Piston damage from not enough ignition timing? 8

[LateApexEE]

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.

 

[Marquis]

Just a quick note as to the OP title. "not enough ignition timing" or not advanced enough- will tend to ramp up exhaust system temperatures but lower piston temperatures and cylinder pressures. When Im on a dyno and the design engineer has specified an upper cylinder pressure limit and a piston temperature limit- I usually use alot of retard to keep within these limits- even though the exhuast temps tend to rise...
Over advancing the ignition timing can damage pistons- with too high temps and too high pressures to say nothing of knock.

http://www.auto-scape.com

 

[TrackRat]

My dyno experience mirrors Marquis' in that the ex temp increases but not the piston temp as a lot of the charge burns in the exhaust pipe. There should definitely not be piston cracking around the valve notches regardless. I have a real doubt about the theory that insufficient ignition timing caused any of this damage, especially if you were even close to MBT.

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.

 

[dicer]

Only one thing to say, the person putting the engine together is just an assembly person, and really doesn't know enough about IC engine principles to be putting blame on the tuner. In all probability the assembly guy goofed up, and is afraid the admit it. I would never let someone else start an engine that I have done. Guessing the assember did the damage when he first ran it before delivery.

 

[evelrod]

This thread suddenly became rather personal last week. As I've mentioned, I've been building my own engines for over fifty years...even built a few for "customers" in years past. Had some really good ones and a few real stinkers. One thing for sure, not a single engine ever left my shop without me personally testing it. I've been pretty lucky as my success rate is pretty good...not too many 'stinkers'.

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

 

[patprimmer]

I was of the opinion that retarded ignition increased temperatures as the exhaust and water became obviously hotter, but what marquis says makes perfect sense. Less heat to the piston, but more lost out the exhaust and possibly to the water via the exhaust port water jacket.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

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for site rules

 

[PriamEngineering]

I have been working on diesel engines for over 11 years. We build over 20,000 engines a year. We have our own test cells here. I have seen the piston scuffing marks on dozens of engines. All were attributed to the piston overheating. 9/10 times this was due to a bent saddle jet or partially blocked lube circuit.

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?

http://www.priamengineering.co.uk

 

[dicer]

Diesel's are different. And yeah scuffing is always caused by overheating. Because that is what happens when the oil flim is lost, friction and heating, galling and friction welding. I have a gas engine that was severly overheated, it dropped valve seats, the cylinders and the pistons are not scuffed.
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.

 

[marcusaurailius]

Intresting thread this, but no one has said BMW has had problems with its cylinder bores,

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

http://s545.photobucket.com/albums/hh369/marcusaurailius/

 

[dicer]

marcus,

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?

 

[TrackRat]

BMW had some Nikasil cylinder issues in their V-8 engines sold in the U.S. about fifteen years ago due to the sulfur present in U.S. fuels that is not in Euro fuels. Since then BMW has not had any cylinder issues that I know of. Like most Euro car makers they specify Longlife oils for 2006 and newer engines but that would not have caused the almost instant failure experienced in the engine being discussed here in my experience.

 

[Marquis]

The BMW engine in question is an S14 4 cylinder from the 1980s with a cast iron block- probably best to read the thread properly and digging deeper before posturising like this.

http://www.auto-scape.com

 

[RACETUNE]

From looking at the picture.

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

 

[evelrod]

I certainly agree that a severe overheat "could" have caused similar problems, RACETUNE. However, should such an overheat have caused the problems as expressed by the OP, the top ring (at least) would have been annealed to the point of total loss of "spring" (That was not noted in the OP). I, too, have also BTDT !

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

 

[TrackRat]

RACETUNE-

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.

 

[dicer]

So then if it was an overheat issue, then the builder must have left a bunch of rags in the water jackets around the one cylinder? If it was a major overheat, then detonation should have wreaked havoc. In all cases its a lack of lubrication period. Most likely from clearancing or lack there of. Unless the piston material is some low grade pot metal!