More cylinders - better? And if so, why?

madaboutcars

I have read a lot of stuff on this forum but I often read something along these lines whenever a BMW comes up "you *have* to get the 6 cylinder model"(instead of the 4 cylinder model).

Why is 6 cylinders so much better than 4?

Performance? I don't think so. Just because BMW hasn't made a 4 cylinder engine larger than 2.0 litres and the smallest 6 cylinder just so happens to be a 2.5 is not a good reason.

I'm sure BMW could make a 2.5, 3.0 or even 4.4 4 cylinder engine if they wanted to.

I agree with people that 6 cylinder BMWs make a terriffic exhaust note, but is that all there is to it? I mean is that all there is to more cylinders - a better exhaust note? There is also the knowledge that I am doing the planet more harm and that way I am pissing off the happies which makes me feel good but I really can't see the need for lots of cylinders. Sure 2 or 3 cylinders would do fine? Like why not have a 3 cylinder Merc E200 Bi-turbo instead of a big huge E500 V8 with the same power?

Why would anyone want to actually own an engine that is less efficent, and heavier? I would have thought that the last thing any self respecting car enthusiast would want is added weight over the front wheels, but that's exactly what you get with more cylinders.

I love the noise of a 6 cylinder or 8 cylinder engine but thanks to the wonders of YouTube I have plenty of access to good sounding engines with lots of cylinders.

Make the case for more cylinders folks:)!

maidhc

A 6 cylinder will always have more torque and flexibility than a blown 4 cylinder.

Also when you go up in capacity, i think it becomes difficult to deal with the various stresses without increasing the number of pistons. Imagine a car with 4+ litre engine and 4 cylinders. I think it would be horrific.

unkel

LIGHTNING wrote: »

Dont porsche have the record for the largest 4pot? I think a 3 litre or something like that.

Dunno if it is the largest 4 pot, but the 3l inline 4 used in the 944 and 968 has 210BHP, which at the time I believe was the highest output per litre for a N/A engine. BTW This engine is half of the V8 engine used in the 928!

madaboutcars

LIGHTNING wrote: »

Plus the larger a engine in general the less work it has to do so reliability can be a benefit as the engine doesnt have to work as hard as a 4 pot.

I can see the benefit of larger engines - I like power in a car too:D!

I learned to drive in a 1.2 litre 3 cylinder Skoda Fabia. I drive a 4 cylinder 1.8 Volvo S40.

I can see the benefit in performance every time, especially out on a dual carriageway where the larger engine was clearly faster. Don't get me wrong, I love the extra power of larger engines, I'm not disputing that!

But the refinement thing puzzles me. The 4 cylinder Volvo is nowhere near as smooth as the 3 cylinder Skoda. Vibrations through the pedals, vibrations in the cabin etc the Skoda is better. I tried my dad's 4 cylinder Avensis *very* briefly recently - it makes the Volvo feel as if it's running on stones but the Skoda does the same thing. Now the Volvo would be the GDI engine - and that I'm told is a heap so fair enough you might say.

But IMHO the Skoda is no less smooth than the Toyota - which brings me back to my OP - what's the point of it having an extra cylinder that is needlessly wasting fuel? There isn't even the noise benefit either - I don't like the sound of 4 cylinder engines, especially above 4,000 rpm when they get quite irritable, while the sound of 3 cylinder engines is quite nice actually. They even do a bit of an imitation of a 6 pot at higher revs - and I love the sound of a 6 cylinder engine.

It's not power because you can make a 3 cylinder 1.8 if you wanted to too. So what is it?

Anyway, that's what balancer shafts are for - to make the engine run smoother.

peasant

There is a limit as to maximum cylinder bore and stroke beyond which bigger doesn't become better but worse.

Increase the bore too much and the ignition of the fuel becomes messy and uncontrollable (unless you want to add a ring of spark plugs and about 10 valves per head), make the stroke too big and the piston speed increases to insane levels, the involved forces on the turn become so big that your engine simply disintegrates (unless you slow the engine down and then you have a tractor)

So beyond this limit, more power simply has to mean more cylinders.

The biggest single cylinder motorbike to my knowledge was the Suzuki DR 800 (779 cc) (bit of a tractor) ...put four of those in a row and you'd have a 3.1 liter four cylinder. That size seems to be about the maximum do-able without ending up with an actual tractor.

maidhc

peasant wrote: »

The biggest single cylinder motorbike to my knowledge was the Suzuki DR 800 (779 cc) (bit of a tractor) ...put four of those in a row and you'd have a 3.1 liter four cylinder. That size seems to be about the maximum do-able without ending up with an actual tractor.

