Z28x

You have 6.0L (.75L per cyl.) and 8.1L (1L per cyl.) V8s and 8.3L (.83L per cyl.) V10s that all have a high displacment per cyl. This seems common in V8 engines but not in 4 and 6 cyl. engines. The biggest I4 on the market right now seems to be 2.5L (0.625L per cyl.) and the biggest car V6 a 3.8L (0.63L per cyl.)

Why doesn't someone build a 5.0L V6 or a 3.0L+ I4????

it would seem like a big block V6 would put out as much power as a small V8 but be cheaper to build. why not chop 2 cyl. off of the GM 8.1L V8 and build a 6.0L V6?

WERM

I'm not an engine expert, but this is what I've picked up. In the case of inline 4 cylinder engines, vibrations start to become a problem around 2.3-2.4L. This is why you rarely see a four with a larger displacement. I suspect that the same thing is true for V6's, and I'll take a wild guess and say 4.3L is close to the limit. (In the case of inline 6's, ford did make a 5.0L inline 6 for many years and I think chrysler slant 6's could be bored/stroked to a pretty decent size).

I also think there are some emission concerns with larger piston diameters.

Burmite

Maybe size. Why make a bigger 4 cylinder when it becomes way bigger than a V6? Make the cylinders smaller and tack two more on in a V. You have a V6. Same goes with a V6. When the displacement gets bigger because of the block, what's stopping you from making a smaller V8? In the case of the big block 8.1L, I think has to do with the fact it only exists in truck engines where big stroke means big torque. So the big block will make more sense in the truck application. Dodge when faced with making a big block V8 for the Ram 3500 decided to just make a V10. With Chevy, I think the development costs were smaller by modifying a racing crate motor for truck use was easier than developing a V10. There were experiments in making a V10 LS1. I read the article and I believe it ran in GMHTP. Ford when faced with a big block V8 or a V10 went with the V10.

Also for car use, smaller strokes means higher revs and more high end power. Cars need this as opposed to the Silverado 3500HD. Vibrations I would imagine would play a part in this too, but I'm no engineer.

Z28Wilson

Even GM's 4.3 V6 was known as a pretty crude piece if I remember correctly....there were a lot of issues with vibration and balancing in that thing.

Z28Marcus

A big displacement I4 would likely mean bigger chamber and cyclinder bore. Bigger bore means bigger == heavier pistons = more stress because of increased momemtum. Then add balancing issues in which would be exagerated by the increased mass. Also packaging & block strength would possibly be affected given a bigger bore.

With a bigger combustion chamber - it's probably harder to achieve high volumetric efficiency too.... more space to fill. With more, but smaller chambers maybe it's easier to fill the chamber. Like someone else suggested above, I've heard it has to do with emissons to. This could be something to do with the cylinder fill issue and burning *all* the fuel that gets into the combustion chamber. When there is more unburnt stuff left over - you get higher emissions right?

Also isn't one reason big displacement v10s and v12s run smoother than big diaplcement engines with fewer cylinders because it's like more smaller explosions making power as opposed to fewer big explosions [in the case of a big I4, 4 big explosions]?

Just educated guesses - I'm no engine expert either, but I have pondered the same question myself.

R377

The reason the 4.3 is so rough is that it's a 90º V6 (out of necessity, since it's made from a V8) and such a design is not inherently balanced. Even the balance shaft it received in later years is not an optimal solution. Balance on such an engine can be improved somewhat by splitting the crankpins (as GM did), but then you introduce friction and wear problems on the bores and pistons.

As for bore size, 4.0 inches is close to optimal from a power standpoint. Larger bores used to be susceptible to emissions problems because of the unburnt gas above the top rings and in the squish area, but engineers have largely licked those issues.

So why don't we see large 4 and 6 cylinder engines?

As WERM said, 4 bangers are hard to make smooth once above 2.2 litres or so. Also, 4 cylinder engines are created mostly as an economy engine, not for power, so it's kind of contradictory to want to design a "large" 4 cylinder. I'm not sure what the reason is for no large bore 6 cylinders (a 60º V6 is a balanced design) other than if you are going to make an engine bigger than 4 litres, people nowaday just expect a V8.

PGR

A 90º V6 is inherently balanced (mass), however does not have an even firing order. This makes for a shaky idle, but things smooth out as soon as you rev it up. Splaying the crank throws allows for a even firing order and smooth idle, at the expense of some introduced mass imbalance. So as you rev it up, you notice more vibration. Adding balance shafts helps reduce this vibration, but means extra parts and cost.

