For the Evo 8/9 folks, balance shaft discussion.

by WANTED Sun Apr 22, 2012 8:44 pm

A question I've gotten many a time from many people is that of balance shaft elimination and the pros and cons of doing it. With a few Evo folks on this board, I figured it might be of some benefit to bring it up here.

First off, the entire purpose of the oem balance shafts is to act as a counter of the natural vibrations/harmonics of the main rotating assembly. In a nutshell, they are there to give the engine a smoother feel to counter the natural imbalance to the rotating assembly. However, the downside to having this is the fact that it is one more thing to fail.

The reason people eliminate them is to free up horsepower and torque, and to reduce the number of things that can possibly fail. Ask any old school dsmer the downside of having a balance shaft belt fail and the destruction caused by having it get caught up in your timing belt and jumping timing. Even a few Evo folks have experienced this. It sucks. An additional side effect of the procedure is an increase in oil pressure.

A common misconception is that because the manufacturer put them there, you absolutely positively must have them or your engine will self destruct. This is far from the truth as inline fours (or really even any design four cylinders in general) do in fact have a natural balance. Case in point is to look at other inline fours such as the honda B-series, D-series, F-series, that don't run them yet still feel rather smooth. Amusingly enough, their rotating assemblies are about as balanced as ours. In truth, they are only there for added driver comfort. In truth, you can completely eliminate the balance shafts using one of two methods and the car will drive perfectly fine. While Honda is one of the few I am familiar with, there are several other manufacturers whose four cylinder (or bigger) engines do not use a counter balance system to offset harmonics, to include Mitsubishi.

There are really only two ways to eliminate your balance shafts. Both of which actually do involve retaining one shaft of some sort on our oil pump/front cover assembly as well as spinning/replacing the balance shaft bearings to block off the oiling holes. Either a stub shaft (from an oem Mirage or ???) or a machined oem balance shaft (from AMS or another shop) will be required. With both methods, there are two schools of thought.

The first school of thought is that the stub shaft will cause premature oil pump failure. In a few big builds (or supposed big builds), multiple oil pump failures were experienced until someone came up with the idea of machining down the oil pump shaft just enough to not make contact with the balance shaft bearing. The reasoning behind this is because some believe that the oil pump requires more weight than a stub shaft can offer to keep it balanced because the rotating will remain imbalanced.

The second school of thought is that the oem stub shaft is fine and that it will cause no premature failure regardless of horsepower.

I subscribe to the second school of thought personally but recognize that both schools of thought are valid. However, I also believe in offset.
Having done the elimination prior to building my motor, I have a few personal observations and myths to debunk.

Myth 1: This is an easy job. This is not a terribly easy mod despite having been done for quite a while. It is by no means an advanced mod, but the act of doing it isn't exactly an afternoon affair. This mod can be done with the engine in the car (which I did), but I don't recommend it at all. Expect to spend a weekend if you do it this way. It's honestly more expedient if you're already doing an engine rebuild.

Myth 2: The engine vibration is unbearable. Absolutely false. On a stock rotating assembly, I did in fact observe more pronounced vibration as the rpms climbed. However, it was by no means violent or unbearable. having driven a couple of cars where this was done, I do agree that some rotating assemblies are better balanced than others. However, if there was a rotating assembly that caused violent shaking to the point of being undrivable, this would have been observed even with the balance shafts equipped. The oem rotating maintains some measure of balance by design.

Myth 3: Balance shaft elimination causes excessive oil pressure to the point of catastrophic failure. I have mixed thoughts on this, but overall I don't agree with this at all. IMO, you can't go wrong with the increased oil pressure caused by this mod because the stock oil pressure isn't enough. I also feel that the increased oil pressure is not excessive, but where it should be from the get go. However, I recognize that it is possible under certain conditions to have so much oil demand due to the increased oil pressure that it's possible to run "dry" under extreme conditions. We're talking extreme road race and autocross to create these conditions those.

Myth 4: There is absolutely no reason to have to undergo a balance shaft elimination of any sort on a built motor. Absolutely false. Unfortunately, depending on the rotating assembly or rebuild parts you decide to use, your rods won't necessarily clear the balance shafts (or worse to have them just barely clear) on a built motor and you will have to perform this mod
in one fashion or another. Now just because you decide to swap out your rods and pistons doesn't automatically mean you will have to lose your balance shafts. It is a matter of whether or not the rotating assembly clears which determines that. However, you need to be open to the possibility should you decide to build the block.

Now there are a couple of ways to offset the potential cons and side effects. First one is how to address the increased oil pressure. The easiest and cheapest way is to add an extra half quart to a quart of oil. This is something road racers do even without the balance shaft elimination on account that the stock oil baffling in the stock oil pan and windage tray does't do the best job of keeping enough oil available under extreme cornering conditions (something you won't experience outside of an autocross track).
Another fairly inexpensive way is through a part made by Kiggley Racing. They offer a replacement HLA which is a direct replacement to the oem one. What it is is essentially an oil restrictor for your cylinder head, which doesn't need or benefit from excessive oil pressure. By restricting the oil flow to the head, it keeps more oil in the block where is really need to be.
Finally there are modified windage trays and oil pans available designed to keep oil more oil in the pan.
As for how to address any excess vibration, there is only one way to address it. Balance your rotating assembly. There are a number of machine shops that can do it, and it's not terribly expensive to do in the scheme of things. However, it involves the removal of your entire lower rotating assembly to get it done.

I personally am using a combination of all of the methods above in my build. You don't have to do that. It is ultimately up to you either by personal decision or with consultation with your engine builder as to how this will be accomplished.

I hope this helps a few of you.

Jon