- In order to raise the compression ratio without having another set of custom pistons made, I welded the domes to fill in unneeded valve pocket clearance.
- Despite the welding and re-machining, the piston dome thickness remained at a reasonable .200".
- The engine was not run too lean, in fact just the opposite.
- Rear cylinder piston failure is common on Harleys with dual fire (aka wasted spark) ignitions at Bonneville.
The small holes were drilled after the fact to check dome thickness
Exhaust pocket had also begun to "sag"
To clarify, for those not up to date on Harleys old wasted spark ignition system, it works like this. On all but the latest offerings from Milwaukee, the timer, whether it be a battery/points ignition or a magneto, turns at half the speed of the engine. This timer has two lobes (or notches for later electronic ignitions) which open the points, initiating the spark. One lobe is set to open the points at the correct time for the front cylinder and the other at the correct time for the rear cylinder. In the case of the Goose, that time was at 42 degrees BTDC (before top dead center) on the compression stroke.But since these lobes both open the same single set of points, each cylinder gets a spark from each lobe, once at 42 BTDC on the compression stroke, and once on the exhaust stroke.
Now you need to remember that the two cylinders on a Harley form a 45 degrees angle (thus the term 45 degree V-Twin). That means that at any given point in time, the front piston will be at a point 45 degrees behind the rear piston in crankshaft rotation (I know that sounds backwards, but its not). For instance, when the rear piston is at TDC on the compression stroke, the front piston will be 45 degrees away from reaching TDC, but it will be on the exhaust stroke. This 45 degree offset is what makes things interesting.
Obviously if the rear piston is the one that normally takes the hit at Bonneville, there must be something that differs front cylinder to rear which is the culprit. And if it seems to be exclusive to wasted spark ignitions, then that would be a good place to look.
First lets look at the front cylinder. After plotting the SS Cycle KN420 camshaft that the Goose employs, we find this: At 42 degrees BTDC on the compression stroke the rear cylinder spark plug fires, and the wasted spark is produced at that same instant in the front cylinder. However, the front cylinder is not at 42 BTDC on the exhaust stroke, but rather 87 degrees BTDC (remember it trails the rear by 45 degrees). At this point in time the front intake valve is still on its seat, in fact still about 6 degrees before it comes to the opening ramp on the cam. The exhaust valve is just starting to close, but still near full lift. OK - no problem. That spark in the front cylinder with the exhaust valve open and intake valve closed won't do much.
Now for the rear cylinder. When the front cylinder spark plug fires at 42 BTDC on the compression stroke, the rear cylinder is in a much different position. Because of the the 45 degree offset, the rear piston is at 3 ATDC (after top dead center); technically not even still on the exhaust stroke, but rather beginning its descent on the intake stroke. The exhaust valve is closing, but still .135" off its seat. The intake valve on the other hand has already started to open to the tune of .185" off its seat. Valve overlap is the common term. Now that gives one something to think about doesn't it?
So her is some other pertinent info gleaned from PipeMax that may shed some light on the issue at hand.
Assume the Goose's engine were running 5500 RPM. At the point where the rear cylinder receives its "wasted spark" (3 degrees ATDC in overlap), the piston has already started down on the intake stroke, accelerating to 325.5 feet per minute providing a "piston demand" of 10.1 cubic feet per minute of intake air flow. Raise the RPM to the target 7500 and that becomes a piston speed of 443.8 feet per minute with a piston demand of 13.8 cfm. All that is at a mere 3 degrees after top dead center.
If you put stock in David Vizard's theories (and I do), you may recall that he puts a large amount of emphasis on the overlap portion of the cam timing. One of his conclusions is that the low pressure area caused by exhaust outflow results in the single strongest action initiating intake flow during this overlap period (hopefully I have paraphrased him properly). So what effect does it have when you throw a spark into the middle of that overlap period? A spark which incidentally does NOT occur during overlap on the front cylinder. Hmmm.
So here are a few thoughts. Obviously the rear cylinder is subjected to a spark during overlap which the front cylinder does not. Since that cylinder is in overlap, there will be a fuel air mixture present to burn. Now, that fuel air mixture is not compressed, but certainly it can burn none the less. And what naturally comes along with burning fuel and air? That's right - heat; heat that the front cylinder is not subjected to. I have no way of knowing or even estimating how much extra heat the rear cylinder gets this way, but the evidence would suggest that it may be just enough extra heat to melt a piston dome.
