Engine Balancing

Trev · Post by **Trev** » Wed Jun 15, 2005 9:33 am

Have a balanced assembly that needs a piston change

The original piston, pin and rings weigh 756 gm

The new piston, pin and rings weigh 747 gm

10 gms lighter

What would you guys do

Trev

bill jones · Post by **bill jones** » Wed Jun 15, 2005 10:54 am

-I'd run it

speedtalk · Post by **speedtalk** » Wed Jun 15, 2005 11:23 am

Same, run it.

Post by **SchmidtMotorWorks** » Wed Jun 15, 2005 11:56 am

Run it, once when I used to do balancing I mixed up some balance sheets and balanced a super stock crank by a lot more than your difference and the customer never noticed. I found it later when another parts change was made.

CNC BLOCKS · Post by **CNC BLOCKS** » Wed Jun 15, 2005 12:02 pm

I would run it as that would put at an over balance which is fine as long as its not under balaced you will be fine.
Good luck

Walline · Post by **Walline** » Wed Jun 15, 2005 12:15 pm

Not to side track the post, but can someone tell me where I can get some good info on balancing engines. I have heard contradicting info, and I am not sure the local shop that does balancing is doing it correct. I would love to learn more about the subject.

Post by **SchmidtMotorWorks** » Wed Jun 15, 2005 3:14 pm

I have looked but never found any books that explain the goals of crankshaft balancing for the balancing technician.

The most important issue to understand about crankshaft balancing for common American V8 engines is that there is not enough room in the block to make the counterweights large enough for any of the counterweights except the first and last counterweights.

If you look the the end counterweight from an end view (looking along the axis of the snout), in most cranks you will notice that the counterweight is biased to the direction oppsite of the 3-4 rod pin. The reason for this is that since there isn't enough room to make the counterweights surrounding the second main, the first counterweight is enlarged and offset to make up for it.

This means that a crankshaft already has a lot of built-in compromises for how it is balanced. Basicly most cranks are balanced to minimise vibration not bearing loads . Much of the shape is designed around what is easy (inexpensive) to forge.

If you are thinking that you want to reduce bearing loads on the 2nd and 4th mains, you can accomplish this by modifying the end cws to make them symetrical (across the plane of the first rod pin) then adding weight to the 2nd & 3rd cws with heavy metal. Be careful there may not be enough room in the 2nd and 3rd to add enough heavy metal to compensate for a completly symetrical end cw.

Another thing to keep in mind, most balancing shops won't have the long drill and reamer tools and press fixtures to add weight to the 2nd & 3rd cws. Not that these tools are big deals, it's just that most people don't have them.

Trev · Post by **Trev** » Wed Jun 15, 2005 8:40 pm

Thanks heaps guys
Trev

CNC BLOCKS · Post by **CNC BLOCKS** » Wed Jun 15, 2005 9:58 pm

Walline wrote:Not to side track the post, but can someone tell me where I can get some good info on balancing engines. I have heard contradicting info, and I am not sure the local shop that does balancing is doing it correct. I would love to learn more about the subject.

Try this link

http://www.centuryperformance.com/../../balancing.asp

bill jones · Post by **bill jones** » Wed Jun 15, 2005 10:18 pm

-type in "crankshaft balance factors" in the search window or search "victorylibrary.com/mopar/crank-bal-c.htm"

bobqzzi · Post by **bobqzzi** » Wed Jun 15, 2005 10:53 pm

bill jones wrote:-type in "crankshaft balance factors" in the search window or search "victorylibrary.com/mopar/crank-bal-c.htm"

That's a good link, thanks.

I do have a question though- virtually every reference takes a 90degree V8 a typical example, but I typically work with inline 4 cylinders. Anyone care to give me a quick primer on the relationship between the weight of counterweights and the rod/piston combo?

