2.5 liter I4 vs 2.5 I6 - What am not getting?

[Gatsby]

Hi, all. I have a general question about the dynamics of a given liter size in different cylinder number engines and the power that can be produced in both 6 and 4 cylinder engine designs, which are both naturally aspirated, both feature DOHC 4 valve per cyl, and both are high compression:

A Porsche 944S 2.5 liter I4 produces 188hp @6,000 rpm and torque of 170 @4,300 rpm. Yet, a similar era 2.5 liter I6 (M50TU) by BMW produces 189hp @5,900 rpm with torque around 180 @4,200.

So, what gives here? I mean why is a Porsche engine producing essentially the same power with 4 cylinders? My common sense tells me the BMW engine should be producing considerably more power with 6 cylinders and a very similar design?

Thanks for your help!

 

[bepperb]

There are too many variables for your comparison.

Similarly, a TSX 2.4 I4 and an IS250 V6 2.5 both produce about 205 hp.

Sure the extra cylinders should result in lower piston speed, giving them a higher max rpm and more horsepower with similar torque.... but really the reason these small v6's are made is for NVH reduction, not for performance.

 

[Shannow]

Simply, torque is piston area x stroke x BMEP (Brake Mean Effective Pressure).

Power is torque x revs.

So for any given volume engine (piston area x stroke), torque is going to be similar.

You move the torque and volumentric efficiency around with ports, valves, and cams, which dictates where the power is produced, and how much.

 

[Pablo]

My 2.5L I5 Volvo produces similar numbers (a little less)....why would you think more cylinders would produce more power?

Displacement always rules. Sure you can get some amazing power out of some low displacement engines, but if you did some of those things to say 3X the displacement, you would get HUGE power.

 

[Eddie]

Remember the English racing engine BRM 16. A screaming 16 cylinder engine that put out amazing power for it size.

 

[Quattro Pete]

Originally Posted By: Gatsby
A Porsche 944S 2.5 liter I4 produces 188hp @6,000 rpm and torque of 170 @4,300 rpm. Yet, a similar era 2.5 liter I6 (M50TU) by BMW produces 189hp @5,900 rpm with torque around 180 @4,200.

Peak horsepower and peak torque don't tell the whole story. Compare how they're delivered over the entire rpm range. My guess would be that the 6-cylinder engine will have more usable torque down low.

 

[Gatsby]

Originally Posted By: Pablo
My 2.5L I5 Volvo produces similar numbers (a little less)....why would you think more cylinders would produce more power?

Displacement always rules. Sure you can get some amazing power out of some low displacement engines, but if you did some of those things to say 3X the displacement, you would get HUGE power.


That's just what I'm not getting, Pablo. My understanding is that 50% more cylinders with an I6 means that 50% more combustion strokes are producing power at any given time or rpm, so the number of cylinders should be as much a factor as liter size - I mean they're both 4 stroke engines. Right?

 

[d00df00d]

The I6 will have more internal friction, all else equal.

 

[javacontour]

Originally Posted By: Gatsby
Originally Posted By: Pablo
My 2.5L I5 Volvo produces similar numbers (a little less)....why would you think more cylinders would produce more power?

Displacement always rules. Sure you can get some amazing power out of some low displacement engines, but if you did some of those things to say 3X the displacement, you would get HUGE power.


That's just what I'm not getting, Pablo. My understanding is that 50% more cylinders with an I6 means that 50% more combustion strokes are producing power at any given time or rpm, so the number of cylinders should be as much a factor as liter size - I mean they're both 4 stroke engines. Right?


Sure, you have more cylinders, but you have smaller cylinders. Since both engines are essentially the same displacement, you can put roughly the same amount of fuel in each of them over two revolutions of the crankshaft.

Sure, you have four combustion events in the four cylinder compared to the six events in the six cylinder, but each of those can do 1.5x the work of a combustion event in the 6 cylinder, so it balances out.

