odonnell

Question: are BOTH balance shafts needed to turn in unison to achieve balance? If only ONE shaft were to be driven, would it be: 1) half of the balancing effect, or 2) completely out of balance and worse than zero balance shafts?

I want to use the balance shaft drive cog for something else, and I have a choice about leaving the upper balance shaft as functional or not. Lower shaft will not be in use.

V2Rocket

run your belt off the crank cog, leave the balance shafts out of the equation. just one would make the vibrations worse than not having them.

or...

get a b-shaft and machine all the weights off and just run it as a "jack shaft" supported at the back end by the bearing.

thomasmryan

You could/would need to double the weight of the remaining balance shaft.

odonnell

Ok thanks. I was just gonna have a machinist buzz off the lobes if it will make the balance even more messed up, I'm pretty sure you know what my intentions for this belt routing would be

Dougs951S

what remains to be seen is if the bearings for B shafts are sufficient to support the load of a blower. I agree that if this is to be pursued, the weights on both shafts need to be removed and turned into essentially jack shafts.

V2Rocket

I'd just cut/ditch the belt cover and run the belt straight off the crank.

odonnell

I'm going to be running no belt covers altogether. I like the idea of using the balance shaft drive gear because it provides a 2:1 ratio with the balance shaft pulleys (I have an extra one I'm going to mount on the whiny thing). And, no belt slip to worry about. My biggest concern is the tensile strength of the belt I use. An Accord timing belt is the right pitch and approximately the right length, there will be adjustment on the bracket itself. I think the loads experienced by a timing belt to compress valve springs at high RPMs should more than qualify it.

I'm not planning on putting a ton of radial load on the balance shaft itself...actually I expect it to be less than what it originally experienced with the stock belt setup (where it had much more capstan force).

Noahs944

Air pump for filling water dingeys & life jackets & hopefully bicycle tires I presume. Great idea!

StoogeMoe

Here study this

http://www.tonyfoale.com/Articles/En...ineBalance.pdf

to know what the balance shafts do.

Now the real question is why is one balance shaft counter rotating, and the other is not on the 2.5 Porsche?

odonnell

I understand their job, I just am unsure about how they commingle in terms of their time-domain response. It's a weird question, and admittedly it's best that I nixed them altogether and just used the upper shaft as a take-off for an idler. Or leave them functional and have a machine shop modify a power steering pulley to be wider, or something, and use that as a drive.

V2Rocket

To counterbalance eachother while "sideways".

odonnell

My understanding is that as soon as you modify the rotating assembly's balance (i.e. knife edging crank in my case) the balance shafts are basically useless because they were designed for a different rotating mass to work against.

So my other question is this: so without going completely crazy, is there a routine way to 'balance' a 4cyl only by machining the crank or flywheel? My current motor has the balance shafts deleted (welded up) and runs smooth as silk. Is there some machine work that could possibly have been done by the PO to re-balance the motor? It seems like if there was a way, Porsche would have done it the first time. But then again you don't see balance shafts too often and there are a lot of 4cyls on the road.

I just want to know because I'm 100% cool with how my current motor is with no balance shafts, it's my daily most days and has been for years. I'm worried about having NVH with this new motor, that my current motor doesn't have because of a custom rotating assembly. Sorry if I am not phrasing this question well.

StoogeMoe

If you read that link, you would realize it is not possible with an inline 4. The reason why you don't normally see balance shafts on other 4's is because the second harmonics don't start becoming annoying until you go above 2L.

StoogeMoe

Yes but why aren't they directly opposite each other instead of one high up on one side and the other down low without the crankshaft being on the intersection between the two?

V2Rocket

In some cars they are, like the current Ford 2.3/2.5 Duratec...the B shafts are right next to eachother, counterrotating, but inside the oil pan.

In the case of the 944...who knows?
Maybe just packaging reasons.