bill jones · Post by **bill jones** » Thu Jun 16, 2005 12:21 am

-nothing scientific here but we race a lowly 2000 (122ci) pinto at Bonneville so when I built the engine I had the crank only balanced---without any bobweights.
-Then I figured up a 50% bobweight off the balanced rods and pistons etc just like what you'd run on a normal V/8 and we checked the balance again with that and there was virtually no difference, something like needed to remove about 6 grams off the crank in a couple of places---so that's what we ended up with.
-We did balance the flywheel and the clutch and the front snout parts using the crank with the bobweights as a mandrel.
-We have run this engine to 9400rpm and we think it's pretty good.
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-But we have this engine bolted solidly to the chassis, and the stainless steel fuel tank is also solidly mounted to the chassis, and we have serious issues with what we think is the power pulse frequencies agitating and aerating the fuel.
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-So I believe that the power strokes of the 4 bangers do bad things that you'd never see with an 8 cylinder engine.
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-Here's another deal:
-I have a friend who runs a VW rabbit pickup truck at Bonneville with a 2000CC (122ci) cross mounted front wheel drive water cooled engine.
-He decided that he wanted to run in a class that allowed 1000cc's or 61ci so he took the rods and pistons out of two cylinders that went up and down at the same time, ground the lobes of those two cylinders off of the camshaft, then he installed a pair of radiator hose clamps on those two empty rod journals to cap off the oil holes.
-This engine shakes super bad at idle and below about 4000rpm but at 4000rpm he says it smooths out and he run it up to 8700rpm during two consecutive runs and he established a record at 96 and something MPH.
-We had talked about it that's what I told him I'd do.
--------------------------------------------------
-Then he put the 4 cylinder engine back in and went out to run and everything broke, like the bellhousing broke, then after replacing the the bellhousing the stock clutch came undone on the next run.
-It's like that 2 banger's power pulses are so far apart that it acts like a jack hammer.
-Now for sure we had no idea what to do about the two empty rod journals but I personally do not believe that the crank balance had anything to do with the breakage of the parts.

Post by **SchmidtMotorWorks** » Thu Jun 16, 2005 12:31 am

-nothing scientific here but we race a lowly 2000 (122ci) pinto at Bonneville so when I built the engine I had the crank only balanced---without any bobweights.
-Then I figured up a 50% bobweight off the balanced rods and pistons etc just like what you'd run on a normal V/8 and we checked the balance again with that and there was virtually no difference, something like needed to remove about 6 grams off the crank in a couple of places---so that's what we ended up with.

No change in bob weight on a flat crank (up_down_down_up) should change the balance. If you found a change, it was due to inaccurate bobweights, attachment of the bob weights or unequal stroke.

Determining the correct size of CW for a flat crank can be done by cutting up the crank and determining the dimensions of the weight that balances to a given bob weight for a one cylinder. Or if you don't have a spare crank, and you do have CAD, start measuring and modeling.

bill jones · Post by **bill jones** » Thu Jun 16, 2005 1:46 am

-We tried to find why the 6 grams showed up, removed the bobweights and double checked back and forth several times, moved the bobs around and it kept showing us the 6 grams with the bobweights so I decided to drill it away.
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-One thing I have never found around here is anyone who could ever prove to me that they knew what they were doing when it came to calibrating their balancer machines.
-There's about two guys that I've had balance cranks that can give me something I sort of trust and I won't let'm touch anything but the crank.
-The horror stories about balance jobs has been here since I paid for my first balance job in the late 1960's and it hasn't gotten any better.
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-So I don't think it's anything to do with perfection, it has to do with what we can and will tolerate.

bobqzzi · Post by **bobqzzi** » Thu Jun 16, 2005 2:37 am

SchmidtMotorWorks wrote:
-nothing scientific here but we race a lowly 2000 (122ci) pinto at Bonneville so when I built the engine I had the crank only balanced---without any bobweights.
-Then I figured up a 50% bobweight off the balanced rods and pistons etc just like what you'd run on a normal V/8 and we checked the balance again with that and there was virtually no difference, something like needed to remove about 6 grams off the crank in a couple of places---so that's what we ended up with.
No change in bob weight on a flat crank (up_down_down_up) should change the balance. If you found a change, it was due to inaccurate bobweights, attachment of the bob weights or unequal stroke.

Determining the correct size of CW for a flat crank can be done by cutting up the crank and determining the dimensions of the weight that balances to a given bob weight for a one cylinder. Or if you don't have a spare crank, and you do have CAD, start measuring and modeling.

Okay, thank you. Given that I have no CAD expertise, and don't really want to cut up a crank...from a practical standpoint- if a 180 degree inline 4 is reasonably well balanced from the factory (like a watercooled VW), and the crank itself proves to be good, then is it likely to cause a balance problem using rods and pistons that are lighter than stock?