KILRWAIL

It's a Bursch, two in, one out, on early seventies headers.

Steve Weiner-Rennsport Systems

Hi Pete,

I'm thinkin' either a machine shop/set up error in deck height, CR measurements, and/or cam timing. I'd be measuring the the heights of all six cylinders as I've seen variances.

OR,....that bank may have had some vacuum leaks between the carbs, intake manifolds, heat insulators, and heads.

This is why we oftentimes install a wide-band sensor on each header collector to make certain that both sides produce the same AFR. Further, we sometimes also install cylinder head temp thermocouples under each plug to observe 6 cylinder head temps simultaneously as that shows load.

KILRWAIL

FYI, I have wideband O2 sensors on both headers, feeding a single Innovate gauge. There was no apparent difference between sides, although the readout fluctuates constantly.

MUSSBERGER

Fred C has a nice motor for sale.

http://www.youtube.com/watch?v=i7XShGg9eg4

Ed Hughes

Just reading this thread....my condolences. The thought that there was some unknown machining on the one bank causing a variance from one side to the other in deck height crossed my mind. With the lean condition and less deck height on one side, could that be why there was meltdown on that side? Were all engine parts original?

I'd say that the autopsy will need to be thorough.

Steve Weiner-Rennsport Systems

Excessive deck height will create a stronger propensity for detonation, rather than too little. Insufficient deck height simply raises the possibility of piston-to-head contact.

Yessir, thats very true,....

Ed Hughes

I think I meant that, but it sure doesn't look like it!. It was a 15 hour work/travel day and I probably should have collapsed when I hit the hotel instead of logging on. Forgot to include the word "other".

My gist was if the one side had more clearance than the other, and there was detonation or lean enough condition, could the burnt plugs result? Or would the pistons have gone before that point.

ivangene

YES... very good thread.

Enjoying Steve and Peter (and others)
I like the idea of temp sensors and thermocouplers on each cylinder. So you collect the data and have some sort of range that is acceptable, but what can you do (On a CIS or injected) motor if one or more are seeing higher temps? I understand if its carburated you can change each ones settings.

Aside from fuel/air what are other factors to cause a hot lung (or 2)

and if you could explain in simple terms why too much head room can be an issue (I get why too little is)

whalebird

I suspect that too little squeeze is problematic if it is different from side/side, lung/lung. It just does not carry the pending doom that to much squeeze does. These engines are sensitive to compression/combustion. If you follow the factory's development, they fiddled with pistons, compression, and developed twin ignition early on because the flame-front suffered as compression and displacement increased. The plug's offset position in the head became a limiting factor which propogated the development of the water cooled/multi cam heads in Porsche motorsports in the 70s.
there is a lot to learn in this area - for me at least. We all benefit from the discussion here.

Steve Weiner-Rennsport Systems

Hi Ed,

Call me,.....

Its not a simple issue and my fingers cannot last through a thorough, written explanation.

You might also read this http://www.rennsportsystems.com/2a.html

KILRWAIL

So here’s what we found.

1. Cylinders 4, 5 and 6
a. All spark plugs look normal
b. All valves are very light grey
c. All combustion chambers and piston tops are lightly coated with carbon deposits
d. All rings are intact
e. Deck height is between 1.27-1.35 mm

2. Cylinders 1, 2 and 3
a. All valves are dark grey or chocolate
b. All combustion chambers and piston tops are lightly coated with carbon deposits
c. Deck height is between 1.23-1.48 mm

3. Cylinder #1
a. Spark plug cathode burned off by 50%
b. Spark plug anode ceramic missing to a depth of 7-8 mm
c. Core of spark plug (metal) is hollowed out to a further depth of 2-3 mm
d. Remains of core ceramic is heavily crystallized and shows a blue colour
e. All rings are intact
f. No apparent damage to piston or cylinder wall

