Problems with lightweight flywheels and underdrive pulley?

czo79 · Post by **czo79** » Sat Nov 08, 2003 9:41 pm

Heya...
I remember hearing something about it maybe not being good to run a lightweight flywheel with a lightweight/underdrive crank pulley....that it throws a CEL, and maybe some other stuff, can't remember. I heard someone say they havn't had problems with the exedy flywheel and a pulley. I think exedy makes flywheels for sti, so one would think they would be ok.
any thoughts?
thanks
Micum

entirelyturbo · Post by **entirelyturbo** » Sat Nov 08, 2003 10:32 pm

There is one problem that lightened flywheels and other rotating components have: a reduction in momentum. That's good and bad. A lightened flywheel takes less time to be revved up, especially good for double-clutching and rev-matching, since there is not as much inertia. However, this also means that the flywheel will take less time to slow down. So let's say you go to pull away from a stop at 1000 rpm (my average takeoff RPM) If you don't give it the proper amount of gas, or engage the clutch just a little too quickly, you will most likely stall, because the engine doesn't have a heavy flywheel to help keep it rotating... Many Honda people, who don't have gobs of low-down torque, put light flywheels on their cars and report a stumbling situation up a hill, since the engine doesn't have momentum to keep it spinning...

But if you're pretty leadfooted, I don't think it will be a huge problem

THAWA · Post by **THAWA** » Sat Nov 08, 2003 11:23 pm

Isn't there also a problem with engine balancing if you get them too light? Since we don't have harmonic balancers and such.

Post by **Legacy777** » Sun Nov 09, 2003 2:22 am

dave's got both.

You can argue both ways.....in actuality.....I wouldn't worry too much.

I'm hoping to soon get a light weight stock sized pulley for crank and smaller alternator pulley to help with my alternator issues.

Nigel · Post by **Nigel** » Sun Nov 09, 2003 3:57 pm

The Legacy's do have a harmonic ballancer. Before you consider replacing it with a solid pulley, or replacing any of the other pulleys with underdrive units, you should read this article by Steve Dinan of Dinan BMW:

http://www.toyotacelicaonline.com/pulleys.htm

Nigel
'94 Legacy Turbo
'73 240Z Turbo

Post by **Legacy777** » Sun Nov 09, 2003 5:08 pm

Our engines are inherently more balanced then an inline engine. While there is a possibility of still having problems, I don't think it's as bad on our engines.

czo79 · Post by **czo79** » Sun Nov 09, 2003 11:48 pm

I guess that the exedy flywheel itself isn't a problem, since I think they make STI ones....and they are heavier than many others. I remember mike sheilds from SPD telling me something about needing to modify the pulley and belts when using really light flywheels...I guess they have to do it on WRC cars. I also remember a lot of people saying that the crank pulley doesn't serve as a harmonic damper on subaru engines. I also remember other people debating this, and can't remember who "won". After reading that Dinan article...it does seem like this wouldn't be nearly as much of an issue in our engines, but I can't say that it wouldn't be any issue. Did anyone else read that? I think I am probably going to go ahead and put on the flywheel, if I end up having to do the clutch.
Thanks all
Micum

evolutionmovement · Post by **evolutionmovement** » Mon Nov 10, 2003 7:22 am

I have the same dilemma as my engine came sans flywheel. Do I get the stock one cheap or a lighter one? I wonder what driveability issues may ensue if I go light. Of course, there's also my budget to consider...

The article about the dampers was talking specifically about BMW's which use an inline six - one of the most inherently imbalanced engine types. Man, but a nice I6 does sound amazing.

Steve

scottzg · Post by **scottzg** » Mon Nov 10, 2003 9:33 am

Dinan is tailored to individuals with infinitely deep pockets who know nothing about cars except theirs needs to be fasterer. You got an e30? Clearly you're not frickin' loaded, so go away. Wanna have something to discuss at your cocktail party? Read this. Ptooey. Dinan sucks.

About the pulleys. I have not investigated them very much for our cars in particular, but here's what i gather about them.

Never replace one that has the built in balancer/absorber. They look like a spiral with lots of rivets and are used to control crank vibration as load is increased.

czo79 says the flat engine is better for the crank- do flat 4s fire 2 at a time? i dont think they do, but if they do, that sounds like it would be internally balanced, we really dont have anything on a V engine. An inline has a longer crank. Come to think of it, our flat 4s must have damned short cranks, can anyone confirm?

Josh, i think, claimed a 5 mpg improvement from pulleys. wow, free mod! Can this be confirmed by anyone?? This is my main interest.

Finally, it stands to logic that as the power goes up, the vibration problem becomes larger as well. I doubt us "NA 140hp on a good day" guys need to worry about it as much, but resonant frequencies can do their harm while cruising using 20 hp.

Any thoughts?

entirelyturbo · Post by **entirelyturbo** » Mon Nov 10, 2003 3:16 pm

Scott, the cylinders fire one at a time, our firing order is 1-3-2-4. And yes, we have very short cranks, that is why we have such durable bottom ends...

