P&C/Head damage diagnosis wanted

[KFJ_SC]

After discovering one broken head stud about a month ago, I began the teardown for a rebuild on my 78SC track car (95k miles). Late last night I finally had a chance to pull the heads and Ps&Cs. Turns out there was only one broken head stud - #1 cylinder, intake stud. From what I have read this is pretty rare, but the good news in the mystery of a broken intake side stud was solved. This engine was rebuilt around '84 (more on that later) and when it was rebuilt the mechanics replaced one of the exhaust (dilivar) studs with a steel stud - yes, just 1 on the whole engine. So, cylinder #1 had 3 steel, 1 dilivar stud.

I knew from the PO that this engine had been short-stroked around '84 when it was imported from Europe. PO believed it was a short stroke 3.2 - the car pulled very well, was dyno'd 3 years ago with 192hp at the wheels and last year (when I bought) had leakdown of 4-5% each cylinder.

After getting the heads off I finally had a chance to get a good look at the Ps&Cs. They measured at 97mm, yup, from a 3.3L 930 engine. I haven't mic'd the rods yet, but according to Henry Schmidt on this thread (930 pistons without a turbo?), different pins and an offset bushing result in a 3.12L short stroke.

So, now to the fun part - the good news I thought at the time is that these cylinders are Nikasil, meaning they can be reworked. The 930 cylinders are odd though, having cooling fins on only one side! Once we got the whole thing apart though, we found significant pitting/corrosion on cylinders 4 and 6 at the head/cylinder mating surface and on the pistons - oh, and all the top rings were broken on all pistons.

Here are some pictures:
Cylinder mating surface pitting:

Head Pitting:

Piston Pitting:


Anyone seen anything like this before? Anyone have any theories on what caused this?

My theory is this: you'll note the pitting is along the exhaust side, as it is for all damaged cylinders. The exhaust side is also the side that lacks cooling fins. I'll bet the additional heat on that side over the years has continued to eat the material.

I'm also assuming at this point that the Ps&Cs and the affected heads (just 4&6 really) are toast - I realize it is tough to get an accurate feel from the pictures, but if anyone thinks differently, I would be interested to hear their thoughts.

[KFJ_SC]

So, some more info based on research indicates:

1) The damage is textbook detonation.
2) #4 head is likely toast - pitting is probably 1mm in spots
3) Pistons are not 930 or CIS pistons

Now to figure out what's next...

[Steve@Rennsport]

Hi Kirk:

For some reason, I was unable to see the pics you posted but if you send them directly to me, I'd be happy to take a look at everything.

Those 97mm Turbo cylinders with the half fins were NOT their best idea and they finally scrapped them in 1992 going to a fully-finned version. I never reuse them due to their poor & uneven cooling.

Send me the pics so I can help.

[KFJ_SC]

Thanks Steve - was wondering why no one responded. Pelican image upload is broken so I linked to somewhere else, but forgot that it's members only. I sent you an email too.

[Steve@Rennsport]

Hi Kirk:

Much better, thank you!...

Detonation Sir. That damage is all caused by long-term detonation.

Those damaged heads are eroded too deeply to save and you need new cylinders, too. Those are the Max Moritz version Mahle pistons and you need some new ones.

It won't be cheap to do properly but you should have a good motor when you are done. The most important thing is determining WHY it was detonating and get a solid, permanent fix on that.

I see evidence of heavy oil consumption that really triggers detonation by lowering the octane of the air-fuel mixture.

[cgarr]

Steve, you ever weld up heads like that and re-surface them? or am I way out of line?

[Eagledriver]

One thing you mentioned was that the exhuast side of the cylinders have no fins. This would indicate that among your other problems the cylinders were installed upside down!

-Andy

[KFJ_SC]

Steve - thanks, makes sense. I would assume that with those pistons that CR was actually pretty high - 10+. It is single plug so presumably this would be the source of detonation. Does this make sense to you?

Andy - you are correct, the detonation is primarily on the intake side (it was late when I pulled them off and my brain was working backwards ).

[Steve@Rennsport]

Craig:

Yessir, I have welded them up for different reasons,... Its very time consuming so in this case, its more cost effective to source some used heads. The only exception might be some exotic heads such as RSR or 935 ones that are worth saving.


Kirk:

Even though the MM Mahle piston domes were designed for single-plug operation as noted by the wedge-shaped domes, insufficient octane can easily cause the damage you've pictured. One really needs 93 or better gas to run at 10:1, especially in hot weather.

[KFJ_SC]

Thanks for you input Steve. This engine was built in 1984 and the care has changed hands several times since the work was done - even IF the builder passed along the minimum octane it surely got lost along the way and has seen plenty of tanks < 93. Now on to the rebuild .