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Let's think about the revolutions of BS. It spins at double the speed of crankshaft. Crankshaft goes through a power stroke every 180 degree rotation (1/2 rotation). Each 1/2 rotation, BS makes a full rotation. It looks like the BS is trying to counter balance the power stroke. The rear balance shaft doesn't look as off-centered as the front balance shaft, so that's probably why it doesn't seem as important. I think the front is left in when the rear BS is deleted.
Thinking more about race engines, most will upgrade to lighter pistons, so the rotational mass decreases, so it actually makes sense to delete the rear BS which has a smaller weight offset. Very interesting!
Update: It definitely is possible since front and rear BS are rotating at different rates to counter the vibrations of power strokes. So now, I don't have an opinion about deleting BS on engine builds. I think it can be machine for new aftermarket engine builds.
Look at post #24.
He describes BS as a counter balance to reduce harmonics from piston affects on crankshaft. It sounds good but I don't believe it. Why? If it is to counter the forces of the 4 strokes, it doesn't make sense to spin it double speed. It should be spinning at 1/2 speed because 4 strokes takes 2 revolutions at crank, so BS spinning at 1/2 speed makes more sense. The weight would be at 4 different off-centered locations (like each pair of lobes on camshafts). Still a good explanation and he could be right. When I have free time, I'll look up the shape of the BS, now that I'm curious about it.
Front and rear balance shafts are the same?
Here is another picture of front BS sprocket. It has 18 teeth. And the outter crank sprocket has 24 teeth. So the front BS turns 1/3 more than crankshaft. Rear BS sprocket has 19 teeth while inner crankshaft sprocket has 38 teeth. Oh wow!!
Last edited by 2006EvoIXer; Feb 16, 2018 at 09:41 PM.
Front and rear balance shafts are the same?
Here is another picture of front BS sprocket. It has 18 teeth. And the crank sprocket has 24 teeth. So the front BS turns 50% more than crankshaft. Rear BS sprocket has 19 teeth while inner crankshaft sprocket has 38 teeth. Oh wow!!
They both turn at the same speed, double crank rpm. There's a second gear behind the oil pump.
There's two gears inside the pump that mesh. One is driven by the timing belt and the other drives the balance shaft. Notice they have timing marks. That's to index the balance shaft.
There's two gears inside the pump that mesh. One is driven by the timing belt and the other drives the balance shaft. Notice they have timing marks. That's to index the balance shaft.
So counting the teeth on these gears, I get 21 teeth on input from timing belt, and 14 from gear to front BS.
So the timing belt sprocket to front BS sprocket increases rotation by 4/3 and oil pump gears increases it further by 3/2 to give us 2x the crankshaft speed!! So front and back BS do spin at double crankshaft.
I don't know the position of rear BS when timing is set, but the rotation is clockwise, which is the same as the crank. The front should be with the weight down and rotation is in opposite direction of crankshaft (which is counterclockwise, thanks to the oil pump gears). This tells me that the weight is designed to balance when ignition occurs when fuel pushes the pistons down. By half way down of the power stroke (90 degrees), the BS is rotating back up to by the time the next piston is about to get spark, BS is ready to go up again. My best guess is the front BS balances the pistons as fuel ignites.
The Japanese engineers that built the car and engine knew what they were doing. With proper service intervals, RPM limits,moderate power and a STOCK longblock the balance shafts will do exactly what they were designed to do. Changing the bottom end will 100% change the balance and harmonics of the engine and the balance shafts won't be able to do the job they were intended to do. Adding a stroker crank exaggerates this. Anytime the shafts are deleted or the rotating assembly is changed the engine will need an ATI or Fluidampr to help with harmonics. Bearing wear and crankshaft flexing are also something to keep in mind at higher power levels. Here is a helpful link. http://www.fluidampr.com/what/why-do...aft-damper-53/ http://www.atiracing.com/products/dampers/101/index.htm
Thinking deeper into it , changing cams and valve springs MAY have some type of effect on the harmonics but since none of us are engineers or have expensive testing equipment we will never know. This isn't as important but worth mentioning in this discussion.
Cliff notes, stock engine at moderate power keep them. Built engine, delete them and add a good dampner.
The Japanese engineers that built the car and engine knew what they were doing. With proper service intervals, RPM limits,moderate power and a STOCK longblock the balance shafts will do exactly what they were designed to do. Changing the bottom end will 100% change the balance and harmonics of the engine and the balance shafts won't be able to do the job they were intended to do. Adding a stroker crank exaggerates this. Anytime the shafts are deleted or the rotating assembly is changed the engine will need an ATI or Fluidampr to help with harmonics. Bearing wear and crankshaft flexing are also something to keep in mind at higher power levels. Here is a helpful link. http://www.fluidampr.com/what/why-do...aft-damper-53/
Thinking deeper into it , changing cams and valve springs MAY have some type of effect on the harmonics but since none of us are engineers or have expensive testing equipment we will never know. This isn't as important but worth mentioning in this discussion.
