4 cyl vs 6 cyl

[Cobalt60]

I am wondering what the differences between a 4 cylinder and 6 cylinder are, concerning power and fuel economy. Most people will say a 4cyl gets better fuel economy, while 6cyl has more power. I believe this is a generalization and is assuming the 6cyl has more displacement. For example, I want to say a 2.0L 6cyl will get better fuel economy than a 2.4L 4cyl, but will have less power.

 

But, I want to ask you guys. Heres a few points Id like to make first:

 

Frictional losses - Since a 4 cyl has 2 less pistons (and less of other parts) causing friction, this would be an advantage for both power and fuel economy. But how much of an advantage?

 

Power overlap - The power strokes of 6 cyl overlap, while a 4 cyl does not. My best guess would be this again gives the 4 cyl advantage, is this true?

 

Balance - An inline 6 will not need balance shafts, so it will not lose and power there as a 4 cyl (with balance shafts) would.

 

Turbocharging - I am very curious to know how a turbo would be affected since it will receive the exhaust from overlapping power strokes.

 

So any thoughts would be appreciated. Thanks.

 

-Chris P

[Cobalt60]

Oh and just to be clear, I am most interested in comparing to inline 6 cyl as opposed to a V6. This is why, for example, I say overlapping power strokes for the turbo (1 turbo not 2). An inline 6 will probably also have less frictional losses to a V6, and a V6 will require balance shafts. So personally Id like to know I-4 vs I-6.

[patra_is_here]

the v6 or I6 you would need to split the exhaust pulses to the turbo, it's common on turbo inline 6 cylinder cars like the supra. so, not really an issue. just clever exhaust manifold design.

 

really, though, straight 4, 6 or V doesnt make much of a noteworthy difference. at the end of the day, there's still a piston inside a cylinder compressing gas and lighting it off. stroke, bore, compression, cam lobes will all alter the economy and power greater than just a comparison of 4 vs 6 , straight vs V.

Edited July 17, 2009 by patra_is_here

[patra_is_here]

v6 or I6 don't need balance shafts. the 6's have a natural balance because the crankpin journals are 120 degrees apart rather than 180 like a 4 banger. Edited July 17, 2009 by patra_is_here

[Cobalt60]

Actually V6s do require balance shafts. Heres quotes from wikipedia:

 

"Due to the odd number of cylinders in each bank, V6 designs are inherently unbalanced, regardless of their V-angle. All straight engines with an odd number of cylinders suffer from primary dynamic imbalance, which causes an end-to-end rocking motion. "

 

"Concentrating on the first order rocking motion, the V6 can be assumed to consist of two separate straight-3 where counterweights on the crankshaft and a counter rotating balancer shaft compensate the first order rocking motion. At mating, the angle between the banks and the angle between the crankshafts can be varied so that the balancer shafts cancel each other completely and the counter weights cancel each other to some degree. Two examples are the odd firing 90° V6 (larger counter weights) and the even firing 60° V6 with 60° flying arms (smaller counter weights. The second order rocking motion can be balanced by a single co-rotating balancer shaft.)."

 

And yes I know other design differences will make a bigger difference. But I am wondering if everything is designed basically the same, other than the fact one is an I-4, and the other an I-6, how would they compare. A good example of such similar engines may be Nissans CA20, and their RB20. The CA20 is a 2.0L 4cyl, while the RB series was design by basically adding 2 pistons to the CA; The CA20 is also 2.0L. I havent actually looked at the ratings of these 2 motors though...

[patra_is_here]

i've yet to come across a V6 with balance shafts at the shops i've worked at.

[PDX87Starion]

The more cylinders the less stress on each piston (and parts) per HP. There is less time between each explosion with more cylinders, but also more moving parts.

 

All things being equal.. more cylinders =more power, and less fuel efficiency.

 

Typically more cylinders equals more torque and less RPM's, but bore and stroke have a big influence. Ferrari has a small v12 like 3 liters that have a short stroke and can rev really high, but not a ton of low end torque, as apposed to the G54B which has a long stroke only 4 cylinders and has lots of mid range torque but not much up top. Also your average VW VR6 2.8 does not have a lot of low end torque but revs up to 7000 + like nothing, while a chevy 2.8 V6 is only good as a boat anchor. lol So its really hard to compare because its never really apples to apples.

 

PS "Actually V6s do require balance shafts" I think require isnt the correct word. "Benefit from" would be better.

[patra_is_here]

Typically more cylinders equals more torque and less RPM's

 

not true. F1 has V8 cars the rev to 15,000 rpms.

 

all power, fuel economy, RPM, etc are really going to be determined by some combo of:

 

rod to stroke ratio

bore

compression

mass/weight of moving parts

and obviously airflow.

 

christ, BMW was making 1200HP on inline 4's back in the early 80's with high compression, turbo charging and ethanol.

 

things really aren't apples to apples. sorry.

