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.

 

[patprimmer]

It looks to me like the piston got to hot or the piston to bore was to tight or there was insufficient lubrication of the piston skirt or maybe the bore finish was to coarse.

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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[LateApexEE]

It is a wet sump engine (BMW S14). For crank case ventilation, we are using the standard BMW oil/air separator which has served well in other applications but I am looking into several other options including an electric vacuum pump and controlled inlet orifice.

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.

 

[ivymike]

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?

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.

 

[WGJ]

From your picture, there is plenty of evidence of hard contact around the crown of the piston and even on the gudgeon pin axis where there is often least contact seen.

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

 

[romke]

for one reason or another the piston got so hot that it expanded more then the bore could accept and hence it scuffed.

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.

 

[LateApexEE]

Thanks to all for your input. The piston to cylinder clearance was .003" as per CP pistons recommendation (this CP spec of .0025-.003" has been sited by the builder). There seems to be a lack of documentation when it comes to this engine build and it is concerning to say the least.

 

[Tmoose]

got a picture of the underside of the dome, and the sparkplug electrode tip, plus insulator way down inside?

 

[LateApexEE]

None of those, but I have the sparkplugs still. They looked normal including a cut down inspection of one insulator (this is SOP for engines like this). No traces of det that I could see under the jewelers loupe.

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.

 

[franzh]

From my experience, ignition timing affects all cylinders, air to fuel ratio variations may affect local cylinders, but damage as indicated to one cylinder seems to be a mechanical defect on that one cylinder.

I would put my money on a shifted cylinder wall casting or a localized hot spot as mentioned above.

Franz

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[Lou Scannon]

Agree with consensus above, localized mechanical cause for skirt clearance to be inadequate at max operating conditions. Oil jet failure, cylinder wall anomaly (distortion, thickness), coolant flow), malformed skirt (e.g. manufacturing defect or mis-handling prior to installation)...

 

[CH5OH]

combustion failures/timing would show on top of piston.
skirt failure is caused by lack of clearance or lubrication
It wasn't by any chance the cylinder closest to the cooling water inlet ?

 

[evelrod]

I have a stack of pistons that look just like yours. All caused by a variety of oiling problems, not timing or overheat. I don't buy the "bad piston" theory, CP has always been of superior quality, in fact, we are getting a new set for our TC "as we speak".
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

 

[thruthefence]

But then you can't blame the 'tuner'. The 'builder' must take responsibility.

 

[evelrod]

Absolutely! Difficult to prove. Even if you are totally NOT at fault, your reputation as a "tuner" takes a hit.

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

 

[patprimmer]

A slightly bent rod just might jamb up a skirt if it cocked the piston a little in the bore, but I still go with lack of lube and/or insufficient piston to bore.

Regards
Pat
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[thruthefence]

what happened to that wrist pin bore? pic #1

 

[LateApexEE]

My goal in posting this was not so much as to assign blame but rather establish a root cause so as to avoid it happening again. But yes, these situations are always difficult to deal with at best. I appreciate everyone's responses.

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.

 

[dicer]

I'm with Pat he covered things I thought of.
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.

 

[LateApexEE]

The oil used is SAE 15w50 synthetic oil with a rather high zinc content and low detergent content.