What Is This Shaft In 3800 Series Chevy Engine - Page 1
Forum section: electronics / general

author: bostonman
author_rank: Super Contributor
timestamp: March 24, 2025, 02:27:15 am
content: Years ago I replaced the intake manifold gasket in a Chevy with a 3800 series engine (actually have done this a few times on different cars).The shaft the red arrow in the attached picture is pointing to has baffled me. I thought it was a counterbalance, but it's too small I think to generate enough rotational force. It can't be something to slosh oil because the fins are only at one end.Does anyone know what this shaft is and/or what it does?

author: Poroit
author_rank: Frequent Contributor
timestamp: March 24, 2025, 04:35:28 am
content: It certainly is a strange shape.Is it hollow?

author: Addicted2AnalogTek
author_rank: Frequent Contributor
timestamp: March 24, 2025, 06:13:07 am
content: Detonation counter balance, aka balance shaft. Its purpose is to dampen the vibrations caused by the detonations, reducing felt engine vibrations and prolonging the life of some engine parts.

author: BradC
author_rank: Super Contributor
timestamp: March 24, 2025, 10:05:23 am
content: Because this is a technical forum and we pride ourselves on correct terminology :Yes it's a balance shaft. Most Buick derived V6's came from a V8 with 2 cylinders chopped off and as a result were a 90 degree V. A 90 degree V6 is terribly balanced, so as rough as guts. To present a more "refined" drive they fitted a balance shaft. It's not so much about the combustion, but the intrinsic balance of the rotating assembly. The FWD adaptation of the 3800 used a new 60 degree block and was a lot more refined.To be accurate, it's not there to balance "Detonation". Detonation is a phenomena whereby the compressed charge ignites at an earlier time than desired and creates a shockwave in the cylinder. Normally sounds like someone rattling a tin of ball bearings and has the ability to destroy engine internals quickly.

author: Addicted2AnalogTek
author_rank: Frequent Contributor
timestamp: March 24, 2025, 02:55:59 pm
content: Quote from: BradC on March 24, 2025, 10:05:23 amIf you wanted to be condescending towards someone who knows enough, but not the technical details, Cudos. You could have simply chosen to offer a more detailed explanation. Look up " Pre-detonation ", since you've confused that with " detonation ". Have yourself a fantastic day.

author: bdunham7
author_rank: Super Contributor
timestamp: March 24, 2025, 03:46:50 pm
content: Quote from: BradC on March 24, 2025, 10:05:23 amThe original Buick V6 models (in the 60's) were rough because they were "oddfire", meaning the compression and combustion events didn't happen at 120 degree intervals, they happened at 90 degrees but with two missing from the sequence. This was because they retained the single rod journal for opposing cylinders from the V8 that they chopped off. The first refinement was to use split journal crankshafts so that the compression and combustion occurred at even 120 degree intervals. This results in a pretty smooth engine with only slight imbalances, not as good as straight or flat six, but better than a four cylinder or many V8s.The 3800s (starting in the 80's) are all 90 degree blocks, the only big thing that changed is that they used the small bell housing pattern that was the same as other V6 models that were 60 degree . They added that balance shaft to smooth out some of the remaining harmonics as the new blocks became lighter and eventually smaller. Since the imbalances are small, the balance shaft doesn't need to be very big to damp out the small torsional mode vibration.

author: langwadt
author_rank: Super Contributor
timestamp: March 24, 2025, 04:52:18 pm
content: Quote from: bdunham7 on March 24, 2025, 03:46:50 pmJaguar has a 90 degree V6 made using a V8 block with two cylinders blanked off, apparently they didn't want to do another crash test with a different block

author: bdunham7
author_rank: Super Contributor
timestamp: March 24, 2025, 05:22:39 pm
content: Quote from: langwadt on March 24, 2025, 04:52:18 pmI've never seen one torn down but they must have an interesting crankshaft solution.

author: langwadt
author_rank: Super Contributor
timestamp: March 24, 2025, 06:53:00 pm
content: Quote from: bdunham7 on March 24, 2025, 05:22:39 pmhttps://youtu.be/yi_IXsBSO1Q?si=Svd5lxFBbJFCUSAs&t=2433

author: bostonman
author_rank: Super Contributor
timestamp: March 24, 2025, 10:22:08 pm
content: QuoteI sensed it was either for balancing or splashing oil, but I'm amazed such a small shaft adds balance.What is the reason for its odd shape and does it spin opposite direction of the crankshaft?

author: bdunham7
author_rank: Super Contributor
timestamp: March 24, 2025, 11:37:57 pm
content: Quote from: bostonman on March 24, 2025, 10:22:08 pmI've never worked out the exact mechanics on the 3800 balance but it rotates backwards at 1x crankshaft speed and counteracts a specific rocking motion with matching but out-of-phase offset weights on either end. Most balance shaft systems work by converting a torsional vibration mode to a vertical one, the weight of the motor and the design of the motor mounts are much more easily able to deal with the latter than the former.

author: BradC
author_rank: Super Contributor
timestamp: March 25, 2025, 09:37:20 am
content: Quote from: Addicted2AnalogTek on March 24, 2025, 02:55:59 pmYeah, sorry. Must have been my "let's be an arse day". Apologies.