For the sake of simplicity, assume these are 6 cylinder engines with identical bore, stroke, cams, etc etc etc.

If you have an I6 and a V6 and maybe even a VR6, is any one of them a "superior" design in terms of power output? Disregarding the physical size and weight differences and focusing just on their power, and assuming fuel delivery and all that junk is equal.

 

Ever drive a BMW with the I6? SMOOTH
All things being equal I would prefer a straight Six. But “V” configurations gives the automaker more options / cost savings.

 

IIRC, the differences in cylinder configuration are mostly made for space (V's being the most compact), but the resulting factor that ends up affecting power output are the relative vibrations of each configuration.

I think straight-sixes are a sort of gold standard... they're vibrationally near perfect and I would guess would therefore be able to rev higher, and so volume for volume, stroke for stroke, be able to generate more power.

This is why BMW, the R32-R34 Skyline GTR's, Supras???, and TVR have all used them, and probably is not coincidental with the fact that they are some of the greatest cars on the road.

V6's I think on the other hand have inherent vibration deficiencies (IIRC, they create 2nd order vibrations, although Im not sure if its 2nd order or even what 2nd order vibrations means... someone please explain). They therefore incorporate counterbalances which reduce vibration but add inertia, which probably limits power compared to straight 6's

www.autozine.org has a good explanation

 

Well ok, but what if you ignore the whole vibration thing? Assuming same red line and all that as well.

 

The vibration thing is overhyped anyways. Not a single F1 V-10 engine is anywhere near well balanced yet they rev to 18k rpm.

I would say that the I6 and V6 have an advantage over the VR6 because the cylinder head port routing is simpler and thus likely more efficient. Maybe this gives these engines a 5% advantage at the power peak?

Hey, you forgot to throw in the Flat Opposed 6 cylinder engine into the mix! I'd say this one is equal to the I6 and V6.

 

How can you ignore Vibration & Drivability?

Here are the specs on the I6 in the BMW M3
333 HP SAE @ 7,900 rpm; 262 ft lb , 4,900 rpm.
Is there a N/A V-6 that does that and doesn’t feel like a riding lawn mower?

 

I for one like the I6 becuase its also a simpler design, has only one set of headers instead of two, which saves weight, VVT is easier since you only have one bank of cylinders to cover.

The VR6 is an interesting engine cuz its almost a I6, it only has one set of exhaust headers rather than two, and is still compact.

with newer alloy tech., I would think you could built a I6 block thats no much longer than a V6 block, because, at the end of the day, the real issue with an I6 is space constraints.

 

Honda's C32 series, high revving V6 found in the NSX. Although the horsepower is not 333, but you get the idea. This motor is a dinosaur compared to the M3's motor though.

 

You can't conclude that since the BMW M3's I6 generates the most power of any NA 6 that it's the best design. Most manufacturers would likely pick a cheaper and more reliable alternative like a V8 when needing 300+ hp. Then there's also the realm of forced induction which is the route that MB has taken rather developing high rpm, expensive, NA engines.

 

ok no one is taking my question seriously. I said, assuming EVERYTHING is equal (flow characteristics, porting, WHATEVER, does one engine have the potential to make more power? I'm just asking theoretical, so I can ignore vibration and driveability.

Or is it that ANY 6-cylinder (or 8, or 16, I don't care) engine with IDENTICAL characteristics will make the same power as any other?

 

I understand your question, but fail to see its purpose. Any answer you get assuming everything is identical, when in reality it is not, is meaningless. I would liken it to a 1st year physics course where you ignore friction, sure it makes life easy, but its answer has no meaning in reality.

 

Did you not read my earlier post that I repeat here?:

 

I've always thought that an inline configuration generates more torque and at lower revs (given equal flow, etc etc etc). But that may be true for 'old school' and not current engine technology.

 

To quote an often used source http://auto.howstuffworks.com/question366.htm


“If you built all three of these six cylinder engines to the exact same specifications -- same displacement, same valves, same intake and exhaust systems, etc. -- they would likely perform nearly identically. Displacement is displacement.”

So to pick the better car you would have to base it on other factors.

 

I don't think you'll ever be able to get an I6 block down to the length of a similar displacement V6 block. The problem is not with the materials used, but with the layout of the cylinders. Look at it this way:

ooo
.ooo vs oooooo

The I6 is clearly longer, no matter how thin you can make the cylinder walls using advanced materials. The V6, on the other hand, is clearly wider, but occupies a more space-efficient squarish shape than the I6's long, thin rectangle. This, along with the fact that most V6s are shorter than an I6 of similar displacement, is why V6s are more compact than I6s, making them layout of choice for transverse engined 6 cylinder cars where engine size is an important design parameter.

 

I've thought about this before (like if I were to build a sports car, what engine layout would I use?).

Quick question...I thought the VR6 was a V6??? Or, a V layout??? Please explain???

So, we have now:

F
I
V
W
R-Rotary...lol

Layouts right? But, I hear the W-8 is one vibration loving monster. Please explain the VR6...thanks...

 

For a very good lay man's explanation go to http://www.autozine.org/technical_school/tech_index.htm and then scroll down to the "Engine Packaging" and "Engine Smoothness" sections.

 

Thank you.

 

the autozine quote doesnt take into account maximum rpm as limited by vibration (assuming we're talking equal NVH here... for purposes of the comparison I'm assuming end-result vibration as well as driveability are equal for the V6, I6, etc that we are comparing.) So I still think that because I6's have less inherent vibration, they have less rotating inertia, can rev higher, and therefore make more power.

Although maybe the answer lies with HP/liter and we should make a list. The acura NSX (though very old, I'm sure Acura can do better with the upcoming if they decide for some reason to keep the same displacement and configuration) is I think V6 300hp and 3.2 liters (I guess even though it revs pretty high). The I6 M3 S54? engine is 343hp and 3.2 liters.

So
93.75 hp/L for V6's
107.19 hp/L for I6's

feel free to add calculations/results for road/sport cars, but I think a new list should be created if you want to consider all-out race cars.