Engine Balancing. Whats acceptable?

[Nasty Wendy]

I've seen 36.6% and 50% balance discussions. What if you are somewhere between those numbers? Do those numbers represent an acceptable range? What should be done with a 46.5% balanced engine? Is it OK?

 

[turbodave231]

For a street engine running stock or HR poly mounts, 36.6% is generally accepted as the best balance factor. A race only engine with solid mounts, 50% will work, but some engine builder use 36.6% on race engines too.

It usually takes a considerable amount of heavy metal to balance a crank at 50%. Heavy metal and the labor to install it = $$expensive.

I have built engines with 36.6 and 50% and I cannot honestly tell the difference. I have not heard of anybody using balance factors other than the 36.6 or 50% on the Buick V6

 

[Nasty Wendy]

Thanks for the response Dave. My question is more about the numbers between 36 and 50. Would 46.5% work or would the engine want to fly apart?

 

[Nick Micale]

I would agree with Dave's comment that it is doubtful you could tell the difference.

From what little I know about these numbers, the closer you get to the 50% mark, the higher the engine RPM it would be running.

With the lower value, that would be for street engines in the 6000 RPM range. The higher values would be in the 9000 RPM range.

This is what I understood when I was working on some cars/engines that were done by Ken D. We were running small displacement turbo Buick V-6's to 9000+ RPM balanced closer to the 46% number!

Most of our race engines are close to the lower number, and my engine close to 36.6% number has been North of 8000 RPM more times than I like, with no affects I have been able to determine.

 

[charlief1]

The 50% mark is more for internal balanced engines that are running high RPMs. If you're not running over 6-6500 RPM go with the 36.6% method.

 

[Nasty Wendy]

Thanks for the info guys. This engine is already at 46.5. In order to change it would require a complete tear down and obviously rebalancing. If it'll work at 46.5 with no self destructive adversities then thats where its staying. I just wanted to see if there was anyone out there that had any info on why it would be a horrible idea to run it balanced at 46.5%. Thanks Dave, Nick, and Charlie.

 

[turbodave231]

I now understand your question better! I don't have any good feedback to give you on that specific balance factor. Being that close to 50%, You will probably be just fine. The engine will never know the difference.

I you really wanted to take it to 50%, you would need to re-spin the crank. You will need to know the rotating and reciprocating weights. That should all be on the balance card. You don't have to tear the whole engine down to get the crank out.

The best info I have found on balancing these engines comes out of the Buick Motorsports Manual. The book has a chapter on balancing and it provides some insight into what the engineers were doing to control/shift the natural imbalance that these engines exhibit.

I can't tell the difference between the two balance factors they describe.

I'm interested to know how it works out.

Dave

 

[Just a Six??]

So it doesn't have to be Exactly 36.6% or 50%???
It will run at other numbers or percentages as long as its all balanced properly?
Good info to know either way.

 

[charlief1]

The prefered balancing percentage is 36.6% of reciprocating weight but you can do it at 50%. The difference is in the type of vibration the engine will experience. If the engine was a 120 degree bank angle, an inline, or a 180 degree bank angle it wouldn't be an issue, but since it's a 90 degree bank engine there's some draw back to it. There will always some sort of imbalance due to the bank angles being set at 90 degrees.

 

[ijames]

A 90 degree V6 without a balance shaft always has some residual imbalance. If you balance the crank at 36% then all of this imbalance is in the horizontal plane so the motor shakes side-to-side but not up-and-down. Buick chose that, and then used motor mounts shaped to absorb motion in that direction. As you move above 36% the motor will shake less in the horizontal plane and more in the vertical plane. At 50% the total imbalance is at a minimum and the motor is shaking about the same amount side-to-side and up-and-down. From memory this total imbalance is about 30% less than the total at 36%, so it's not a real big change. From what I've read (not from building lots of high hp motors ) this doesn't really change the power output and for drag racing won't affect the durability, it just affects what the motor mounts have to deal with. For endurance racing I think there may be a small benefit to 50%.

 

[SloGN]

For a street engine running stock or HR poly mounts, 36.6% is generally accepted as the best balance factor. A race only engine with solid mounts, 50% will work, but some engine builder use 36.6% on race engines too.

It usually takes a considerable amount of heavy metal to balance a crank at 50%. Heavy metal and the labor to install it = $$expensive.

I have built engines with 36.6 and 50% and I cannot honestly tell the difference. I have not heard of anybody using balance factors other than the 36.6 or 50% on the Buick V6

X2


My engine is balanced @ 36.6

My fianl balance @ 6k netted me 14lbs on the rear main and 18 lbs on the front main.

the motor run pretty dang smooth with poly mounts.


if you are using a stock balancer and flywheel you will only be able to balance @ 36.6 due it being externally balanced. unless those items are nuetral balanced.

 

[750H.P.V6]

I've built a few that balance out around 40-42% and they work fine. Like Carl stated it's just a trade off between horizontal and vertical imbalance. I'd run it.

Neal

 

[Nasty Wendy]

Thanks again guys. With the knowledge of the folks who've chimed in here I feel much better about running it.