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/cgi-bin/YaBB.pl General Category >> Rubber Side Down! >> Meltdown /cgi-bin/YaBB.pl?num=1581222575 Message started by DragBikeMike on 02/08/20 at 20:29:34 |
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Title: Meltdown Post by DragBikeMike on 02/08/20 at 20:29:34 I was just finishing up on a carburetor comparison test. I was testing four different mixers for a report I wanted to post, and this was the final phase of the test. I was anxious to finish up. My wide-open throttle (WOT) air/fuel (A/F) ratio was just a bit leaner than I wanted (about 13.5 vs 12.5), but this engine had been taking everything I could throw at it so I figured it would be OK. After five or six 4K to 7K pulls in second gear it felt really good, so I decided to stretch it out. I took it up to 7250 in 4th gear. The A/F hung in there around 13.5 and all seemed well. I looped around for another pull. I took it up in 5th and right about 6400 I could sense things weren’t right. A quick glance at the A/F meter showed it was dead rich, 10:1. I chopped the throttle and could tell immediately that something was wrong. The mixture was waaaaay rich and it had zero power. I was testing an Ultimate Flow Optimizer (UFO) and figured there was a good chance that maybe the contraption had come loose and was choking off the venturi. No such luck. |
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Title: Re: Meltdown Post by DragBikeMike on 02/08/20 at 20:31:31 My engine is a little different from some of the hotrods other forum members have built. I raised my compression by machining about .130” off the top of my cylinder instead of installing a pop-top piston (Wiseco). I used the stock flat-top cast piston and machined four valve reliefs in the piston. My intent was to provide a good tight quench area between the cylinder head and the piston. It worked too good. It had whopper compression and the cranking pressure was too high (245 psi). I was able to mitigate that problem by installing a Web 340b grind cam, retarded 5°. It seemed to be working fine on pump gasoline (92 octane E10). I put several thousand miles on it that included hundreds of full-power pulls and some dyno pulls. All sorts of torture. There were tests on the camshafts, tests on the breather system, tests on air filters, tests on carburetors. All the tests involved full power ops to the max. It was spending way too much time at engine speeds well above what any normal person would consider reasonable. In the middle of all that testing, I sucked in a cylinder base gasket, so I had to tear it down. At that time, the piston assembly looked great. Not a hint of trouble. |
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Title: Re: Meltdown Post by DragBikeMike on 02/08/20 at 20:32:27 Aside from the pretty brown hue from the octane booster I had been running, the integrity of the piston assembly was stellar. Looked great, solid as a rock. It was lovin the tight quench. |
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Title: Re: Meltdown Post by DragBikeMike on 02/08/20 at 20:34:16 Fast-forward about 1000 miles to Super Bowl Sunday. I end up with this after getting a little lazy and anxious. I should have waited until I had that WOT mixture deep into the 12 zone before trying to set the land speed record (LSR). Good lesson for this old dog. I think you will find these photos interesting, and I am also hoping I can pry some good comments and advice from the experts on this forum. You can see from the first picture in this post that a section of the piston top is missing. The section extends from the left-hand intake valve relief to just a little past the middle of the piston. The photo below shows the piston from the side, note how the top of the piston appears to be pulled up. |
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Title: Re: Meltdown Post by DragBikeMike on 02/08/20 at 20:35:24 Fast650 (The Fastman) sort of zeroed in on the ring gap before I even had the cylinder off. He saw the first photo from the top (with cylinder still in place) and said your ring gap is too small. I checked my records and when I assembled the engine my ring gaps were .020” top and .030” 2nd. With one exception, all my reference material recommends gaps in this range, about .005” to .0055” per inch of bore diameter (that’s for a hot-street or mild race engine). I have a 3.700” bore so the first ring should have been gapped to .019” to .020”. I was right there. But that one exception reference book calls for .006” per inch of bore if the engine is air-cooled. That would mean my top ring gap should have been at .022”. BTW, the factory manual specifies .018”. The Fastman figured it’s the speed that I’m operating at. I must admit that I have become complacent. This thing has spent so much time above 7K without so much as a hiccup, that I simply lost sight of any sort of redline. I’m guilty, I bury the tach every time I ride it. It’s almost like second nature. Swing a leg over it and run its guts out. I have been justly rewarded. The piston skirt got scuffed up on the exhaust side. |
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Title: Re: Meltdown Post by DragBikeMike on 02/08/20 at 20:36:28 And of course, the intake side is a mess. Why the intake side? I thought that side would run cooler. |