I wouldn't think any HP increases would cause more vibration, just maybe a difference in timing curves perhaps, or the obvious, stiffer mounts or something like that...

Brat4by4 · Post by **Brat4by4** » Mon Nov 10, 2003 3:50 pm

The exedy flywheel does not cause driveability issues. It's more about the "clampiness" of the clutch that will stall the engine. Road going STi stuff is meant for everyday driveability. I love my flywheel, you can't really tell it's there until you blip on downshifts... then wow!

Sorry about making up the word there. But it works, i think.

evolutionmovement · Post by **evolutionmovement** » Mon Nov 10, 2003 7:12 pm

Inline cranks flex more than the compact V or flat motors due to their length (ever wonder why they don't make straight eights anymore?), but there are other factors to consider as well. Various engine configurations have differnt balanced forces and moments. These can be altered by changing firing order, crankshaft design, angle between the cylinders in a V, etc. Generally a flat motor will have balanced forces, but unbalanced moments, while a flat crank inline has unbalanced second -order forces, but balanced moments. Ah! Thre's too much to write here and I'm lazy. Suffice it to say flat motors are well balanced as well as having an inherently stronger crank due to it's short length. The rigid main bearing supports and the fact that there are five main bearings is also a factor in it's strength.

Steve

DOA · Post by **DOA** » Mon Nov 10, 2003 10:46 pm

The only problem with lightened flywheels (assuming theyre done properly!) are that the engine will lose revs quicker when you let the throttle off.
Other than that theyre one of the cheeper ways of reducing your off the line acceleration (wont do much for higher gear accel though). Since the gearbox ratios in the lower gears have an effect of making the engine see less actual mass to accelerate, lowering teh weight of ANY of the components IN the actual engine has a massive effect on the ratio of masses to be accelerated. Since the engine effectively has to accelerate itself before seing the mass of the car itself once the torque has been through the gearbox (and its reducing ratio) lowering the weight of reciprocating/rotating parts has a large effect. It just happens that the flywheel happens to be one of the heavier components to be accelerated and one of the slightly easier parts to swap easily.
The only reason I would see for the engine throwing a CEL is if their is a sensor flag missing on the new crank.
Hope that helps (and makes sense

).

Nigel · Post by **Nigel** » Tue Nov 11, 2003 12:22 am

I would have to disagree that an in-line six is an inherently imbalanced engine type. In fact, the complete opposite is true. An in-line six produces nearly zero vibration. Strength and durability are also non-issues as there are plenty of I6's producing a reliable 500+ lb/ft of torque on a stock bottom end.

Boxer engines are also inherently balanced. Yet, boxer and in-line 6 engine manufacturers still feel that it is necessary to include a harmonic balancer on these engines despite the obvious cost savings that could be had by eliminating them. I would tend to trust their judgement in this case.

As for lightened flywheels, it's a very subjective issue with regards to responsiveness and drivablility. Most people that I've met with them seem to like them. I had 5 lbs shaved off the flywheel in my Datsun, and it makes double clutching in the Z far easier than in my Legacy. One thing to keep in mind with turbocharged engines is that the engine needs to be under load for boost to build. The mass of the flywheel can help to get boost to build faster.

Nigel
'94 Legacy Turbo
'73 Datsun 240Z Turbo

Post by **Legacy777** » Tue Nov 11, 2003 1:06 am

I think the word "balanced" is being misused here.....at least IMO.

Regardless of firing order, etc. A longer crank will most likely have more higher order vibrations, simply due to the length and more forces acting upon it. Also, you will have a greater deflection between ends with the longer cranks.

To balance the cranks to reduce any chance of vibration counter weights must be very meticulously added.

The smaller subaru cranks need much less balancing weight to counter the frequencies that can arise.

Personally I would say that if you took both cranks and put them on a test jig to see which one produced less vibrations....the smaller crank would probably have less. Also, computer modeling & FEA will play a big part in the design of crankshafts.

Overall I think the subaru motor design is just a better design.....but that's my opinion.

Nigel · Post by **Nigel** » Tue Nov 11, 2003 1:46 am

Well, I don't want to turn this into a "my engine is better than your engine" debate

I think both are brilliant designs and I own vehicles with both types. However, there do appear to be some misconceptions about I6 engines. The fact is that all forces generated by individual cylinders in a straight six are cancelled out, just as they are in a boxer engine, making them both virtually vibration free. The I6 has the added advantage of only requiring a single block, head, and valve train. But then, there's the lower center of gravity of the boxer...

I don't believe that either can be considered better than the other.

Nigel
'94 Legacy Turbo
'73 Datsun 240Z Turbo

czo79 · Post by **czo79** » Tue Nov 11, 2003 1:57 am

Nigel...
You mentioned that both I6's and B4's require harmonic balancers (or have them, can't remember which you said) but I thought the general opinion was that subaru boxers do not have a harmonic....hmm, is balancer and damper the same thing? And although I thought subarus don't use the crank pulley as a harmonic damper, are there ones somewhere else in the engine? As you can see, I'm getting confused here....
Thanks
Micum

PS: Oh yeah...do I6 engines (bmws in particular) have balance shafts? I know a lot of high displacement I4's have them...

