To Delete or Not Delete Balance Shaft

[geoffbaltz]

My buddy and I are building a 3 Liter motor and still can't decide what to do with the factory balance shaft setup. We have spared no expense on this motor and will be running new JE pistons, Pauter rods, and have had extensive work done on the crank.

Everything is done and all the parts are in so we are at the stage of having the motor put together. With all of the changes to the internals of the motor, doesn't it make sense to delete the balance shaft setup?

I know Porsche designed it originally for harmonics and to counteract the stock internals. But now that the motor will be completely different on the inside, isn't the stock balance shaft setup not ideal at this point? I mean, if Porsche designed it for what they knew they were putting in the motor, aren't we now going to have a balance shaft setup that isn't ideal for the new guts?

Would appreciate some thoughts and insight.

Thanks.

[gpr8er]

FWIW I built my 3L using Mahle pistons and a FULLY lightened crank, Fully lightened flywheel, and KEP aluminum PP. Pauter rods etc.All reciprocating parts were balanced. IMO you need the balance shafts for a big 4 banger.

[geoffbaltz]

Originally Posted by gpr8er

FWIW I built my 3L using Mahle pistons and a FULLY lightened crank, Fully lightened flywheel, and KEP aluminum PP. Pauter rods etc.All reciprocating parts were balanced. IMO you need the balance shafts for a big 4 banger.

Thanks for the input. So do the balance shafts counteract the ignition of each cylinder and it doesn't matter what internals are? Is it a timing thing?

[nynor]

the balance shafts counteract, so to speak, the rotating mass of the rest of the engine to reduce vibration.

[DannoXYZ]

Originally Posted by geoffbaltz

Thanks for the input. So do the balance shafts counteract the ignition of each cylinder and it doesn't matter what internals are? Is it a timing thing?

No, the 2nd-order harmonics is caused by the difference in swept-area of the crank-journal + big-end of the con-rods at the botttom of the stroke vs the top of the stroke. The area and distance moved will always be different regardless of stock vs. aftermarket parts or whether the parts are within 1gm or 0.001gm of each other. The only way to balance this difference without balance-shafts is with a flat-4 engine.

[geoffbaltz]

Thanks Danno. Hope you are doing well. I knew one of you engineer types would chime in.



I just fix and drive these things.

[rick13]

My thoughts...I would be inclined to put them in.

[blade7]

For Porsche to pay a licence fee for every set they fitted they probably explored every viable option. Fit them.

[AaronM]

Originally Posted by DannoXYZ

The only way to balance this difference without balance-shafts is with a flat-4 engine.

And even then only if it's a boxer arrangement, and you'll still get a slight rocking couple because the "opposite" pistons are very slightly out of line with each other.

If it's a shared crankpin (180-degree V) flat-4, it won't be balanced at all.

https://en.wikipedia.org/wiki/Flat_engine#Boxer_engines

For a good basic explanation of why an inline-4 needs balance shafts, check out this link: AutoZine Technical School - Engine

[geoffbaltz]

AaronM



Thanks for the link. Good stuff on balance shaft setups. We are keeping them for sure. I see the light!

[guru944]

Geoff;

By removing the balancing shafts you also increase the possibilty of crank shaft walk which would not only damage the thrust bearings but also wipe out your front crankshaft oil pump.
Any engine 2.0 or larger and spinning over 5000 rpm is prone to this.....regardless of power level. Besides they only consume 3-5hp max off the crank. Engine would eventually reach resonance and shake it self to death.... Honda, Mitsubishi, Toyota and Porsche have been uses them since 1980.
Later

[DannoXYZ]

I remember JME recommends not removing the shafts. A lot of racecars without balance-shafts end up with cracked oil pickup tubes from the vibration. Even with the shafts, I'll lay down a brass fillet at the ends of the tube to smooth out the transitions.

[333pg333]

Something that I've also been considering and have come to the conclusion that we will keep running them. Had some massive vibration or harmonic issue last weekend which killed our car in the most important event this year. Very disappointing but stuff happens when you stress these motors. We do run the balance shafts on this motor but see what happens when something gets out of alignment! First the supplied dry sump pulley sheared in half. We machined a better version and replaced within hours and then the crank pulley sheared off the motor. Also created a 4" tear in a weld of the intake we made. May have all started at the rear of the car. Looking at building up a special 16v motor but we will almost certainly run balance shafts with this too.

This site is too hard to load pics on. Sorry.

[Pete R]

My car had balance shaft leaks and I really wanted to delete them for simplicity. I was told that the only guys that delete them are racers who rebuild the motor every year. So leak repair it was.