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Title: Re: Meltdown Post by DragBikeMike on 02/08/20 at 20:37:28 The wrist pin bore in the rod had a little galling as did the wrist pin itself. The wrist pin will be headed for the trash bin, but I gotta try and save that rod. I’m not so sure that the rod galling is directly attributable to this meltdown. It might be my reward for the constant high rpm ops. |
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Title: Re: Meltdown Post by DragBikeMike on 02/08/20 at 20:38:23 I’m happy to report that a few quick passes with a ball-hone cleaned it right up. It’s only .0001” over design specs, and still .0009” below the wear limit. |
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Title: Re: Meltdown Post by DragBikeMike on 02/08/20 at 20:39:23 The combustion chamber looked OK. It almost looked as though nothing bad had occurred, simply the standard layer of carbon deposits. |
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Title: Re: Meltdown Post by DragBikeMike on 02/08/20 at 20:40:20 I was wondering where that chunk of piston top went. As I was cleaning up the combustion chamber, I figured that out. The combustion chamber had a nice layer of aluminum flame-sprayed all over, especially the intake side quench area. |
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Title: Re: Meltdown Post by DragBikeMike on 02/08/20 at 20:41:12 Look at the beautiful flame spray job on the combustion chamber pocket. I don’t think I’ve ever seen anything like this. A good 55 years of wrenching and never seen a piston liquify and get deposited so nicely over the combustion chamber. |
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Title: Re: Meltdown Post by DragBikeMike on 02/08/20 at 20:42:03 It flame-sprayed the exhaust valves. Cool (or should I say hot, as in JetHot?). |
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Title: Re: Meltdown Post by DragBikeMike on 02/08/20 at 20:42:58 If any of you doubt that there is intake reversion when using a big cam with a late closing intake valve, look at the aluminum specs on the intake. There’s only one way for that aluminum to find it’s way back up the intake tract, reversion. Those aluminum specs are firmly adhered to the valve, things were warm. |
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Title: Re: Meltdown Post by DragBikeMike on 02/08/20 at 20:44:02 After I cleaned up the valves with scotchbrite and oil, there was still aluminum forge-welded to the seating surface. I will probably have to replace the exhaust valves. I think the intake valves can be cleaned up with a Neway Gizmatic. |
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Title: Re: Meltdown Post by DragBikeMike on 02/08/20 at 20:44:50 Even the intake valve, with all that cooling from the fresh charge, had aluminum forge-welded to the seating surface. Mama Mia, that’s a one spicy meatball. |
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Title: Re: Meltdown Post by DragBikeMike on 02/08/20 at 20:45:33 The cylinder looks OK. I think I can probably clean it up with a rigid hone. It will be too loose for a stock cast piston, but I think I could keep it to about .0030” to .0035” clearance with a forged Wiseco. That would be OK. I would have to machine the pop-top off the Wiseco. It’s already 52 grams lighter than a stock piston so I think I will forgo that mod. It’s probably a good candidate for an overbore, but it’s certainly not ready for the trash bin. |
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Title: Re: Meltdown Post by DragBikeMike on 02/08/20 at 20:46:42 When I was disassembling the engine, I noticed that the center forward head nut (the one accessed from the bottom) was finger tight. Like I really mean finger tight as in no wrench needed. I suspected that was not good. It wasn’t too long ago that I had checked those two lower nuts and they were both good and tight. I have lapped three of these heads now. Each one displayed the same low spot just above the exhaust port. Even under optimum conditions, it looks like the heads give a bit due to the high operating temp in that area (just above the exhaust port). When I started lapping this head, it was evident that the area just above the exhaust port was low in relation to the rest of the sealing surface. This is an obvious weak link in the LS650 head. |
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Title: Re: Meltdown Post by DragBikeMike on 02/08/20 at 20:47:40 Even after quite a bit of lapping, it still didn’t clean up entirely. Eventually, I was able to completely eliminate the low spot to the point where you couldn’t detect it. This is what it looked like after going through 3 sheets of 180 grit. The 4th sheet finally got it entirely cleaned up. |
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Title: Re: Meltdown Post by DragBikeMike on 02/08/20 at 20:48:48 Generally, a good indication that you’ve been getting things too hot is coking on the underside of the piston. Mine didn’t exhibit any coking. I am guessing that’s because this failure was a one-time deal. It’s not something that has been degrading over time, not repeated operation at elevated temperature. |
