unorthodox crank pulley

[jmauld]

Read the link I posted. The harmonic balancer is not there for rotational stresses, but to counter stresses that come from the actual combustion process. If that combustion is a whole lot stronger, those stresses are higher.

yep, I follow that completely.

Here's an excerpt from the DSM Technical manuals, just to provide a view of why the designing engineers put them there.

"The crankshaft pulley not only drives the water
pump and alternator, but also functions as a
torsional damper to reduce torsional vibration of
the crankshaft."

So, is the problem the turbo kit? No. The problem is the combination of the underdrive pulley and the turbo kit. It's possible to claim that the problem is a badly designed oil pump, but they do work very well when they're operating as designed. Since the turbo kit will double your horsepower and the underdrive pulley might give 5, it's pretty easy to choose

Exactly. If you want 5 extra hp out of a turbo car, then there are PLENTY of other things you can do.

Is the miata oil pump driven from the pulley, or is it driven from a seperate pulley or gears?

Keith, I'm glad that you didn't feel I was flaming you. That wasn't my intentions. I like to discuss things and try to cover all possible cases, especially things that I (or others) may be overlooking.

 

[Keith@FP]

Is the miata oil pump driven from the pulley, or is it driven from a seperate pulley or gears?

It's driven directly by the crank - it goes through the oil pump then on to the pulley. Vibration at the end of the crank (ie, no dampening) causes chattering on the faces that drive the oil pump until failure occurs. The Protege has the same oil pump arrangement.

Keith

 

[jmauld]

It's driven directly by the crank - it goes through the oil pump then on to the pulley. Vibration at the end of the crank (ie, no dampening) causes chattering on the faces that drive the oil pump until failure occurs. The Protege has the same oil pump arrangement.

Keith

Does a lightened flywheel not have the same effect?

 

[Keith@FP]

No, because the flywheel is at the other end of the crank where the dampener is about 1" away from the pump. There are 5 bearings between the flywheel and the pump. The flywheel is also attached to the tranny.

The power gain from a lighter crank pulley is the same as the gain from a lightweight flywheel - it's rotating mass. However, the flywheel is not intended to dampen the same sort of stresses as the harmonic dampener.

I'll put it this way - most turbo Miatas have lightweight flywheels and they don't have oil pump trouble.

Keith

 

[Traveler]

Quick class. The harmonic damper is called that for a reason. Your crank flexes torsionally everytime a cylinder has a power cycle. The piston pushes the connecting rod down which in turn pushes on the rod journal of the crank. Since there are other things hooked to the crank, the first thing that the cranks tries to do is resist rotation. This causes the crank to twist ever so slightly as the power cycle overcomes the parasitic and rotational drag. It stays flexed well into the power stroke until the combustion pressure drops off and then rebounds. Your crank is doing this with every power stroke on every cylinder.

This causes harmonic resonance as the crank flexes along its entire length the whole time it is running. Some of these harmonics are damped out by the parts connected to the crank. There are certain RPM's however, where the harmonic resonance CANNOT be damped out by the masses connected to the crank. These are called 'critical rpms'. These are the points that the crank is most likely to fail as the harmonics will self amplify unless they are damped out. That's where the harmonic damper comes in. Basically it's a thin piece of blended rubber (the durometer depends on the engineering of the motor it goes to) that is stuck between two masses. The part that attaches to the crank nose, and the outer ring which the pulleys are usually part of on newer cars. On older cars the damper and the pulleys were seperate. The pulleys bolted onto the front of the damper.

Now, on higher output motors, the critical RPM's have more amplitude since the crank is flexing farther than it was with the stock output. You can strengthen the parts (forged crank and rods, cryogenic treatment) to help combat this. What you DO NOT want to do, is take a line of defense away from a stock crank. It's one of the things keeping it all together. If you plan on making power on a stock bottom end, don't even bother. It's not worth the couple of HP. There are better ways to make power.

As for flywheels, they are an energy storage device. They help smooth out the rotation of the engine by having a mass that wants to keep rotating between power cycles. This makes your engine run smoother. If you use a lighter flywheel, you will have to use slightly more throttle to get the car moving since less energy will be stored in the flywheel and the weight of the car will have an easier time slowing down the smaller rotating mass. Once you get moving however, you'll have more throttle response and an engine that can accelerate faster. It won't 'make' more HP, but it will provide less resistance to rotation so less of the HP at the crank is lost to accelerating a larger mass. The same thing applies to wheels and tires. If you use lighter wheels and tires, you will see an increase of HP to the wheels.