An I4 engine has a even firing order, but has primary and secondary mass imbalances. They become noticable above 2.0 liters. Mitsibishi made a 2.6 liter I4 with twin balance shafts. GM will produce a 2.8 liter I4 with balance shafts, based on the 4.2 I6.

A 60º V6 has a even firing order, but is not a balanced design, in terms of mass (unless its crank throws are splayed, but then it loses its even fire)

When a I4 or I6 becomes too large, the amount of cost or weight needed to make them smooth makes a V engine more practical.

GM's I4 may be expensive compared to a V6, however it will share much of the development costs, parts, and tooling of the I6 and I5.

Sixer-Bird

Porsche made a 3.0 liter inline four based off of their flat six. They used this engine in the 968. Bentley also made a 3.0 liter inline four back in the 1920's. I believe the Bentley "3 Liter" won Le Mans.

Z284ever

Actually, the I4's in the 924/944/968 were one half of the 928's V8.

Sixer-Bird

Ah! Damnit! You're right, and I knew that too. I've had flat sixes on the brain lately since my cousin came into town a couple of weeks ago with his 911 Turbo.


BTW, speaking of the 928's V8, check out what my cousin and one of his friends were able to achieve out of the DOHC 5 liter:

Z284ever

Very Cool! I'll probably get afew flames for this....but I've always thought that the 928 was the GT that Camaro should aspire to .

I remember once at a cruise night, this mint red 928 pulled in. It's exhaust note had a menacing and yet familiar burble to it. When he popped the hood, the familiarity of the exhaust note was explained. He had an electronic fuel injected smallblock under the hood. It looked and fit great in there...with plenty of room to work on it.

I remember thinking how awesome it would be if Chevy built a Camaro like that.

R377

A 90º V6 without split pins is only partially balanced. While it's true that you can balance the mass of the crankshaft and its throws by adding weights to the crankshaft, this crankshaft counterbalancing cannot cancel out the force of the pistons and conrods moving at different speeds and in different directions within each bank. The result is an end-to-end vibration, which is what the balance shaft tries to address.

A 60º V6 must have its crank pins split by 60º in order to maintain even firing. I'm not sure anyone even makes a modern 60º V6 where the crank pins aren't split. Like the 90º design, it suffers from end-to-end vibration, though it tends to be smoother overall than a 90º V6.

An I6 is actually a perfectly balanced design, in effect two I3s mirrored together where the end-to-end vibration inherent in a 3 cylinder engine (or half a V6) is cancelled out. I6s, V12s, and all boxer engines are the only perfectly balanced designs. Except for the V12, no V engine is perfectly balanced.

SageofKnight

The answer is simple really. Why make a large displacement 4cyl when you can have an equal displacement 6cyl that would be much easier to engineer? The same goes for big V6s vs an equal displacement V8. Once you start getting into V12s and V16s the engines start to become too physically large, so V8s end up getting some wide range of displacements.

Z28x

A V6 has less parts than a V8 so it should be cheaper to build a 4.5L V6 than a 4.5L V8

PGR

A 90º V6 with common crank throws is similar to three Ducati 90 V-twins connected together. Although a Ducati motor has a shaky idle, once reved up, it is a very smooth engine, despite the fact that it is a larg displacement twin. The 90º common pin design provides nearly perfect promary balance, with some minor secondary forces. Adding a pair of high speed (2x crank) counter-rotating balance shafts will address the secondary imbalance.

I6 engines still have a torsional imbalance that makes the ends of the engine orbit. Careful motor & transmission mount design helps reduce this vibration, but it is still there, and can be quite noticable.

All boxer engines are not perfectly balanced. BMW claims that thier Boxer twin motorcycle engines have perfect balance. Once you ride one, you realize they vibrate quite a bit. This is due to the offset to the two crank throws along the axis of the crank. So in two axis, the design is balance, but once you consider the third axis, it is obvious the design is unbalanced.

I guess I wasn't very clear about the 60º V6. It has an even firing oder due to the 60 degerees of crank splay, but as a result is not balanced. If the crank throw splay was different (30º I believe?), it would have perfect primary balance, but an odd firing order, and a shaky idle.


I'm more familiar with motorcycle engines, there seems to be loads of different engine configurations available, with their corresponding characteristics.