RustRocket

While navigating this forum, I've come across the mentioning of odd-fire and even-fire several times. What does this mean? I have an 82 Cutlass Calais with a 231 V6, and I was wondering how to tell if it is an Odd-Fire or Even-Fire, as well as the advantages/disadvantages and specs of each. Any input is great, thanks

techg8

Oddfire had a three throw crank, each throw separated by 120 degrees. This resulted in the pistons not getting to TDC at regular intervals, and power strokes therefore were also uneven, resulting in vibration

Evenfire was an improvement on the oddfire design. The crank was changed to "split" the throws, essentially making a 6 throw crank by splitting each original crank throw into two separated by 30 degrees. This gets the pistons to TDC at regular intervals, and thus powerstrokes follow suit and vibrations are eliminated.

Oddfire is the older design. Other than the crank there werent many differences. In 1979 heads were redesigned to a better flowing port design.

My opinion, Oddfire feels like driving a tractor.
I prefer evenfire.

RustRocket

Do you know of a way to tell the difference, or is there a specific model year in which they switched?

joe_padavano

OK, let's be a little more specific. The odd-fire/even-fire distinction refers to motors where the crank throw separation angle does not match the bank angle between the cylinders. Buick started this with their original 198 cu in V6 back in 1961. For cost reasons, Buick wanted to be able to machine the V6 engines on the same line as their 215 V8 motors. In fact, the V6 and V8 have a lot of dimensional and architectural similarities.

V8 motors with a 90 degree bank angle and crank throws 90 degrees apart have firing pulses evenly spaced every 90 degrees of crank angle. Similarly, V6 motors with a 60 degree bank angle and 120 degree crank throws have a firing pulse every 120 degrees of crank angle. Unfortunately, when you have a V6 with 90 degree cylinder bank angle and 120 degree crank throw angle, the firing pulses occur at uneven intervals (90 deg, 150 deg, 90 deg, etc). These odd-fire pulses cause vibration.

Buick redesigned the crank to split the crank throws such that each rod was at a different crank angle. The result was that the firing pulses evened out, smoothing the engine considerably but increasing production cost. Since firing pulses are now evenly-spaced, this is the even-fire motor. Buick made this change in the 1977 model year, so your car certainly has the even fire motor. One easy way to tell is the distributor cap.

Here is the odd-fire cap:



Here is the even-fire cap:

RustRocket

Thanks for the information and Cap pictures, it's an evenfire...Now i just have to figure out how the get the plugs out, as the headers are right where they don't need to be...