When I had my balance shafts in I was never too sure about how tight the balance shaft belt needed to be. On top of that, it was another 3 or 4 steps during the timing belt change. I'm neither here nor there about weight savings and i'm all for having a smoother running engine. Less vibrations also mean lower chances of things starting to back out. Still, not worth keeping though.

With Balance shafts:

Pros:
-Less vibration into cabin
-Oil pressure is more towards OEM levels
-Less noisy interior

Cons:
-Extra steps when doing a timing job
-Extra cost of balancer pully and belt
-Chance of belt coming loose and screwing with timing gear
-Another two bearings that could get wiped out from debris and destroy your engine
-If balance shaft breaks at high RPM it could window the block. Remember that both shafts are actually unbalanced in one direction to balance the block itself when they spin. The shafts spin unbalanced and have unequal stresses like a video game controller vibrating motor weight.

Without Balance shaft:

Pros:
-timing belt is safer
-feels more like race car
-less weight
-less parts to buy for timing belt job
-less moving parts to break stuff

Cons:
-increased interior vibrations
-higher oil pressure (could be a pro if you actually need it)
-one time cost of balance delete it kit. ( or just punch out and spin the oem bearings and pick up an oem stubby shaft with case plug)


Balance shafts are not worth keeping.

 

what I mean is that the forces that create the vibrations are still acting on the engine. The imbalance remains and is acting on the bearings and the block, its just that by creating even more vibration, in just the right way, you don't experience the vibrations. To make a crappy analogy, think about wearing noise canceling headphone at a concert. You have one speaker blasting away making sound and by blasting away another speaker you can cancel out the sound you hear. What you still have in the end is two speakers blasting away, they themselves are experiencing all the forces they are generating creating that sound. Disclaimer, I'm not a physicist this is just my understanding of how this works.


To keep things in context, the amount of vibration you feel deleting the shafts doesn't come close to what happens if you use poly mounts. Those don't have any effect on the amount of vibrating the engine is doing, just how much you experience external to the engine.

 

Does a 2500hp 35 psi engine count? How about a destroked Ecotec that turns 9k? In the middle of a flat plane crank LS engine build that we are hoping to see make power over 9000 rpm. Me personally, have built over 300 engines that landed in real race cars. None of these engines had balance shafts. I havent built an Evo engine but as I have said before, an engine is an engine at the end of the day. They all work in the same way, except those stupid rotarys.

The engine I built for my race car came with balance shaft from the factory. It now has none. I cant tell the difference in vibration. Revs smoothly up to 8500.

 

If you're gonna just throw the challenge out there...........

There's a way around that: I am not running balance shafts and my oil pressure is normal. I removed the front and took the belt off the rear. No worries about flipping bearings, higher oil pressures, etc. The car has been this way over a year now, with no problems.
FWIW I decided to do this after noticing the balance shaft belt was shredding itself, somehow. I didn't really care how or why it was shredding... just that it was a possibility and could end catastrophically.

Our cars basically rattle like a bag of aluminum cans anyway... anyone REALLY notice the difference with no balance shafts?

 

Dont think its anything youre going to "feel". It about potential high freq fretting damage

If this fretting damage is real maybe its the diff between a motor lasting 30K vs 100K, as a rough example

 

My opinion on this is that BS only produces up/down vibrations to counter the power stroke. I not sold on anything else Jacks is claiming about other vibrations. I can see poor clutch contact (inconsistent) on entire surface from the vibrations, but it should seat over time since the BS vibrations are at TDC and BDC to counter power. Without BS, I would only blame the extra wear at those loctions for not having BS. Anything else could be cause by a number of other things.

"Buzzing" noise is possible from the main bearings taking a little beating since there is a loss of clearance benefit from removing BS.

As for torsional whip, I agree with what Jack is saying. We need to understand that at TDC, the explosion is most violent because volume is so small from compression. As crankshaft turns and pistons start going down, the energy drops significantly. So when you think about it, your pistons help to absorb that power spike at ignition and transfers energy to rods and then to crankshaft, which then goes to tranny and transfer case to wheels.

Let's start at pistons. The heavier the piston, the better it stabilizes the power to transfer to rods. Am I saying get the heaviest pistons you can find? Absolutely not because the mass turns into massive rotational energy the rods and crankshaft hast to withstand at high RPMs. What I'm saying is the lightest pistons on the market will transmit most of the power spike to rods. That means you will need strongee rods AND crankshaft to withstand that huge spike. When will these spikes matter? My opinion is during a launch with heated up slicks where tires don't give much by spinning. That 7,000 RPM 500 ft-lb launch will test rods and cranks to their limit. Why foes cracks mostly happen at #4 journal? Take a look at the distance of crankshaft metal between the flywheel mount to the nearest rod connection. There is very little metal between them to absorb the internal torque since wheels are holding and engine is making all that power.

Jacks also mentioned lightened flywheel. I agree with that too. Heavier flywheels will increase the rotating toque of the engine and helps to transfer to wheels. This heavier moment of inertia gives the crankshaft and rods more buffer by storing extra energy when using 2 step or launch control and transfer more energy to wheels to help accelerate car. The difference in engine power and wheel rotating energy is less so crank and rods have less to equalize. If this doesn't make sense, I can try a different angle to explain.

