MENUThe main areas for engine balancing.
"A balanced article."
We all know what happens when a wheel is fitted to a car without being balanced.
At low speeds you do not notice it but at higher speeds noticeable vibrations start to appear.
Inside an engine the rotational speeds typically reach 6-7000 rpm and, the higher the speed, the more critical it is to ensure that vibrations do not occur.
As you can see from this cross section of a piston there are a number of parts which move in each stroke of the engine, in varying directions and speeds, and if these are imbalanced you will end up with vibrations and internal stress.
The benefits of engine balancing
A balanced engine will generally be more efficient and reliable and will certainly tolerate much higher engine speeds.
We must remember that no engine can be perfectly balanced due the amount of subtle factors involved but we can certainly improve upon the balance of a mass produced engine. Torquecars members will typically want to maximise the power delivery of their engine.
They will have already modified many other aspects of the engine but we should not overlook the benefits of balancing the engine first, as this enables us to sustain higher engine speeds and gives us reliability.
When balancing an engine there are 2 main areas to focus on, firstly the components inside the engine, their weight and how accurately machined they are and secondly the forces created within an engine under combustion.
When each cylinder reaches combustion ideally the amount of energy released should match that of the other cylinders. Firstly though we will look at the physical engine components from a balancing perspective.
Blueprinting an engine
Balancing the physical engine components is often referred to as "Blueprinting", as the components are carefully matched and machined as a set with each engine being unique.
Just as a tyre installer has a special machine to balance your wheels a machine shop have a dedicated engine balancing machine onto which the components are fitted, rotated and a computation is made of where adjustments need to be made.
The main components in an engine to be considered for balance are the flywheel, bearings, pistons & rings, the piston rods, pins and the crank. Weight is removed from the heavier side of a component to achieve balance but sometimes weight has to be added to the lighter side.
The pistons and rods are machined as a matching pair (with drilling away excess metal & again occasionally adding metal around the pin boss and the inside of the crown) and the weight and dimensions of each is also closely matched you will be well on the way to having a nicely balanced engine.
The counterbalance in the crank usually has metal cut away and sometimes weld added to add metal to adjust the weight and in extreme cases, a tungsten alloy is added, which is substantially heavier than most other metals although this is an extreme solution.
It is worth remembering that weight incrementally increases imbalance so using lighter components can assist with the aim of getting a nicely balanced engine although we should stress that often lighter components are weaker.
The crank, located at the bottom of the engine has quite a hard job as it converts the vertical motion of the pistons into rotational forces. Crankshafts are generally forged from steel although they can also be cast from iron but the forged steel ones are lighter and are more compact.
A machined crank from a billet of steel will be stronger as the metal has not been subjected to heating and cooling cycles of casting or the additional heavy pounding of forging. In some performance applications the crank shaft is balance lightened to reduce its rotational mass and then fitted with counterweights made from a tungsten alloy (or similar) to help balance the rotation of the shaft.
Crank shafts will suffer from sideways motion as they convert the vertical piston action into rotation and the bearings combat this, so getting good quality well machined bearings is also an essential in creating a balanced engine with high performance engines potentially carrying a larger number of bearings.
The crank also endures a twisting motion so can be subject to fatigue. Various engine configurations attempt to address the problem of the forces on the crank in different ways, ranging from the v6 and v8 to flat 4s and boxers but no engine is perfect and they all suffer from an amount of vibration the best we can hope for is to maintain the engine balance throughout our usable rev range.
Balancing the engine dynamics
The next area of balance involves the dynamics of the engine itself. If the compression ratio is out between cylinders you will have an imbalance and vibration happening as the engine turns over.
The compression ratio can be affected by a large number of factors, cylinder wall damage, gasket imperfections, poor valve seating and even a badly machined spark plug.
It is a good practice to take compression readings from each cylinder at regular intervals and check that everything is within an acceptable tolerance.
Check the camshaft also as uneven wear will cause the valves to open at different intervals and amounts and this can upset the delicate balance between cylinders.
Assuming that each cylinders compression matches you now have to balance the fuelling to each cylinder. Again the injectors should be matched and need to deliver the same amount of fuel to each cylinder and the fuel to air ratio also needs to matched.
A gas flowed head (see the TorqueCars article on port matching also) will help to ensure equal air delivery to each cylinder although getting the compression ratio matched is more important. When an engine is balanced you will hear a purr as it ticks over.
Get used to the sound of the engine exhaust note on tick over and if you lose the regular throbbing purr it will generally indicate that something has moved out of balance in the engine.
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4 Responses to “Engine Balancing – balancing pistons, rods and crank.”
I’m building a sbc 400 , will b an 406 with my purchase of Scat crank…then later bought Scat “H”-Beam rods, with speedpro flattop piston’s…. OK I know this crank was balanced at Scat, an the sbc 400 is externally balanced… An I’ve weighed my piston’s with pins an circlips an the ring’s installed= matched weight…rods with bearings installed= matched weight…. An have a nice pricy balancer with Scat flywheel…. My question is do I need to get balanced ?…everything is pretty much equal matched weight, just need sum advice from someone who’s experienced this b4 ?…all answers will be took inconsideration…thanks
When fully assembled you need to balance it rotationally. Component weight is obviously a large part of this. It’s just like doing a wheel balance I suppose, any vibrations caused under rotation need to be smoothed out. Parts can all weigh the same but engineering, different tolerances in assembly and any other variations come into the picture and need to be addressed. Bear in mind that most modern engines are balanced well already but if you wish to raise the red line it is a good idea to make sure it’s in perfect balance dynamically, we haven’t yet seen an engine which couldn’t be improved.
If I replace pistons with ones that are significantly different from the original one, do I have to re-balance the crank shaft or the complete assembly?
If all the pistons and components are balanced and matched, you should get away with it but I’d always feel safer ensuring the entire assembly is balanced.