 

[jmauld]

Quick class. The harmonic damper is called that for a reason. Your crank flexes torsionally everytime a cylinder has a power cycle. The piston pushes the connecting rod down which in turn pushes on the rod journal of the crank. Since there are other things hooked to the crank, the first thing that the cranks tries to do is resist rotation. This causes the crank to twist ever so slightly as the power cycle overcomes the parasitic and rotational drag. It stays flexed well into the power stroke until the combustion pressure drops off and then rebounds. Your crank is doing this with every power stroke on every cylinder.

This causes harmonic resonance as the crank flexes along its entire length the whole time it is running. Some of these harmonics are damped out by the parts connected to the crank. There are certain RPM's however, where the harmonic resonance CANNOT be damped out by the masses connected to the crank. These are called 'critical rpms'. These are the points that the crank is most likely to fail as the harmonics will self amplify unless they are damped out. That's where the harmonic damper comes in. Basically it's a thin piece of blended rubber (the durometer depends on the engineering of the motor it goes to) that is stuck between two masses. The part that attaches to the crank nose, and the outer ring which the pulleys are usually part of on newer cars. On older cars the damper and the pulleys were seperate. The pulleys bolted onto the front of the damper.

Now, on higher output motors, the critical RPM's have more amplitude since the crank is flexing farther than it was with the stock output. You can strengthen the parts (forged crank and rods, cryogenic treatment) to help combat this. What you DO NOT want to do, is take a line of defense away from a stock crank. It's one of the things keeping it all together. If you plan on making power on a stock bottom end, don't even bother. It's not worth the couple of HP. There are better ways to make power.

As for flywheels, they are an energy storage device. They help smooth out the rotation of the engine by having a mass that wants to keep rotating between power cycles. This makes your engine run smoother. If you use a lighter flywheel, you will have to use slightly more throttle to get the car moving since less energy will be stored in the flywheel and the weight of the car will have an easier time slowing down the smaller rotating mass. Once you get moving however, you'll have more throttle response and an engine that can accelerate faster. It won't 'make' more HP, but it will provide less resistance to rotation so less of the HP at the crank is lost to accelerating a larger mass. The same thing applies to wheels and tires. If you use lighter wheels and tires, you will see an increase of HP to the wheels.

Couple of issues with what you stated.

1) "Some of these harmonics are damped out by the parts connected to the crank."

With the failing oil pumps on the miata, it should be obvious that the accessories are NOT intended to dampen harmonics. Another point of the rubber material is to isolate the engine accessories FROM these vibrations.

2) The flywheel is a BIG weight stuck on one end of the crankshaft. While you are correct in it's primary functions, it WILL dampen shocks to the crankshaft. Thump a little nail and then thump a big nail. Which one vibrates the most? I hate to keep pointing to this, but since the engineers who build engines hopefully know more then you and I do, the DSM Tech Manual also states that the weight of the flywheel was chosen for increased driveability (see your explanation), and the reduction of engine vibrations. In this case engine vibrations is specifically referring to vibrations on the crankshaft.

I'm not suggesting not to use a lighter flywheel.. As Keith pointed out, they are on the other side of the engine from your accessories and their contribution to removing vibrations is probably small, especially while retaining the stock dampener. But, the potential still exists, just as it does with underdrive pulleys.

 

[Keith@FP]

With the failing oil pumps on the miata, it should be obvious that the accessories are NOT intended to dampen harmonics. Another point of the rubber material is to isolate the engine accessories FROM these vibrations.

I believe the items attached to the crank to which Traveler refers are items such as flywheels, rods, etc. Not alternators. The rubber is not there to isolate the accessories at all - the rubber belts likely do a far better job. No, it's to dampen harmonics in the crank.

....the DSM Tech Manual also states that the weight of the flywheel was chosen for increased driveability (see your explanation), and the reduction of engine vibrations. In this case engine vibrations is specifically referring to vibrations on the crankshaft.

No, the reduction of engine vibrations refers to the roughness at idle. A lighter flywheel will not smooth out the power pulses as much as a heavier one, making the car vibrate more. Most people don't like engines that are "rough" like this, so the engineers will spec a heavier flywheel than is ideal for sheer performance. There are shades of grey in here of course - it's possible to take some weight off the flywheel without affecting the idle noticeably.

The flywheel is not intended to dampen crank harmonics. Certainly not to anywhere the same extent as the dampener. It's intended for a different use. A different mass on the flywheel may change the resonant frequency of the crank somewhat, if it's vibrating along the right axis. However, it won't absorb vibration. Once you're done hitting nails and watching them vibrate, hit a chunk of rubber.

Lightweight flywheels have not proven to have the same destructive effect as removing the dampener. 100,000 miles with a lightweight flywheel in a high-power Miata is not a problem.