Dampener allows crankshaft to spin easier at ignition and absorb that energy to carry the energy rotation after the spikes. Crank dampening helps but remember, it is connected on other end of clutch. This won't help clutch as much as what many thinks. Dampener would be awesome if they can incorporate into the flywheel/pressure plate. It doesn't help at the other end where there is so much crankshaft length to twist.

When I build my engine, I will balance out the internal forces so that stresses are balance out as much as possible.

 

Literally most of this wrong.

 

Good points. I'd like to know if anyone has torn apart high power engines with and without BS. I expect the one with BS to have healthier main bearings. Nothing more since other things are independent of BS. Same with stub shafts vs full length replacements. I would expect full length to have less wear on oil pump bearings, especially on those on antilag. Most race engines don't last 60k miles so BS won't matter since it gets rebuilt before any wear shows. I think race engines are refreshed every 30k miles or so.

 

I will clean it up later. This is brain dump so far and I am thinking about this in between breaks at jury dervice.
I know the acceleration of pistons is another force to account for but haven't found a good spot to add it (or if that is needed since I may shorten everything and get to the point).

Feel free to explain since we want to gather all views.

 

1. No, explosion is not most violent at TDC, actually you want most pressure quite a bit later than TDC..
2. You are introducing crank harmonics into discussion which is not the point of the topic...
3. Heavy fly is good for transmission as it filters out crank harmonics... other way of doing it is using twin mass fly (which are complicated POS).... Clutch plates with springs also help.... however none of this helps the crank.... To lessen the crank harmonics you need a crank dampener which you get standard from mitsubishi (rubber main pulley)
4. Heavy pistons are no good as the crank and rod needs to stop and accelerate that lump of metal twice per revolution..

 

Thank you for supporting my point. I wasnt calling out everyone in the thread I was looking for those that knew what was going on like yourself

General-

A balance shaft is for NVH not the engine, if it mattered at all we wouldnt have odd degree engines through out the history of F1 which have turned higher than any Mitsu motor currently used (I am not going to discuss that aberation in the old world).

We tear down race engines, we tear down high powered street motors. I have a 2.2 in circulation that has ran since 2014, made 1080whp on a 7285 is "down" to 934 on a 6870. It hasnt ever had an issue without its balance shaft. It runs a max of 9600, this thread seems to have been started based on theory and not application.

tl;dr - dont need balance shafts. next.

 

Kiki layed it out pretty well.

I'll add that there is no explosion. If there is an explosion, that is considered knock. There is a controlled burn as the flame front propagates from the ignition source (spark plug) across the cylinder, this burn starts before TDC (hence timing ADVANCE). And peak cylinder pressure actually occurs shortly after TDC when the burn is completing and the power stroke has started. If the burn was complete and we saw peak cylinder pressure at or before TDC, you'd be wasting energy. Hence why the term MBT exists.

There are no "views" on this subject. You either think the engine needs the bshafts and you're wrong. Or you know the engine doesn't need the bshaft and you're correct.

 

OE stub shaft ok in your experience?

 

How often do you guys tear down full race engines for maintenance? For the pro drift guys, road course and the people who run stuff like the Baja 1000, we like to refresh the engine once a year, just to be safe. Most of the time the engine still looks new. The bearings typically show little wear, most of the wear being on the lower bearing. I think the pistons skirts get the most beating out of everything. But as far as I can tell, none of that came from the lack of balance shafts.

And I am not a fan of the stubby balance shaft delete. At least on the Ecotecs, I have seen some pretty bad wear on the the balance shaft housing since its aluminum with no bearing when using a stubby delete kit. When running full length neutral balance shafts, the wear doesnt exist. However, what I have noticed in taking apart some high mileage engines with balance shafts, is that the balance shafts rear bearings are down to the copper almost every time. On the ones with neutral balance shafts, the bearings still look good.

 

We dont pull them apart until they fail typically. The only exception is the 4B11 in the Drag X so we can see what is going on. The 4G63 based stuff is dry sumped and never typically has a problem related to the mechanical side, when we do its something stupid like a sensor malfunction or something caused it drop fuel pressure. The oil pumps in the 4G63 fail because of oil starvation though in the wet sump motors. Like I said there is a 2.2 that is going on year 5 of street abuse and race use, maybe 40k? We havent had any problems with the stub shaft causing the galling though, its cavitation/starvation in the pan because of most pan designs. It sucks air and its all over. Our bearings always look great as long as there has been no decel in gear or money shifts.

Since this thread seemed to be about vibration because of the lack of a balance shaft I didnt really touch on the causes of failures. There are probably 50 more theories out there why the oil pump fails, but its always been air in the system. 1-3rd pressure will drop to 35psi in a stock pan and not come back until the top of 3rd. Build a drop sump with extended pickup helps, dry sump obviously fixes it, running it a little higher on oil level, accusump is nearly pointless because of the difficulty in plumbing it properly into the 4G63 to make sure there is never air in the system.

We only run balancers because of the need for a drive pulley to run the mechanical fuel pump/dry sump down low. You can run a cam drive for the mechanical and a stock pulley if you wanted to.