Nissan 370Z Forum - View Single Post

wstar · 06-17-2014, 02:58 PM

Yeah, so someone asked on the "visitor messages" thingy here: Nissan 370Z Forum - Conversation Between 12nismo and wstar about whether the engine failure was pulley-related and whether I moved my crank pulley to my new block. The TL;DR answer to that is: I *think* the failure was unrelated to the pulley, but I also did *not* move the pulley over to the new block.

The long explanation, if the above is important to you and/or confusing:

Personally, I don't think the (Stillen, light/underdrive) pulley was a significant factor. My best guess is the flywheel probably started cracking a long time ago, adding some imbalance/wobble to the crankshaft and beginning to cause excess wear in the crank bearings, and then when the flywheel finally got bad enough (when the last bit cracked on that fateful Saturday), we had some major imbalance/wobble/whatever going on that really tore up the bearings, along with the flywheel striking the crankshaft position sensor, which is what caused the engine to go into limp mode at that point.

Even then I think the bearings were merely damaged at that point, but not seized/spun, initially. They didn't really come apart until test-fires and moving the car around around later, off-track (but that was inevitable at that point, they were shot one way or the other)... which brings us back to the Great Internet Pulley Debate (which rages forever on almost every car forum), where pulley-detractors tell us that lightweight pulleys without dampening rings on them will destroy crankshaft bearings over time.

To recap the basic internal/external balancing stuff, open the spoiler (or you can skip this as a TL;DR reduction if you get this part already):

( Click to show/hide )

Our engine is an internally-balanced one, meaning the pulley doesn't play a role in basic rotating-assembly balance issues on this car. It *does*, however, contain a rubber vibration-damping ring on the outer face of it. The core of the real debate here is whether the purpose of that vibration damping ring is purely for aesthetic NVH reduction, or whether it has a real dampening effect that partially reduces some mechanically-important crankshaft vibrations from combustion (and if so, how much longevity are you losing if you eliminate this? It could be 5%, it could be 95%!).

Anecdotally, I don't think it's much of a loss, if any, because the pulley kits from NST and Stillen (and others) have sold tons, and we're not hearing about massive lawsuits or even complaints about destroyed engines. It's most likely that the ring is mostly about NVH. Maybe it also reduces some mechanical vibration that matters, but maybe that's not very significant to begin with. For this sort of reasoning, I'm generally of the opinion that you're not risking any kind of large or short-term damage by properly installing a pulley kit on this engine. That and my engine made it through a considerable number of very abusive track miles on such a pulley (and even then, I don't think it was the cause).

However, I can't in good conscience say that without adding 3 important disclaimers/caveats:

My car is a 7AT, not a 6MT. Vibrational issues could be different on the two. I tend to suspect that, if there is any issue, 7AT's would fare better than 6MTs. This is because a 7AT has a torque convertor full of transmission fluid bolted to the flywheel at the rear of the engine. I suspect the torque convertor acts like a giant fluid damper on the rear of the engine (e.g. The Original Fluidampr ), offering far better vibration damping than any silly rubber ring on a pulley ever could.
Regardless of all the logical weaseling above, you can't escape a few basic facts: Vibration hurts engines. All engines vibrate. There's a small rubber ring on the stock pulley that may help with some unknown vibrations to some unknown degree. You're tossing that out for a <1% difference in the engine's performance capabilities at the end of the day. Maybe that's worth it, maybe it isn't. How much stress you're putting on the engine to begin with is a big factor. If I were building a 750rwhp turbo monster, I'd probably go in the *opposite* direction and install a heavier replacement pulley that does some serious damping to increase longevity. On an N/A street engine, I doubt you'll ever know the difference (in performance *or* longevity), making the upgrade pointless.
Jason (the mechanic who did the swap, and who I think has a trustworthy opinion about all car things) recommended against moving the pulley over to the new engine when moving all my other stuff over, because he thought it *could* have been a factor, and it's not worth the risk if it was. While, academically, I may believe it to not be a major contributing factor in the failure, I don't believe that strongly enough to tell him "No, put it on anyways", especially given that I could end up destroying this replacement block in a short period of time via something totally unrelated and coming right back to him with it, at which point he'd get to say "I told you so" about the crank pulley whether it was the issue or not, because that's only fair at that point. So he put my stock crank pulley on the new block, and that's what I'll be running for the foreseeable future (until I kill this engine in some new and imaginative way!).

