Will I have to balence the rotating assembly on forged rods and pistons?

[1990Z71Swede]

You don't HAVE to, but you probably should.

A 90 degree V8 is balanced somewhat similar to a single cylinder engine, so in the following example, just think of it as just a single cylinder engine

The idea of balancing is to counteract the g force of the rod and piston changing direction at TDC and BDC to reduce bearing load and engine vibration.

Now if you try to do this by letting the counterweight have the same mass as the piston and rod, it will be perfectly balanced at TDC and BDC but terribly out of balance mid stroke. Thats why Single cylinder engines and 90 degree V8's use a "balance factor" of normally 50%. That means only 50% of the reciprocating weight is balanced out. Making it equally out of balance at mid stroke & top and bottom dead center.

Changing the rod and piston combo will most likely change the balancing, but as long as your new bob weight end up close to that 50% balance factor you are fine... l would assume 48-52% is probably close enough with some margin.. (High rpm engines are sometimes deliberately overbalanced).

A smooth running engine is soooo much more than just a perfect 50% dynamic balance at a constant rpm in a balancing machine.

Just remove a plug wire and see what happens

 

[Hipster]

You don't HAVE to, but you probably should.

A 90 degree V8 is balanced somewhat similar to a single cylinder engine, so in the following example, just think of it as just a single cylinder engine

The idea of balancing is to counteract the g force of the rod and piston changing direction at TDC and BDC to reduce bearing load and engine vibration.

Now if you try to do this by letting the counterweight have the same mass as the piston and rod, it will be perfectly balanced at TDC and BDC but terribly out of balance mid stroke. Thats why Single cylinder engines and 90 degree V8's use a "balance factor" of normally 50%. That means only 50% of the reciprocating weight is balanced out. Making it equally out of balance at mid stroke & top and bottom dead center.

Changing the rod and piston combo will most likely change the balancing, but as long as your new bob weight end up close to that 50% balance factor you are fine... l would assume 48-52% is probably close enough with some margin.. (High rpm engines are sometimes deliberately overbalanced).

A smooth running engine is soooo much more than just a perfect 50% dynamic balance at a constant rpm in a balancing machine.

Just remove a plug wire and see what happens

It can get extremely complex for sure, and definite compromises involved. A crank being a bit heavy is not the end at be all, better than too light, but balancing is done end to end. Front at the snout might be heavy while different counter weight needs weight. In large part about getting rid of harmonics. Race balanced tend to smooth out, more to it than just the balance, while climbing into rpm, where a street engine wants to come undone. Piston speeds, internial mass,rod/stroke ratios, etc. Street balance for a street engine. Along the lines of what Schurkey said, way out of balance, wrong balancer/flexplate situations you'll know it because it'll shake the car apart and damage will be evident on journals and bearings.

 

[Erik the Awful]

I should have put a caveat on my previous post - I balanced the rods and pistons against each other, but that's a far cry from a dynamic balance.

Your rods and pistons can be perfectly balanced to each other, but the geometry and timing of the engine means that it will still vibrate. The crank, harmonic balancer, and flywheel are weighted to counteract this, but by their nature they're also balanced to the rods and pistons. If you take the time to balance your rods and pistons to each other, they may not be balanced to the crank, balancer, and flywheel.

You can balance your rotating bits, and that's a very good idea, but it's not a full balance.
You can dynamically balance everything after the fact, and that's a very good idea, but it's not a full balance.
Balancing your rotating bits and then paying a shop to dynamically balance the rotating and spinning bits together is the preferred way to go.

 

[Supercharged111]

I should have put a caveat on my previous post - I balanced the rods and pistons against each other, but that's a far cry from a dynamic balance.

Your rods and pistons can be perfectly balanced to each other, but the geometry and timing of the engine means that it will still vibrate. The crank, harmonic balancer, and flywheel are weighted to counteract this, but by their nature they're also balanced to the rods and pistons. If you take the time to balance your rods and pistons to each other, they may not be balanced to the crank, balancer, and flywheel.

You can balance your rotating bits, and that's a very good idea, but it's not a full balance.
You can dynamically balance everything after the fact, and that's a very good idea, but it's not a full balance.
Balancing your rotating bits and then paying a shop to dynamically balance the rotating and spinning bits together is the preferred way to go.

In the past, I've mixed and matched pistons and rods to have the assemblies be closer.