Frank@Work

Almost two years ago I’ve rebuilded my 951 engine. I have replaced the bearings, rings, seals, etc. etc. The reason for the rebuild was that I wanted to get some more power out of the engine and wanted to make sure everything was ok.

In the last two years I’ve bought several parts to get more power out of the engine (MAF, 3” exhaust, chips, injectors, …). The only part missing is a bigger turbo. I had planned to buy a new turbo next month. Because I was having some heavy blow by issues I wanted to take the head of the engine to check everything. A compression test showed a somewhat lower compression on the #2, but it was (just) within the margin.

Today I’ve pulled the head and noticed cylinder wall scoring on the #2 and #3. The scoring isn’t deep, but it is over a large area. It looks like the scoring in this picture.

Cylinder wall scoring

QUESTION 1: What causes scoring like this?

At this moment I’m not in for pulling the complete engine and want to get the car back on the street (it has been sitting in my garage for almost half a year). I consider replacing the rings on the #2 and #3 and putting back the stock turbo (after a rebuild) and keep the maximum boost at 15 psi.

QUESTION 2: Am I stupid considering the above?

I also have an ’86 924S Automatic for daily driving, but I wanted to convert it to a track car by putting a 3.0 16V S2 engine in it with a manual transmission. I already have all parts for the S2 engine rebuild. I wanted to start this project after finishing work on the 951. I now consider building a 3.0 16V Turbo engine for the 951. I think I’ve most the parts needed. I know I have to lower compression and have to build a custom intake and perhaps exhaust manifold.

QUESTION 3: What engine management should I use on a S2 Turbo engine (S2 / 951 / custom)?

Any other comments will be appreciated!

Tanks.

mark944turbo

1- there are many opinions, but i think it is caused by a combination of overheating/detonation that both cause each other. Piston expands too much, and scores block.

2- new rings wont seat unless you have the block honed and silicon re exposed, you will make it worse putting new rings in without doing this.

3- you will probably save money going full standalone of some kind. That way you can use whatever flywheel you want, etc.

Tom M'Guinn

I've got about 30k on my motor since first finding similar scoring. Not sure what caused mine -- it was like when I changed the HG at 40k, before I made any mods! I've had the head off several times and confirmed it does not seem to be getting worse. In your case, with a new motor, my concern would be that it will just keep eating away the walls until something really bad happens. Were your're piston to wall clearances checked carefully? Did you re-ring used pistons? If so, were the piston grooves within spec? Do you run rich enough to wash the cylinders?

DDP

Really? I know a lot of ppl that have re-ring a block without honing with no problems. Great compression.

mark944turbo

I would be interested in their leakdown numbers, in my experience compression tests dont mean much at all. The s2 I just bought had perfect compression, and when I took the head off there was major cylinder wall scoring, and the piston was visibly rocking back and forth in the bore when I put force on it. The piston skirt had completely collapsed (i measured it).

It will depend somewhat on what rings you get, some are made to seat on crap surfaces, but they dont last as long or perform as well.

Do it once, do it right...

MichelleJD

Aren't these cylinder walls lined with alusil? Honing them removes it.

cruise98

Looks like a broken ring, but there is no way to know for sure without taking it apart.

It is perfectly acceptable to rering as long as the bores/pistons are within spec. Re-polishing the bores is not required. The "honing" is not really honing like iron blocks get, but is really polishing with felt pads and special silicium paste to expose the silicon in the alloy. I would only use the factory Goetze rings in these alloy blocks.

ehall

proper honing allows the "sil" to be exposed from the "alu" It's basically a composite of sorts, and the silicon content allows the pistons to move on a more relatively slippery service. The honing exposes it, but has to be done with specific tools for the job.