How about a 10.9L single cyl?

Here you go:


(it is a tractor, but kind of a road orientated one!)

Quazzie

LIGHTNING wrote: »

One word smoothness- 6 cylinder engines have overlapping power strokes so less vibrations are transmitted to the passengers. A V6 has a power stroke for every third of a revolution and with the cranks set at 60 degrees intervals the engine is smooth running and resonably balanced. Plus the larger a engine in general the less work it has to do so reliability can be a benefit as the engine doesnt have to work as hard as a 4 pot.

And this is the reason why V8's are the perfect smoothest engine*


*excl Wankel engines

copacetic

madaboutcars wrote: »

I have read a lot of stuff on this forum but I often read something along these lines whenever a BMW comes up "you *have* to get the 6 cylinder model"(instead of the 4 cylinder model).

Why is 6 cylinders so much better than 4?

Performance? I don't think so. Just because BMW hasn't made a 4 cylinder engine larger than 2.0 litres and the smallest 6 cylinder just so happens to be a 2.5 is not a good reason.

I'm sure BMW could make a 2.5, 3.0 or even 4.4 4 cylinder engine if they wanted to.

I agree with people that 6 cylinder BMWs make a terriffic exhaust note, but is that all there is to it? I mean is that all there is to more cylinders - a better exhaust note? There is also the knowledge that I am doing the planet more harm and that way I am pissing off the happies which makes me feel good but I really can't see the need for lots of cylinders. Sure 2 or 3 cylinders would do fine? Like why not have a 3 cylinder Merc E200 Bi-turbo instead of a big huge E500 V8 with the same power?

Why would anyone want to actually own an engine that is less efficent, and heavier? I would have thought that the last thing any self respecting car enthusiast would want is added weight over the front wheels, but that's exactly what you get with more cylinders.

I love the noise of a 6 cylinder or 8 cylinder engine but thanks to the wonders of YouTube I have plenty of access to good sounding engines with lots of cylinders.

Make the case for more cylinders folks:)!

If you rule out the small petrols then normally people are comparing the 2.0D 4 cylinder engine with 2.5 or 3.0 6 cylinder engines.

I've been in a lot of both over the last few months and for most driving it is not that easy to feel the difference between the lastest say 320d and a 325d or a 330d. The sound isn't that different either.

However you can feel the difference between a 320d and a 325i or 330i, there is a big difference in sound. You can maybe feel a difference in 'smoothness' too, but imo it is very hard to quantify if this is actually a physical feeling or is it the sound that infuences you more than anything else. Audio really affects our perceptions. It's not a big difference but it is noticeable.

The 320d has 177bhp and a turbo and can be rempapped easily to 220bhp, the current 325i has 218bhp. It would be cheaper to buy a 320d and remap it than a 325i and you get more torque and low down pull. Some people would say that the 2.0d won't be reliable etc, but I'm not sure I agree.

In the end it is a matter of what you can afford and what you feel after driving the various engines. In the end I went for the 325i, maybe a lot of it is the nice feeling of history/tradition in a 6 cylinder 3.0l BMW. Maybe it is a bit of looking down your nose at the 2.0l versions. I'm not sure, plenty of people look down on the 325s and 330s now as they aren't 335s and people look down on the 335 cos they aren't M3s.

unkel

Q2002 wrote: »

And this is the reason why V8's are the perfect smoothest engine

V12s are the smoothest, but V8s are a close second. I love them

Dartz

Good 4 cylinder engines are statically balanced. No matter what angle you turn the crank to, the engine will remain at that angle. But, they are not dynamically balanced. The act of rotating the engine causes forces along the length of the crank from the balancing weights and con-rod/piston assemblies. Also, there is the fact that both sets of pistons are accelerating and decelerating at the same time. Long story short, 4cylinder engines have a vibration frequency that's about double the current RPM. Most modern engines have a set of balancing shafts, which smooth these vibrations (but also sap power)

6 cylinder engines have both static and dynamic balance (provided they use an even firing order), in addition to the fact that the power strokes overlap.

peasant

unkel wrote: »

V12s are the smoothest, but V8s are a close second. I love them

What about V16's then?

unkel

peasant wrote: »

What about V16's then?

