Odd-Fire V6's...How bad are they really?

Technically a 90 degree odd-fire V6 could have better mechanical balance of the rotating assembly than either a 90 degree or 60 degree even-fire V6. Whether or not the production engines were balanced as such, I have absolutely no idea. Thus the vibration in an odd-fire engine should come primarily from the cylinder combustion events acting torsionally about the crankshaft axis. The effect of this should be most noticeable at idle, and apparently smooth out with additional rpm, as opposed to even-fire V6's (especially 90 degree w/o balance shafts) which should start smoother and increase in harshness with RPM. There is no shortage of internet lore and hearsay regarding such engines, but I'm wondering if anybody here actually has first hand experience with these engines to be able to support or refute these claims?

I've got experience with both odd and even fired Buick V-6's. I owned examples of both at the same time. The odd-fire engine ran rougher at every RPM. That's my story and I'm sticking to it.

bravenrace wrote: I've got experience with both odd and even fired Buick V-6's. I owned examples of both at the same time. The odd-fire engine ran rougher at every RPM. That's my story and I'm sticking to it.

I agree. I had a 225 in a Jeep CJ-5. To try and smooth it out they tuned it a bit more for torque and included a heavy flywheel.

It always ran well, but sounded like it was never running right.

I just drove a nearly pristine time capsule grade 76 Skyhawk with the odd fire 231. Real funny sounding idle, to a point I thought it needed the carb gone through till the HEI distribution cap with two dummy terminals was pointed out, but otherwise it drove like a slower 3800 powered car, nice linear acceleration. Didn't seem to vibrate much or anything like that, even when I wrung it out shifting manually, smooth cruiser ride. Give this was through a non lockup TH350, might be a bit rougher with the stick.

Then again I'm used to that classic 90s Toyota idle that shakes the whole car.

I was constantly chasing timing and fueling "problems" that turned out to not be problems at all.

In reply to Fueled by Caffeine:

Those old farts at Buick probably did the odd fire engine as a practical joke just to trick people into looking into everything like you did....

They're probably rolling in their graves/nursing home beds laughing about it as we speak.

The PRVolvo V6 shook like a bastard at idle in its odd fire B28 guise, and smoothed out significantly with the B280 redesign. They also ran oil up to the cams, so they would survive for more than 2 years, and put EFI on it. They forgot about fixing the oil leaks, though. The big old GM compressor bolted to the valve cover was still a bit of an issue, too.

Every 90 degree Chevy V6 (200/229/262/4.3/?) was odd fire. They weren't REALLY ODD fire like the pre-77 Buicks but they weren't even fire either. I'd like to say the firing intervals had a 108/132 degree pattern but could be wrong.

Could someone define the difference odd and even fire V6s?

I've always found that confusing. It seems to me that if you have a 90 V6, there should be 3 con-rod journals spaced 120 apart, each with 2 rods. That should have good natural balance, but very few V6 cranks look like that.

There used to be a great animated gif on earlycj5.com to explain the difference. can't find it now.

HappyAndy wrote: Could someone define the difference odd and even fire V6s? I've always found that confusing. It seems to me that if you have a 90* V6, there should be 3 con-rod journals spaced 120* apart, each with 2 rods. That should have good natural balance, but very few V6 cranks look like that.

The 'natural' angle of a V engine is 720 (number of crankshaft rotation degrees in a 4-cycle engine) divided by the number of cylinders. For a V6, this would be 120 degrees between cylinder banks. This leads to an engine almost as wide as a horizontally opposed engine. By going with a narrower V angle, the cylinders fire at alternating shorter/longer intervals. An "odd fire" 90 degree V6 with shared crank pins will fire at 90-150-90-150-90-150 intervals. In order to make it an even firing engine, you have to split the crank pins to force the pistons to align with the cylinders at the correct time. To figure this out, you subtract the actual V angle from the 'natural' V angle, and space the split crank pins that far apart. Thus on a 90 degree V6 the crank pins are split by 30 degrees, and on a 60 degree V6 the crank pins are split by 60 degrees, to get even 120 degree cylinder firing events.

In reply to HappyAndy:

The odd fire V6 engines were literally a V8 with the #3 and #7 cylinders lopped out of the block, cam, and crank. Right down to using a V8 distributor with a cap that has two plastic dummy terminals where the #4 and #7 wires would have gone. So they run a like a V8 with two dead cylinders.

Knurled wrote: Every 90 degree Chevy V6 (200/229/262/4.3/?) was odd fire. They weren't REALLY ODD fire like the pre-77 Buicks but they weren't even fire either. I'd like to say the firing intervals had a 108/132 degree pattern but could be wrong.

Pretty sure the 4.3 went even fire at some point.

http://archive.today/mS6ch

found it!

oddfire 4.3s have way stronger cranks, theyre the hot setup (what they ran in the bush north series IIRC). I broke a crank in my evenfire 4.3 swapped toyota 4x4.

Kenny_McCormic wrote: Pretty sure the 4.3 went even fire at some point.

I do know that, up until the end, the 4.3 Vortecs had a "one way only" distributor hold down so you could absolutely, positively not clock it wrong. If they did go to even fire, I'd just chalk that up to corporate inertia.

I had a talk with someone who opined that the offset crankpins in the Buick even-fire engines were scary. To me they aren't as scary as a 60 degree V6s crank. They don't even remotely share throws, they look like a four main inline six's crank. And because of the way the crank has to be laid out, 60 degree sixes tend to have much greater bank offset than you can get with a 90. Need the room for the crank since the pins can't be right up against each other.

Kenny_McCormic wrote: In reply to HappyAndy: The odd fire V6 engines were literally a V8 with the #3 and #7 cylinders lopped out of the block, cam, and crank. Right down to using a V8 distributor with a cap that has two plastic dummy terminals where the #4 and #7 wires would have gone. So they run a like a V8 with two dead cylinders.

Yes and no. Just so that there is no confusion, the crank throws were changed to 120 degrees and the cam lobes were necessarily positioned accordingly as well. So it also did not actually have the firing intervals of a V8 minus two cylinders. I'm not sure exactly how they got the ignition timing to work with the V8 minus two distributor cap though.

Knurled wrote:

Kenny_McCormic wrote: Pretty sure the 4.3 went even fire at some point.

I do know that, up until the end, the 4.3 Vortecs had a "one way only" distributor hold down so you could absolutely, positively not clock it wrong. If they did go to even fire, I'd just chalk that up to corporate inertia. I had a talk with someone who opined that the offset crankpins in the Buick even-fire engines were scary. To me they aren't as scary as a 60 degree V6s crank. They don't even remotely share throws, they look like a four main inline six's crank. And because of the way the crank has to be laid out, 60 degree sixes tend to have much greater bank offset than you can get with a 90. Need the room for the crank since the pins can't be right up against each other.

In 1980 I took a '79 Olds Starfire (even fire 231cu.in.) and built it with a blower to make close to 500hp. It was just a street engine, but I never broke anything on that engine during the time I owned it.

Yeah, and I have a 1100hp Grand National a couple yards behind me. The split cranks work just fine.