Friday, August 08, 2008
8/9 Sat Dan Schuler PS800 cancelled, ignition not working
8/9 Last year Sean Ray replaced the Aerocharger on NASCAR sort-of-retired Buddy Parrott's HD Softail turbo S&S 113 with a GT25 Garrett for more HP and better longevity. Now Greedy BP wants more so Sean is installing a GT28. Since Dan cancelled, and BP is in Watkins Glenn this weekend Sean is tuning for big HP on the roller dyno Sat PM. tweaking up to 202 (on the way to 250 djrwhp) heard rattle in cam area, turned out to be bad cam bearing, so sean yanked it off, is rebushing cam cover, putting in a better turbo friendly cam from Zipper's, will dyno again later probably 8/15, maybe cams off because Sean doesn't want anyone to see his plenum/ intercooler/ scoop on the CV carb)
8/12-13 Private session, cams off
Sunday, August 03, 2008
One sign of detonation in a sled engine is spark plug(s) loosening during operation. When I was involved with sled turbo kits, we had a 700 Wildcat turbo that would loosen plugs every time we tried to ride it on pump gas. We even tried red loctite to no avail. Running half and half 100LL av gas and pump gas cured the loose plugs. Tim Bender said recently that loose plugs on their 800 mod SnoX engines meant they had to retard timing a whisker or jet up, and no more looseness. When Glenn Hall was tuning the F1200 turbo to beyond 500 HP, he listened for detonation on the dyno copper tube deto sensor. When he heard lots of midrange "clicking" he would wave off the dyno test, and go out to tighten the loose plugs on the offending cylinder.
What causes plugs to loosen? Is it some high frequency vibration doing strange things, or are the aluminum threads being stretched slightly beyond the aluminum's yield strength?
Justing Fuller drives a supercharged Ford Lightning pickup, with (of course) an undersize driven pulley on the blower. He recently had a sparkplug blow completely out of one of the aluminum heads. Was it super-high pressure or violent vibration, or both, that caused Justin's catostrophic thread failure?
One of my favorite basement shop tools is a Jet 15 ton hydraulic press with a pressure gauge showing tons of pressure exerted by the press. For an experiment, I used the press to pop the sparkplug threads out of a cast cylinder head (7.5 tons of pressure required), and a billet head (10 tons required). Considering the actual plug area, 83,000 psi was required to destroy the threads in the cast head, and 110,000 psi to destroy the threads in the billet head. There was very little "yield" before failure. Sort of like when you just begin to pull the threads of a 6mm aluminum bolt hole, in just an 1/8th turn too much you get that sick feeling. You know you are done, as torque suddenly drops by even one lb/ft as you continue to turn the wrench slightly as threads go beyond yield, threads are now useless with no strength.
Now the steel piece I was using to push the sparkplug out was slightly smaller than the OD of the plug, causing the plug to mushroom a bit before it popped, maybe wedging into the aluminum hole meaning higher pressure to pop. I may try one more using my solid head turning mandrel instead of a hollow steel sparkplug.
Is it possible that severe deto, early in the compression stroke, could cause pressure spikes that high? Don Emery who loosened four plugs on the dyno making 525hp N2O assisted not only loosened the plugs, but he discovered later that all four combustion chamber domes were deformed, permanently bulged upwards allowing fire to burn the center orings. That must have take lots of pressure to achieve deformation. But 110,000 psi?
While Sean Ray and Tim Bender were dyno testing last week, part of my lovejoy rubber coupler and a 3/8 x 2" bolt came loose and went through the ceiling, causing shaft vibration. This buzzing at 9000 rpm loosened the 1/2-20 bolts that attach the closest 1.5" pillow block bearing to the aluminum dyno drive bulkhead. I had just aligned the toothed belt, and had a solid 80 lb/ft of torque on the pillow block bolts into the aluminum fine threads. The vibration caused the 1/2-20 bolts to become loose. I replaced the rubber coupler and missing 3/8 bolt (there's a new T-shaped hole in the ceiling tile where the bolt disappeared), and retightened the 1/2 20 bolts back to 80 lb/ft. This has happened at least once a year for 22 years, same aluminum fine threads, still hold 80lb/ft so I doubt thread deformation there from the out-of balance (that out of balance has been high enough on occasion with shaft failure to explode the cast iron pillow block frames!) vibration. In the case of my dyno drive system, the loose bolts are probably caused just by vibes doing strange things I think, not thread deformation.
So assuming detonation-created peak pressure is lots lower than 100k psi, probably just continued vibration somehow loosens the plugs, and maybe hot gases erode the threads after time and ultimately the plugs gets blown out. Or maybe in Justin's truck, continued impact of the loose steel plug male threads against the weaker alum female threads created the failure and plug blowout? I'll ask Justin Fuller if the plug dangling from his plug wire had aluminum threads on it, or maybe aluminum slag from a melt-out.
For sure, loose plugs are a good indicator that you must cool combustion chamber temps by adding fuel, reducing timing, reducing boost, reducing coolant temp, or increasing octane.
