Piston damage from not enough ignition timing?
Piston damage from not enough ignition timing?
(OP)
I'm going to leave this a little vague, so bear with me.
A naturally aspirated, 13:1 big cam BMW race motor was brought to me for mapping of its Motec M4. The engine was built by a reputable builder. The car was started and the timing set using the CRIP page and verified using a timing light as normal, base fuel pressure, etc (all normal first startup checks). I mapped the car as I usually would on our Dyno Dynamics dyno using 109 octane VP fuel. I finished the fuel map and was working on the timing map, steady state in the lower load regions then doing sweep runs at higher RPM and loads to keep coolant temps in check. I was using the torque feedback from the dyno to determine MBT while doing the sweep runs. The timing was 18* starting the pull at 3k RPMs ramping up to 22* by redline, all in by 6500 or so RPMs. The engine made great power (~240 HP on these early pulls), had no blowby, and was otherwise well behaved. The WOT mixture was .86 lambda. The spark plugs were all an even, normal light brown color and they were removed and inspected several times during the day. After 4-5 power runs the engine developed a noise at idle. It was shut down, removed, and inspected.
What was discovered was a scuffed piston skirt on one cylinder. The top of the piston looks perfect, bearing is perfect, rings good, no damage at all on top of the piston, but the skirt is scuffed along with the cylinder wall. The engine builder claims that running the timing this low will damage the piston in this manner. The builder claims to run 10* more timing across the board, which is totally believable. So how did he get to this timing value? I assume by testing until no more power could be achieved. He further claims that high EGTs are to blame for the piston damage. If this is true, then so be it I am prepared to accept responsibility but I have never seen an engine fail in the manner and it puzzles me. I could see damage being done if the timing was retarded severely, but the engine would not have good power or response. The exhaust ports, valve guides, and manifold show no sign of heat damage. All other cylinders look perfect.
Is it possible for an engine to be damaged in this manner by running timing that may be 10* retarded from optimal? Thanks in advance for any responses.
A naturally aspirated, 13:1 big cam BMW race motor was brought to me for mapping of its Motec M4. The engine was built by a reputable builder. The car was started and the timing set using the CRIP page and verified using a timing light as normal, base fuel pressure, etc (all normal first startup checks). I mapped the car as I usually would on our Dyno Dynamics dyno using 109 octane VP fuel. I finished the fuel map and was working on the timing map, steady state in the lower load regions then doing sweep runs at higher RPM and loads to keep coolant temps in check. I was using the torque feedback from the dyno to determine MBT while doing the sweep runs. The timing was 18* starting the pull at 3k RPMs ramping up to 22* by redline, all in by 6500 or so RPMs. The engine made great power (~240 HP on these early pulls), had no blowby, and was otherwise well behaved. The WOT mixture was .86 lambda. The spark plugs were all an even, normal light brown color and they were removed and inspected several times during the day. After 4-5 power runs the engine developed a noise at idle. It was shut down, removed, and inspected.
What was discovered was a scuffed piston skirt on one cylinder. The top of the piston looks perfect, bearing is perfect, rings good, no damage at all on top of the piston, but the skirt is scuffed along with the cylinder wall. The engine builder claims that running the timing this low will damage the piston in this manner. The builder claims to run 10* more timing across the board, which is totally believable. So how did he get to this timing value? I assume by testing until no more power could be achieved. He further claims that high EGTs are to blame for the piston damage. If this is true, then so be it I am prepared to accept responsibility but I have never seen an engine fail in the manner and it puzzles me. I could see damage being done if the timing was retarded severely, but the engine would not have good power or response. The exhaust ports, valve guides, and manifold show no sign of heat damage. All other cylinders look perfect.
Is it possible for an engine to be damaged in this manner by running timing that may be 10* retarded from optimal? Thanks in advance for any responses.





RE: Piston damage from not enough ignition timing?
I am surprised there is no coating on the skirt to help it bed in without scuffing.
If it was making good power that indicates it should not have been so retarded so as to overheat the piston.
Most likely wrong piston to bore clearance for that piston type.
Was it dry sumped or did it have crank case evacuation?