Nigel · Post by **Nigel** » Tue Nov 11, 2003 2:45 am

Neither I6 or B4's have nor require balance shafts. They both have harmonic "dampers" though. (Perhaps someone with a B4 crank pulley handy can 100% confirm that for me). The harmonic damper is made up of a heavy ring mounted over a thin layer of rubber attached to a center hub that attaches to the end of the crankshaft (opposite to the flywheel). It's needed to cancel out the vibrations that do occur in the crank (nothing is perfect). I'm not sure if harmonic 'balancer' is another term for "balance shaft" or "harmonic damper."

Uppon further reflection, I would have to conceed that there is probably less strain on the shorter B4 crank than on the longer I6 crank, making a harmonic balancer more important on an I6. But like I stated earlier, crankshaft length doesn't seem to have any impact on durability.

Nigel
'94 Legacy Turbo
'73 Datsun 240Z Turbo

THAWA · Post by **THAWA** » Tue Nov 11, 2003 7:22 am

Wouldn't firing order also help balance an engine out, like if two cylinders fired at the same time in an inline engine or would that make it worse?

evolutionmovement · Post by **evolutionmovement** » Tue Nov 11, 2003 7:25 am

I've owned a Datsun 240 and 260Z, so I'm also a fan of the I6 engine. I wasn't trying to denigrate such a beautiful thing, but in any engine, there are first order and second order forces to balance as well as moments. Various engine configurations balance these all out differently, and I don't believe there are any that cancel everything out without the work of a balance shaft or damper to counteract or suppress them. I was referring specifically to a flat crank in my example. A multi-plane crank found in some sixes would change the effect of balancing out these vibrations as well as some other factors I listed and I don't want to think about how that would change stuff (I hate it when my head hurts).

I believe that in a flat crank six, the first order forces are cancelled as well as moments, but second order forces (which rotate at twice the crankshaft speed) would be imbalanced.

Then again, I could be completely off and my dad was right when he said I'd never be smarter than him (he's a hairdresser).

Steve

scottzg · Post by **scottzg** » Tue Nov 11, 2003 7:50 am

well, now that my noodle is thoroughly aching, time to ask the questions that matter to me:

Will a lighter pulley harm lifespan?

And

Is +5 mpg a real number? its almost too good to be true.

mTk · Post by **mTk** » Tue Nov 11, 2003 9:53 am

I saw no noticeable increase in mpg from my pulley set.

MK

Post by **Legacy777** » Tue Nov 11, 2003 3:11 pm

when i initially replaced my pulley, I saw a bump in fuel economy.....over long run I think it has been a wash.....but without putting the stock pulley on, I couldn't tell you.

I've had my pulley on for a good 60+k miles......I haven't seen any direct damage due to it.....however I haven't torn down the motor either. Lastly.....can someone really tell me exactly how a lighter weight pulley can damage the engine? I hear this all the time..."it'll damage your engine" Tell me exactly how, and what's going to fail. I don't recall anyone being able to do this. The only issue I know of with the lighter weight pullies is with the WRX, and the fact that there are some added harmonics that freak out the crank sensor, due to the loss of the harmonic damping affect of the stock crank pulley.

entirelyturbo · Post by **entirelyturbo** » Tue Nov 11, 2003 5:40 pm

Legacy777 wrote:can someone really tell me exactly how a lighter weight pulley can damage the engine? I hear this all the time..."it'll damage your engine"

I've heard it too and I don't believe it. How could it possibly damage your engine? Heck, I would think it would only help it. Simple logic: A lighter pulley means the engine has to work less to spin it, which means, well, the engine doesn't have to work as hard.

I do think that the only way for a lightened pulley to be effective is for a whole set to be used, for alternator and PS pump too, because if you don't, then the lower momentum the crank pulley has, the more the engine will have to work to spin the other heavier pulleys...

Also, Josh, as far as your underdrive goes, if nothing has happened over 60k miles of spirited driving that I know you do

, then I don't think anything will happen.

evolutionmovement · Post by **evolutionmovement** » Tue Nov 11, 2003 8:35 pm

I personally don't believe a lighter crank pulley would be much of an issue (at least on our engines), but it's possible that aside from any frequency damping, the point of the heavy pulley may be to better balance the strain from having to spin the flywheel on the opposite end of the crankshaft which could maybe reduce torsional strain on the crank. I don't know, that one's just a guess, plus there are varying loads when taking into account whether or not the clutch is engaged on one end and the number of accessories on the other end. Then there's variance in the accessories' resistance to turning... I'm sure a lighter pulley would be fine, I'm probably putting too much thought into it and any difference would likely be marginal.

Whew, that one still smells from pulling it out of my a...

Steve

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Problems with lightweight flywheels and underdrive pulley?

Problems with lightweight flywheels and underdrive pulley?