Balancing Engine
Thread Starter
Safety Car
Joined: Mar 2009
Posts: 3,558
Likes: 4
From: Oakland California
Balancing Engine
I have always wanted to ask this question, but wasn't sure if it's a dumb question or not. When are you suppose to balance a rotating assembly? Are you suppose to do it every time you rebuild an engine? I know you're suppose to balance any new rotating assembly.
Team Owner
Joined: Apr 1999
Posts: 21,953
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From: Reno Nevada
2024 C3 of the Year Finalist- Modified
let's say you buy a so called balanced rotating assembly....... It might be close.
Let's say you have a shop take all your parts and balance it???? Is the balance job within 5 grams or .5 grams. Did the shop ask you if you had a balanced flywheel, flex plate or damper?
I was building a blown up race motor last week. All the titanium rods were within 1/100th of a gram. The Cosworth custom pistons were 1.9 grams apart after we machined the domes down to 13.8 compression. and the piston pins were .3 grams apart. The crank was balanced to 12,800 rpm We matched the pins and pistons to have to machine the least amount from the piston skirts.
You would not waste that time and money on some American V-8
OH, The car in avitar blew it's motor and turned into an oil fire ball going around the track.
As rpm go up the closer to perfect the better
Last edited by gkull; Feb 3, 2010 at 01:56 AM.
Drifting
Joined: Aug 2004
Posts: 1,883
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From: Lubbock TX
Factory motors are balanced...kinda. If they get each piston/rod assembly to within about a pound of each other, it's good 
There are different ways to balance with over and under balanced, etc. Balancing is part science, part art and part voodoo. Everyone has to decide how much of the rod is rotating and how much reciprocating, how much oil to allow, etc.
If you're replacing major parts of the rotating assembly and this is an engine you plan to beat on, have it balanced. If it's just a daily driver, it's optional. I can tell you that whatever the use, your bearings will appreciate a good balance. I have all of mine done, even the replacement engine in my Tahoe. It's smooooth, you'd think the accelerator pedal was hooked to an electric motor.
There are different ways to balance with over and under balanced, etc. Balancing is part science, part art and part voodoo. Everyone has to decide how much of the rod is rotating and how much reciprocating, how much oil to allow, etc.
If you're replacing major parts of the rotating assembly and this is an engine you plan to beat on, have it balanced. If it's just a daily driver, it's optional. I can tell you that whatever the use, your bearings will appreciate a good balance. I have all of mine done, even the replacement engine in my Tahoe. It's smooooth, you'd think the accelerator pedal was hooked to an electric motor.
Team Owner
Joined: Jan 2006
Posts: 37,637
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From: Crossville TN
I think the auto factories do a much better job than "one pound per piston". The engines that require it (most modern engines do, because they can turn 7000 or 8000 rpm) had some parts balanced for rotation and others (like pistons and rods) balanced by weight to minimize vibration at higher rpm. The need for your engine to be balanced depends on rpm...and with higher rpm, better balancing is required...and that costs more money. Everything that rotates with the crank (inculding the torque converter, if you have an automatic) needs proper balancing.
Burning Brakes
Joined: Feb 2009
Posts: 780
Likes: 1
From: Shelby NC
I think the auto factories do a much better job than "one pound per piston". The engines that require it (most modern engines do, because they can turn 7000 or 8000 rpm) had some parts balanced for rotation and others (like pistons and rods) balanced by weight to minimize vibration at higher rpm. The need for your engine to be balanced depends on rpm...and with higher rpm, better balancing is required...and that costs more money. Everything that rotates with the crank (inculding the torque converter, if you have an automatic) needs proper balancing.
My first experience was on a new 67 GTX 440 and one piston was 80 grams heavier than any other piston in it--glad I did it!
Thread Starter
Safety Car
Joined: Mar 2009
Posts: 3,558
Likes: 4
From: Oakland California
Oh, so if I just add some aftermarket rod bolts, there's no need for re-balancing anything? I thought that you needed to balance the rotating assembly even if you did something as small as replacing some bolts. This is good info
.
.
Le Mans Master
Joined: Dec 2006
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From: Oquirrh Mountains
St. Jude Donor '09
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