[Cloud9...68]

This is something I've been wondering about for awhile. Given that these enormous four-bangers of ours are justifiably notorious for being inherently poorly-balanced engines, requiring counter-rotating balance shafts for which Porsche paid Mitsubishi royalties (a first for Porsche), a crankshaft that weighs an eye-popping 59 lbs (I weighed mine), and we get a rattle in the transmission when anything but the massive dual-mass flywheel is installed, I have to wonder if installing a lightweight single-mass flywheel is really a good idea. The thing that really got me thinking about this is that when I asked Karl Poeltl of Racers Edge if he recommended getting the crank lightened and knife-edge, he replied with a very emphatic "Absolutely not!" His reasoning was the need to dampen the harmonics of these engines. Also, Pete didn't lighten the crank of his 300+ hp NA engine.



My car will primarily be used as a track car once it's put back together, so I'm not worried about the noise at 2300 rpm per se, but I am beginning to wonder if the rattle is a sign that the DMF should be retained. I hope this isn't the case, because I'd love to see some improved throttle response to help pull the car out of the corners, and of course, many people have used SMF flywheels in these cars for years, apparently with no ill effects, so I guess I'm just looking for some confirmation that I'll be doing the right thing by installing a SMF (I'm planning to use RS Barn's 18-lb unit). Thanks.

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[flash]

now you're starting to use logic - lol - you know that will just get you into trouble



maybe remembering that everything comes at a price might help - a light flywheel will indeed make the harmonics change - you really don't feel it, but the math says it's there - there is no long term data i know of to say whether or not it does any damage, but i have to think it will - i ran a 12lb for a long time and am now running an 18 - i balanced both of them as individual parts and as assemblies, so i did what could be done to mitigate those harmonics, and now i have added the S2 harmonic balancer, so hopefully that will help, but still i expect that the bottom end won't last as long as it would with the DMF



in a race car, where you plan to tear the motor down every season anyway, this is not a problem - in a street car, where you are looking for longevity, it becomes a real issue



tough call - this is exactly why i recommend against a dual purpose car - too much compromise - i do understand the situation though, and it is a common one - been there done that



if i had to do it over again, i think i would have left the DMF in there - i still might go back

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[Monstrous4Banger]

Should be ok with the vibration damper. Part # 944 102 123 06 . Its what the s2 uses. The s2 used a single mass flywheel, and compensated for the harmonics with it. Im using one in my 951. I have a 3.0 built up from a s2 block(same bore, and block minus piston squirters, that I had Lindsey install into), with a 968 crank and a 930 disc that has never rattled. I shied away from lightening the crank when I did my project also. I was told the same reasons when I poked around about it. I would imagine compensating for a light crank would be more difficult than slapping on a vibration dampener.

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[Monstrous4Banger]

[quote name='flash' timestamp='1310877166' post='112702']

in a race car, where you plan to tear the motor down every season anyway, this is not a problem - in a street car, where you are looking for longevity, it becomes a real issue



tough call - this is exactly why i recommend against a dual purpose car - too much compromise - i do understand the situation though, and it is a common one - been there done that



[/quote]



Yeah true. My car is pretty much 100% street. In the situation of a dedicated track car, I could see how it would be a tough call. I have no experience pushing it like Cloud would be.Just how light one goes would be a factor too.

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[Cloud9...68]

Flash, Monstrous,



Your responses are pretty much what I had expected, and feared. Obviously, there is no free lunch; practically every modification has some sort of side effect. Yes, I could add a harmonic balancer, but of course that would add some rotating mass, counteracting some of the benefit of the lighter flywheel. Plus, I understand harmonic balancers are pretty pricey.



I see my car evolving from a track car that I drive on the street periodically (at least to get me to and from the track) to eventually more and more of a pure track car (maybe I'll end up buying a second 968 as a daily driver...). I'm still leaning toward the 18 lb SMF, understanding the trade-offs.

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[flash]

adding the mass of the balancer was definitely noticeable, but it did smooth things out a bit - i'm not inclined to get in there and change it out again at this time, but only because i did so many clutch jobs trying to develop the new disk (which in the end was a complete failure)



if you add the balancer, which isn't that expensive (i think i got mine for just under $300) you will need to modify the front plastic timing belt cover to accommodate the rear flange of the balancer, as well as modify the lower guard retaining bolt

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[PhilG968]

I've had my RS Barn single mass flywheel in for years now. I'm a heavy daily driver, and have had no ill effects from the vibrations, that I'm aware of . Motor currently has 231,000 miles on it.