Frank@Work

I have rebuilded the engine myself, with the exception of main, rod bearings and reringing the pistions. Those jobs where done by a professional rebuilder who measured everything. However, he talked me into using aftermartket rings, because there was no point paying a lot more for the genuine rings. (Perhaps that was a mistake). He also told me not to worry about the oil used for the break-in period. I was under the impression that I had to use non-synthetic oil for break-in, but the rebuilder told me I could use synthetic. It seems (now) he is probably the only rebuilder with that opinion.

After rebuilding the engine I have been playing around with the AFR ratio. It took me some time to get it where I wanted it to be (± 13.0:1 during spool-up, 11.8:1 between 3k and 4k and after that 12.2:1). There is a spot between 5000 and 5400 where it is a bit leaner 12.8:1. I couldn't get it richer at that spot without creating problems at different rpm's. The exhaust valves indicate a lean condition throughout all cylinders.

I've been thinking about reginging the pistons and see what it brings, but I'm not feeling comfortable threating the engine that way after all time and money spent on it.
At the moment I'm thinking of pulling the engine and replacing it with the stock S2 engine I'm rebuilding. That way I can drive the 944 this summer and take care of the Turbo engine (probably replacing the short block if the scoring can't be fixed).

Thanks for your input (and keep it comming!)

fast951

How deep are the scratches, can you feel them with your finger. If so, then you must hone/fix it.

For the 3L project, not a problem, we can assist you with engine management. My advise is to use a 951 DME/KLR to get the knock control.

Frank@Work

@fast951: I can feel the scratches.

I have been reading a lot of 3.0 8V/16V threads on rennlist the last couple of hours and I'm not sure what to do.

fast951

If you feel the scratches, time to rebuild it. However, before boring it, evaluate all the options as you might need to go to oversize pistons, which are hard to find and expensive.

I personally would go with the 3L/8V it's simpler, less expensive.. However we have kits running on both.

hosrom_951

It is quite normal to feel some scratches here and there, but if you can catch you're finger nail on the scratches, they are very deep and they should be measured and checked if the block neds to be bored to next overside or not.

I have one or two scratches on my cylinders, i can barely feel them but they are far from catching my fingernail, which is fine.

I know that Oili had scratches on his block, similar to what you ahve Frank, but the shop just hones the block and they were gone, you can get away with it sometimes.

Frank@Work

I will pull the engine and have it checked out.

The 3.0 8V options seems very affordable considering the parts I already have.

To do:
1. spent a bag of dollars on pistons
2. modify the lower balance shaft cover for the turbo oil return line
3. use the 951 head, intake and exhaust
4. spent another bag of dollars on custom chips

Am I missing something? Oh, do I use the turbo flywheel on the 3.0 crank?

Will it be a problem to use my current Guru chips with a 3.0 8V (using a SplitSecond for tuning).

Tom M'Guinn

"seems reasonably affordable" -- yes, it seems that was doesn't it. It's not.

1. Pistons. With Andial out of business, the 104mm Mahle pistons will be harder than ever to find. I think they only had "one-over" sizes available when they shut down (104.5). If you hunt, you can find a set though. There are a few places selling aftermarket pistons also. I have a set of never used set of "two-over" pistons (105mm) from Tom Charlesworth if you are planning to bore -- but I'd need to share the whole story about them before selling them to you (cheap). If I were starting over, I'd probably go with Chris Whites sleeve system so that you can use regular JE pistons.

2. Also may need to tap oil pan for turbo oil return banjo -- unless yours is already tapped (some are) or unless you plan to use the 951 pan.

3. You need to modify the 951 head to match the front cooling port on a 104mm block. Lindsey has pics of its site. An 8v head from an 89 2.7 motor is better, but costs more than my first car did.

4. I'll be using a Vitesse kit for off the shelf tuning. (You mentioned split sec -- are you using an ARC2? I am about to swap out mine for a digital fuel controller. The ARC2 is a crystal radio in an iPod world.)

5. You'll need forged rods (from the 951 motor or Pauter, Carrillo, etc.)

6. You'll need a big bucket of miscellaneous stuff that adds up fast.

7. Clutch? Pressure plate?