Are balance shafts needed or helpful once the engine pistons, rods and crankshaft have been either replaced by aftermarked lighter ones and the crankshaft has been lightened?

The way I see it the purpose of the balance shaft is to counter balance the vibrations on the big bore 4 cylinders and are built accordingly to the stock pieces weight. Once you replace these parts with aftermarket the weights are no longer the same.

Are functional balance on a modified engine actually worst as they will create additional unbalance and vibrations?

Trying to figure out if I'm better off just taking out the bs belt and leave the bs in place without rotation. Any effect on the oil supply to the turbo?

A friend's modified engine top bs went bad and destroyed his engine block. I might be better off taking the bs belt out and preventing the same situation of happening to my engine.

My friend bs failure might have been from the unbalance vibration fatiguing the bs bearing eventually leading to bs failure. Or might have been just a bs fatigue failure.

What you technical guys think?

Angel

 

The balance shafts are there to dampen harmonics inside the engine at certain RPMs. There have been reports by very reputable engine builders on this site that imply that an unbalanced engine will accelerate fatigue in certain engine parts, i.e. the oil pick up in the oil pan which can lead to total engine destruction from lack of oil pressure.

Items like lightened cranks have there own problems.

The engine in my car does not have the balance shafts. Knock on wood I have not had any problems, but it is a track car and I go through the bottom end every couple of years to replace bearings and I do a visual on all the various bits that might suffer from vibration fatigue. I will also be changing out to a dry sump in the next couple oy years.

I would not disconnect the balance shafts if I were you. I would do a visual on all of the belts and replace as necessary. There are a ton of cars on this website that run them with no problems.

How did his balance shaft go "bad"?

 

For some reason the black plastic attached with two bolts to the bs broke destroying the housing and causing a small oil fire, quickly extinguished. This failure cracked the block behind the bearing mount.

Some vibration that must have fatigued this part of the bs causing it to break away.

Would the change in weight affect the engine harmonics and thus render the purpose of the bs useless or even create an even worse problem?

Do we know how cars with modified engine and bs compare to modified engines without bs?

 

Did a search. Lots of info to answer my original question. Thanks.

 

What was your conclusion?

 

+1

 

The actual weight of the balance shafts was determined by the actual rotating and reciprocating mass in 944 motor. If you change the piston and/or crank counter weights the mass the balance shaftes are tuned to counter act has changed. As such they will not be as effective. However I don't know how much mass change it takes make it better to remove them entirely. In an ideal build you would install new shafts matched to the new engine weight. However I have never heard of anyone doing that.

 

I have my answer.. They have nothing to do with harmonics of the motor..(sorry Bob) Strictly there for comfort. It has been proven time and time again on all the Mitsubishi's and a crazy large number of 2.2 or larger Honda motors. Zero issues with tons of mileage. I also got my supporting argument straight from a Toyota design engineers mouth. (not that there are many Toyota balance shafts lol. But his word is damn compelling in my book, and NOT because of his title, but because of his experience) Take my word for it or dont.. on this topic, I'm not going to argue with anyone..

Or.. ask Weston Dillard.. been racing for years (seven?) on his 3.0 liter turbo, no balance shafts. thats.... Three... Point.. OH. no issues. other than headgasket failure and all the other common BS with our motors.

Sure things have fallen off like the oil pick up.. but ask yourself, are you sure its from lack of balance shafts? or just regular ole use and a poor design... 2 sides to every coin...

Example; (off topic, but showcases thought without understanding facts)

Should a rural county in Texas invest a large sum of money to stop drunk driving accidents that rose almost 50% over the past 2 years!? If you say, no matter what, YES! of course... well guess what. there was only one accident the previous year and two the next.. big deal.

I want to rant today.. thank you for allowing me..

 

balance shafts, on a street car , in traffic , good. on a race car, balanced and rebuilt motor, almost never at idle, not really necassary. but the delete must be done the right way.

 

Having seen my share of motors in many configurations I can tell you that balance shaft delete engines , IMHO, are obnoxious….but that’s just my opinion.

