another engine failure article

[James Brown]

found this on the web, good reading:

Another Porsche Engine Failure « Fearless Garage

the mystery is why just one side. Maybe there wide band 02 sensor was only on one side?

[tom1394racing]

It would seem to me that somehow the LHS bank of carbs had a fuel restriction that caused them run lean.

[MBruns]

What do you have for ignition now, single plug ? what spark plugs, do you have a fuel curve from the dyno, Do you know what you have for a compression ratio. If you can fill in some more blanks I will give it some thought
Mike Bruns

[tom1394racing]

Hey Mike

Read the link in the first post. It gives a detailed description. Although not my engine, I would be interested to hear your diagnosis.

[MBruns]

It sounds like a combination of several things mostly on the tuning side but aggravated with spark plug heat range, who knows what kind of fuel quality or octane if its street fuel, the AFR we shoot for on the dyno at peak tq/hp is 12.9 - 13.0 but with all the proper tuning if we ran 13.2 it would not have caused the above. It is odd that it effected one bank and no damage on the other unless there is a cam timing or grind issue from side to side, the likelyhood of trash in the carbs on one side is hard to believe, the combination above and the WOT at a place like Elkhart or Daytona where its run harder than most all places it shows these problems quickly.

My .02, Mike Bruns

[HawgRyder]

I tried to find out what the overall elevation changes are for the track, but haven't found them yet.
So...my first guess for the lean or very lean condition on the affected bank would be the tuning (effective A/G ratio) on that bank.
Perhaps the engine was tuned on a dyno at a higher elevation and the resulting difference on the track put the ratio at critical.
Just my first impression.
Also...I have run into conditions where the torque of the engine and the side gravity of the car add up to fuel starvation in the carb system.
I have not worked with PMO's....so the question would be about the orientation of the fuel bowls....do they tend to fuel climb on one side?
The track appears to be right handed...so the majority of the turns will send the fuel up the left side of the bowls.
Just my $.02
Bob

[James Brown]

each bank can be thought of as a separate engine. comparing measurements might shed some light on this failure

[daepp]

Wouldn't the AFR gauge have shown this problem?

[Steve@Rennsport]

I see a discrepancy in deck height, side-to-side, with one bank having more than the other.

Excessive deck heights invite detonation, especially at full load, WOT with slightly lean AFR's for sustained high RPM operation. Too much fuel/air mixture is left at the circumference of the piston domes that tends to spontaneously ignite approaching TDC and the evidence was clear in the forensics. Hemispherical-head engines have little natural swirl and poor squish properties which do not tolerate such conditions as well as other type engines.

In other words, the failure occurred from the combination of factors.

[snbush67]

I don't understand how an engine could end up with a difference in deck heights between 0.25 mm on the same bank of cylinders.

"deck height is between 1.23-1.48 mm"

Isn't that a large difference? Wouldn't that large of deck height differences result in a different Compression Ratio (CR) for each cylinder? Could this CR imbalance have caused the failure?

[Walt Fricke]

I share Shane's puzzlement over this, but I have always been lazy and just measured piston to head clearance with solder on one hole, and assumed that, with matched cylinders, new copper base seals, pistons, rods, and heads, plus a true case, that all would be identical.

On my latest motor, with 12/1 CR, I started with NGK 9s to be safe. These were clearly too cold after a dyno run, so switched to 7s. They seemed to be fine, though the run was short for other reasons. I can see a plug which is too hot causing the plug to break. Would that also be a source of pre-ignition/detonation? Still hot enough for the next compressiin cycle to cause the fuel to light off before it was spark time? Still leaves unanswered the question of why one bank only, with only small deck height differences between the two.

[AlfonsoR]

Could mis-adjusted throttle linkage, one side more lean versus the other cause this? Isn't the afr an average?

[Eagledriver]

The advance is too much for pump fuel. We in the spec 911 world run 9.8 compression and 29 degrees advance at most. He could run less timing or use race fuel.

-Andy