Ferdinand might not be a Porsche but a superhero he is

Quazzie

unkel wrote: »

V12s are the smoothest, but V8s are a close second. I love them

We were thought in our college in becoming a mechanic that V8's were the smmothest because of the perfect overlapping of power strokes. That might be because he didn't consider V12's because of their rarity!

unkel

Q2002 wrote: »

We were thought in our college in becoming a mechanic that V8's were the smmothest because of the perfect overlapping of power strokes. That might be because he didn't consider V12's because of their rarity!

Indeed! And in the same way I didn't consider the V16 (Bugatti Veyron) as peasant kindly pointed out

unkel

Just an afterthought. The Veyron engine isn't really a V16, is it? Just 2 side by side V8s. If that's right, the V12 still wins the smoothest engine competition, does it?

Quazzie

what about this?

bazz26

I'm just waiting for E92 to give us the real reason for choosing more cylinders.

CJhaughey

The skoda Fabia has a lot of refinement it is a very new design and has benefited from the cabin being soundproofed a lot better than older designs of cars like the S40, vibration through the pedals could be due to a multitude of factors, wheel balance etc. I don't think you can make a judgement based on S40 vs Fabia, there are many more factors than those involved.
You wondered why MB didn't build a biturbo E200? I am sure that they could get horsepower close to the V8 E500 but in terms of engine longevity it wouldn't come close, al things being equal a large capacity engine running at a relatively low engine speed will last a high revving small capacity engine by a large factor.
Do you remember the mid eighties turbo'd F1 cars? They had 1.5 litre 4cyl engines but produced huge horsepower the BMW B186 had a rumoured 1400hp and lasted just long enough to qualify.
Power is one thing and torque is another, large engines with lots of cylinders produce lots of torques (I quote Clarkson) and this makes for a good towing vehicle and a more relaxing drive.


In a petrol engine 100mm is about the maximum piston dia, this is due to the inability of the flame front to completely burn the fuel charge before the piston commences downward motion and the cylinder pressure reduces.
In practice this means that an engine with a piston of over 100mm is likely to have a lot of problems with unburnt charge remaining and affecting emissions etc.


I still think a straight 6 is the smoothest engine configuration and likewise the variants such as the Jag V12 engine smooth as silk but incredibly thirsty,
TBH 3l seems to be the practical maximum for a 4cyl engine in modern times.
The Toyota landcruiser 90 and 120 series have a 4 cylinder 3litre engine with balancer shaft.
Have a look at this page

Steve

unkel wrote: »

Just an afterthought. The Veyron engine isn't really a V16, is it? Just 2 side by side V8s. If that's right, the V12 still wins the smoothest engine competition, does it?

Isn't it a 'W16' (same thing, 2 V8's)

mike65

Audi did a "fake V8" with thier Uber-cruiser in the late 80s/early 90s, which had two VW 1.8 engines bolted together. Dunno if that compromised its charactisitics.

http://www.cruisersspecialtyautos.com/prodimages/14AQEng1820.jpg

niceirishfella

LIGHTNING wrote: »

One word smoothness.

+1 X 1000:)

RobAMerc

copacetic wrote: »

If you rule out the small petrols then normally people are comparing the 2.0D 4 cylinder engine with 2.5 or 3.0 6 cylinder engines.

I've been in a lot of both over the last few months and for most driving it is not that easy to feel the difference between the lastest say 320d and a 325d or a 330d. The sound isn't that different either.

I can't agree with this, I have an e90 320d, my brother in law has an X5 3.0d and my mate has a 525d - there is a significant difference in smoothness IMO - the six cylinder is noticeably smoother, particularly at idle and at start up - OK when your on an M-way maybe but that could be said of diesel vs petrol even, but anywhere else there is a noticeable difference.

I would agree that the 6 cylinder petrol engines are also way smoother than the petrol fours - there is no way the e46 320 I had would have been anywhere near as nice a car if it was a 4 pot - no way

Darragh29

It's not so much to do with BMW's as it it to do with one feature of a 6 cylinder engine. If you have 6 cylinders, you have an overlap in power strokes, effectively you have two power strokes occuring at the same time, although they are at different stages, whereas in the case of a 4 cylinder engine, you only have one power stroke occuring at any particular time.

This obviously gives you more power output...

Edit: And it also accounts for the smoothness you get from a 6 cylinder....

copacetic

RobAMerc wrote: »

I can't agree with this, I have an e90 320d, my brother in law has an X5 3.0d and my mate has a 525d - there is a significant difference in smoothness IMO - the six cylinder is noticeably smoother, particularly at idle and at start up - OK when your on an M-way maybe but that could be said of diesel vs petrol even, but anywhere else there is a noticeable difference.