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[DaveN]

I definitely noticed a difference when I added my S2 balancer before the SC install; my wife stopped stalling the car <img src="/forum/images/smilies/968/wink.gif" class="smilie" alt="" /> I thing Bob is close, I think I paid around 298.00 from Sunset for the pulley.

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[Cloud9...68]

PhilG - thanks for the input. I'm strongly leaning toward getting the RS Barn flywheel - I can't see replacing my likely-worn-out DMF with another DMF, seeing as the car's primary venue once it's put back together will be the track.

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[flash]

if i were not going to actually race (where rules come into play), AND i were never going to supercharger, AND the car were primarily a track toy, i'd run the 12lb flywheel



if any one of those 3 conditions were not met though, i'd run the 18 with the dampener



if i were to do it all over again, knowing i was going to supercharge, i think i'd keep the DMF



does that help? or does it confuse you more?

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[gbarrier]

What does the S2 flywheel weigh and is it an option for our car with balancer?

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[Cloud9...68]

I'm not positive, but I believe it's 18 lb, and that RS Barn used it as the model for their 18 lb SMF.



I think I'll go with the 18 lb flywheel, and skip the harmonic balancer for now. I'll double-secret-probation balance everything, deck the block, etc, and see how it goes. If it's too "buzzy" for me, I'll add the harmonic balancer.

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[RS Barn]

The S-2 flywheel is 16Lbs and bonded rubber crank pulley adds less than 1 lb. You can see why Porsche had someone make a 18lb solid flywheel for 968 Motorsports applications. There have been many methods used to reduce driveline vibrations on 944-968 series.

Lighter rotating mass is usually a good thing. Driveline vibration is downside to quicker revs.

I'm working with Nascar suppliers for possible Harmonic Dampener solution.

Pete

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[flwbyu]

I have to say that I have had the RSB lightened flywheel in my daily driver for quite some time without any ill effect. These cars have so much noise to begin with what is a little more from the flywheel. <img src="/forum/images/smilies/968/laugh.gif" class="smilie" alt="" />



I think in the end the more you alter any car from the factory specs the more time you are going to spend dealing with maintenance. Which is why I am more cautious about what I change in a car. Plus, you better have a good mechanic to back it up if you cannot due the repairs yourself.

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[968norway]

Well, interesting discussion. I recently put in an RSB flywheel, needed a new flywheel, heard this was good idea.

I can pretty much drive without the bad rattling in the transmission. But it concerns me that people are saying using one could lead to damage to the engine.

One has to think that the bang in the cylinders is a pretty violent action, so I wonder if a little extra vibration from the FW would have such a large effect.

Have to say the car goes a lot better.



Cheers,

Tore

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[ds968]

Tore's post prompted a reply, but I edited it out of this thread and posted again on a new non-968 related topics segment.

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[Dimma]

Just removed my new SPEC Alu flywheel ; for a street car it's just not good enough by far. If I would be deaf and paralysed I might consider putting it back ; shaking and stalling @ take off, vibrations @ 2200 and 2800 rpm in first 3 gears. I've taken it out ( after 1 week ) and remounted the stock dual mass flywheel though I'am curious how long it will last with 500+ hp ...



So Cloud9, I have 1 for sale ; has done 500 km ....

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[Cloud9...68]

Thanks, but I already purchased one from RS Barn. Do you know the weight of the Spec? I'm guessing it's 12 pounds. From what I've gathered in talking to knowledgeable people about this, 12 lbs is just too light for these cars. It appears that RS Barn's 18-pounder is a reasonable compromise between livability and throttle response.



As far as your questions about the spring-center disk on your other thread, Flash and Pete have tried to create one, but there just isn't enough room between the flywheel bolts and the springs (I think...), so it just isn't possible, unfortunately.

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[flash]

correct. i went through a number of different variations, and could not find a spring that did not create more problems than it solved. very frustrating.

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[Dimma]

Thanks Cloud9, but I'm not convinced so easily ; why not change the holes and mount drowned bolts then ? would that create enough clearance ?

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