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The decision to delete b/shafts was made in concert with the previous builder. The thinking was that the dynamics of a race engine designed to be able to rev to 8800rpm would not benefit from b/shafts that are there for lower rpm harmonic issues. From my understanding you can't just add b/shafts and covers from another motor as they're paired for each motor individually? Hence we didn't consider retro fitting a set for the current build. I mean, given a choice again I'd probably opt for them however people who have run them on race motors have cited throwing belts as a negative. Also seen enough people run without them with no disadvantages.
The crank was modified for offset grind to try and get a little more tq than a 16v typically produces. Because we only have a factory 968 H pattern gearbox vs sequential gb that pretty much all our competitors have, we thought a bit more tq might be help close the gap. The old 3.1 8v had great low down tq which was a plus on the tracks we drive on. It was not knife edged though. Not going down that path again.
Patrick my knowledge of the balance shafts is that they are there for higher rpm. Their function is to offset the 2nd order vibrations caused by differential piston speed. Because the piston speed is different travelling from TDC to the midpoint as from BDC to midpoint, the opposing piston doesn’t quite balance out. It is a function of stroke length, rpm and reciprocating mass, rod ratio also plays a role. But essentially once the engine is built it becomes purely a function of rpm.
With the increase in stroke and rpm of your engine I would expect the effects to be exaggerated, even if the piston mass has been reduced.
I asked Jon Milledge the same question when I purchased my camshaft and his response was to use the balance shafts as the 5hp loss outweighs the reliability problems if removed. I also researched the daylights out of the subject because I couldn’t find reliable information on the topic. Studying pictures of 944 gtr’s and factory race cars and all still appear to retain balance shafts.
http://en.wikipedia.org/wiki/Balance_shaft Balance shafts are most common in inline four cylinder engines which, due to the asymmetry of their design, have an inherent second order vibration (vibrating at twice the engine RPM) which cannot be eliminated no matter how well the internal components are balanced. This vibration is generated because the movement of the connecting rods in an inline engine is not symmetrical throughout the crankshaft rotation; thus during a given period of crankshaft rotation, the descending and ascending pistons are not always completely opposed in their acceleration, giving rise to a net vertical inertial force twice in each revolution whose intensity increases quadratically with RPM, no matter how closely the components are matched for weight http://en.wikipedia.org/wiki/Harmonic_balancer A harmonic balancer (also called crank pulley damper, crankshaft damper, torsional damper, or vibration damper) is a device connected to the crankshaft of an engine to reduce torsional vibration. Every time the cylinders fire, torque is imparted to the crankshaft. The crankshaft deflects under this torque, which sets up vibrations when the torque is released. At certain engine speeds the torques imparted by the cylinders are in synch with the vibrations in the crankshaft, which results in a phenomenon called resonance. This resonance causes stress beyond what the crankshaft can withstand, resulting in crankshaft failure. To prevent this vibration, a harmonic balancer is attached to the front part of the crankshaft. The damper is composed of two elements: a mass and an energy dissipating element. The mass resists the acceleration of the vibration and the energy dissipating (rubber/clutch/fluid) element absorbs the vibrations.
There is a big key right there. Balance shafts run at double the crank speed to cancel 4th harmonics, while a harmonic balancer runs at crank speed to cancel lower order harmonics
The decision to delete b/shafts was made in concert with the previous builder. The thinking was that the dynamics of a race engine designed to be able to rev to 8800rpm would not benefit from b/shafts that are there for lower rpm harmonic issues. From my understanding you can't just add b/shafts and covers from another motor as they're paired for each motor individually? Hence we didn't consider retro fitting a set for the current build. I mean, given a choice again I'd probably opt for them however people who have run them on race motors have cited throwing belts as a negative. Also seen enough people run without them with no disadvantages.
The crank was modified for offset grind to try and get a little more tq than a 16v typically produces. Because we only have a factory 968 H pattern gearbox vs sequential gb that pretty much all our competitors have, we thought a bit more tq might be help close the gap. The old 3.1 8v had great low down tq which was a plus on the tracks we drive on. It was not knife edged though. Not going down that path again.
I'm probably missing something here, but aren't you tempting fate by building the same motor spec again? Did I also read something about a redesigned stronger crank girdle idea.
There is a big key right there. Balance shafts run at double the crank speed to cancel 4th harmonics, while a harmonic balancer runs at crank speed to cancel lower order harmonics
This isn’t correct. Because the frequency of the vibration occurs twice per revolution it is referred to as second order. By spinning the balance shafts at twice engine speed and phasing them 180deg from each other they cancel out the piston once on the upstroke and once on the downstroke.
The harmonic dampener is for torsional load and can in fact help 2nd order vibrations as they are loaded to the crankshaft. It is not limited to 1st order vibrations. The harmonic dampener is absorbing all 4 cycles of all cylinders as well as any other vibrations that is loaded to the crank, it does this constantly many times per revolution.
When I bought an S2 crank I made sure to also get matching crank dampener. When I changed the belts recently I did think about fitting it to the 2.5 engine, but as Porsche didn't do that, I didn't bother.
