If this post shows up in bold it's cause I cant turn it off/my apoligies. Ive dynamically balanced several thousand aircraft engines as part of my business so I can speak to the general principles of it. An engine that is dynamically balanced is more confortable, exhibits less wear on components and is actually slightly more powerful and fuel efficient. This is because power that goes into vibration is wasted energy that does not contribute to spinning the crank. An engine that is balanced has it's center of rotation and power exactly in the middle of the crank. If it's not balanced it is slightly outside the middle of the crank where it is not doing it's best work. My customers usually see 3-4 mph increase in speed all other things being equal. Pardon if it sounds like a sales pitch but engines love to be balanced. It's a good thing to do.

 

Balance it, even stock ones. Balancing also diminishes the harmonics that are inherent in engines. These can cause problems at any rpm. For a stock balance job, the price is around 120.00 to 150.00. They should match all the pisons, rods seprately. Then weigh the rings and bearingings. It's best if they balance the components internally, then add and zero the balancer and flywheel / clutch or flex plate. This way if either of those items need to be changed, you can have them just zero them and all will match. I know you can possibly have the old ones matched, but that's more work and sometimes you cannot spin the parts of the old ones.

 

Ford years ago sent out alot of FE shakers or what they called
"within spec" but you can feel it.
Even after you've spent hard earned money on a balance job you can end up with a shaker if you replaced any one component like a pressure plate, disc, torque converter, flexplate or damper even if the part is brand new and "factory balanced". Get it checked before you install.
I had a converter that was custom built and balanced, it was a shaker, installed another custom one from a different manufacture and had a turbine smooth motor. The battle was getting my money back as the first rebuilder refused to check his balance work after it was installed. Question I asked the judge was how could I of known it was out of balance without installing plus I had all my motor build and balance records in court. Yes I won as the second shop checked the shaker converter and his findings on paper also helped my case.
The counterweights on a crank are not 100% of the rod and piston's combined reciprocating and rotating mass. Some motors have a balance factor as low as 52%.
With pistons all the same weight, rods the same weight and they have the same balance ratio big end to small end then you'll have a smooth motor.
Only with equal recipical and rotational weights will you have a smooth motor.
A little harder to do on a 30 year old FE motor if your scratch building with rods from many different motors and rebuilds. Bottom line do your homework.
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The OEM's have learned to match build their engines with matched sets of components. They have refined their formulas and specs over the years to assure decently running engines but you can still get a lemon or a running beast. The big problems I see with aftermarket reman engines from a shop is that they build them with mismatched rods out of the big pile in the corner bin. They make absolutely no attempt to balance the rotating assy and as a result their engines are mediocre at best and most are far worse. Many people will try to rebuild one of those engines starting with mismatched components which is a waste.

I balance ANY engine I build, it is money well spent.

 

I had a 390 core supplied by a friend that had three rods factory stamped number 5. I'm confused, which one to use for cylinder number 5?
Want to see a mess, look inside a GM Target motor, it can have different oversize bores in the same motor, now that's cool!
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Are you sure the 390 hadn't been rebuilt b4? That is the kind of stuff remanufactured engines end up with. I have seen different bore sizes also but usually just .001-.003" or something like that. I have never considered what that did to the compression ratio but then if they matched the oversize pistons with shorter rods... -Still sounds like a mess but computers could solve that problem on the assy line.

 

A friend with a few old musclecars swears by balancing every engine that comes apart. He said he got bit once by not having it done, and he will never let it happen again.

 

Unless it's just a ring and bearing freshing up job balance it.
I like it were people have $8K in a motor then use old worn rockers with springs instead of solid spacers to keep the rockers against the stands.
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