Here is something else to consider though. What happens when that wasted spark fires off and the fuel air mixture is richer than ideal? The BTUs are in the fuel, not the air, so I would assume that you would be releasing even more heat than with a correct mixture. Hmmm. Remember that I said that the Goose was not run too lean. It actually had the baseline jetting that it was dyno'ed with here in Minnesota. The reason I was pretty sure that the piston did not fail from a lean condition was that I believe that it was "pig rich" (as I like to call it). Could this be a case of a rich mixture giving the opposite results that one would expect?
But why does this happen at Bonneville, but not on the drag strip or on the street? Well, my guess would be that it has everything to do with length of time spent under a heavy load. Remember that aerodynamic drag becomes a huge factor at high speeds. There are plenty of horsepower/MPH calculators available on line. Plugging in some estimates (guess-timates?) for weight, frontal area and drag coefficient, we find that if it takes a mere 29 HP to hit 100 MPH, the same bike would need 55 HP to get to 125, and 77 horses to reach 140. And if that is not enough of a wake up call, if you want to raise the MPH from 140 to 150 you better be ready to call up an extra 17 HP to wring out that 10 MPH. Bottom line is that high speed puts a tremendous load on a motor, and the longer that load is present, the better the chance for heat build up.
Now we know that a drag motor will not see much time under full load and even less time at high speeds. If you hit 100 MPH in the 1/8 mile, then you will likely see the 1/4 mile finish line in another 4 seconds. And on the street? Despite thousands of bar room stories to the contrary, most street motors will never get more than a few seconds at full throttle and high speed before law enforcement rains on that parade.
So where do we go from here? Obviously a single fire ignition system is in order. The exact form that will take is still up in the air, with part of the team leaning toward keeping things as simple as possible, and part leaning toward as hi-tech as possible. The other obvious bit is that new pistons are needed (you didn't really think I would weld them back up, did you?). That part has been settled.
Four new pistons from Arias arrived this week, with the domes finished as per my sample (the undamaged front piston). They are down right beautiful, and hopefully the two spares will remain in the box as spares for a long, long time.
13 comments:
I know you're not supposed to look a gift horse in the mouth, but I feel that I need to point a couple glitches with "Blogger" once again.
I cannot write "S&S Cycle" in a post without it being published as some gibberish. My guess is that Blogger reads the "&" sign as an html command.
The other pet peeve is that both a numbered list or bullet points always become girly looking flower blossoms (at least in my viewer). Neither of these are big issues, but I wanted everyone to know they are not by my choice.
I'm enjoying your description and just thinking out loud as I reflect on your thoughts.
I don't know the dimensions of the motor, but if a stock EL, I think it would be 3.5" stroke and 7.468" rod length. With those dimensions at 5500 and 7500 rpm wouldn't this give instantaneous piston speed at 3 deg ATDC of 6.3 ft/sec and 8.6 ft/sec, with a maximum instantaneous piston speed at 7500 rpm of 86.3 ft/sec occurring at 77 deg ATDC ? Your numbers seem way too big.
The rear cylinder obviously faired differently in this build than the front, and I'm not disputing the fact that they are different, or that heat caused the failure. But I wonder if the welded piston is a significant contributor to the "holing". Once welded I believe the filled area would be soft, fully annealed material and of significantly lower strength than the virgin piston. I note from the pictures that the area adjacent to the "hole" appears collapsed where it was softening. I wonder if it could have survived with fresh 2618 alloy pistons? Did the front piston show any piston deformation in this region?
Well, you are right that I had the piston speed wrong. It should have read feet per minute not feet per second. I misread the data from Pipemax. I will correct it ASAP.
Pipemax also shows the max piston speed of 86.2 ft per second at 77.5 degrees, so we are on the same page.
I agree about the lower strength of the welded piston, and that was my initial thought as well. However that does not explain others with non-welded pistons experiencing the same problem on the rear cylinder only. The front piston looks as good as when I installed it. If the rear piston had not sagged I would attribute the hole to the welding, but as far as I know the annealing and lower strength should not affect melting temperature.
Very interesting stuff Lee,I'm glad your taking us along for the ride. Also wanted to know if you follow the "Quartermile Knucklehead" blog.
Tim
I like girlie looking flower blossoms.....
