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.

 

[larrylcoyle]

LateApexEE

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

 

[Redtriangle]

Hello everyone, Long time reader but first time poster. My experience is almost exclusively with BMW only. 10 years, factory trained, dealer master tech. I've owned two E30 M3 S14's, rebuilt 4, and worked on many. I've personally built a 2.5L S14 from a standard U.S. spec 2.3L. No oil squirters installed. The piston to cylinder wall clearance does get a bit thin when boring out to 95.0mm but I beat the crap out of my 2.5L and no internal problems. WISECO pistons and Pauter rods.

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.

 

[LateApexEE]

I appreciate your insights and thanks to all who have offered their expertise. This has truly been helpful even though there isn't much I can do to convince the builder.

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!

 

[thruthefence]

Send a link to this thread to the builder, and one to your Lawyer.

 

[dicer]

Looking at the last photo.
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?

 

[LateApexEE]

The builder will provide absolutely zero information regarding build specs. He said that they used the CP Pistons recommendation but then went on to say it was ".002 or so". He has provided no details regarding the use of assembly lube on the cylinder walls either.

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.

 

[dicer]

So did the builder know that someone esle was going to tune the first engine?
If those were forged pistons .002 is not enough.

 

[patprimmer]

I would have said not quite half enough.

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

http://eng-tips.com/market.cfm

for site rules

 

[evelrod]

Piston clearance is not a given, guys. I use 0.0015" on my 73.5mm Mini JE pistons and 0.002"/0.0025" on the CP 84mm Lotus. Torque plate for final hone, skirt finish, lots of things make up what will live and what will not.

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

 

[thruthefence]

evelrod, Can you give a ballpark estimate on a big valve twin cam "hot street" overhaul?

 

[dicer]

Piston to cylinder clearance is a given. It depends on the expansion of the materials, and the lubrication used.
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.

 

[evelrod]

dicer, Yes, I think that is the consensus.

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. (

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

 

[dicer]

Rod, Gotta keep thinking your still a kid. Keep busy. Just port smaller heads.
I'm 14 behind you, things are rough at this age.

 

[evelrod]

I'm still thirty something until I look in the mirror.
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 (

 

[LateApexEE]

More information regarding the pistons. We got them back from the builder last week and there was evidence of cracking around the intake valve reliefs. 2 of the pistons look perfect, with the normal shiny spot on the skirt. The other 2 are heavily damaged on the skirts and one is damaged on the pin axis all the way up to the ring pack (pictured earlier in the thread).

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.

 

[evelrod]

Hmmmm. I've speced my own pistons before and been called on the "too thin crown" deal. I cannot imagine a prominent piston mfgr NOT refusing to make such a piston. Reputations are damn hard to come by in this sport and not easily surrendered. Be that as it may.

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

 

[dicer]

I have some questions. And maybe I already asked it, how did you establish the ignition timing point? And after you got timing and A/F set, did a change in timing one way or other cause a dip? And what about BSFC?
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.

 

[evelrod]

Setting up an engine, any engine requires time and patience...often a lot of $$$$, too. It usually requires a lot of "tail chasing"!!!

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

 

[dicer]

I had asked who originally started the engine.

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. ???????????????????????

 

[LateApexEE]

I believe that we started the engine first. It came separate from the car and we put it in. The ignition timing was set up in the ECU by shutting off the injectors and cranking the engine while observing the timing marks on the crank pulley. The CRIP angle in the ECU was adjusted until commanded ignition timing was the same as actual ignition timing. I worked the timing up slowly while observing torque output. The engine was not knock limited on the fuel used and ignition timing was left at MBT. Reducing timing would reduce the observed tractive effort. Increasing timing wound not make more torque nor would the engine knock. I went as far as 6* past MBT with no knock apparent on the plugs or through my knock listening rig.

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.