Engine balancing question

69427

69427

The Artist formerly known as Turbo84
Joined
Mar 30, 2008
Messages
3,077
Reaction score
38
Location
Clinging to my guns and religion in KCMO.
Now that I've got the suspension stuff about done, I'm looking to start on the new engine. My plan is to build a new shortblock, and then take the top end and accessories off the present (running) engine. My question: I'll be taking the block and rotating assembly components into the machine shop soon. Given that this is an internally balanced big block (427), is it necessary to bring along a flywheel and harmonic damper? I've got a spare L-88 flywheel if needed, but the damper is on the current engine. It's been a few years since I built the present engine, and I can't remember what the process was last time.

Thanks,
Mike
 
Mike -
You're going to want to give the balancer all of the rotating (moving) parts, including:

Crank
Pistons
1 complete set of piston rings
Rods
1 set of rod bearings
Harmonic balancer
Flywheel (or flex plate if auto)
Pressure plate & clutch (if manual)

It doesn't matter if the engine is internal or external balance - each part has an individual spec, but a good balancer will fine-tune the combined balance after setting the individual balance. You can do it without the parts you mentioned, but performance is all about precision - the closer you can get it to "perfect" the more power you will obtain. For as inexpensive as they are, just buy a new SFI-approved balancer and have that balanced with the other components.

We demonstrate the "get it close to perfect" concept on the "Tuning for Beer Tours" all the time... "Close enough" ain't close enough...

Lars
 
Last edited:
I agree 100% with Lars - the closer to perfect the more power and reliability you will have.

If you have full floating pins you need a set of the locks for one piston you are going to use too.
 
I appreciate the info, Lars. I forgot about the rod bearings. I'll have to put them in the pile of parts. I've got a low mileage Fluidamper on the current engine, so I hate to spring for another piece. I'll have to figure out what I'm going to do there. The spare flywheel is not a problem, as I suppose I could sell the current one along with the old shortblock.
 
I used a fluid damper too, my machinist hated it because he had to do more work to the crank since he could touch the damper.
 
Why can't you touch a fluidampr? You can simply drill them to balance w/ the crank. I can take pics of one if needed to prove it. Just don't drill through the outer shell.
 
Twin is correct - FluidDampers can be balanced with no problem.
Lars
 
Maybe my machinist didn't know what he was talking about or didn't want to screw it up. We do have a language barrier here in south texas:wink:
 
lars said:
Twin is correct - FluidDampers can be balanced with no problem.
Lars
Click to expand...

If you try balancing with fluid dampner they do not repeat and the outter ring has to be taken off on exteranl balanced engines.

We do a lot of balancing and have not seen a fluid one repeat yet!!
 
Fluidamper tells you not to use their balancers when you balance the assembly because a balancing machine will not center the inertia ring the way a running engine will.

Everything that I have ever been told or read about balancing said not to drill the balancer or the flywheel. As long as they are in spec, you balance the internal parts. That way if you need to replace one or the other you dont have to rebalance everything.
 
CNC BLOCKS said:
lars said:
Twin is correct - FluidDampers can be balanced with no problem.
Lars
Click to expand...

If you try balancing with fluid dampner they do not repeat and the outter ring has to be taken off on exteranl balanced engines.

We do a lot of balancing and have not seen a fluid one repeat yet!!
Click to expand...

Clue me in on what you guys mean by "repeat".

Thanks

Jake
 
Jake, repeat is just that. When they balance an engine, they spin the parts and find the heavy spot, corrrect that to more closely match the light spot, then run it again and again until all the heavy & light spots are as close to each other as possible. With a viscous fluid (Fluidamper) you spin it once and find a heavy spot, stop it and spin it again and the heavy spot is in a different place.

Think of a common room fan- add a hunk of clay to one of the blades and it shakes, put a hunk of clay exactly opposite to correct the weight of the first piece, but before you run it again, move the first piece 2 blades to the right.
Kind of a rough description, but I think it demonstrates repeat.
 
I totally unnerstand a Fluid damper as a harmonica balancer, but I presume I am correct in that it can never act as part of the engine balance....?? unless they make provisions somehow for some external weight with one....I have never used one, only stock ones....

:fishing:
 
You are confusing 2 things, the damper is not a balancer, it dampens crank harmonics caused by crank throw acceleration when a cylinder fires. This actually bends the crank and accelerates the throw a good 1-2 degrees. Coupled with the cranks natural frequency (dep. on crank type, cast, forged, billet, material, journal overlap, counterweight thickness blablabla) this can have dire results if left unattended.

This is something completely different from spin balancing a crank w/ bob weights/damper/flywheel or flexplate combo.

As for ext. weights, all ext. balanced engines have an external weight on the damper and on the flywheel/flexplate, you can even buy bolt on ones for the rear of the crank.

The reason for ext. balanced engines is because the stroke is so large that the counterweights on the crank don't realistically fit within the crank case. You could add heavy metal slugs to the counterweights to make them heavier but that costs $$$, for a factory a cheaper option is a simple external balance.
 
Twin_Turbo said:
If you want more details about crank harmonics & why you need a damper:

http://www.vettemod.com/forum/showthread.php?p=44988
Click to expand...

Jesus, think I stick to electronics.....:hissyfit: looking at those vibration graphs, I just shake my head and say, 'they gotta be joking'....crazy shit man, never gave it much thought....

still wonder why there is so much of a difference in the fit of the balancers on a given crank, like old Pontiac engine just slipped on and off, no sweat, where SBC if force fit on there, wonder why the force fit, ?? you have any clues??

:huh2:
 
Twin_Turbo said:
Why can't you touch a fluidampr? You can simply drill them to balance w/ the crank. I can take pics of one if needed to prove it. Just don't drill through the outer shell.
Click to expand...


I'd fire anyone that did this.

Sometimes you have to replace the balancer.........balance the crank only.
 
TimAT said:
Jake, repeat is just that. When they balance an engine, they spin the parts and find the heavy spot, corrrect that to more closely match the light spot, then run it again and again until all the heavy & light spots are as close to each other as possible. With a viscous fluid (Fluidamper) you spin it once and find a heavy spot, stop it and spin it again and the heavy spot is in a different place.

Think of a common room fan- add a hunk of clay to one of the blades and it shakes, put a hunk of clay exactly opposite to correct the weight of the first piece, but before you run it again, move the first piece 2 blades to the right.
Kind of a rough description, but I think it demonstrates repeat.
Click to expand...

Thanks for that explanation; gottcha!

Jake

West Point ROCKS!
 
You must log in or register to reply here.

Latest posts

Back
Top