The six will be smoother, but I wouldn't expect a significant difference in power with like displacements.

 

[Nick R]

For comparison, the AMC/Chrysler 2.5L I-4 made 120HP MAX. I think the fact that despite having more cylinders, it was the same displacement, power is going to remain relatively the same. However, there is a huge advantage that I-6s have over I-4s. Vibration. Inline fours are inherently unbalanced, there is a secondary up-down vibration at twice crankshaft speed. An inline six on the other hand, is completely, naturally balanced.

 

[Gatsby]

Originally Posted By: javacontour
Originally Posted By: Gatsby
Originally Posted By: Pablo
My 2.5L I5 Volvo produces similar numbers (a little less)....why would you think more cylinders would produce more power?

Displacement always rules. Sure you can get some amazing power out of some low displacement engines, but if you did some of those things to say 3X the displacement, you would get HUGE power.


That's just what I'm not getting, Pablo. My understanding is that 50% more cylinders with an I6 means that 50% more combustion strokes are producing power at any given time or rpm, so the number of cylinders should be as much a factor as liter size - I mean they're both 4 stroke engines. Right?


Sure, you have more cylinders, but you have smaller cylinders. Since both engines are essentially the same displacement, you can put roughly the same amount of fuel in each of them over two revolutions of the crankshaft.

Sure, you have four combustion events in the four cylinder compared to the six events in the six cylinder, but each of those can do 1.5x the work of a combustion event in the 6 cylinder, so it balances out.

The six will be smoother, but I wouldn't expect a significant difference in power with like displacements.


Thanks for that info, Java! I never realized that the cyclinders were smaller in 6 cyl engine vs a 4 cyl with the same liter designation. I just now realize how displacement is calculated. That clarifies A LOT. Thanks to everyone else too for all the great input.

 

[mechanicx]

Originally Posted By: rudolphna
For comparison, the AMC/Chrysler 2.5L I-4 made 120HP MAX. I think the fact that despite having more cylinders, it was the same displacement, power is going to remain relatively the same. However, there is a huge advantage that I-6s have over I-4s. Vibration. Inline fours are inherently unbalanced, there is a secondary up-down vibration at twice crankshaft speed. An inline six on the other hand, is completely, naturally balanced.


Yeah engine smoothness is really the only reason a production car would use an I-6 of the same displacement as an I-4. Balance shafts will get rid of the 4-cylinder's secondary imbalances (but also soak up some power and revability, but not as much as 2 additional cylinders probably would). Also a 6-cylinder will give you 3 smaller power pulses per revolution vs. the same engine-sized 4-cylinders two big ones. You get a much smoother power deliver with a six cylinder, especially an inline 6. The 4-cylinder has more power potential per liter, a 6 cylinder slighly robs it's cylinders more when sharing a plenum, and more friction.

 

[dwendt44]

A 6 cylinder engine of the same size is CAPABLE of more horse power than a 4. It doesn't follow that it will be offered with more though.
Balance, smoothness seems to be the key here.

 

[hillclimber]

Originally Posted By: Quattro Pete
Originally Posted By: Gatsby
A Porsche 944S 2.5 liter I4 produces 188hp @6,000 rpm and torque of 170 @4,300 rpm. Yet, a similar era 2.5 liter I6 (M50TU) by BMW produces 189hp @5,900 rpm with torque around 180 @4,200.

Peak horsepower and peak torque don't tell the whole story. Compare how they're delivered over the entire rpm range. My guess would be that the 6-cylinder engine will have more usable torque down low.


I'm not sure about that. I had a Mazda 626 with a 2.5 six cylinder and it didn't make any power until you got it revved up. I was always sorry I didn't get that car with a manual tranny -- would've been a lot more fun.

 

[Gary Allan]

In the past, more cylinders just gave the engineers more latitude in where the power was produced. The 70's Ferrari engines had relatively small displacements and at idle half of the cylinders were misfiring. They barely developed enough power at idle to stay running. You probably couldn't pull one into the garage if there was a slight incline if you didn't light the tires off. After they got Beverley Hills'd ..the threw in a creeper gear for practical reasons.