I don't think we're concerned so much about the piston alloy melting point, but rather the difference in yield stress of the alloy in the annealed and hardened state (perhaps 5X ?). This ultimately becomes a circular discussion, because if there is excessive heat in the system and the piston is exposed to it, the piston can become softened in use, and then it makes little difference whether it started soft, or was compromised during use, as it fails either way in the softened state.
In doing blown engine post mortems where the piston is suspect, one of the 1st things a piston vendor does is check the hardness of the piston, which provides a clear picture of the thermal history the piston has experienced, where they are always soft if they've been overheated.
This is an intriguing discussion and I'll be sure to stay tuned for the next chapter.
Hi Tim, yes I do follow that blog. What he is doing and has done with that Knuck is very impressive.
WZ - Thanks for the comments, I appreciate your input. If I understand correctly, you are suggesting that the "sag" may have started from cylinder pressure on the annealed aluminum at a temperature which it may have withstood if it had been unmodified. That is certainly possible, I suppose, but testing it would involve running next year with new pistons and wasted spark.
The anecdotal evidence seems to show that rear piston was doomed to fail either way - perhaps the Goose would have lived longer (to a higher speed and temperature)with an unmodified piston, but we may never know for sure.
Good reading Lee!
The Blog would not allow the posting of my comments due to the length (a few hundred characters over the limit), so I split it into 2 consecutive comments. Hope it makes sense in split form.
I'm still puzzling over the Goose's rear cylinder instability and can't get a grip on cause and effect of the failure other than the obvious - it over heated and detonated. But why? I certainly don't have an answer but since feedback is encouraged here, I offer the following speculation in the matter.
Assuming there are no mechanical issues with the engine I think overheating can be distilled down to 2 critical parameters - air/fuel ratio and ignition timing. If there were no issues with either parameter I don't see things going south and we wouldn't be having this conversation.
It has been suggested that the bike was run in an excessively rich condition, so fuel doesn't seem likely to be the smoking gun here, and if it was a factor at all, one would think the dyno testing would have identified it straight away. Therefore, IMHO the prime suspect here is ignition related.
I do appreciate the point about wasted spark and agree that it is a bad situation that needs to be eliminated. I personally can't see wasted spark lighting any mixture in the absence of compression (near TDC at overlap), but to play devil's advocate, lets assume it does momentarily light some minor fraction of the charge, which serves to "spoil" the real ignition/combustion event later in the cycle by contaminating the charge with some minor fraction of partially combusted hydrocarbons (like minor EGR in a flathead causing black combustion). If so, the spoiled combustion event might be of poor quality (slow), and hence convey more heat (where heat eventually leads to detonation) to the system via the prolonged combustion time. Who knows? At any rate I'm glad "wasted spark" is now in the past tense so this subject can be checked off the list of possible ignition related issues.
Another aspect of ignition timing relates to the relative temperature of the engine and the ability to run in a detonation-free state. As noted by you and other contributors here, there is night and day difference between drag racing and LSR racing, i.e., both events are WOT, but one event occurs in 10 seconds with a lukewarm engine, the latter goes on for well over a minute (which is nearly an eternity relative to anything we'd normally do to an engine) with an engine that is typically at full operating temp at the start. Thus, ignition timing that is tolerable for the drag strip may be inappropriate for the long haul on salt, due to the difference in engine operating temp. I'm suspecting that as heat builds during the run, the rear cylinder, for whatever reason (aerodynamics of the fairing, wasted spark/EGR, or whatever?), is not cooling like the front cylinder. This leads to detonation in the rear cylinder which in short order destroys the engine. If this is so, what could one do about it?
From your recent post it sounds like The Team is up in the air right now with respect to which particular ignition system to fit. My $0.02 would be to go for the "as high tech as possible" version that allows independent timing of each cylinder (for whatever reason the rear may not tolerate the timing of the front cylinder), has a knock sensor interfaced in real-time with the ignition module, and can automatically retard the timing if/when detonation is detected. Given the time and energy that goes into building an engine (and bike) it seems to me that if such an ignition system kept the engine together, it would end up being reasonably priced insurance.