06-17-2014, 02:58 PM	#811 (permalink)
wstar A True Z Fanatic Join Date: Mar 2009 Location: Houston, TX Posts: 4,024 Drives: too slow Rep Power: 3594	Yeah, so someone asked on the "visitor messages" thingy here: Nissan 370Z Forum - Conversation Between 12nismo and wstar about whether the engine failure was pulley-related and whether I moved my crank pulley to my new block. The TL;DR answer to that is: I think the failure was unrelated to the pulley, but I also did not move the pulley over to the new block. The long explanation, if the above is important to you and/or confusing: Personally, I don't think the (Stillen, light/underdrive) pulley was a significant factor. My best guess is the flywheel probably started cracking a long time ago, adding some imbalance/wobble to the crankshaft and beginning to cause excess wear in the crank bearings, and then when the flywheel finally got bad enough (when the last bit cracked on that fateful Saturday), we had some major imbalance/wobble/whatever going on that really tore up the bearings, along with the flywheel striking the crankshaft position sensor, which is what caused the engine to go into limp mode at that point. Even then I think the bearings were merely damaged at that point, but not seized/spun, initially. They didn't really come apart until test-fires and moving the car around around later, off-track (but that was inevitable at that point, they were shot one way or the other)... which brings us back to the Great Internet Pulley Debate (which rages forever on almost every car forum), where pulley-detractors tell us that lightweight pulleys without dampening rings on them will destroy crankshaft bearings over time. To recap the basic internal/external balancing stuff, open the spoiler (or you can skip this as a TL;DR reduction if you get this part already): ( Click to show/hide ) The balancing of the rotating assembly (crankshaft, connecting rods, pistons, counterweights, and yes, also the flywheel and pulley) is critical in all engines. This is the same "balancing" you hear about when people talking about blueprinting and balancing a custom/race engine. Factory engines are balanced at the factory. If the rotating assembly isn't balanced to within a fine tolerance, it will destroy itself and/or the bearings it rides on fairly quickly. Some engines are designed to be internally balanced. This means the crankshaft and attached internal components (counterweights, connecting rods, pistons) are balanced against each other for a net of zero. At that point you can stick whatever you want on the ends (pulleys/flywheels) and things remain balanced so long as those components are also evenly balanced. Some are externally balanced (including many notable V8's people like to work on), meaning that they rely on offset weights in the crank pulley and/or flywheel to come into full balance. If you were to replace the pulley or flywheel with a different model that didn't have the exactly-correct balancing weight on it (e.g. 50 grams of excess weight at 5 inches radius at 120 degrees or whatever), your whole rotating assembly is no longer balanced correctly and you're going to have problems. Our engine is an internally-balanced one, meaning the pulley doesn't play a role in basic rotating-assembly balance issues on this car. It does, however, contain a rubber vibration-damping ring on the outer face of it. The core of the real debate here is whether the purpose of that vibration damping ring is purely for aesthetic NVH reduction, or whether it has a real dampening effect that partially reduces some mechanically-important crankshaft vibrations from combustion (and if so, how much longevity are you losing if you eliminate this? It could be 5%, it could be 95%!). Anecdotally, I don't think it's much of a loss, if any, because the pulley kits from NST and Stillen (and others) have sold tons, and we're not hearing about massive lawsuits or even complaints about destroyed engines. It's most likely that the ring is mostly about NVH. Maybe it also reduces some mechanical vibration that matters, but maybe that's not very significant to begin with. For this sort of reasoning, I'm generally of the opinion that you're not risking any kind of large or short-term damage by properly installing a pulley kit on this engine. That and my engine made it through a considerable number of very abusive track miles on such a pulley (and even then, I don't think it was the cause). However, I can't in good conscience say that without adding 3 important disclaimers/caveats: My car is a 7AT, not a 6MT. Vibrational issues could be different on the two. I tend to suspect that, if there is any issue, 7AT's would fare better than 6MTs. This is because a 7AT has a torque convertor full of transmission fluid bolted to the flywheel at the rear of the engine. I suspect the torque convertor acts like a giant fluid damper on the rear of the engine (e.g. The Original Fluidampr ), offering far better vibration damping than any silly rubber ring on a pulley ever could. Regardless of all the logical weaseling above, you can't escape a few basic facts: Vibration hurts engines. All engines vibrate. There's a small rubber ring on the stock pulley that may help with some unknown vibrations to some unknown degree. You're tossing that out for a <1% difference in the engine's performance capabilities at the end of the day. Maybe that's worth it, maybe it isn't. How much stress you're putting on the engine to begin with is a big factor. If I were building a 750rwhp turbo monster, I'd probably go in the opposite direction and install a heavier replacement pulley that does some serious damping to increase longevity. On an N/A street engine, I doubt you'll ever know the difference (in performance or longevity), making the upgrade pointless. Jason (the mechanic who did the swap, and who I think has a trustworthy opinion about all car things) recommended against moving the pulley over to the new engine when moving all my other stuff over, because he thought it could have been a factor, and it's not worth the risk if it was. While, academically, I may believe it to not be a major contributing factor in the failure, I don't believe that strongly enough to tell him "No, put it on anyways", especially given that I could end up destroying this replacement block in a short period of time via something totally unrelated and coming right back to him with it, at which point he'd get to say "I told you so" about the crank pulley whether it was the issue or not, because that's only fair at that point. So he put my stock crank pulley on the new block, and that's what I'll be running for the foreseeable future (until I kill this engine in some new and imaginative way!). __________________ 7AT Track Car! Journal thread / Car setup details Last edited by wstar; 06-17-2014 at 03:13 PM.