The V8 Owners Forum -

All in the tittle.

When you change of piston for higher CR the weight of piston generally changes. Up to what point it affects engine balance ?

A commum practice to increase CR in 4.6 is to instal 4.0 piston.
But 4.6 piston weighs 601g with pin and 4.0L piston weighs 581g with pin http://www.v8forum.co.uk/forum/viewtopic.php?t=7731.
I done many time this "upgrade" and never had balancing problem after.

There is a factory tolerance during engine balancing ?

You can fit whatever weight piston you want to. As long as they are all equal weight within maybe 1-2 grammes the balance will not be affected!
I weigh my pistons with pins and rings fitted, and trim them to within 1-2 grammes of each other. I balance my rods the same way. Then when I have the pistons fitted to the rods I weigh again and trim from inside the piston skirt if required. My digital scales may not be 100% accurate, but they will allow me to keep the piston and rod weights even.

I cant afford to pay a machine shop to do this as it is simple to do at home.

Perry

Wotland wrote:There is a factory tolerance during engine balancing ?

Very unlikely as factory-built engines are generally not balanced at all.
Production processes and casting accuracy have improved over the years, so modern-built engines are often not too bad, but with older Rover V8 engines, the variations in weight between pistons on a factory engine can be huge - and why some engines just don't feel smooth when revved.

mgbv8 wrote:You can fit whatever weight piston you want to. As long as they are all equal weight within maybe 1-2 grammes the balance will not be affected!
I weigh my pistons with pins and rings fitted, and trim them to within 1-2 grammes of each other. I balance my rods the same way. Then when I have the pistons fitted to the rods I weigh again and trim from inside the piston skirt if required. My digital scales may not be 100% accurate, but they will allow me to keep the piston and rod weights even.

I cant afford to pay a machine shop to do this as it is simple to do at home.

Perry

This the theory I have rolling about in my mind, but are the counter ballance weight of the on the crank weighs a specific amount to counteract the weight of the rod/piston combo and crank pin, so if you reduce or increase the weight of the piston or rod, you are putting the dynamic ballance of the engine out,??? so im now thinking the combo I am using will put the dynamic ballance of the engine out by 800g as my combo is about 100g per piston lighter than the factory settings,
Or am I talking out of my posterior [as usual]

I'm with Ian. The counterbalance weights on the crankshaft are 'matched' to the weight effective at the little end. So fitting pistons with different weights will affect balance.

I had my 4.6 balanced after switching to 4.0 pistons, and a fair bit came off the crankshaft.

Chris.

5000SE wrote:

Wotland wrote:There is a factory tolerance during engine balancing ?

Very unlikely as factory-built engines are generally not balanced at all.
Production processes and casting accuracy have improved over the years, so modern-built engines are often not too bad, but with older Rover V8 engines, the variations in weight between pistons on a factory engine can be huge - and why some engines just don't feel smooth when revved.

Rover engines were balanced. To start with they knew the weights and tolerance of the internal componants so the cranks were designed and balanced to suit before assembly. Then the engines were rotated on balancing machines and adjuctments made to the pulley if needed.

"the counter ballance weight of the on the crank weighs a specific amount to counteract the weight of the rod/piston combo and crank pin, so if you reduce or increase the weight of the piston or rod, you are putting the dynamic ballance of the engine out"
Hi
Ian you are not talking out of your posteria, but you are a foctor of 2 out. When ballancing the crank the big end journals have a mass added to the journels equivilant to the mass of the big end of the con rod (plus bearings) plus half the mass of the little end of the con rod plus half the mass of the piston assemblies. If you remove 100 gramms off each piston then each journel mass will be 100 gramms out so a full reballance will be needed (this is still a good chunk less metal overall to accelerate by the time you removed all this lot!) Remember only the middle two big ends are opposiete each other, the outer pairs of big ends are at 90 deg to each other, this makes a pretty complex set up to ballance.
Best regards
Mike

V8 engine balance:
http://www.autozine.org/technical_schoo ... mooth4.htm

Chris.

when i had the 3.9 balanced ,it was the crank that had to be altered. I did read somewhere about the effects and weight difference at certain rpm of a unbalanced engine, a 1oz difference became lbs at certain rpm rich

bones wrote:when i had the 3.9 balanced ,it was the crank that had to be altered. I did read somewhere about the effects and weight difference at certain rpm of a unbalanced engine, a 1oz difference became lbs at certain rpm rich

I was also told, when asking about getting my piston pocketed, and they said if they pocketed them, then they would have to re-balance the whole system.
I did not ask why. but does seem if the rotating mass changes, even if its even over all piston, then a re-balance is needed.

It's interesting to note that when a v8 is put on the balance machine they put on bob weights equal to 100% of the rotating mass and 50% of the reciprocating mass. This 50% is a magic number established by experience throughout the industry. I ask, why not 45% or say 55%.
It's hard to believe that given the laws of physics an exact number of 50% gives the best balance to an engine.
Anybody know why 50% just happens to give the best balance?
Naturally, pistons/rods etc need to be matched and of equal weight +/- 1 grm.
Regards Denis

Isn't this because of the 90 degrees angle between the cylinders?

Grt

Frank

From Eagle website:

The second thing to note is the 50% value used for the reciprocating factor. This number deals with the geometry of the engine itself. A 90 degree bank angle "V" engine will use 50% here. A V6 or a narrow or wide bank angle "V" engine will use a different value (again, consult the balancer manufacturer). Some engine builders will perform what is called "underbalancing" or "overbalancing". They will use slightly differnet values here such as 48% or 52%. This is done to help compensate for dynamic effects at extremely high or extremely low rpm operation (again, beyond the scope of this discussion). Eagle uses 50% because this value is required for almost all common street or racing engines.

How is it possible that 4.0 piston is slightly lighter than 4.6 piston? Same bore, same compression height, but piston dome is smaller.? I will estimate it is another way..?

I have got my friend 4.6 engine in my garage, and it will need new pistons, and i´m looking for 4.0pistons in it. So is re-balancing necessary?

Or is it possible to grind some metal for piston pin and piston skirt?