Internal vs. external balancing
Registered User
Joined: Nov 2004
Posts: 55
Re: Internal vs. external balancing
please correct me if I am wrong.
an engine shop will do something like...
weigh the rods twice, big end weight is 'rotating',
small end is 'recip' weight, add piston weight to recip weight,
...formula...
bobweight is --> rotating weight, plus 50 percent of reciprocating weight.
add bobweights to crank, spin, and do various things
to get the crank w/bobweights to be balanced.
this is different that a car factory, that can afford
an engine test stand, and is making thousands of the same
thing. Total engines are tested for vibration.
I assume they do something like, match and weigh pistons,
match and weigh rods, with certain weights of
rods and pistons, use slightly different cranks.
note that the '50 percent' formula for a 90 degree Vee,
is just an empiracle result that seems
to work out most of the time, other factors are sometime used.
an engine shop will do something like...
weigh the rods twice, big end weight is 'rotating',
small end is 'recip' weight, add piston weight to recip weight,
...formula...
bobweight is --> rotating weight, plus 50 percent of reciprocating weight.
add bobweights to crank, spin, and do various things
to get the crank w/bobweights to be balanced.
this is different that a car factory, that can afford
an engine test stand, and is making thousands of the same
thing. Total engines are tested for vibration.
I assume they do something like, match and weigh pistons,
match and weigh rods, with certain weights of
rods and pistons, use slightly different cranks.
note that the '50 percent' formula for a 90 degree Vee,
is just an empiracle result that seems
to work out most of the time, other factors are sometime used.
Banned
Joined: Oct 2002
Posts: 6,518
Re: Internal vs. external balancing
Originally Posted by Stephen 87 IROC
Normally, with heavy pistons and rods, weight is removed from the crankshaft counterweights to neutralize the balance. However in a high reving engine like a race engine, you want lightweight components to reduce the mass that's moving up and down inside the cylinders. When this kind of rotating assembly is balanced, they add a heavy mallory weight into the crankshaft counterweights to bring it back into a neutral balance.
contactpatch.... the factory can spin balance the whole motor itself IN THE BLOCK! It's been going on for a long time now.
Bret
Registered User
Joined: Mar 2000
Posts: 2,037
From: 51°N 114°W, 3500' elevation
Re: Internal vs. external balancing
Internally balanced BBC stroker Eagle crank 445442526385 crossdrilled, center counterweight, must use 6.385" rod or longer.
This is not one of their cranks listed as lightweight. My rods are H-beam which will be lighter than I-beam rods. My JE pistons are lightweight. The balance shop had to add weight. That's all I know. I can only assume the pistons and rods are too light for the crank. He had to fill one of the drilled out holes in one of the counterweights and install some mallory weight in a different location to get the balance perfect.
This is not one of their cranks listed as lightweight. My rods are H-beam which will be lighter than I-beam rods. My JE pistons are lightweight. The balance shop had to add weight. That's all I know. I can only assume the pistons and rods are too light for the crank. He had to fill one of the drilled out holes in one of the counterweights and install some mallory weight in a different location to get the balance perfect.
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