Regards
Pat
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RE: Piston damage from not enough ignition timing?
I was also surprised about the lack of skirt coating as well as the fact that there are no under-piston oil squirters. This was originally a 2.3L which has had it's crank swapped and bored to 95mm. The factory 2.5L has oil squirters, could this be a contributing factor? Based on my research, it is common for racers to retrofit this block with squirters.
RE: Piston damage from not enough ignition timing?
yes.
the first thing I thought when you described the failure was "someone must've bent a jet"... but Pat had some good ideas too, so I thought I'd wait a bit.
RE: Piston damage from not enough ignition timing?
I'd go with the theory of insufficient piston to bore clearance at completely soaked and stabilised operating temperature.
How about gettig some accurate measurements on a good parts of the bore and piston and then consulting the piston supplier for advice?
Bill
RE: Piston damage from not enough ignition timing?
oil squirting may prevent that, but the fact that the other pistons show no damage indicates a local cause.
apart from the piston not having the correct size as already mentioned by patprimmer, also a local hotspot in the cooling jacket may be the reason. when that happens locally vapour may build up with the result that the heat is not sufficiently carried away from the bore resulting in piston scuffing.
the fact that it is the skirt that jammed and not the piston head also indicates that the cause is not excessive heat stemming from the combustion or the changed ignition timing but that it was a local problem outside the cylinder.
local hotspots in the cooling jacket can go unnoticed because it will not show on a meter reading before all the coolant has risen in temperature.
RE: Piston damage from not enough ignition timing?
RE: Piston damage from not enough ignition timing?
RE: Piston damage from not enough ignition timing?
I find it interesting that the damage starts at the skirt rather than the top. Engines that I have seen killed by lots of detonation and preignition were four corner scuffed or plain seized in the bore at the top of the piston.
Attached is another view of the piston.
RE: Piston damage from not enough ignition timing?
I would put my money on a shifted cylinder wall casting or a localized hot spot as mentioned above.
Franz
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RE: Piston damage from not enough ignition timing?
RE: Piston damage from not enough ignition timing?
skirt failure is caused by lack of clearance or lubrication
It wasn't by any chance the cylinder closest to the cooling water inlet ?
RE: Piston damage from not enough ignition timing?
I've had a sleeve shift...It looks different, not as severe.
I've had a faulty cooling system failure...Stuck pistons look different.
I've had a rod bearing failure that put bearing material between the piston skirt and cyl wall...It looks similar.
I've had two failures that look exactly as you picture...One was from no oil jet holes in the rods (Cosworth) combined with too little side clearance. The other was from loss of oil pressure that took out the bottom end quickly, only found the piston on tear down.
I totally agree with all previous posts, especially the "mis handling" aspect.
Rod
RE: Piston damage from not enough ignition timing?
RE: Piston damage from not enough ignition timing?
We just sent one of our vintage engines to a specialty tuner in Norcal last week. I am confident in my work (and his, I've known him for years) and not too worried.
Every time I put a "customer" car or engine on the dyno I was terrified. A good enough reason to not do "customer" cars any longer.
Rod
RE: Piston damage from not enough ignition timing?
Regards
Pat
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RE: Piston damage from not enough ignition timing?
RE: Piston damage from not enough ignition timing?
We are working with the owner and our usual machine shop (not the previous builder) to put together something better with oil squirters, coated pistons, and adequate block preparation. I will research coolant flow of this particular engine to be sure of even cooling of the cylinders.
The wrist pin bore in the 1st pic is fine. It is out of focus and the notch shows up strangely.
RE: Piston damage from not enough ignition timing?
One more, bore distortion.
What was the oil viscosity?
Depending on who assembles the engine, many tails of how to are everywhere. And especially when it comes to rings and pistons and what magic lube or not to lube they use.
Its either a fitment problem, or a lubrication issue. I see no over heat signs. Its not the tuners fault.
Maybe a rush it friday assembly job. Or twisted rod. Like in china after market hole alinement.
Please let us know what the findings are.
RE: Piston damage from not enough ignition timing?
RE: Piston damage from not enough ignition timing?
