3400 Crankshaft

[caffeine]

What do you mean without balancing the rotating assembly? Of course its been balanced. If you look at a crankshaft you will see some drilling marks on the counterweights just like on a flywheel.

[SwiftDrift]

the pistons are not equal weight, i heard the rods are not balanced either

[geoffinbc]

The engine is balanced. If it was not it would vibrate like mad. There is an acceptable amount of variance from perfect that is acceptable and in no way perceivable even if your hand was on the engine. Sure you could balance it to within 1g if you want to pay for it but no factory engine is going to be that close.

There are several types of balance that need to be considered when dealing with a rotating part connected to reciprocating parts. To say a 90* v6 and 4 cylinder are not balanced is partly true but not in the same way an incorrect balance job would yield. There is first order and second order vibrations along with that you have rocking couples both horizontal vertical and rotational then you have vertical and horizontal shaking. 4 cylinder engines with a flat crank all exhibit a vertical shake. But this can be eliminated by making a cross plane I4 crank which now makes it an odd fire 4 which may sound funny and not idle as smooth but at high rpm it is smooth and does not need a balance shaft to remove the buzz of a flat plane crank.

All boxer engines with 4 or more even number of cylinders, I6's and V 12's are all what most would call "perfectly" balanced.

[caffeine]

I'm pretty sure the rotating assembly would be balanced as a whole. I don't know where you get the idea that the pistons and rods aren't equal weight; it's not like there are different part #s for different cylinders. Any differences would probably be caused by casting imperfections and such.

[caffeine]

Ya I meant that 60*V6s are naturally balanced and smooth running vs the other engines I mentioned which naturally have vibrations that are typically masked with the use of a balance shaft or two.

[SwiftDrift]

some factory engines in foreign cars have rods and pistons that are almost identical in weight. yes i was talking about the imperfections. pay to get the pistons within a gram? when i get access to a shop again ill balance the pistons and rods myself, and probs get them closer than a gram. then get the crank balanced. i didnt mean that they werent balanced at all, just not as well as they could have been.

[caffeine]

3400 Crankshaft

It's more efficient to mass produce the parts and then balance the whole rotating assembly together. Honestly it would be a waste of time to do otherwise maybe unless you were buying aftermarket forged parts.

3500s don't seem to really break much until over 400whp. And when they do it always seems to be rod related. Pistons seem to survive unless they're damaged by other shrapnel.

[SwiftDrift]

fair enough. the 3500 must be better balanced than previous engines, my brothers 2.8 vibrated noticeably at idle.


ps: look at my post in pushrod perf.

[caffeine]

Perhaps it wasn't running correctly?

[SwiftDrift]

haha well she was tired..

[caffeine]

In my friends truck with the 3.1 it also has solid mounts and you wouldn't even know unless you were told because its so smooth.

[SwiftDrift]

the 2.8 wasnt like jumping out of the bay or anything, but i noticed it and pointed it out and my bro didnt think much of it

[3400-95-Modified]

I'm building an engine for a friend come spring time... 3400 bottom end with H-beam rods and forged pistons... I'm getting the full rotating assy balanced at the machine shop so I will report back what is done to it as well as photos of the crank before and after so you guys can see what is done, or how much changes. I think that will better help people understand what happens when you do forged and different rods vs the stock stuff.

[SwiftDrift]

sounds good, thanks. so is there any difference in weight between forged and cast?

[caffeine]

Forged parts are usually heavier I believe.