I would agree that the 6 cylinder petrol engines are also way smoother than the petrol fours - there is no way the e46 320 I had would have been anywhere near as nice a car if it was a 4 pot - no way

I think we are basically in agreement Rob, I mentioned in 'most' driving the difference isn't that noticeable. i.e once you are moving along with the windows up. You are bang on that the time you would notice it most is at idle/start up or from outside the car. However comparing the 6 cylinder diesels to the 6 cylinder petrols is a similar experience imo.

peasant

unkel wrote: »

Indeed! And in the same way I didn't consider the V16 (Bugatti Veyron) as peasant kindly pointed out

I wasn't actually thinking of the Veyron but of some luxury liners of the distant past ...Cadillac did a V16, Duesenberg, if I remember correctly and Maybach and some other cars for the super-super rich of the time.

kuro_man

for smoothness, a inline-6 will be best because it is the only naturally balanced configuration - it needs no balancing rods. A V12 is essentially two I-6's bolted together, but a much more complex arangement.

One things that nobody's picked up on: there is no direct correlation between fuel economy and engine size or piston count. I have a 4.3 V8 that easily does 30mph+ - the same as the 2.8L alternative. Why? The Block is made from aluminmum and it's an efficient design (variable intake manifold, dual spark, variable valve timing etc.) and I keep it well serviced. Difference is even bigger in diesel, my old 1.2L petrol fiat used almost the same amount of fuel as the 3.0 I6 Merc diesel!

A larger engine running a low revs will be more efficient than a small engine that has to be trashed to do the work asked of it. Also, small engines have steeper torque curves: if you want to overtake that oil truck doing 50mph, then acceration from 50-70mph is the key, not 0 - 60. Not a good time for the torque to evaporate.

In other words, match the engine to its use: small city cars will be fine with a 3 or 4 cyclinder - it won't be expected to pull a lot of weight or provide blistering acceration. It would want to tow a boat, use a big diesel. If you want to go fast, get a big petrol I6 or V8 (or a tiny car like a suzuki cappucino).

The future, though, is electic. 1 forward gear, no torque curve , no pistons.

smash

peasant wrote: »

What about V16's then?

you mean W16's

D20903

The most famous V16 - turn your speakers up! :

http://www.youtube.com/watch?v=fZMPDCNyQxE

B11gt00e

unkel wrote: »

Just an afterthought. The Veyron engine isn't really a V16, is it? Just 2 side by side V8s. If that's right, the V12 still wins the smoothest engine competition, does it?

The term 'double-six' used by daimler to describe their v12 engines to me seem to imply that their v12 is just 2 v6's working in tandem!?

B11gt00e

kuro_man wrote: »

I have a 4.3 V8 that easily does 30mph+ - the same as the 2.8L alternative. Why? The Block is made from aluminmum and it's an efficient design (variable intake manifold, dual spark, variable valve timing etc.) and I keep it well serviced.

This man sounds like he knows his motors... was just chuckling at how one letter can make such a difference in the point one is making.

While 30mpg+ might be impressive for a V8, surely one would be a bit disappointed to find tha 30mph+ was something that only V8 drivers had the pleasure of experiencing!!

E92

B11gt00e wrote: »

The term 'double-six' used by daimler to describe their v12 engines to me seem to imply that their v12 is just 2 v6's working in tandem!?

No, it's 2 straight 6's.

A V12 is basically 2 straight 6's with a common crankshaft.

To answer the OP's question, more cylinders gives more smoothness, and of course as alluded to it p!$$€$ off the Greenies too, so yes of course it is important.

Big petrol engines with lots of cylinders sound great too.

The straight 6 is so much better than the standard 4 cylinder engine to drive (in petrol engines at least, not sure about diesel) it's not even funny. Even an average 4 pot is a lot nicer than a good 3 pot.

The S40 GDI is probably one of the least smooth if not the least smooth petrol engine you're ever likely to drive, trust me most 4 cylinder engines are a lot better. Those GDI engines are not representative of what a standard 4 pot is like you'll be pleased to know:). If you drove a good one you'd see why it is better than a 3 pot. I would not have thought a Skoda Fabia 1.2 was smoother than it thouh, if anything I would have said the Fabia is rough compared to the Volvo so yours must have a fault with it.

You need to drive a straight 6, if you're the type of person who understands what cylinders are and are open to persuasion about then benefits then all it takes is a quick drive of one and you'll find out straight away to pardon the pun:)! They are a teriffic engine configuration, and it's the smoothest engine this side of a V12 too.