Keith

 

[1FastMP3]

Ok guys quick question
I understand not changing out the crank pulley, but what about replacing the other two. The waterpump pulley, and the alternator pulley? I know the guys on the Ford Focus website are putting all three on with no problems. If I'm not mistaken though I think they said that there engine is internally balanced. Anyways back to the question would it be ok to change out the other two pulleys

 

[Keith@FP]

Go read back a bit- it's the third time I've posted this in the same thread. Changing the pulleys on the accessories will have no effect on the crank. It may affect the accessories themselves depending on if they have to work harder to do their job - ie, the alternator. But no catastophic failures will be expected. Neither would any significant gains.

Keith

 

[jmauld]

I believe the items attached to the crank to which Traveler refers are items such as flywheels, rods, etc. Not alternators. The rubber is not there to isolate the accessories at all - the rubber belts likely do a far better job. No, it's to dampen harmonics in the crank.



No, the reduction of engine vibrations refers to the roughness at idle. A lighter flywheel will not smooth out the power pulses as much as a heavier one, making the car vibrate more. Most people don't like engines that are "rough" like this, so the engineers will spec a heavier flywheel than is ideal for sheer performance. There are shades of grey in here of course - it's possible to take some weight off the flywheel without affecting the idle noticeably.

The flywheel is not intended to dampen crank harmonics. Certainly not to anywhere the same extent as the dampener. It's intended for a different use. A different mass on the flywheel may change the resonant frequency of the crank somewhat, if it's vibrating along the right axis. However, it won't absorb vibration. Once you're done hitting nails and watching them vibrate, hit a chunk of rubber.

Lightweight flywheels have not proven to have the same destructive effect as removing the dampener. 100,000 miles with a lightweight flywheel in a high-power Miata is not a problem.

Keith

Think of the flywheel, crankshaft and dampener as a system. Not as individual components.

You are correct that the flywheel does not serve the purpose of a dampener. But it does effect the amplitude of the shock wave on the crankshaft, which directly effects the damage capability of those shock waves.

Look at what you said here: "A lighter flywheel will not smooth out the power pulses as much as a heavier one."

And then here:
"The harmonic balancer is not there for rotational stresses, but to counter stresses that come from the actual combustion process. If that combustion is a whole lot stronger, those stresses are higher."

These stresses, during the combustion process, that you refer to, come directly from the power pulses that the heavier flywheel smooths out better. So they are the same thing. If the flywheel is smoothening the pulses out better then the need for a harmonic dampener is reduced (not eliminated). Thereotically, the flywheel could smooth the pulses completely, but who wants to run a 200lb flywheel. I would say that with a lighter flywheel, the need for a harmonic dampener increases.

okay, okay, the nail thing was close, but not perfect. Stick the nails in the chunk of rubber and it will more closely duplicate the system we are talking about.

Anyway, I like this discussion. I wish this board would see more discussions like this.

 

[Keith@FP]

Since we're abandoning actual real-world data and just waving our hands in the air now...

Every time the crank gets a power pulse, it accelerates, being twisted by the cylinder(s) that is firing. Meanwhile, another cylinder is under compression and is twisting the crank as well. This can set up a vibration at certain engine speeds if a resonant frequency is reached. If this vibration is not absorbed, it can cause hightened wear. Thus the dampener.

Meanwhile, the flywheel is smoothing out the natural acceleration/deceleration caused by the intermittent combustion events. It's not absorbing the vibration, but smoothing out rotational changes. Different problem, different solution. Actually, a heavier flywheel means that the crank will resist acceleration more and may actually cause greater localized forces on the crank. May.

But this has all been posted before in this thread and in the link I provided. The real-world result is:

- cars with light flywheels do not exhibit problems because of the flywheel, no matter how much power they make.
- cars with no harmonic dampener and increased power can suffer from catastrophic oil pump failures.

I'm done, seeing as how I'm just posting the same thing over and over. If you're that freaked out about unproven but imaginary potential problems about lightweight flywheels, then don't get one.

Keith

 

[BryanPendleton]

I hate to beat a dead horse, but. . . . .

What is the construction of the Pro5 crank pulley? What about the miatas?