 

[PDX87Starion]

not true. F1 has V8 cars the rev to 15,000 rpms.

 

all power, fuel economy, RPM, etc are really going to be determined by some combo of:

 

rod to stroke ratio

bore

compression

mass/weight of moving parts

and obviously airflow.

 

christ, BMW was making 1200HP on inline 4's back in the early 80's with high compression, turbo charging and ethanol.

 

things really aren't apples to apples. sorry.

 

Did you read the rest of what I said? obviously you did not. "typically" does not mean F1 or even Ferrari in my book.

 

and I covered the apples part already, sorry.

 

Patra.. hypothetically, all things being equal like bore & sroke ratio, comparing a 2.0L 4cly and a 2.0L 6cly what would "typically" be the difference between the two? that is what colbalt was asking.

[patra_is_here]

ohhh. i was agreeing with you PDX87, hence the apples and apples comment. the F1 statement was just a factoid of a relevant atypical example.

 

anyhow, all things being the same. there'd be no difference between the two. =)

[PDX87Starion]

anyhow, all things being the same. there'd be no difference between the two. =)

 

LOL, all things except what I mentioned and HP & torque & efficiency. =)

[patra_is_here]

meaning, if they are both 2.0L, with equal redline and powercurves (all things being equal), they they would make the same power. they are both moving the same amount of air, 2.0L

 

horsepower and torque formulas don't depend on the number of cylinders.

[patra_is_here]

let me rephrase that.

 

horsepower is

 

(torque x RPM)

divided by

5252

 

so, then you ask what is torque?

 

torque is force x length

(i know it sounds weird, but trust me)

 

so, what is force

force = mass times acceleration

 

mass would be the weight of the engine components

so, what determines acceleration?

 

acceleration would be determined by how explosive a force you can produce

which is directly related to how much air and fuel you cram in there.

 

in this example. 2 liters.

 

so, both engines are in theory the same. =)

Edited July 17, 2009 by patra_is_here

[Shelby]

for a 4 cyl and a 6 cyl to have the same cids , there are many differances in the two engines, one has a much smaller piston that = a higher rpm range due to weight,, also the major point is the vehicle both are use'd in,, in todays cars we have such a wide rage of styles and designs that many cars have an engine spacifily design'd for that body , many have such a poor tourqe out put that they have to rely on rather low diff final drive ratio's , some into the 4:56 range , now that makes it nessarry to have more then the normal number of gears available,,ergo the 6-7 speed trannys , of late

 

also are these engine iron block or alum block, whats the indend'd use,, expect'd power out put ,, traget mpg range , are you looking to see which design offers the most power potential,, whats your reason for asking the question and it's intend'd use

[PDX87Starion]

let me rephrase that.

 

horsepower is

 

(torque x RPM)

divided by

5252

 

so, then you ask what is torque?

 

torque is force x length

(i know it sounds weird, but trust me)

 

so, what is force

force = mass times acceleration

 

mass would be the weight of the engine components

so, what determines acceleration?

 

acceleration would be determined by how explosive a force you can produce

which is directly related to how much air and fuel you cram in there.

 

in this example. 2 liters.

 

so, both engines are in theory the same. =)

 

 

No way you would have the same torque. With two more cylinders firing per RPM the 6cly would inherently have more torque assuming the bore and stroke ratio is the same but equally smaller to make 2 liters. HP would depend on the torque curve which would also be different.

 

If having more cylinders would not change anything why aren't there any 7.4 liter 2 cylinder hot rods around? According to your "math" they would have exactly the same the same HP & TQ as a BB Chevy 454 V8.

[Zack_Morris]

i've yet to come across a V6 with balance shafts at the shops i've worked at.

 

GM 4.3L V6 has a balance shaft. I believe it is only the "W" code engines.

[patra_is_here]

No way you would have the same torque. With two more cylinders firing per RPM the 6cly would inherently have more torque assuming the bore and stroke ratio is the same but equally smaller to make 2 liters. HP would depend on the torque curve which would also be different.

 

If having more cylinders would not change anything why aren't there any 7.4 liter 2 cylinder hot rods around? According to your "math" they would have exactly the same the same HP & TQ as a BB Chevy 454 V8.

 

a 6 cyl doesn't have 2 more complete combustion cycles per RPM.

 

and THAT is irrelevant anyhow, since combustion per RPM isn't even part of any of the calculations.

 

the reason there arent 7.4 liter 2 cylinders is because the MASS completely changes. a 454 v8 would have significantly LESS mass that a 7.4 liter 2 cylinder.

 

think of it this way. if you double the size of a piston, you quadruple it's mass. simple physics.

 

 

also, if anyone can show me wrong on any of these points, i will gladly admit i was incorrect.

Edited July 17, 2009 by patra_is_here

[Ernie88TSi]

GM 4.3L V6 has a balance shaft. I believe it is only the "W" code engines.

 

 

3800's as well.

