Ray III said:
The degree offset of the cylinder banks of a V configuration engine affects its balance. 90 degree V6s are imbalanced, but 60 degree V6s run very smoothly with no balance shafts.
Actually, the fact is when you overcome the length issue of an inline 6, it has much more space to the sides and is no taller than a comparable V8. You forget that some of the height of a V engine is because it has to have its intake manifold sitting on top of the engine, whereas inline engines can have nothing above the valve cover. Manifolds are pretty straightforward with one intake and one exhaust manifold, usually located to opposite sides but they can be located to the same side of the block. If you look in an older 300 powered F-150 there is a huge amount of space in the engine bay. A 351 truck fills the bay up pretty good.
Can you explain some more about the torsional rigidity of V blocks? Never heard of such a thing being an issue in 2000 ft-lb torque inline engines.
A 90 degree v6 can be made to have good balance. It means you either have to add a balance shaft, as GM did on it's 4.3L vortec v6, or split the crank thows to get back to 60 degree firing intervals.
The vortc v6 engine essentially was a 350 cu in SB V8 with 2 cylinders lobbed off. It was a relatively large displacement V6 with bad primary balance so vibes weren't that good. I vaguely recall initial versions of that engine with a manual trans having bearing issues (or something). In the late '90's GM added a balance shaft, in the "V", right above the camshaft. This smoothed it out a lot, but I don't think they ever got it back to the smoothness of say, a 305 V8. Balance shafts also have the disadvantage of robbing a bit of power and they generally work best in a specific RPM range.
Splitting crank throws has the disadvantage of resulting in more expensive and weaker crankshafts. Lamborghini's new 5.0L v10 engine has a split crank, allowing even 72 degree firing intervals, but the lower CG of a 90 degree block. F1 engines have generally migrated to wide angle blocks but I think they are probably doing a combination of splitting crank throws to some degree and just figuring out how to handle the exess vibes. Renault's engine last year was 110 degrees. This year, with the "one engine rule" a lot of manufacturers are going back to 72 degrees.
As for "2000 lb-ft inline engines", I assume you are talking about highway truck engines. I can only make educated guesses:
1. These engines don't have to deal with High RPM/resonant frequencies you generally see in a high performance car engine.
2. These things aren't just heavy, they are
F@#&ing Heavy. Here are the specs for a C15 CAT highway diesel.
Cylinders In-Line 6
Bore/Stroke 5.4 x 6.75 (137mm x 171mm)
Displacement 15.2 L (928 cu in)
Weight 2890 lb (1311 kg)
Horsepower 435 to 550 @ 2100 rpm
Torque 1350-1850 lb0ft @ 1200 rpm
This engine weighs more than most new cars.
3. These engines tend to have a more undersquare bore/stroke ratio and very long con rods. This contributes to a realtively tall deck, shorter length block, which would be much stiffer than a shallower, longer one.