The reason I ask is the 2.0L FS in the 93-97 Probe/MX6/626 is a solid cast iron piece weighing in a just over 9lbs. Now while its mass alone effects the harmonics and natural frequency of the bottom end assembly, I am curious how much of a sacrifice or rather change in harmonics is made by swapping to the UR solid aluminum pulley weighing in at about 14-15oz (just under 1lb). Polymers and rubbers offer significant dampening properties, but solid cast iron? Thats like the difference between solid motor mounts and OE rubber mounts. In the case of miatas it appears that harmonics from the bottomend rotating assembly are being transfered to the accessories (when using UR pulley) which causes them to fail. In the case of the old FS motor, I don't see a cast iron pulley isolating the accessories from bottom end harmonics very much. If accessories did not fail on the miata (when using pulley and F/I) would people use them. Naturally, bottom end harmonics and natural frequencies have still changed, perhaps causing increase wear on main bearing, etc, but would people then not hesitate to install the pulley on there miata. Ultimately, anything that increases torque and power increases wear on the motor. Question is where do you draw the line. Obviously, failing accessories in 2000miles is not acceptable.


I'm curious about the new VQ25DE motors from Nissan too. . . They use some sort of balancing mass that rototes just below the crank. . . Jim Wolf Technology designed or is designing a replacement assembly for this to effectively eliminate that balancer. I believe inline fours are inherently poor configurations for balancing out reaction forces, but with most being small displacement, the forces are probably not the great. Now Nissan's 2.5L I4 is pretty good size for an I4 engine, perhaps nessitating the balancer to ensure a smooth running motor, but curious as to how JWT's new "toy" effects engine wear and longevity.

Good discussion though. . . I have always promoted the pulley on the 1st gen FS motors, because the stock pulley is solid cast iron unit, so I figure harmonics from the bottom end must not be significant to cause any problems with accessories. If the OE pulley had any kind of rubber or polymer dampener on it, similiar to chevy V8's, I would never had promoted it. . . Thats why I trying to get some background into on the miata harmonic dampener and the Pro5's. . .

Enjoy,

 

[jmauld]

I hate to beat a dead horse, but. . . . .


I'm curious about the new VQ25DE motors from Nissan too. . . They use some sort of balancing mass that rototes just below the crank. . . Jim Wolf Technology designed or is designing a replacement assembly for this to effectively eliminate that balancer. I believe inline fours are inherently poor configurations for balancing out reaction forces, but with most being small displacement, the forces are probably not the great. Now Nissan's 2.5L I4 is pretty good size for an I4 engine, perhaps nessitating the balancer to ensure a smooth running motor, but curious as to how JWT's new "toy" effects engine wear and longevity.

Enjoy,

I'm not familiar with the nissan engine, but this balancing mass sounds like a device that just offsets the vibrations made in 4cyl engines, and making the engine "feel" balanced. Otherwise, the vibrations would reach the chassis and annoy the driver. The point of the mass is to make the engine feel smoother.

The harmonic dampener on a crank pulley has a different job. "IF" my above assumption is correct. Which I don't know for certain, so this is just my opinion.

 

[BryanPendleton]

I'm not familiar with the nissan engine, but this balancing mass sounds like a device that just offsets the vibrations made in 4cyl engines, and making the engine "feel" balanced. Otherwise, the vibrations would reach the chassis and annoy the driver. The point of the mass is to make the engine feel smoother.

The harmonic dampener on a crank pulley has a different job. "IF" my above assumption is correct. Which I don't know for certain, so this is just my opinion.

The intent of it was to reduce engine vibrations which is the result of both harmonics and reaction forces created by the rotating assembly, primarily offseting the reaction forces of the rotating assembly though.

The more I think about it, removing that balancing mass from the VQ should not significantly effect the harmonics of the bottom end, but it will effect it. . . .

Still curious about the construction of the Miata and Pro5 crank pulley. . . .

 

[YAK Motorsports]

The more I think about it, removing that balancing mass from the VQ should not significantly effect the harmonics of the bottom end, but it will effect it. . . .

I agree completely!! Just like anything that you do that changes the rotating mass of the engine. Pullies, balance shaft removal, lightened flywheels, light rods. All of that will change the characteristics of the harmonic vibrations, good or bad.

 

[alec172]

So, basically, the crank pulley is part of the dampening system on the engine. The flywheel is too but has less impact on engine premature wear. Is that a fair assumption?

I always though that changing for a lighter flywheel to liberate pwr was a great mod to do.

 

[YAK Motorsports]

So, basically, the crank pulley is part of the dampening system on the engine. The flywheel is too but has less impact on engine premature wear. Is that a fair assumption?

I always though that changing for a lighter flywheel to liberate pwr was a great mod to do.

If the crank pulley contains a harmonic balancer, then it IS the dampening system on the engine. The weight of all of the various engine components, amoung other things, just effect the frequency and strenght of the harmonic vibrations.

 

[Keith@FP]

I don't have an FS front pulley handy to look at. The Miata one appears to be cast iron, but it's not. It's actually a sandwich. I suspect the same is true of the FS pulley.

Counter-rotating balance shafts are for NVH control. A different problem.

Keith