Originally Posted By: meep
ok I had to google all of this. so the 3.7 fires every 120° due to the "split pin" crank. This implies there are 4 main bearings on the crank, right? and that each of the three pair of crank pins is offset between the main bearings, right? (I've never been into a 90° V6 before.) That requires a very sturdy crank design IMO!
M
V6 engines are odd beasties. You can build a 60-degree v6 just like you do a 90-degree v8- with three connecting rod journals with a rod from each bank sharing the journal. It be even-firing... but it has a horrific "rocking" imbalance. So they build both 60- and 90-degree v6 engines with the rod journals slightly offset from each other for 6 independent throws. In the 60-degree v6 case, its *solely* to get a firing order that gives better balance. In the 90degree block case, its done both to bring the firing order closer to evenness and to improve balance.
I don't think the 3.7 actually fires every 120 degrees, although it might. The 3.8 certainly does, and all inline sixes do. But many 90-degree v6 engines retain a slight odd-firingness because splaying the crank journals far enough to make them truly even firing (30 degrees of offset) would weaken the crankshaft too much. Some, like the Chevy 4.3 Vortec do splay all the way to an even-firing configuration, but that means the rod journals need to be big and heavy which has its own drawbacks. No splay at all results in firing alternately at 150 degrees between two cylinders then only 90 between the next two (kinda like two 3-cylinder engines running slightly out of phase!). A more typical compromise is to splay the pins by 18 degrees and fire at 132 and 108 degrees.
By the way, 90-degree V10 engines like the Dodge Viper engine are also semi-odd firing, which is part of why the Viper exhaust note was always so dang weird.