I believe the balance shafts are phased alike in the vertical plane. They are both down at each of the top dead center crankshaft positions.
its more clever than that!
the balance shafts rotate in opposite directions at twice the speed of the crankshaft.
The point of imbalance in the reciprocating assembly is 90 degrees form TDC. You would think that the pistons cancel each other out but due to the geometry of the crank and rods the pistons are not exactly half way down the bore at 90 degrees (and 270 degrees) from TDC. This creates a situation where the pistons do not balance each other. The balance shafts are designed to go in and out of phase due to rotation in opposite direction. When the balance shafts are at mid rotation the offset weights create equal and opposite forces and cancel each other. When the offset weights both point up or down they create a force that is opposite the inherent imbalance is a four cylinder engine.
Clear as a bell - right?
Sort of .
I'll add to my earlier thoughts that at 90 degree CS and 270 degree CS, both balance shaft weights are up.
At 0 degrees CS and 180 degree CS, both balance shaft weights are down.
Fabulous!
The force calcs are way beyond my science. Do the quantities in the "position" column represent degrees of rotation for a balance shaft?
That would confirm that max downward force (negative 1.89) would be at each crankshaft TDC, 0 and 180 degrees and the max upward force (positive 1.89) would be at the mid piston stroke crossover, 90 and 270 degrees. I'm trying to graph the piston travel relative to this for better understanding.
Are the balance shaft weights to be considered when changing reciprocating mass with lighter pistons / rods?
Fabulous!
The force calcs are way beyond my science. Do the quantities in the "position" column represent degrees of rotation for a balance shaft?
That would confirm that max downward force (negative 1.89) would be at each crankshaft TDC, 0 and 180 degrees and the max upward force (positive 1.89) would be at the mid piston stroke crossover, 90 and 270 degrees. I'm trying to graph the piston travel relative to this for better understanding.
Are the balance shaft weights to be considered when changing reciprocating mass with lighter pistons / rods?
Close! the max force is 2 at TDC, the 1.89 was for 19 degrees before TDC (341 degrees). the first and last rows are TDC.
when the balance shaft's are at 90 and 270 degrees (thats crank degrees) the vertical forces will be 2 and at 0 and 180 they cancel each other and the additive force is 0.
You are correct - if you make the pistons lighter you should make the balance shaft counter weights lighter. that would be really annoying to figure out - except there is a easy 'cheat'. Figure out how much lighter the pistons are (in percent) as well as the small end of the rod (also in percent) and then cut off that percentage of length of the balance shaft counter weights. You can lose 20% of the weight before you get close to the mounting point of the plastic part of the counter weight (thats just there for windage purposes). one of the reasons I did the calculations is that you can 'fudge' the change by making the balance shaft gears one or more teeth off.
Here is an example of advancing 1 gear 2 teeth and retarding the other by 2 teeth
You can see that is reduces the vertical element so that you could make up for lighter pistons - but you add a lot of horizontal forces that will probably make up more vibration that you fixed with the vertical forces!
Thanks all for chiming in. However Chris, would you run a very custom race motor without balance shafts? Dry sump. Twin billet fuel rails. All the usual suspect items have been removed or redesigned. I've read enough people who have run race motors without balance shafts with no issue.
Thanks all for chiming in. However Chris, would you run a very custom race motor without balance shafts? Dry sump. Twin billet fuel rails. All the usual suspect items have been removed or redesigned. I've read enough people who have run race motors without balance shafts with no issue.
I guess the question is what’s the reason to delete them? 3-5hp gain is nothing to a built turbo engine. I have found that most people delete them because of leaks….usually caused by crank case pressure. Most well built engines don’t have balance shaft leaks, and dry sump engines usually run a little crank case vacuum so really not likely to leak. So I don’t see any positives to removing the balance shafts on a good engine and Porsche tried it and went back to balance shafts on their endurance engines.
I don’t think it’s a big deal for sprint engines, but I would nut and bolt the engine more often!!!
The point of imbalance in the reciprocating assembly is 90 degrees form TDC. You would think that the pistons cancel each other out but due to the geometry of the crank and rods the pistons are not exactly half way down the bore at 90 degrees (and 270 degrees) from TDC. This creates a situation where the pistons do not balance each other.
What would the stroke or rod length need to be to make them balance at 90* from tdc? From looking at the upper and lower collums in your chart I'm guessing one half the difference at 720*, being 38, is needed. Just not sure what those collums are measuring.
I could maybe make a solver in excel if you could help me with some particulars.
What would the stroke or rod length need to be to make them balance at 90* from tdc? From looking at the upper and lower collums in your chart I'm guessing one half the difference at 720*, being 38, is needed. Just not sure what those collums are measuring.
I could maybe make a solver in excel if you could help me with some particulars.
can’t be done! Basic geometry can’t be out smarted. The only way to cure the imbalance is to make the throw 0 - and that makes for a pretty crappy engine.
02-05-2023, 04:08 AM
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