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Title: Re: Meltdown Post by DragBikeMike on 02/08/20 at 20:51:01 So why did it fail? I have a theory and I’m hoping I can get some good comments and ideas from you folks. I think I sort of created the perfect storm. First, I raised my compression to a level that exceeds anything that could be reasonably expected to survive normal street operation. Pump gas, sustained freeway operation, stop-&-go traffic, a large bore, all work against cranking pressure in the 240 psi range. I got away with it at first by adding a healthy dose of octane booster. Then I brought the cranking pressure down to around 220 psi with the 340b cam. But 220 psi is still too high for street operation on pump gas. At a minimum, I should have continued the octane booster. Essentially, I was always running on the ragged edge. It was fun but you know what happens when you dance with the devil. Second, I got lazy and careless. I developed a bad case of complacency and didn’t follow best practice. I know better than to do WOT pulls with the A/F ratio above 12.5:1, but I was in a hurry to finish up my big carb comparo. Dang, I was inches away from finishing that shootout, it was goin so good. Third, when I set up the engine with .060” quench, I fly-cut valve reliefs in the stock cast piston. Thinking ahead, I cut the intake reliefs for 35mm valves. I intend to eventually try 35mm valves in place of the stock 33mm valves. That meant the reliefs had to have a radius 1mm larger. Increasing the relief by 1mm reduces the wall between the top ring groove and the relief by 1mm. I think that was too thin and leant itself to the failure. Now that the chunk of piston is gone you can see how thin it was. I also think I should have applied a larger radius in the corner of the relief pockets. I used a tool bit with a 1/64" radius, I think it should have been more like 1/16" or 3/32". |
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Title: Re: Meltdown Post by DragBikeMike on 02/08/20 at 20:53:41 Forth, I’m just windin this sucker up too tight. At 8000 rpm the piston speed is 4933 feet per minute. Recommended threshold is like 4500 feet per minute for a street engine. On each one of my LSR pulls I had the thing above 6K for around 25 seconds. That top ring is the heat sink for the piston crown, the hottest component in the engine. It’s soakin up heat like a dry sponge soaks up water, and that heat must transmit through the rings to the cylinder and out to the cool air. I think the failure occurred on the intake side of the piston because that area of the cylinder gets the least cooling air. I know the fresh charge is on that side, but I think that’s mostly cooling the head. The piston is mostly relying on the cylinder to shed heat. It also might not get as much oil thrown up from the rod bearing. All you have to do is look at the failed piston to see which side runs hotter. Since I was runnin on the ragged edge, and I didn’t have the benefit of the additional fuel to help keep things cool, that poor piston simply went past the limit and started getting soft. The ring got so hot that the gap closed and butted, then all hell broke loose. The Fastman was right about the ring butting. It’s not that it didn’t have enough gap, I just took it past it’s allowable limit, generating so much heat that it couldn’t handle it. The ring butted and bound up and as the piston moved down the ring stayed put and ripped the top off the piston. I guess I could set it up with a lot more gap to provide a margin of safety, but maybe it would be better to set it up with sufficient fuel and a reasonable redline, or maybe all three (fuel, redline & butt gap). All along I’ve been concerned about detonation with this tight-quench setup. But I don’t think this failure was detonation. Nothing else is beat up. I’ve reviewed a ton of photos showing detonation damage. There’s plenty of pistons with similar damage, but all of them show ring lands folded down. None show the land folded upward. And there is generally lots of other areas pock marked and beat up. I don’t doubt that there may have been some detonation going on in my engine (due to the heat), I just don’t think detonation initiated the failure. The big carb test will have to be done over. I’ll be putting it back together with a Wiseco. The test data I’ve collected so far on the carburetors is nice to have but won’t lend itself to a fair test. I have to look at the bright side, now I will be able to do a comparative test of the Wiseco vs machining the cylinder to increase compression. That should be interesting. Hope you find this report informative. I haven’t given up on my tight quench motor, just have to figure out a few things before I try it again. I learned a lot from this experiment, and I look forward to learning more as I forge ahead. Knowledge is power. Best regards, Mike |
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Title: Re: Meltdown Post by hotrod on 02/08/20 at 22:02:49 DBM, Sorry about your engine. I know you put many hours time and thought into it. None of us know the exact number of what the rpm limit is or should be. As we search for more power, it seems the power band moves up, and the sweet spot is in a higher rpm range. Maybe with this engine it might be better to settle for a lower rpm . It will just want to tear itself apart at some point. |
