Editing V6 engine (section)

==== 90 degrees ====
[[File:ZS KV6.jpg|thumb|1996-2005 [[Rover KV6 engine|Rover KV6]] 90-degree engine<ref>{{cite web |title=1998 Rover 75 2.0 KV6 specifications, fuel economy, emissions, dimensions 79673 |url=https://www.carfolio.com/specifications/models/car/?car=79673 |website=www.carfolio.com |access-date=21 December 2019}}</ref>]]

Many manufacturers, particularly American ones, built V6 engines with a V-angle of 90 degrees based on their existing 90-degree V8 engines. Such configurations were easy to design by removing two cylinders and replacing the V8 engine's four-throw crankshaft with a three-throw crankshaft. This reduced design costs, allowed the new V6 to share components with the V8 engine, and sometimes allowed manufacturers to build the V6 and V8 engines on the same production line.

The downsides of a 90 degree design are a wider engine which is more vibration-prone than a 60 degree V6. The initial 90 degree V6 engines (such as the [[Buick V6 engine#Fireball V6|Buick Fireball V6]] engine) had three shared crankpins arranged at 120 degrees from each other, due to their origins from the V8 engines. This resulted in an uneven firing order, with half of the cylinders using a firing interval of 90 degrees and other half using an interval of 150 degrees. The uneven firing intervals resulted in rough-running engines with "unpleasant" vibrations at low engine speeds.

Several modern 90 degree V6 engines reduce the vibrations using split crankpins offset by 30 degrees between piston pairs, which creates an even firing interval of 120 degrees for all cylinders.<ref name="caranddriver.com"/> For example, the 1977 [[Buick V6 engine#231|Buick 231]] "even-fire" V6 engine was an upgraded version of the Buick Fireball engine with a split-pin crankshaft to reduce vibration by achieving an even firing order.<ref name="Nunney"/>{{refpage|page=16}}<ref>{{cite web |title=The Strange Tale of the Buick Special, Buick-Rover V8, and 3800 V6 |url=https://ateupwithmotor.com/model-histories/buick-special-skylark-rover-v8-3800-v6-history/ |website=www.ateupwithmotor.com |access-date=24 December 2019 |date=9 April 2008}}</ref> Such a 'split' crankpin is weaker than a straight one, but modern [[metallurgy|metallurgical techniques]] can produce a crankshaft that is adequately strong.

A balance shaft and/or crankshaft counterweights can be used to reduce vibrations in 90 degree V6 engines.<ref name="Nunney">{{cite book|last=Nunney|first=M J|title=Light and Heavy Vehicle Technology|edition=4th|publisher= Butterworth-Heinemann|year=2007|isbn=978-0-7506-8037-0}}</ref>{{refpage|pages=14–44}}