Precision Engine Balancing
The Final Lap - Feb. 2009 Newsletter ver. 1.0
To all of our patrons and supporters,
This month, I will write a little about one of the most overlooked aspects of engine building.... engine balancing. Just because it's a Honda engine and they are known to last well beyond 200,000 miles in factory trim, does not mean that it will do the same for a high-performance build. The only part of the rotating assembly that is "pre-balanced" so to say, from the factory are the crankshaft and crank pulley. The piston and rods are not; because Honda has very accurate casting and machining processes that the tolerances are so close, that they are only off by "x-amount" of grams. This variance is so minimal; that it has almost no effect for daily commuting and driving within the range for what the engine was designed for. Now, when stepping into the realm of high-performance parts and engine building, the table is completely turned. Especially, when you have hybrid builds such as LS/B20 VTEC and rev limitations exceeding 8.5 and 10k rpm's. We all know that revs kill a motor, and if it is not balanced correctly, then the engine will fail much sooner than it normally would. Which is why we offer "Precision Balancing". Our precision balancing of rotating assemblies have a variance of only .01g to .5g of a gram (1.0g). What this means is that pistons and rods, if not exact, will have a variance from a hundredth of a gram to half a gram. This variance spectrum is near perfect, thus allowing our engine builds to see high rpm and perform at that rpm as needed. There are only a handful of specialty machinists in the balancing field that can perform such precision balancing. Our private machinist has over 35 years of precision balancing experience! Now on to the good stuff...
Balancing goes hand-in-hand with performance clutch and flywheel sets. Balancing reduces internal loads and vibrations that stress metal and may eventually lead to component failure. A smoother-running engine is also a more powerful engine. The crank wastes less energy as it thrashes about in its bearings, which translates into a little more usable power at the flywheel. Reducing engine vibration also reduces stress on motor mounts and external accessories.
As previously mentioned, no engine is going to survive long at high rpms if it is out of balance. And no engine is going to last in a high-mileage application if the crank is bending and flexing because of static or dynamic imbalances in the flywheel.
To better understand the mechanics of balancing, look at the theory behind it. If you didn't already know, a rotating object generates "centripetal force". Centripetal force is an actual force or load generated perpendicular to the direction of rotation. For example, tie a rope to a brick and twirl it around and you'll feel the pull of centripetal force generated by the "unbalanced" weight of the brick. The faster you spin it, the harder it pulls. In fact, the magnitude of the force increases exponentially with speed. Double the speed and you quadruple the force.
The centripetal force created by a crankshaft imbalance will depend upon the amount of imbalance and distance from the axis of rotation (which is expressed in units of grams, ounces or ounce-inches). A crankshaft and flywheel with only two ounce-inches of imbalance (9.5 grams at 6" radius) at 2,000 rpm will be subjected to a force of 14.2 lbs. At 4,000 rpm, the force grows to 56.8 lbs.! Double the speed again to 8,000 rpm and the force becomes 227.2 lbs.
This may not sound like much when you consider the torque loads placed upon the crankshaft by the forces of combustion. But centripetal imbalance is not torque twisting the crank. It is a sideways deflection force that tries to bend the crank with every revolution. Depending on the magnitude of the force, the back-and-forth flexing can eventually pound out the main bearings or induce stress cracks that can cause the crank to snap.
Centripetal force should not be confused with "centrifugal" force, which is the tendency of an object to continue in a straight trajectory when released while rotating. Let go of the rope while you are twirling the brick and the brick will fly off in a straight line (I do not recommend trying this, because it is difficult to control the trajectory of the brick. =)). Ok, back to centripetal force. As long as an equal force in the opposite direction offsets the amount of centripetal force, an object will rotate with no vibration. Tie a brick on each end of a yardstick and you can twirl it like a baton because the weight of one brick balances the other. If we are talking about a flywheel, the flywheel will spin without wobbling as long as the weight is evenly distributed about the circumference. A heavy pot at any one point, however, will create a vibration because there is no offsetting weight to balance out the centripetal force.
Ok, back to high school and college...This brings me to another law of physics. Every object wants to rotate about its own center of gravity. Toss a chunk of irregular shaped metal into the air while giving it a spin and it will automatically rotate about its exact center of gravity. If the chunk of metal happens to be a flywheel, the center of gravity should be the flywheel's axis. As long as the center of gravity for the flywheel and the center of rotation on the crankshaft coincide, the flywheel will spin without vibrating.
But, if there is a heavy spot on the flywheel, or if the flywheel is not mounted dead center on the crank, the center of gravity and axis of rotation will be misaligned and the resulting imbalance will create a vibration.
If you are rebuilding an engine that is internally balanced (all Honda engines and most foreign engines), the flywheel and damper (crank pulley) have little, to no effect on engine balance and can be balanced separately. Which is why when we balance a rotating assembly, we ask that you provide us with the flywheel, crank pulley and the clutch pressure plate that you intend on using with your build, along with their respective bolts. Whenever replacing a clutch or flywheel, we can also have the pair balanced as well. To insure exactness, we also mark the flywheel and clutch pressure plate, to insure that it is installed on the vehicle, just as we had it installed during the balancing process. And during our crankshaft balancing process, aside from adding the weight of the pistons and rods to the crankshaft, we also account for oil that is picked up by the crank during operation.
I hope that I have shed some light on the subject at hand and have educated those that did not know the importance of balancing. Majority of end users drive their cars daily and also take part in spirited driving on a daily basis as well. With this in mind, they put their engines together and drop them into the car, not expecting to take them apart to check wear, clearances and evaluation, like we do or how race teams do. This makes balancing even more important. Like I mentioned in the January newsletter on preventative maintenance, balancing is like preventative maintenance. With all the money spent on your engine build, the parts, the labor, the time and the effort. You want to make sure that it will run as intended, as reliable as can be and to maximize the potential and output of your engine.
I also understand that many of our customers do not have the luxury of a specialized machinist that can provide such a service. Therefore, we can provide the service for you. Don't hesitate to send us your rotating assembly and related components for a precision balance. We've been balancing our customer’s parts for the last 10 years, all over the United States and abroad. Sound crazy?? Well it isn't when you add up all the money spent on a high profile race engine that is making big power and only meant to last six races. We want to make sure it lasts all six and performs as expected. We also want to keep you ahead of the competition and in the winner’s circle...any way that we can.
Give us a call at 714-903-1898 for pricing and info on other services such as connecting rod service, crankshaft service and custom piston machining.
In the next newsletter, I will discuss why it's sometimes better to use OEM Honda parts in place of aftermarket parts for your naturally aspirated engine build.