I have had some experience at deliberately retarding the ignition timing on the dyno for testing purposes. The first thing that is impacted is the exhaust. Generally the exhaust headers will start to glow an orange color and eventually we stuck an exhaust valve. All of this and still we had no damage to the piston.
As with the others here, the problem lies elsewhere and I would lean toward either lubrication or piston to wall clearance.
Larry Coyle
Larry Coyle
Managing Partner
Cylinder Head Engineering, LLC
CNC Porting
De Soto, KS 66018
RE: Piston damage from not enough ignition timing?
I've seen failures from unmetered air leaks over the number 2 cylinder. Evidence of detonation on the top of the piston and the rod bearing being wiped out. There is a pressed in air fitting on the throttle body assembly that "vibrates" itself loose over time. It starts letting unmetered air in over cyl. #2. Two cars I have seen this happen. Rod bearing is wiped out, detonation ensues and the rod bearing spins.
My experience dealing with S14's and overbored S38's leads me to think this issue was not air/fuel or timing related. My own personal next step would be to get some accurate piston and cylinder wall diameter measurements of all cylinders. Best of luck. No one wants to assume blame in these circumstances.
RE: Piston damage from not enough ignition timing?
Unfortunately, the builder now has the engine and is not budging from his story. He still adamantly claims that 18* of ignition timing at 100% engine load (ie. ~100 KPa) will damage the pistons (even though only one was damaged). He claims the high EGT will cause damage. It seems to me that retarded ignition timing that less heat will be transferred to the piston as the burn happens so late that a lot of combustion is taking place around the exhaust valves and in the exhaust port (hence high exhaust gas temps). We do not have the engine anymore so there is no verifying the bores.
It's an unfortunate situation, but the builder can provide no documentation of anything regarding clearances. My datalogs and map showed how the engine was treated and the builder says that is the cause of the failure. Not much left to do but to not take the job again!
RE: Piston damage from not enough ignition timing?
RE: Piston damage from not enough ignition timing?
I've seen overheated engines, from loss of coolant, and not seen that sort of damage. On the other hand I have seen many 2 cycle Oil/fuel mix engines that were run with no oil mixed in the fuel that had pistons that looked just like that.
The key word here is no oil.
I would guess that builder is one of those guys that does not lube the cylinder during assembly thinking the rings will seat faster. Or again a fitment problem.
What were the bearing clearances?
So who originally started up this engine?
As far as the timing goes, even the builder should know
that is set to were the engine runs good and makes power, you just don't guess at it. That is one of the reasons to dyno it. To establish such things as timing. Too many variables to guess at it. When NOS is used it is standard practice to retard, and run close to same F/A ratio.
If timing is late then power suffers and exhaust heats up.
So as you were tuning it, what was the power doing vs the timing?
RE: Piston damage from not enough ignition timing?
I originally started the engine here and it was a normal new engine startup. I had a wideband lambda sensor on the engine from startup and I calibrated the timing at several RPM points to be sure the CRIP setting was correct in the ECU and that there was no timing error.
Torque was increasing with each degree of timing but the gains were getting smaller and smaller per degree, so I knew that I was closing in on MBT. The excuse that the timing was too retarded was given by the builder as a way of deflecting blame in my opinion.
The owner of the car has decided to pay the same builder to do another engine with lower compression. I have opted to not take the job and the builder is going to tune it himself.
RE: Piston damage from not enough ignition timing?
If those were forged pistons .002 is not enough.
RE: Piston damage from not enough ignition timing?
Regards
Pat
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RE: Piston damage from not enough ignition timing?
Yeah, on my old Venolia forged slugs it was 0.007"...Old school. You don't get a reliable 206hp from a two valve DOHC 1600cc engine with "old school"...not with any degree of reliability/longevity.
Rod
RE: Piston damage from not enough ignition timing?
RE: Piston damage from not enough ignition timing?
The style of cylinder ie wet liner, dry liner, no liner cast in aluminun liner etc. will all be different. So yeah what works in one will not work as good in the other.
No matter, the problem was not caused by running the ignition SLIGHTLY retarded. All signs say the builder goofed up and he's trying to pass it on to a guy that tried to tune the mess.