Cliff notes, stock engine at moderate power keep them. Built engine, delete them and add a good dampner.
Thats one way to think of it. But there are also other possibilities that make sense. Just because a part exists on the car doesnt mean it is because its a necessity for longevity. Example EGR, EVAP canister, catalytic converter, CAM profile for MPG/smooth idle and not necessarily best power, possibly even balance shafts etc... They add certain parts and designs to pass emissions laws, attract majority of consumers, luxury, etc..
We enthusiasts might buy a car that has vibration at certain RPM range or idle with heavy cams etc.. but the majority which are the actual profit targets wont buy. The RS is supposed to be a bare bones race car version of the evo. No CD player, No power windows, No sound deadening etc.. but yet Mitsu still added AC on the US cars? Why is that ?
One side of the coin assumes BS’s are needed for longevity of the motor. The other side says there are similar motors that dont have them. If we use the oem Mirage motor parts as delete, is the mirage motor better than the evo to not have it ? Is the Mazda CX-5 2.0L skyactiv-G motor better than the 2.5L skyactiv-G for not having it ?
All inline 4 motors from my understanding produce these secondary vibrations . Perhaps, it is a luxury addition, based on a few things such as Power output, displacement etc.. that determines if BS’s are needed.
I’m not saying I'm right. Just sharing a few things for healthy discussion.
Thinking about where the weight is on the BS, it looks like the plan was to balance the first 90 degrees (first 1/2) of the power stroke at the 3 & 4 cylinders before the energy gets to the flywheel. It makes sense since you have the clutch and then the tranny and transfer case. The biggest vibration would be from fuel ignition and this varies greatly since some of us make 500whp out of the stock block. Fortunately, the increase in forces isn't proportionally more than stock (otherwise, we wouldn't net any more power) because most of the increased energy just spins the engine faster. There is a slight increase in lateral forces that the bearings have to deal with but I think it's the twisting forces we should be worried about.
Cams and valve springs will create more resistance in timing belt. I can't imagine it causing more harmonics than a lightened flywheel since the timing belt will stretch and contract to reduce it. One good side effect of a lightened flywheel is that the crank isn't as prone to twisting because the lightened flywheel moves easier.
I agree with what you're saying. We are here to understand what parts appear to do and by better understanding, we can make an informed decision.
My thinking is the engine is balanced when stock. We make it a little more off-balance with each mod. There will be a point where the off-balance is so much that removing the BS wouldn't make engine worse. I also think this point is when we change the bottom end.
Rotational off balance isn't as bad as lateral vibrations. The clutch is designed to transfer rotational energy from engine to transmission, so the pulses of power won't be a problem. Now, if we get vibrations where clutch is moving sideways (forward and backwards if sitting in car) from the clutch's view, your clutch is forced to clamp disk at angle that isn't rotational. This is what Jack's Transmission was showing in pictures of worn clutch disks where the outside material started chipping off since nothing is resisting that sideways force, so the weakest areas start falling off.
Again, this is likely from shifting at high RPM ranges. I use my 2-step launch control and then shift at 8000rpm, so I have no plans on removing my BS.
Originally Posted by BluEVOIX
Thats one way to think of it. But there are also other possibilities that make sense. Just because a part exists on the car doesnt mean it is because its a necessity for longevity. Example EGR, EVAP canister, catalytic converter, CAM profile for MPG/smooth idle and not necessarily best power, possibly even balance shafts etc... They add certain parts and designs to pass emissions laws, attract majority of consumers, luxury, etc..
We enthusiasts might buy a car that has vibration at certain RPM range or idle with heavy cams etc.. but the majority which are the actual profit targets wont buy. The RS is supposed to be a bare bones race car version of the evo. No CD player, No power windows, No sound deadening etc.. but yet Mitsu still added AC on the US cars? Why is that ?
One side of the coin assumes BS’s are needed for longevity of the motor. The other side says there are similar motors that dont have them. If we use the oem Mirage motor parts as delete, is the mirage motor better than the evo to not have it ? Is the Mazda CX-5 2.0L skyactiv-G motor better than the 2.5L skyactiv-G for not having it ?
All inline 4 motors from my understanding produce these secondary vibrations . Perhaps, it is a luxury addition, based on a few things such as Power output, displacement etc.. that determines if BS’s are needed.
I’m not saying I'm right. Just sharing a few things for healthy discussion.
Feb 16, 2018, 05:12 PM
