And from a Jeep board:
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 Citroėn SM, the PRV V6, Chevrolet's 4.3 L Vortec 4300 and Chrysler's 3.9 L (238 CID) Magnum V6 and 3.7 L (226 CID) 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.
Odd and even firing
Many older V6 engines were based on V8 engine designs, without altering the V angle or using a more sophisticated crankshaft to even out the firing interval. One characteristic of these engines was a notorious odd-firing behavior.
Purpose-built V6 engines use one crankpin per cylinder for an even 120° ignition pattern. In contrast, most V8 engines share a common crankpin between opposite cylinders in each bank. That is, the crankshaft has just four pins for eight cylinders, and a cylinder fires every 90° for smooth operation.
V6 engines derived from V8 engines often have three shared crankpins arranged at 120° from each other, similar to an inline 3-cylinder, with two pistons per crankpin. If the cylinder banks are arranged at 90° (as they commonly are in V8-derived V6s), this leads to a firing pattern with groups of two cylinders separated by 90° of rotation, and groups separated by 150° of rotation.
An example is the Buick 231 odd-fire, which has a firing order 1-6-5-4-3-2. As the crankshaft is rotated through the 720° required for all cylinders to fire, the following events occur on 30° boundaries:
can understand? Photos would be great as well.