..but getting back to the original question, how would it appear if you just had said:

2.5 vs. 2.5 ..like power output. Why? The friction advantage, as was stated, goes to the fewer cylinders. I'm sure piston speed and all kinds of other stuff goes into the mix.

..btw..the DC/Jeep 2.5 produces 147hp iirc. It basically falls into the common 60hp/liter format. The 4.0 I6, otoh, oil develops (iirc) 190+/-.

 

[OVERKILL]

Originally Posted By: Gatsby
Originally Posted By: javacontour
Originally Posted By: Gatsby
Originally Posted By: Pablo
My 2.5L I5 Volvo produces similar numbers (a little less)....why would you think more cylinders would produce more power?

Displacement always rules. Sure you can get some amazing power out of some low displacement engines, but if you did some of those things to say 3X the displacement, you would get HUGE power.


That's just what I'm not getting, Pablo. My understanding is that 50% more cylinders with an I6 means that 50% more combustion strokes are producing power at any given time or rpm, so the number of cylinders should be as much a factor as liter size - I mean they're both 4 stroke engines. Right?


Sure, you have more cylinders, but you have smaller cylinders. Since both engines are essentially the same displacement, you can put roughly the same amount of fuel in each of them over two revolutions of the crankshaft.

Sure, you have four combustion events in the four cylinder compared to the six events in the six cylinder, but each of those can do 1.5x the work of a combustion event in the 6 cylinder, so it balances out.

The six will be smoother, but I wouldn't expect a significant difference in power with like displacements.


Thanks for that info, Java! I never realized that the cyclinders were smaller in 6 cyl engine vs a 4 cyl with the same liter designation. I just now realize how displacement is calculated. That clarifies A LOT. Thanks to everyone else too for all the great input.


I should clarify this and state that the DISPLACEMENT per cylinder will be less with a higher number of cylinders. This does not mean the BORES are going to be smaller. They could in fact be LARGER if one were to reduce the STROKE.

I will give you an example of two V8 engines with very similar displacement, yet have two very different performance potentials due to this difference:

GM 305
Ford 302

The 305 3.736-inch bore, 3.4803-inch stroke
The 302 4.000-inch bore, 3.0000-inch stroke

The larger bore allows for the use of heads with larger valves, which increases the breathing ability of the engine allowing it to make more power.

This is more of an issue on push-rod engines than it is on OHC engines however.

 

[brianl703]

Doesn't the shorter stroke also mean the engine can rev higher?

 

[Gatsby]

Originally Posted By: brianl703
Doesn't the shorter stroke also mean the engine can rev higher?


My understanding is, yes, shorter stroke engines usually have more rpm potential, as well as, horsepower potential, since more hp can be generated at higher rpm's generally. Moreover, a longer stroke engine generally will produce more torque at lower rpm, so the torque band becomes more usable at low to mid rpm levels, yet it comes at the expense of ultimate horsepower and rpm potential.

 

[Gatsby]

Thanks for all the great posts, everyone. Much appreciated.

 

[Gatsby]

Originally Posted By: Gary Allan
In the past, more cylinders just gave the engineers more latitude in where the power was produced. The 70's Ferrari engines had relatively small displacements and at idle half of the cylinders were misfiring. They barely developed enough power at idle to stay running. You probably couldn't pull one into the garage if there was a slight incline if you didn't light the tires off. After they got Beverley Hills'd ..the threw in a creeper gear for practical reasons.


You know that is something I have long wondered about, Gary. Why were the early supercars by Ferrari, Aston Martin, Jaguar, etc usually using such small displacements in such high cylinder engines - I mean earliest Ferrari '125 Sport' was equipped with 1.5L V12engine! Why?? What was the benefit, in their eyes, of running so many cylinders with such small displacements?