The need for balance shafts is to dampen a high order harmonic vibration due to the basic nature of a 4 cylinder inline engine. A simple example may give light to why they are needed – At TDC you have 2 pistons at top dead center and two at bottom dead center. Half way through the cycle (1/4 revolution) you might think that the pistons are all in line at the half way point….nope. They are all inline but they are below the midpoint due to rod angle geometry. The pistons actually move faster during the top down movement than the bottom up movement – this creates a dynamic imbalance even though the parts weigh exactly the same. You can’t fix this with any amount of static balancing.

The frequency of vibration caused by this dynamic imbalance is quite high, it doesn’t feel like an imbalanced engine, it ‘buzzes’. Above idle it feels like an electric shaver on crack. Because we have large 4 cylinder engines (2.5 – 3.0) the effect is greater, longer stroke with heavier pistons amplify the problem.

Unlike a static imbalance issue the crank, pistons and rods don’t care about the inclusion or exclusion of the balance shafts – they do not affect the balance of the rotating assembly – the balance shafts damped the vibration of the whole engine assembly. The down side of a large mass that is vibrating at a high frequency is that anything that is bolted to it will want to become unbolted and anything that has a lever arm with not appreciate having its base vibrate at a high frequency. The oil pick up is the best example – bolted to the block with a long arm and a mass on the end. I have built new engines for three people that had oil pick up failures – all 3 had balance shaft delete engines. I have also put stand alones on many engines with balance shaft deletes – you can tell right away by the vibration of the engine - even from the driver’s seat, that the balance shafts are not there. A large 4 cylinder engine is inherently not smooth, but a balance shaft delete engine is even less smooth – reminds me of a farm implement!

If you make a dramatic change in piston weights you may consider modifying the balance shafts. Remember that the balance shafts are only working on a vibration caused by the reciprocating mass, not a rotating mass – so a change in rod weight does not have a great effect on the dynamic imbalance (maybe about ¼ of the total rod weight change – depends on rod design).

BTW – to give you an idea of how noticeable it can be I have the ‘California car’ back here to replace the engine. The previous engine was built as a 2.8 without balance shafts. The project was turned over to me to get it right, I switched to a stand alone, changed turbos and a couple of other things and it made good power and drove well….but after a year or so the Owner so dislikes the vibration that he has shipped the car back to me to build a new engine. Old California car thread - https://rennlist.com/forums/showthre...eferrerid=1956

BTW part 2 – Porsche tried the balance shaft delete on it endurance racing 944 (LeMans)….they quickly went back to a balance shaft design. They found too many vibration related problems.

 

50%? …. Holy crap, the accidents actually went up by 100%.....I think all driving should be banned in that part of Texas…..either that or they should have all there alcohol confiscated and sent to me.

So….is the math teacher also the drivers ed teacher??

 

HAHA Chris. I see the error of my ways. The original story was suppose to be 50% and I embellished a bit. That's what I get.. and it's spelled "their" not "there" but no worries.. mistakes happen.

I have been in many shaft delete cars. (Most, admittedly where 2.3 liters or less) Some shaft delete cars were obnoxious, but most of those had worn out engine mounts. Many others I could BARELY tell the difference. So little that I doubt anyone else would know the difference. In this car, I will never know.. It's strictly for racing, and I have no care at all about a little more vibration that makes me lighter and faster. Clearly a 3.0 street car is just being ridiculous by wanting to remove the balance shafts. I don't see the advantage.

Both the harmonic dampener and the Balance shafts on say a 3.0 S2 clearly have two different purposes.. One is to stop the engine damaging harmonics, and the other is just a nice comfort. As explained to me over and over again. And which makes complete sense to me.

Aw man.. I'm arguing.. I said I wouldn't. I've been fished in!

And for the race team that had vibration problems; was it because the driver couldn't hold his bladder as long? lots of reasons why they decided against it. Maybe it was harder to diagnose other problems. etc. etc.. not enough info or direct feedback from that team to use this example as proof as much as it is completely circumstantial.

DARE I ASK.... anyone ever have a pickup tube breakage issue without a balance shaft delete motor? I'm willing to bet my next months paychecks... yes. yes indeed.