Feedback is welcome from those who have experienced this, or who have ideas about what's going on. firstname.lastname@example.org
1fastxc emailed: “Hey Jim, just got done reading your article on your blog. I can definitely relate. I had
Kelsey from RK Tech emailed: “I think that you are correct and pressures are not
I think there are several differences between your experiment and real
1) The "Heat" involved.. The aluminum head will expand faster (under heat)
2) You also have the fact that the deto will have a direction as its
pressure wave contacts the plug area. Meaning..it may originate from, say,
3) You also have oil deposits (uncombusted) that will act as a lubricant
4) Deto also has its own frequency. So the wave will have some harmonics
Other factors are the constant "pounding" of the deto occuring at the
frequency of the engine and the deto freq, itself.. vs. the constant
(non-repetative) pressure your press would yield.....
Anyway.. just some quick thoughts, off the cuff.
Kevin Cameron Emailed a few times: This is fascinating about the plugs loosening. Are they all .680 or .750" long reach? I will ask in aircraft engine circles to see if they are familiar with this.
One interesting point is that at least some versions of the BMW 1500-cc 4-cylinder turbo F1 engine had a small, spring-loaded relief valve in each cylinder. That indicates that they had some bad events that needed radical measures to control them. I've seen insulators broken by deto.
I think your idea about yielding of the threads is the explanation, but with the added notion that each pressure spike causes some pretty stout sound waves to rattle back and forth many times through the metal. It might be the cumulative result rather than the single spikes. Much the same explanation as why ignition-side flywheels may slowly work their way off the crankpin in engines turning higher-than-stock rpm. The force of vibration isn't enough to push the wheel off the pin, but the general disturbance of the flywheel, being set vibrating, lets it work loose.
Some of the early castings used on testing of the engines destined for the B-29 cracked and in some cases whole pieces of the heads were broken loose so they fell out once the sheet-metal cooling air baffles were removed. They always had piston temperature problems until they adopted (only postwar) toroidal oil jet manifolds in those engines.
Those guys that make the vehicle shakers, MTS, also make a device they call a "gun banger". It is used in fatigue testing gun barrels, and it applies many high pressure pulses that are very steep. I am suspecting that a lesser pressure in psi, but applied to the whole inner combustion chamber surface, would deform it enough to perhaps loosen the threads. BMW must have had some good reason to fit their race engines with those relief valves. Do you suppose they had this trouble? I suspect that only very high BMEP engines have this problem. Imagine combustion proceeding, and the flame front moves out from the central plug. Ahead of it, the unburned charge is being compressed. Because the initial density is about 2X that of a non-turbo engine, the amount of charge that auto-ignites when enough hydroxyl radical population accumulates is also large - not only because of the 2X from the presence of the turbo, but maybe also from the very high pressure pushing that end gas into squish zone and down into ring crevice spaces. When the sneeze comes, it will be a hum-dinger.
Related Q to KC from Jim C— "is hydroxyl radical (gas?) the cause of our eyes watering in the dyno engine room? I joke with sled owners here that even with all of our fine dyno instrumentation, we still stick our noses into the engine room, and when our eyes just begin to water from some noxious smell, then we know all is perfect, and there is very little extra HP to be gained with leaner mixture. Interestingly, we get that smell mostly with max HP NA and N2O engines, very little with turbos even with huge HP. Maybe turbos allow us to make our desired HP with boost and very safe A/F ratio, keeping peak chamber temps lower."
I think the gas you smell is nitrogen oxides making acid with atmospheric moisture, rather like sulfur dioxide makes sulfuric acid with moisture. The hydroxyl radicals are short-lived - I don't know if they even make it out of the engine. They are the trigger for deto, and what lead oxide (product of burning TEL) does is act as a negative rate catalyst. In our old barroom analogy, the lead oxide is the big bouncers who go up to the most excited troublemakers in the room and quietly whisper what they'll do to them if they make ANY trouble. It only works up to a certain level of excitement, after which the trouble starts no matter how many big bouncers are in the room. KC
[537 HP is] a lot of power from one of those [1200cc two-stroke twin] air compressors. How do you make pistons last even a moment at such power? That would make a good setup for 40,000 feet, if the air wasn't too thin up there to cool the rad. That was the big problem with the B-29 - above 20,000 the air was too thin to cool it properly, so on the trips to the Empire they had to run in auto-rich to fuel-cool the engines.
Have I mentioned that the top 125s in GP racing now make a bit over 60-hp? They are singles [operating at 13,000 RPM, equal to a 1200 making nearly 600 HP normally aspirated]. Not many secrets can be left!
Also, I see that your turbo sled bmeps are very high, so I have asked if any of the air racers have had the plug-loosening experience. A wartime R-3350, operating at take-off power and rpm, sees a bmep of only 193-psi. A Merlin in air racing, making 4000-hp at 4000-rpm, is at 480-psi. Those toluene-burners in 1980s F1 were at 1000-psi for qualifying and 700-750 for the races. I guess the 125s are really four-strokes, operating in double-time. I don't know how they do it. KC
Related Q to KC from Jim—“According to Gerhard Schruf, Ferrari F1 turbo engineer during the1980’s, they ran the Formula 1 V6 turbo in testing or qualifying at 100 psi boost on 102 octane spec gasoline !!??!!”