RE: Piston damage from not enough ignition timing?
TTF...You want me to guess what someone else can build into a twincam? Our engine is up at Veloce Mtrs. for rebuild and reporting of the exhaust ports now as I'm just a few months short of 70 and my hands won't hold still long enough for me to do my own stuff any longer. I've ported my last head. :o(
However, in the past I have done several tc's for street that made an easy 120hp at the wheels (perhaps 140/145 at the flywheel) that were really NICE. I mean they did not give up anything in tq over the stock setup...Which, incidentally, is not at all bad. Several camshaft profiles are available from a variety of cam grinders. I've use Isky, Kent, Harmon-Collins on street grinds with success and Cosworth, Omnitech and, "G-3" (Gorilla 3, my own grind) in race engines...One thing that seems to be a given is the valve sizes, 1.625"/1.375" are best for most applications, race or street.
A minor clean up of the ports, a nice set of 40 DCOE's with 30 to 32 chokes using the stock exhaust headers with 0.040" oversize pistons, 10:1 CR, nice valve job with a moderate cam grind and a light weight flywheel/clutch assembly. Match that with a good proprietary electronic ignition system WITH A BUILT IN REV LIMITER and you should be pleased with the result.
Rod
RE: Piston damage from not enough ignition timing?
I'm 14 behind you, things are rough at this age.
RE: Piston damage from not enough ignition timing?
When I drop the visor on my helmet and the green flag flies, all those youngsters that are in the field BEHIND ME couldn't be convinced that I am NOT thirty anyway.
The lie to all this is things that happen to remind me. Like yesterday I fell off a ladder trying to get into the attic to do some rewiring. No damage except my pride...hanging there and calling my son to come move the ladder so the old fat guy can let go!!!
Rod :o(
RE: Piston damage from not enough ignition timing?
The pistons are speced by the builder and made by CP. CP stated to me that the cracking around the intake valve reliefs is usually attributed to the piston top being specced too thin. So this leads me to my next question: Can the piston construction (either too light or too thin) cause the piston to run too hot and damage the bore/itself in the process? I will post more detailed piston pictures this evening.
RE: Piston damage from not enough ignition timing?
The only piston crown I've seen that was so thin as to cause problems in fairly normal (for racing) use was in a Datsun 510. A very high CR L-16/18 that the kid had cut the pistons down for deck clearance and which got hot enough to sag...Really. Sagged like a bowl of pudding! I think this was a "one off"...
Sorry to hear these stories, LAEE. I know it happens and it really pisses me off. Y'all probably cannot be like me, I made all my crappy mistakes myself and in the process, learned how to NOT get taken in by "big shot" reputations. It makes it tough on some of the genuine builders and leaves the competitor with a sour outlook...especially when he must find another builder to re do the crappy work.
Rod
RE: Piston damage from not enough ignition timing?
So the cracked pistons where the same ones with the galling? If lets say the late timing caused heat build up, then why only a select few pistons were involved?
You might ask the piston manufacture if a tight bore clearance could set up stresses to cause the incipient cracking.
And why the intake relief area? Is it the thinest?
Normal shiney spot on the skirt? Not too normal if the piston is fit correctly, sounds like a tight high spot, and lubrication is lacking there. Any idea how the cylinder was finished? What ring widths?
Do any of the pistons show signs of overheating ?
Well yeah they do on the bad skirts from friction.
RE: Piston damage from not enough ignition timing?
Start with what you 'think' is close (based on books, what others have done, etc.) and make small changes until it doesn't get any better (a dyno is a great aid here). Every change in timing may require changes in jetting and, of course, every change in jetting may require a change in ign timing. This "circle jerk" also goes for changes in camshaft setup and timing, also. Change any one thing may require changes in everything else.
I don't do much street stuff except vintage and antique cars. I stick pretty much to what the book calls for and, for me, fuel mileage is something other people lie about.
For race engines, too rich is much better than too lean and slightly less ign timing is MUCH better that slightly more. Most camshaft specs are near ideal compromise but performance can often be altered by slight changes from std.---Common practice with most engine builders.
Rod
RE: Piston damage from not enough ignition timing?
I didn't want to read through this whole thing again.