4. Cylinder #3
a. Spark plug anode ceramic is broken about 4 mm from the end but still held in place by the central wire
b. Spark plug cathode is intact but shows signs of burning at the tip
c. Hole in cylinder wall adjacent to cylinder #2. Hole is teardrop shaped; round end (about ¼” diameter) is centred on the edge of the deposit which marks the end of the travel of the topmost ring. Teardrop extends towards crankshaft.
d. Erosion of circumference of combustion chamber adjacent to hole in cylinder
e. Rings are fused to piston adjacent to hole in cylinder; gouging of piston top
f. Ring end gaps are not close to the hole
g. Spark plug cathode tip is aimed towards hole in cylinder when fully screwed in
h. There are tiny metal deposits on the top of the piston, likely coming from the melted cylinder wall

5. Cylinder #2
a. Spark plug looks normal
b. Top ring is broken in one place
c. Damage (melting) to outside of cylinder adjacent to hole in cylinder #3

6. Ignition: Spark plugs are NGK BP6ES. No damage to inside of distributor cap; plug wires (Magnecor) to cylinders 1-3 have similar resistance levels.

While the carburetor jetting is undoubtedly too lean, based on all available advice and literature, it is unlikely to have caused the above damage. We have a theory about what caused this, but your ideas and questions will be most welcome!

Steve Weiner-Rennsport Systems

Hi Peter,

Its difficult to offer conclusive forensic analysis without inspecting everything for myself so I will not speculate about absolute causes for the damage.

I will offer that from your descriptions, it sounds like preignition followed the round of detonation (its normal) and thats why you found the hole in #3 as well as melted components. One big signature of detonation is shattered plug insulators.

I can tell you that your jetting was too lean and your "6" heat range plugs were too hot for any track use.

Deck heights seem excessive to me and I know how much that contributes to the onset of detonation.

Auto_Werks 3.6

I've been silently following this thread for a while... just a couple observations.

1: Were you using an innovate technologies gauge, or an LM2 with 2 ox data logging?
Unless you were data logging I'm not sure that you have gained much information. A lot of things are happening while you have your eyes up looking ahead. If you were data logging, I assume that you have some reference to RPM and throttle position? 13.5:1 with a nearly closed throttle is not a problem, where WOT 13.5:1 is very lean. I'm also asking because I data log with an LM2, and if you have figured out how to make it work with RPM O2 and throttle position I would love to know how. They recently issued a firmware update which I am planning to try. If that's not a success I will be sending them an e-mail to let them know my (low) opinion of their product.

2: It seems like you are reliving a similar problem over and over. It might be time to replace the mechanic. You're now on engine #3, had you taken it to the most expensive rebuilder you would almost certainly be ahead of the game by now.

3: If you are going to be tracking an even slightly modified 911, 91 octane fuel seems a little on the low side for any safety margin. If it were me, I would be spiking my tank with some track fuel when you arrive. It really is cheap insurance, even at $8 per gallon

KILRWAIL

I was using an Innovate without data logging, but it's easy to see the AFR read-out at WOT - on the straights. It was 13.5 at WOT just before the failure and generally no higher than 13.2 before that.

We have spoken to a number of experts around the US (including contributions here), and the consensus is building that we (a) should use race fuel due to high combustion chamber temperatures in a racing environment, (b) should use a colder race plug (per NGK and others), (c) should tune for an AFR in the mid-12's at 6000 and (d) the timing should be retarded a bit from to 35 to near 30. I won't comment on your other point.

Auto_Werks 3.6

damn, I was hoping that you were going to teach me how to make my LM2 do my bidding!!

Sounds like you're on the right track! Good luck, hopefully you won't have any more threads like this.

as a side point, when people outside the porsche circle talk about AFRs, they typically talk in the high 11.x:1 to keep the heads cool. When I took my advanced EFI class my instructor cringed when I told him I was going to shoot for 12.5:1


Also, regarding the ignition timing, if you want to know for sure you have the timing right, set it on a dyno. That's the ONLY way to get it perfect for your exact application. You want the least amount of advance to still make peak torque. If your engine stops making torque at 29' there is no reason to run higher, etc etc. Be reasonable, don't try for a glory run, and I probably wouldnt try beyond 35 (we already know that doesn't work).