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Jeep I6 & V8's (more)...
My sister used to have a Jeep Grand Cherokee with the 4.0 liter Inline-6. I do not think that she ever opened the hood once in its 60,000 miles. Her dealer did all of the oil changes every 3,000 miles. True, it is on the rough side, but that engine is nearly bullit proof and dead-on when it comes to strength and durability. One really important thing that you have to remember is that high-end cars with V8s like Lexus use electronically controlled engine mounts. That is one of the main reasons why their engines "feel" so smooth. The Jeep 4.0 Inline-6 in from another "era" sort-of-speak, but a classic also.
Almost all V8s are 90°. I have never heard of a V8 that was not 90°. 90° is the only configuration achieving good balance for a V8. However, there are two types of crankshaft arrangements, which deliver very different characters. They are cross-plane crankshaft and flat-plane crankshaft V8s. Most of the world’s V8s are cross-plane V8, including all American V8s and all sedans’ V8s. However, most exotic European sports cars, including Ferrari, Lotus, and TVRs, employ flat-plane V8s.
No matter which kind of V8s, they have 5 main bearings. A cylinder, in bank A shares the same crank pin with the corresponding cylinder in bank B, therefore the crankshaft of V8 is actually simpler than in a V6. Both V8s generate no vibration in vertical, transverse directions or between bank and bank.
For cross-plane V8s, there is vibration from end to end of the engine, this is because the first piston of bank A is not in the same position as the last piston of bank A (the same goes for bank B), unlike an inline-4 engine. No problem, the 90° V8 solves this problem by introducing an extra-heavy counter weight to every cylinder. The counter weight is heavy enough to balance the weight of crank throw, connecting-rod, and piston of that cylinder, thus resulting in lack of vibration. Without going to much further into detail, the 90° cross-plane V8 employs full-weight counter weights to achieve near perfect smoothness.
However, the disadvantage of cross-plane V8s is also about the counter weights. They not only increase the weight of engine, they also contribute to rotational inertia, thus making the engine less responsive and less rev-able, dropping the upper rev limit and top-end power. Moreover, the larger counter weights usually require a larger crankcase to house them, thus raising the height and more important, center of gravity, of the enigne. Now that you know this, you can understand why all V8 Ferrari models, TVR Cerbera AJP V8, and Lotus Esprit V8s employ flat-plane V8s instead.
Flat-plane V8 is named according to the shape of the crankshaft, which is in a flat plane. It is very much like two inline-4 engines mated together. In particular, it achieves end-to-end balance because the first piston and last piston of a bank is exactly in the same position, so are the center two pistons. This is just the same as straight-four engines, therefore the sound of flat-plane V8 is usually somewhat like a pair of four-pot engines screaming simultaneously, unlike the rumble-bumble of cross-plane V8s.
However, flat-plane V8 doesn’t achieve balance between banks, unlike cross-plane V8. This is because the pistons in both banks are not in the same positions. In some angle, Bank A may have two pistons at the highest positions and the remaining two in the lowest positions, while Bank B have all the pistons in somewhere slightly below the middle of the stroke. That cause vibration between banks, hence not as smooth and quiet as cross-plane V8s. To exotic sports cars, less refinement is not a big problem. Moreover, the short stroke of high performance V8s, reduces the level of vibration.
Actually, I own a Volvo 960 with a 2.9 liter Inline-6 and an Infiniti Q45a with a 4.5 liter V8. At 6,000 rpm, the Volvo engine has extremely low constant vibration characteristics that can hardly be felt. This engine is very smooth and quiet with 181hp/201lb.ft. The Infiniti engine has harmonic oscillating vibrational characteristics that can be felt a little more. This engines likes to rumble and rev, but it has ample and generous power with 278hp/292lb.ft. Imagine a coordinate plane, the Inline-6 is like a straight line, and the V8 is like a cosine function when it comes to smoothness.
-Ian Rosdale
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