Re: 4.3 vortec
Another thing to keep in mind:
The 4.3 is a Frankensteined engine to begin with.
By design (for proper balancing) a V6 should be 60 degrees, not 90.
The 4.3 was derived by hacking 2 cylinders off a 350, they did this to be able to use a lot of the same tooling.
At first they didn't even bother to charge the firing order to be evenly spaced, so the early generation 4.3's were odd fire (kind of like a harley V-twin)
Later, they added balance shafts to try and fix this balancing issue as well.
So it makes perfect sense to see two motors with different balancing on the crank, and it makes perfect sense to see different crank designs over-time. GM was constantly making tweaks to try and balance this "unbalanced by design" motor.
If you look up the specs on any modern V6, they're all 60 degrees for a reason.
From wikipedia
Balance and smoothness
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. Each cylinder bank in a V6 has an odd number of pistons, so the V6 also suffers from the same problem unless steps are taken to mitigate it. In the horizontally-opposed flat-6 layout the rocking motions of the two straight-3 cylinder banks offset each other, while in the inline-6 layout, the two ends of engine are mirror images of each other and compensate every 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.).
This is almost the same technique which balances an even firing 90? crossplane V8 in primary and secondary order. A 90? V8 is in primary balance because each 4-cylinder bank is in primary balance, and the secondary vibrations of the two banks can be made to cancel each other using a crossplane crankshaft. However, there is no equivalent of the V8 crossplane crankshaft for the V6, so that the vibrations from the two banks cannot be made to completely cancel each other. This makes designing a smooth V6 engine a much more complicated problem than the straight-6, flat-6, and V8 layouts. Although the use of offset crankpins, counterweights, and flying arms has reduced the problem to a minor second-order vibration in modern designs, all V6s can benefit from the addition of auxiliary balance shafts to make them completely smooth.[5]
When Lancia pioneered the V6 in 1950, they used a 60? angle between the cylinder banks and a six-throw crankshaft to achieve equally spaced firing intervals of 120?. This still has some balance and secondary vibration problems. When Buick designed a 90? V6 based on their 90? V8, they initially used a simpler three-throw crankshaft laid out in the same manner as the V8 with pairs of connecting rods sharing the same crankpin, which resulted in firing intervals alternating between 90? and 150?. This produced a rough-running design which was unacceptable to many customers. Later, Buick and other manufacturers refined the design by using a split-pin crankshaft which achieved a regular 120? firing interval by staggering adjacent crankpins by 15? in opposite directions to eliminate the uneven firing and make the engine reasonably smooth.[6] Some manufacturers such as Mercedes Benz have taken the 90? design a step further by adding a balancing shaft to offset the primary vibrations and produce an almost fully balanced engine.
Some designers have reverted to a 60? angle between cylinder banks, which produces a more compact engine, but have used three-throw crankshafts with flying arms between the crankpins of each throw to achieve even 120? angles between firing intervals. This has the additional advantage that the flying arms can be weighted for balancing purposes.[6] This still leaves an unbalanced primary couple, which is offset by counterweights on the crankshaft and flywheel to leave a small secondary couple, which can be absorbed by carefully designed engine mounts.[7]
Six-cylinder designs are also more suitable for larger displacement engines than four-cylinder ones because power strokes of pistons overlap. In a four-cylinder engine, only one piston is on a power stroke at any given time. Each piston comes to a complete stop and reverses direction before the next one starts its power stroke, which results in a gap between power strokes and noticeable vibrations. In a six-cylinder engine (other than odd-firing V6s), the next piston starts its power stroke 60? before the previous one finishes, which results in smoother delivery of power to the flywheel. In addition, because inertial forces are proportional to the cube of the piston mass and the square of the piston speed, high-speed six-cylinder engines will suffer less stress and vibration than four-cylinder ones of equal displacement.[8]
Comparing engines on the dynamometer, a typical even-fire V6 shows instantaneous torque peaks of 150% above mean torque and valleys of 125% below mean torque, with a small amount of negative torque (engine torque reversals) between power strokes. On the other hand, a typical four-cylinder engine' shows peaks of nearly 300% above mean torque and valleys of 200% below mean torque, with 100% negative torque being delivered between strokes. In contrast, a V8 engine shows peaks of less than 100% above and valleys of less than 100% below mean torque, and torque never goes negative. The even-fire V6 thus ranks between the four and the V8, but closer to the V8, in smoothness of power delivery. An odd-fire V6, on the other hand, shows highly irregular torque variations of 200% above and 175% below mean torque, which is significantly worse than an even-fire V6, and in addition the power delivery shows large harmonic vibrations that have been known to destroy the dynamometer.[9]
[edit] V angles
Chrysler 3.3 In LH form
[edit] 60 degrees
The most efficient cylinder bank angle for a V6 is 60 degrees, which design is very compact and tends to minimize vibrations. While they are not as well balanced as inline-6 and flat-6 engines, modern techniques for designing and mounting engines have largely disguised their vibrations. Unlike most other angles, 60 degree V6 engines can be made acceptably smooth without the need for balance shafts. When Lancia pioneered the 60? V6 in 1950, they used a 6-throw crankshaft to give equal firing intervals of 120?. However, more modern designs often use a 3-throw crankshaft with what are termed flying arms between the crankpins, which not only give the required 120? separation but also can be used for balancing purposes. Combined with a pair of heavy counterweights on the crankshaft ends, these can eliminate all but a modest secondary imbalance which can easily be damped out by the engine mounts.[1]
This configuration is a good fit in cars which are too big to be powered by four-cylinder engines, but for which compactness and low cost are important. The most common 60? V6s were built by General Motors (the heavy duty commercial models, as well as a design used in many GM front wheel drive cars) and Ford European subsidiaries (Essex V6, Cologne V6 and the more recent Duratec V6). Other 60? V6 engines are the Chrysler 3.3 V6 engine, Nissan VQ engine and the Alfa Romeo V6 engine.
[edit] 90 degrees
90-degree V6 engines are also produced, usually so they can use the same production-line tooling set up to produce V8 engines (which normally have a 90-degree V angle). Although it is relatively easy to derive a 90-degree V6 from an existing V8 design by simply cutting two cylinders off the engine, this tends to make it wider and more vibration-prone than a 60-degree V6. The design was first used by Buick when it introduced its 198 CID Fireball V6 as the standard engine in the 1962 Special. Other examples include the Maserati V6 used in the Citro?n SM, the PRV V6, Chevrolet's 4.3 L Vortec 4300 and Chrysler's 3.9 L (238 cu in) Magnum V6 and 3.7 L (226 cu in) PowerTech V6. The Buick V6 was notable because it introduced the concept of uneven firing, as a result of using the 90 degree V8 cylinder angle without adjusting the crankshaft design for the V6 configuration. These engines were often referred to by mechanics as "shakers," due to the tendency of the engine to bounce around at idle speed. More modern 90-degree V6 engine designs avoid these vibration problems by using crankshafts with offset split crankpins to make the firing intervals even, and often add balancing shafts to eliminate the other vibration problems. An example is the 90-degree Mercedes-Benz V6 which, although designed to be built on the same assembly lines as the V8, uses split crankpins, a counter-rotating balancing shaft, and careful acoustic design to make it as smooth and quiet as the inline-6 it replaced.