Perhaps with the new ignition system the rear cylinder heat issue will be gone entirely and it will be business as usual. Nevertheless, I still wouldn't venture onto the salt without conducting steady state dyno testing. This would be the test where fuel mixture and ignition timing are fine tuned and the system configured to assure that if detonation occurs, timing is retarded an appropriate amount to eliminate it. If the exhaust temp can be stabilized at WOT for each cylinder, under a heavy load and over an extended period of time, you should be good to go, as this suggests that all critical parameters are under control. If exhaust temp can't be stabilized, troubled waters lie ahead, and you've got to understand why they can't be stabilized, and assure that they are ultimately stable.
To conclude on an optimistic note, successful steady-state testing should give The Team all the encouragement they need to head to the salt assured, optimistic and enthusiastic. Hard to believe it's only 4 months away!
WZ, thanks for commenting again. I find your insights valuable.
About your comment on the wasted spark not lighting a mixture that is not compressed. That was my initial reaction, but upon further consideration I came to the conclusion that it should burn. After all, if you pour gas on the floor and light it, it will burn absent compression. My take is that the compression adds enough pressure to the burn to make an engine run, but it still will burn as long as there is fuel, air, and spark. Don't know how many BTU's are burned during overlap, but...
The next thing is that I am seeing no evidence of detonation, at least not as I understand the term. The intake and exhaust pockets of the rear piston both sagged, but only the intake side actually melted through. There are no other signs (that I can find) anywhere in the chamber or on the plug indicating detonation.
Word has come from Joe Taylor, the owner, that the Goose will run the mag with the new single fire plug wires from Joe Hunt. Part of his decision is based on simplicity, but also an attempt to remain true to the original purpose of replicating Joe Petrali's 61" land speed record Knuck.
Your reccomendations about dyno tuning, etc. are noted and well thought out. I hope those precautions will will be taken, but time and money have a way of slipping away from all of us...
Lee,
Don't mean to beat detonation into the ground (I think I already have), but…….
I'm not necessarily suggesting that it did or didn't occur, just noting that in my mind, "holed" pistons and detonation (high energy waves in the combustion chamber) go together like fire and smoke. We tend to time engine ignitions for optimum power, which has to be somewhere shy of detonation. However, if anything in the performance equation changes (increased temperature or pressure), the system can be perturbed into an operation regime where detonation does occur. That said, since the piston reached a temperature where the yield strength of the alloy was exceeded (it caved in), it had to experience a very significant temperature that was well beyond the "normal" operating condition. It would therefore be reasonable to suspect that as rear cylinder temperature climbed, combustion could very possibly move from a normal combustion event to a regime where detonation occurred or perhaps dominated the combustion process.
Since detonation is the product of heat, pressure and time, it would preferentially occur at lower, rather than higher engine speed because detonation takes time to occur, i.e., an engine that rattles (detonates) when lugged, may operate detonation-free if engine speed is increased significantly. In light of the foregoing, if detonation were occurring, the highest probability for occurrence would be just after a shift point when the engine is operating at the lowest speed in the recovery phase. Did the driver by any chance note when the piston "holed", i.e., what gear, just after a gear change, at the top of a gear, how far into the run, etc?
I suspect when the piston "holed" the sparkplug was contaminated with oil or carbon, making it difficult if not impossible to read for detonation. I wonder if the plug residue could be gently washed away (gentle rinse with carb cleaner?) to reveal any information the plug might hold. Perhaps if the plug base were cut off in a lathe, to reveal the full porcelain length, some detonation telltale might be visible. Then again, maybe there is no telltale and maybe detonation didn't occur at all. I'll stay tuned.
Wow, I cannot believe how long it has taken for me to get back and answer this last comment - sorry. I finally got around to chucking up the rear plug and cutting off the base. Here is what I found. There are some specs of shiny aluminum, however there are no sign of any black specs. Also there is no damage to the porcelain. I would expect detonation to damage the porcelain before or at least at the same time it holed a piston, but I am no expert.
The reports I have are that the engine sound changed at the top of 3rd gear, which I assume is when the piston became ventilated. On the other hand I know the bike was on and off the throttle several times on the previous run due to being blown off course by cross winds. If I remember correctly, the pilot previously told me that he likes to take it easy through the first couple gears (something I did not necessarily agree with, but then I have never been to Bonneville, so I did not feel qualified to comment).
Anyway, now you have me worried about having too much timing! I'd feel much better if I was still convinced that it was just the wasted spark. Maybe I should back off the timing a little and see what the plugs look like after some dyno time. Wish I was a better tuner and/or that a really first rate tuner would be at Bonneville with the team.
Any volunteers?
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