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Old 09-11-2003, 03:30 AM   #1
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Default Differences in engines with same displacement, but different # of cyl.?

I was just pondering...

What are the differences in engines with the same displacement, but different number of cylinders?

I'm basically thinking of something like your (somewhat high end for a 4banger) 2.5L 4 cyl engine, like our beloved N/A or Turbo 2.5 boxer and your (somewhat low end) 2.5L 6 cyl engine, like the BMW inline used in the 325 or Suzuki's new 2.5L 6 cyl, and I'm sure there are Vees out there as well.

Why would someone choose one packaging over the other?

A couple things I see:

It seems that using 4 cyl would be lighter and lead to a less nose heavy weight distribution.

The inline 6 would have the inherent advantage of cancelling out all orders of motion (or whatever it is )

I guess where I'm getting at is which is the more "powerful" configuration, or what are the tradeoffs. Thanks
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Old 09-11-2003, 10:42 AM   #2
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I have a RS (2.5L H4) and a 325xi (2.5L I6) for the wife...the only thing I have observed so far is that the I6 is smoother?

of course the BMW makes more power than my RS (185hp VS 165hp and 170ftlb VS 166 ftlb)
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Old 09-11-2003, 11:18 AM   #3
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The more cylinders you have, the smoother the engine can be without other measures (counter rotating shafts, etc). Less cyl's with the same displacement has the advantage of less rotating weight and less friction, so more potential power from the same displacement.

Engine design can have a huge contribution to all of this. The E30 M3 4 cyl engine was 2.3L but made 190hp and revs to a 7300rpm redline. Back long ago, my dad had a Chevy Vega with an aluminum 2.3L 4 cyl. I would be surprised if it made more than 75hp.

I was once in the market for a motorcycle and rode 2 different 650's. One was a 4 cyl and the other a 2cyl. The smoothness was obvious on the 4, but also, it had noticable "drag", while the 2 cyl felt more free'd up. I attribute that to the difference in internal weight and friction.

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Old 09-11-2003, 12:34 PM   #4
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the big advantage of having more cylinders it that all of the reciprocating parts are smaller and lighter. this allows the engine to spin faster (higher rev limit). more revs=more fuel/air going through the engine which means more power. that's why F1 uses v tens.
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Old 09-11-2003, 01:14 PM   #5
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Don't forget that the more cylinders you have, the more combustion events you have per cycle.

This means that if you have a 4 and 6 cylinder car making 240hp, the 4 cylinder is making 60hp per cylinder and the 6 cylinder is making 40hp per cylinder.

If you bump the power of the 6 cylinder to match the per cylinder power of the 4 cylinder (60hp/cylinder), you have 360hp

That gives you a lot more potential from the same displacement and same hp per cylinder.

Why do you think Ferrari V12's (like only 3.5 Litres) are so small but make so much power

Last edited by Eric SS; 09-11-2003 at 01:52 PM.
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Old 09-11-2003, 02:16 PM   #6
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Default Re: Differences in engines with same displacement, but different # of cyl.?

Originally posted by NeoteriX
I was just pondering...

I guess where I'm getting at is which is the more "powerful" configuration, or what are the tradeoffs. Thanks
Of course the most "powerful" configuration is going to be a large-bore diesel. Gas engines are practically limited to around 6 inches linearly from the spark plug, while diesel bores can be made the size of a trash can. Nothing like standing on top of a piston while it goes up and down...

In a car, you first have to identify the purpose of the engine, then you identify the design constraints, then work on the design within the constraints. For example, if you want to design an ethanol-powered dragster, you would expect much greater latitude in terms of fuel efficiency than for a gasoline powered rally car.

So, some design constraints:
Overall width
Overall length
Overall height
Center of mass
Phyical relationship of engine output to driveshaft
Fuel efficiency
Torque/speed characteristics dictated by intended use (towing, acceleration, high-speed cruising)
Ease of maintenance

I think what you really meant to ask is "which piston configuration has the best torque/speed characteristic?" Recognizing that it depends on the application, here are a few generalities:

Greater bore displacement = more torque available at lower RPM
Turbocharger = more torque available at higher RPM
Supercharger = more torque available, RPM range depends on fan design
Intercooler = more torque available in "boosted" RPM range
More "oversquare" (bore dia > stroke length - a la Mustang) = "peakier" torque/speed curve weighted toward lower RPM range
More "undersquare" (bore dia < stroke length - a la BMW) = "smoother" torque/speed curve
Smaller bore displacement = smaller moving parts -> higher redline -> more RPM range available to take advantage of engine torque
Greater total displacement = more torque available throughout RPM range
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Old 09-11-2003, 02:27 PM   #7
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don't forget about the Volvo 2.5L inline 5cyl... Or the Daimler Saloon 2.5L V8 (60's....)
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Old 09-11-2003, 03:10 PM   #8
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From my experience with motorcycles:

Displacement being equal (both 1000cc), a 2-cylinder will have more low and mid-range torque, and a flatter powerband. A 4-cylinder will be smoother, higher revving, more peak (total) horsepower arriving at higher RPM's, and slightly better fuel economy.

Each is good for their own purposes. If you want max HP and don't mind revving the bejeezus out of the motor, more cylinders is the way to go. I prefer the driving characteristics (instant torque) of having less cylinders, and they tend to sound cool too (Ducati anyone?).

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