Fuel octane is still measured in a CFR test engine at 1300-rpm, so I think the fuel suppliers have gone back over the original experiments of Dr. Graham Edgar and repeated them, but this time at much higher rpm. I'll bet they found that certain fuel components that tested at 102 ON at 1300-rpm tested much higher at 12,000 and higher yet at 20,000. I think those guys have a little black book with all that useful info in it, and we would love to have a copy! KC
8/6 Smooth combustion is like a linebacker leaning hard against a door. Deto is the same man, backing off ten yards and hitting it at full speed. The door lasts a long time in the former service, and less than 1/10 of a second in the latter. KC
Email from Swedish ME/ sled hotrodder who designs equipment that transfers the forces of vibration to be used in heavy industry:
I just read your text on the subject as well as the email replies.
I believe that you have covered most of the probable causes of the spark plugs coming loose but I want to put light on a phenomenon which has not been covered.
Much like the gas in our two stroke pipes the metal in the combustion chamber and the sparkplugs will have tensile and compressive stress waves traveling through them, and also much like the pressure waves in the pipes, these stress waves can and will superimpose. From experience with stress waves and threads I also know that part of the longitudinal stress wave will be converted into a torsional stress wave which can in fact by sheer torque loosen the thread.
A little text on how the thread can come loose from this:
The very high pressure of deto combustion acts on the bottom spark plug surface and thus induces a compressive stress wave in the spark plug. Said stress wave travels upwards with the local speed of sound (~5200 m/s) propelling the spark plug particles upwards with some particle velocity. When the compressive stress wave reaches the top of the spark plug it will reflect back downwards as a tensile stress wave which propels the particles upwards. This stress wave will lower the normal force in the thread interface effectively decreasing the thread friction. When combining this fact with the fact that part of the longitudinal stress wave energy will be converted to torsional stress waves traveling up and down in the spark plug and for each pass through the plug they will untighten [loosen] the thread interface.
The cracking of porcelain can also be explained by the tensile stress waves as when compressive the metal portion of the spark plug diametrically expand and when tensile it will diametrically shrink. This will induce tensile stress into the porcelain and we all know that ceramics does not like tensile stress.
The stress waves will be dampened out quite fast, escaping into the big structure of the motor, but as long as deto is going on new stress waves will be fed into the spark plug.
This is just a simplified picture of what is going on, there will also be stress waves in the combustion chamber which also can loosen the thread interface.
p.s. The "spark plug particles" is actually the material of the spark plug, i.e. the steel in the spark plug body and the "slug" of particles inside the stress wave thus have an inertia. You can picture the particles moved by the stress wave as a "slug" of steel moving. Imagine a hammer striking a rod, the end of the rod will not move until the stress wave has propagated from the end hitted by the hammer, through the rod length and to the end of the rod. The stress wave has a certain length and the metal particles affected by the stress wave will be pushed in the direction of the stress wave if it is a compressive stress wave and in the opposite direction if the wave if it is a tensile stress wave. The theory is almost exactly the same as for the pressure waves travelling through our twostroke pipes except for the differences in working media.
Kevin Cameron read "Scavenger's" info and added:
I think this sounds pretty good - especially when combined with the higher amplitude of such waves in highly supercharged engines (i.e., why our aircraft engine friends have nothing to say).
Some of the worst injuries in armored vehicles come from what is called "spalling", which is detachment of armor from the inside surface of the armor at high speed. As our Swedish informant notes, the stress wave is actually moving the material axially. When such a wave reaches the inner surface of the armor (from a shell hitting the outside, for example) it becomes a high amplitude wave of expansion, and a chunk of the inner surface comes loose by tensile fracture and flies around the interior of the vehicle, wrecking people and things. The armor is not penetrated at all, but some of the incoming projectile's kinetic energy (or, in the case of a so-called "squash-head" explosive, some of the detonation wave's energy) is transferred to material at the inner face of the armor, detaching it at high speed.
Another thing like this is the harmonic pile-driver. A traditional pile driver uses a steam hammer to send stress waves down the object being driven into the ground. As each one arrives at the tip, the material there actual moves, displacing/penetrating the soil ahead of it. A clever engineer then reasoned, "Why be satisfied with such slow advance? Let's set the entire pile into resonance, by applying sound waves to the top end in step with the time it takes the wave to bounce back and forth from top to bottom to top".
I watched such a machine at work in 1963. The top of the column was tightly coupled to a pair of contra-rotating eccentric weights, supported in heavy bearings. These were driven by a large industrial engine, mounted at the top of the column. Once everything was in place, the engine was accelerated to resonant speed, a powerful hum came from the column, and it sank quickly into the ground to its full length in a very few seconds - not the usual hours of pounding. Very impressive.