for the fuels and materials of the time, the number of cylinders chosen by a rule of thumb like this...if it "needed" 4.3 cylinders, the decision would be made on whether to have a "big 4" or a "small 6"...naturally that choice iterates back to the target weight of the vehicle, the market placement (rough big 4 versus luxury 6)...if you look at the British market or the 60s/70s, you can see how it worked (but they also skewed it with taxable hp ratings which made small bore long stroke taxed less)
 
One reason why I love my 328 so much is the fact that it is a perfectly square engin, with 84mm stroke and bore (alt hough when I built up the motor, I went to 86.5mm bore w 84mm stroke for more displacement and less stress to help with running a very high pressure SC @ 10.8:1 SCR).
In fact, the reason why the E46 and E9x M3's have had issues with main journal bearings is due to the significant stress they undergo from combustion on an over square motor.
Same reason I almost never recommend stroker kits, as they tend to wear out the cylinder walls and piston skirts quickly, especially in very high specific outputand/or high RPM aapplications.
 
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