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Title: Re: Meltdown Post by Dave on 02/09/20 at 04:22:48 DBM: I think you analyzed it right - the failure is located at the thin area near the valve pocket. The small amount of material overheated and liquified. I wonder if it would be wise for you to use one of those thermo coatings on the top of the piston? They all tell a good story about how well their products work - but I have never seen anyone provide a photo of the piston after it has been used. Sorry for your loss. I am sure the next round will go better. I have a couple used stock pistons if you want them......not sure about the quality of them. I believe Muratic Acid will remove the aluminum from the valves without doing much of anything to the valves.....apply it with a Q-Tip to the aluminum. (Flat trackers used to do this trick to remove scorched pistons from cylinder walls). |
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Title: Re: Meltdown Post by Armen on 02/09/20 at 07:19:39 Hey DBM, Once again a well written, photographed and posted thread! Wondering why you didn't go with a taller piston to begin with? I believe you said they are lighter? Once, to scare my self I did the math on the loading on a Big Twin HD piston/rod at redline. Roughly like using an air-cooled Beetle as a Yo-Yo. Loading is insane. Worst is at TDC exhaust when there is less pressure on the piston top. Anyhoo, if I had to guess, the valve pocket cut-out seems like the villain. Maybe with a lighter piston (and pin?) you can keep your stratospheric redline. You did stronger valve springs/lighter retainers, yes? Many moons ago I did horrible things to an XL250 Honda. Bigger bore, bumpier cam, bigger intake valves, bigger carb, stronger valve springs, free flowing exhaust, and so on. No tach, so I just revved the nuts off the bike. All fun and games til the main bearings on the crank blew their lunch. Point is, tweaking the motors sometimes leads to failure of the weakest link. When we raced our SR500 we had to butch up so many parts of the motor as we pumped up the HP. Can't wait to see the next incarnation. Sorry for your loss of time/money/energy. Good on you for sharing. |
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Title: Re: Meltdown Post by DieselBob on 02/09/20 at 09:24:24 Thanks for that thoughtful and thorough analysis DBM. It's a reasonable hypothesis and I loved how you walked us through your thinking on the diagnostic process. It's all to our benefit as interested observers. Albeit at your expense. But, a wonderful education for us all. Thank you! |
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Title: Re: Meltdown Post by DragBikeMike on 02/09/20 at 15:37:46 I appreciate the condolences. I'm not bummed out over the failure, it comes with the territory. The nature of my project is experimentation and I expect failures. All part of learning. Hotrod: I think 4500 fpm is a good number for a street engine in mild state of tune. If I set an arbitrary redline of 7500 rpm my average piston speed will be limited to 4625 fpm, just slightly above 4500. I think that should be fine as long as I maintain proper A/F ratio. My engine made max power at 6240 rpm. Shift point should be 110% of max power or 6865 rpm, well below the arbitrary limit of 7500. I just gotta be reasonable and stop burying the tach. One thing I might consider is changing my performance measurement criteria from 4K to 7K in 2nd to say 3.5K to 6.5K in 3rd, but that would make comparisons to the older setups impossible. I really need to take it just past 7K to do meaningful testing. I believe I can do it safely. Dave: Good suggestion on the thermal coating. I think I will look into that. Do you think the forged piston will hold up better to the heat than the cast piston, or does it just handle more load better? Regarding the acid, I think I will mess around with my laps and the Gizmatic first. Problem with acid is I have to store it and ultimately get rid of it. If I have to resort to the acid I will see if I can find some in a small quantity, maybe I can get a small amount from someone who owns a swimming pool. Don't they use that stuff to clean pools? Armen: Yes, I installed robust springs but the retainers and keepers are stock. DieselBob: Thanks, it's my pleasure. I simply love doing these tests and reports. I'm glad you find it beneficial. |
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Title: Re: Meltdown Post by Mavigogun on 02/10/20 at 13:55:34 7A7C730F0D0A0E3E0 wrote:
I picture you saying this climbing out of the wreckage of some sub-orbital X-plane, pilot-engineer headed aloft to see what all the guff from the test pilots was about. Data collected, back to work, in quest of better. |
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Title: Re: Meltdown Post by Armen on 02/10/20 at 14:00:45 Soichiro Honda used to say "We learn more from failures than from success." |
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Title: Re: Meltdown Post by stewmills on 02/10/20 at 14:56:27 Yes, some use muriatic acid to clean pools/concrete. You can get it at most hardware stores in a gallon jug pretty cheap. |
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