So the installer didn't fire the engine up?
The engine builder didn't fire the engine up?
If you were the first to initially start the engine up, before anyone else did after it was built, you for sure should have verified that you were the first, to run it.
I think the builder started it and ruined it and recomended it be tuned by someone like you. Or maybe the owner started it first. ???????????????????????
RE: Piston damage from not enough ignition timing?
Fueling did not have a major impact on torque (not surprising) and the mixture at 100% load was .86 lambda. Below about 30% load the mixture was as lean as .95 lambda, about as lean as the engine would run smoothly.
Not sure on the cracking around the inlet valves, that is puzzling to me. I will inquire with the piston manufacturer about your theory regarding the cracking.
RE: Piston damage from not enough ignition timing?
Over advancing the ignition timing can damage pistons- with too high temps and too high pressures to say nothing of knock.
www.auto-scape.com
RE: Piston damage from not enough ignition timing?
IMO this is a lousy position to be in and one more reason why all professional engine builders should dyno their own engines before supplying them to customers, even if they must buy dyno time at another facility to conduct the proper break-in and tune. Everyone looses in a deal like this.
RE: Piston damage from not enough ignition timing?
RE: Piston damage from not enough ignition timing?
I posted a while back that we just sent one of our Lotus engines up to a pro shop run by a man I've known for years and respect for his ability to build winning engines. This was a first for us. I'm just too old to do all that is necessary for such a complicated race engine. Well, to make a long story a bit shorter...we had a serious problem on initial start up that required sending the engine back. I have no idea what was wrong (I do have some suspicions) but, it came back just fine. I'm not at all worried about it as I know how easy it is for something so small to slip by....even from a pro shop!
Unlike the OP, my recent experience has been, on the whole, very good. We take the car up next week for tests and I'm sure it will be fine.....fingers crossed.
Rod
RE: Piston damage from not enough ignition timing?
Regards
Pat
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RE: Piston damage from not enough ignition timing?
The cracking on the top of the piston may be due to high in cylinder temperature or actual piston to valve contact. Any sign of contact on the top of the piston?
www.priamengineering.co.uk
RE: Piston damage from not enough ignition timing?
Cracked pistons? I've reused many in old Cat D 342 engines, and they never gave a problem in 10 plus years of operation, sunburst cracks forming on the top and around the heat plug that prevents pre cup flame from melting the top.
The op wasn't talking about diesel engines.
RE: Piston damage from not enough ignition timing?
6 8 and 12 cyl ones, so the ladds in the local BMW garage say, some thing to do with the ..coatings.. on the cyl wall deteriating,.
wonder if this could be it,!!!
Marcus
One does nae have to know how some thing works, to know that it is nae working right
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RE: Piston damage from not enough ignition timing?
Maybe with stock, this was a special performance build I think. Look at the photo in the OP's link. Most likely any such coatings where removed, lots of performance engines with no coatings and no problems.
Hmmm just did a quick look through this thread again, what oil and viscosity? What ever came of this?
RE: Piston damage from not enough ignition timing?
RE: Piston damage from not enough ignition timing?
www.auto-scape.com
RE: Piston damage from not enough ignition timing?
1)Assuming the engine builder clearances the bore correctly to the piston.
2)Assuming the Fuel you used is a good octane gas.
I have tuned and build many engines, that I see is an over heating issue. Not mechanical related or tuning calibration related. You must always watch your EGT if not the piston will get hot and make those marks on your piston.
No need to think so deep. It's common I see this all the time. You need to watch your temps or watch your exhaust manifolds because it got hot really hot, it doesn't take much to get the pistons hot.
-Frank Siharath
RE: Piston damage from not enough ignition timing?
Re read the OP. It indicates the engine was on a dyno while being tested. Certainly ANY severe overheat would have been immediately noticed. Again, BTDT !
Rod
RE: Piston damage from not enough ignition timing?
As noted in the discussion, retarded ignition timing causes the EGT's to rise but the piston temp to be lower due to the burning of the charge in the exhaust system instead of the cylinder. This particular case looks to be an improper piston design and or clearances.
RE: Piston damage from not enough ignition timing?