[TMS]

I am posting this thread in hopes that someone can maybe save themselves from a similar outcome. What follows by no means is scientific but just the best back-yard DIY forensic review I can offer from my own findings.

So, I recently upgraded to an ATI SuperDamper with a 15% OD crank pulley (9.17"). With upgrades done and initial tuning sorted, I proceeded to do some 1500-5000 RPM 3rd gear WOT pulls (with nannies off) to get some good data logging for the tuner. And then this happened… Blower belt broke and most all of the coolant dumped out. I shut it down really quick. After a ride home from the friendly AAA driver and a ½ day that weekend cleaning up coolant from spraying most of the car I began the search to answer the two questions:

What happened (beyond the obvious)?

Why did it happen?

The “what” –
Snapped blower belt, broken thermostat housing bypass nipple, and primary radiator return hose pushed completely off of the thermostat housing. Fortunately, no other damage. When the belt broke, it took out the bypass nipple and pushed the radiator hose off of the thermostat housing causing the massive coolant dump. Good news is I had this all captured in the data log. It seemed like the coolant started spraying out the hood immediately when I let off the throttle. The data log shows full boost climbing up to 5,000 RPM and then no boost immediately following the throttle close. Had the belt broke while under WOT it would have lost boost before TPS% began dropping down from 99.5%. So now that timing of the belt break was confirmed and damage assessment complete, onto the next question.

The “why” -
First, I was concerned that there might be some belt slip so I decided to forego the Gates HD series belt and go with the new Gates RPM series belt. Supposedly, these belts are made for blower applications and have some sort of Kevlar or Aramid type composite that resists stretching. Originally, I was going to use an 080845 size belt. But decided to go one step smaller to an 080842.


With the RPM series belt I was able to barely get the belt on over the tensioner. When I let the tensioner spring back there was almost no movement as compared to the same size belt in an HD series. I thought, wow these new belts really don’t stretch and there is no way this belt is going to slip as the tensioner is under full preload (hold that thought for just a bit).


Theory 1 – Belt was slipping causing it to superheat and snap. Under close inspection I could find no heat marks. Also, no sign of slippage in the datalog.


Theory 2 – I installed a new crank damper and pulley. Perhaps there is a mis-alignment with the pulley. Checking with a straight edge on the crank pulley, all other pulleys line up perfectly. I also straight edge checked the alternator/accessory crank pulley that rides directly on the damper for alignment and it too was dead on.

Theory 3 – The tensioner was fully preloaded given the size of the belt. When I let off of the skinny pedal the tensioner had no place to go and bottomed out against the stop. That coupled with a belt that had almost no “give”, it snapped.

Theory 3 seemed like the most likely cause and would be the focus of my fix and solution.

The “fix” -
At this point I am scrutinizing everything. First order of business, replace the thermostat housing. Done. Second, perhaps having a little belt stretch is a good thing. My new take away is that belt stretch may just provide enough of a safety margin to prevent belt breakage under similar circumstances. Perhaps stronger isn’t always better. Back to the tried and true HD Green stripe series belt. Also, went back to a 080845 (longer) belt which puts the tensioner exactly half way in between the tensioner range. With that done, I started the engine to let the belt find its center on the tensioner. I noticed that where the supercharger belt rides on the tensioner pulley was not centered longitudinally. The pulley is 1 1/4" wide. The belt is 1 1/16" wide. The belt rides flush with the outside edge of the pulley, leaving 3/16" gap to the back side of the pulley with no belt on it. So, the belt is off center by 3/32". How can this be if the pulley is perfectly aligned using the straightedge? I decided that if my Theory 3 was correct, maybe I somehow damaged the tensioner. So, I replaced the factory tensioner with a new GM tensioner only to discover the same problem. Having logged 1500 miles on my car before doing the upgrade I could clearly see where the original factory belt was riding on the tensioner. And it too was riding to the outside edge of the tensioner pulley. A quick call to my buddy in Texas who also has a ZL1 1LE that is factory stock and I had him take some measurements. His belt rides perfectly centered on the pulley unlike mine.



Even though the tensioner pulley was positioned perfectly longitudinally in relation to the other idler pulleys, I found that either because of a machining or casting imperfection in either the block or the bracket mounted to the block that the tensioner mounts to, the pulley was not perfectly square to the belt causing the belt to run out off center of the pulley. I needed to shim out the one mounting point furthest to the passenger side of the 3 bolting points on the tensioner pulley. I started with a washer about .040" thick sandwiched between the bracket and the tensioner bolt holes and it moved the belt on the tensioner from the outside to the inside edge of the pulley by about the same distance (I flip flopped the problem). In the end, it took only a paper thin shim of .015" to get the belt to ride perfectly aligned on the tensioner. It is amazing how just the smallest shim can cause such as wide swing in the movement of the belt.