[BrazilBoy]

Put in the new VW 2.0 TDI and get your torque with a side of 50mpg and be done with it....

[nathan89quest]

3800's as well.

the ford sohc 4.0 and it actualy sits in the same spot where the cam does on push rod motors.

[PDX87Starion]

a 6 cyl doesn't have 2 more complete combustion cycles per RPM.

Yes but the amount of time and the amount of travel each piston does until the next piston fires is less. With 4 cly the #3 piston doesn't fire until the #1 piston is at the bottom of its stroke. With 6cly the next piston fires approx half way down its stroke, and with 8 cly the next piston fires approx 1/4 way down the stroke. Which give you less efficiency because a full power stroke is NOT compleated before the next piston fires.

 

 

the reason there aren't 7.4 liter 2 cylinders is because the MASS completely changes. a 454 v8 would have significantly LESS mass that a 7.4 liter 2 cylinder.

 

think of it this way. if you double the size of a piston, you quadruple it's mass. simple physics.

 

Yeah I dont think you figured mass into your math. It might be equal at 1 RPM but once every thing is spinning its not going to be the same. Your math would have to work comparing a 1 cylinder and a 12 cylinder with the same displacement having equal torque and HP too. I would theorise that the 12 cylinder would have more torque at 3000 RPM, but the 1 cylinder would be more fuel efficient because it does not have over lapping firing cycles.

 

[patra_is_here]

those are 'my math'.

 

those are THE math for figuring this stuff out.

 

look at it this way, lets say the amount of time you're dealing with is one complete revolution. a four banger will have 1/2 a cycle on each cylinder, at some point in the 4 strokes, 2 of those cycles will contain a power stroke. 1 will contain 1/2 power stroke and the remaining will not. soooo, that 2.5 power strokes in one rotation

 

a six banger, will have still have only 1/2 a cycle per cylinder on 1 rotation. 2 of those cycles will be power strokes (but SMALLER power strokes since the pistons are smaller), the remaining 4 will have some combination of partial power strokes that will equal the same total power as the 4 banger.

 

it's still moving the same amount of air. just less air to each cylinder and less power per cylinder. but it makes up for it by having a closer overlap of powerstrokes than the 4 banger.

 

otherwise, you could make a 2 liter 12 cylinder engine and it would be really small yet make 700 HP. but it doesn't otherwise everyone would have just started using tons and tons of small cylinders years ago.

 

the real issue here in 4 vs 6 is the mass of the engine parts like piston and crank etc.

Edited July 18, 2009 by patra_is_here

[Cobalt60]

OK lots of great info here everyone. So to sum up what I think I learned, and please correct anything that is wrong:

 

- Concerning overlapping power strokes, a 6 cyl gains more torque, but less efficiency. One conclusion this brings me to is that this means a 6 cyl will have less potential maximum power, because ultimately that loss of efficiency will come into play.

 

- A 6 cyl will tend to be more reliable since there is less force exerted on each of the pistons and other components.

 

- The pistons in a 6 cyl are smaller and lighter, meaning less power is loss overcoming that additional mass. The lighter pistons will also allow the engine to rev higher, and probably also give it an efficiency advantage.

 

- Oh, no one really talked about the frictional losses of having those 2 extra pistons etc. So any approximate ideas on frictional losses of 6 cyls?

 

So in conclusion, the differences are relatively minimal. but, it seems the age old idea of 4 cyl being more economical and 6 cyl being more powerful, may actually be accurate.

 

Thanks guys. As far as motivation for making this post, well, inline 6s have always been my favorite motors. Having bought a Conquest, i really started thinking more about 4 cyl, and I really started liking 4 cyl. the Conquest also gave me an appreciation for turbochargers I never had before. But yeah, Im just trying to pick my favorite engine layout, I-4 or I-6. Hey even I-5s are nice...

 

-Chris P

[patra_is_here]

dude it all comes down to displacement. if you can fit a 6 cyl in the bay, do it,.

 

[Zack_Morris]

I would say that 4 cylinders being more efficient than 6 is a bit of a generalization. You should really take the type of car, weight, transmission type and gearing into account.

 

Example 1: A 4WD, 4 door Jeep Cherokee with a 4 cylinder is much less efficient than a 6 cylinder Jeep Cherokee purely because the 4 cylinder is under stress all the time pulling around 4k lbs of SUV. The 6 cylinder doesn't work nearly as hard, so is more efficient in that example.

 

Example 2: A Toyota corolla with a 3 speed manual will be much less efficient than the exact same corolla with 5 speed automatic. BUT if you put that SAME engine (which gets about 40mpg hwy) into a 4wd 5 speed Jeep wrangler with big tires and 4:10 gearing, it wont get nearly 40mpg.

 

See where I'm going with this? The harder an engine has to work to propel the vehicle it's in, the less efficient it is.

Edited July 18, 2009 by Zack_Morris