 

The reason I refer to harmonics when talking about the balance shafts is that the vibration caused by the dynamic imbalance is a much higher frequency than the operating frequency of the engine. For example an engine running at 6,000 rpm is actually running at 100hz (cycles per second). A second order harmonic would be at 200hz and so on up the scale of harmonics. The higher order harmonics cause the ‘buzzy’ feel of a balance shaft delete engine and can cause metal fatigue in the long term and non locktited fasteners to back out in the short term.

Harmonic balancers are a very different issue. The purpose of a harmonic dampener is to dampen the torsional resonance in the crank shaft. We tend to think of a crank shaft revolving smoothly as the engine runs. In reality it is being subjected to many violent combustion events as well at stress form compression. At 6k rpm the crank is seeing 200 ‘explosions’ per second with each one unloading all of its force on one journal at a time. At the same time as one rod journal is being forced down by combustion the neighboring cylinder is applying force in the opposite direction due to the compression of the intake charge. This creates a situation where the crank is flexing due to the rapidly changing opposing internal forces. All cranks flex to some extent, they have to to survive. It becomes a problem when the engine rpms get close to a resonant frequency of the crank, then the resonance can increase the amplitude of the torsional vibration – this will add to metal fatigue in the long term. The short term problems are actually pretty interesting. Back in the CanAm days of aluminum big block Chevys the engineers could not figure out why they were having serious ignition timing problems. In the end they finally figured out that the crank shaft was flexing by as much as 3-4 degrees and the distributor was driver off the back end of the crank so the ignition timing was changing under load at certain RPMs .

So both the harmonic balancer and the balance shafts are designed to reduce harmful vibrations – just different types of vibrations.

As for the race team that had problems – it was Porsche and the problems were parts failing from vibration induced stress and other parts that just wanted to fall off the engine before the race was over. Porsche tends to be pretty serious about LeMans and they did a good deal of testing, not just circumstantial proof. In fact the 1981 2.5 liter 16v turbo entry finished 7th overall (yes – including all the prototypes and GTs) mainly on the strength of spending a very minimal time in the pits. Some good info - http://944turbo.net/?page_id=57

If you assemble and maintain the balance shafts they will not be a problem and the parasitic drag from the belt is very minimal. The total weight of the system is also very minimal.

I prefer to build an engine that will last as long as possible – even in racing conditions. If somebody wanted a true sprint race engine that gets torn down on a very regular basis AND you wanted every last HP then I might be swayed to build such a balance shaft delete engine. Most of my customers would not be happy if I told them that they had to pull the engine out at least once a year and tear it down just to check parts for fatigue. If you like that kind of engine then get a 911 GT3 RSR that has to be sent back to Germany after 40 hours of run time for a $40k refreshing….

BTW – since the vibration problem is associated with stroke and reciprocating weight most of the smaller engines are not very symptomatic. Its noticeable in the 2.5 and really noticeable in the 3.0 engines. Not so much in the 2.0 engines.

 

gotta say, the buzzing does kinda sound cool...i was on a 100-mile trip after doing the belts and halfway there it occurred to me that the shafts were out of time (turned out to be 2 teeth off...), but when you womp on it and drive hard the buzzing + engine/exhaust noise is somewhat exciting, yet the known consequences are terrifying.

that said, i did drive without functional BS shafts for some time and it was pretty obnoxious, the difference the vibrations made (even on good motor mounts). happily my oil pickup tube is still in good shape (had pan off in january), so i would guess it takes a longer period of time to do damage.

 

Chris.. you described me to a "T" above. 20-30min races, Time attack, extreme weekend fun. (More like I WISH i were doing these right now) Balance shaft kept on a 24hr race.. well.. DUH!. Although I doubt the front hubs/bearings would last as long as a shaft delete motor. as you stated, most of your customers should keep it.

I still disagree the "harmonics" of a shaft delete are any worse than a motor that just revs all the time with the shafts. Extreme vibrations either way.

AND.. all this silly arguing is making me want to go RACE! I envy you guys. Hell, I'd be HAPPY to prove you right Chris.. That would mean I was actually out there RACING. (but the only way to prove it is to have two identical brand new stock motors. stock vehicle. and remove shafts from one. go race.. and then have two other sets of examples to support in case of random anomalies.) God the fun to be had proving this example! ugh.