In typical fashion, I changed more than one variable at the same time so I don't know for sure what specific change I made actually fixed the problem. Maybe it was the shimming, or maybe it was going back to the HD Green stripe belt or maybe it was the longer belt for better tensioner resting position. What I do know is that I have been floggin’ and loggin’ this car and have had no issues since.

Good luck out there.

[waterman]

Sorry for your troubles.

Informative post. Thanks for the feedback. Great info.

Pics did not post in my browser.

[TMS]

Originally Posted by waterman

Sorry for your troubles.

Informative post. Thanks for the feedback. Great info.

Pics did not post in my browser.

I'll fix the pic links.

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[djfury05]

Good info.. thanks. I have the K080842HD belt and the K080845RPM belt. Was planning to use the RPM series belt first and keep the 42HD belt in the trunk and still will, but I'll be sure to check all tolerances before start up.

[waterman]

Originally Posted by TMS

I'll fix the pic links.

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Got the pics.

[PhreddT]

Great write up and methodology . Thanks for sharing, I think I will give mine a good look-over this weekend.

[Agent K]

I agree with everyone else; great write up. Along with this info and Conrad's road map, I hope my build goes smoothly.

[Yellow6thGen]

Great write up, Thank you...

[closs2sx]

I would say it was the shorter belt that amplified the pulley being out. Your stock belt was on with the pulley in the same condition correct?

[TDS]

Thanks for the heads up Trevor. I just ordered the RPM belt (K080842RPM). My pulley is a 9.10 not a 9.17 and I am already running the K080842HD so thinking the size will be ok. Checking everything this weekend

[TMS]

Originally Posted by closs2sx

I would say it was the shorter belt that amplified the pulley being out. Your stock belt was on with the pulley in the same condition correct?

That is correct. The stock belt had the same problem.

Keep in mind that when I refer to the shorter belt I am referring to the shorter of the two belts being considered for use with the new, larger 9.17" pulley. The factory belt is shorter than either the 080845 or the 080842 which are both long by comparison. The stock pulley is only 7.99" diameter.


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[TMS]

Originally Posted by TDS

Thanks for the heads up Trevor. I just ordered the RPM belt (K080842RPM). My pulley is a 9.10 not a 9.17 and I am already running the K080842HD so thinking the size will be ok. Checking everything this weekend

Travis,

My recommendation is to use whatever size belt it takes to put the tensioner in the halfway spot between both tensioner hard limits. Also, I found that the same size belt in an RPM vs an HD does not equate to the tensioner resting in the same spot with both belts due to the greater elasticity with the HD belt. An 080845 in an RPM puts the tensioner about equivalent to an 080842 HD because of this even though they are technically the same size with no load on the belt.

I have done a number of hard WOT pulls with the new HD belt riding at the 1/2 way sweet spot on the tensioner and have not seen any belt slippage show up in the datalogs and my boost peaks consistently at about 197 to 201 kPa (approx. 14 psi). I'm sticking with the HD mostly out of fear at this point.

Triple check everything as you go through it.

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[closs2sx]

Originally Posted by TMS

That is correct. The stock belt had the same problem.

Keep in mind that when I refer to the shorter belt I am referring to the shorter of the two belts being considered for use with the new, larger 9.17" pulley. The factory belt is shorter than either the 080845 or the 080842 which are both long by comparison. The stock pulley is only 7.99" diameter.


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True but the stock belt would have been “longer” if designed or used with the same size pulley as the new ati pulley allowing the tensioner to move through a range of motion when on and on throttle hard where a shorter belt for the same application does not allowing a brief moment of a looser then designed tension condition and the angle of the pulley allowed belt to walk off

[TMS]

Originally Posted by closs2sx

True but the stock belt would have been “longer” if designed or used with the same size pulley as the new ati pulley allowing the tensioner to move through a range of motion when on and on throttle hard where a shorter belt for the same application does not allowing a brief moment of a looser then designed tension condition and the angle of the pulley allowed belt to walk off

I'm not sure I follow your logic but I think we are saying the same thing as my theory 3. Mama says I was dropped on my head when I was a child so it's probably just me.

When I checked with my buddy on his stock configuration, his tensioner was about in the middle of the throw. So with the 080842 RPM belt (that broke), yes I guess you could say that proportionally it was smaller based on where the tensioner ended up (almost fully compressed). The 080845 HD belt placed the tensioner about where the stock setup has it riding so would be basically the same size proportionally speaking.

I agree, no room for the tensioner to move (at least one direction) and probably the direction it needs to move when I quickly let off the throttle under full boost. The tensioner bottomed out.

As for the belt runout related to the belt size, I really don't think it had anything to do with that as that issue was evident under the stock condition and with multiple size belts. Plus if you study the belt routing, the belt does not directly feed to the tensioner from the crank pulley. It goes around a fixed idler pulley before it makes its way to the tensioner normalizing the length of the run between between each pulley directly next in line both directions to the tensioner.




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