3.2 Diagnostic help - broken rings
 

I could use some diagnostic help.
3.2 Carrera (RSR flywheel, ported intake manifolds, Autothority MAF and chip, has seen a fair amount of track time, engine rebuilt 40,000 miles ago in prep for track work – JRP valve springs, ARP head studs and rod bolts, etc.)
Developed a heating problem about a year ago, accompanied by a lot of blow-by out the oil tank.
Compression – cyl 1,2,3: 115-120 PSI; 4,5 6: 155 psi
Leakdown: - cyl 1,2,3: 35-40%; 4,5 6: 2-3%

I have dismantled the engine.
Compression rings broken cyl 1-2-3, cyl 4-5-6 OK

So the problem is clear, and a new set of rings is in order (cylinders, pistons are in nice shape)

But what could have caused the problem? Presumably detonation, but what would damage three cylinders on one bank but not the other?
There is a vent line connected to the left side of the manifold – could a leak there cause a lean condition?

Or can three broken rings be chalked up to the luck of the draw?
I hate putting things back together without finding a cause.

I have a bunch of pictures if they might be useful

Sorry to hear that. Pics would help. Proper inspection top end rebuild, and in three weeks max she is refreshed/tuned.

Good luck!

Regards,

I have attached some photos.
My concern is the root cause of broken rings in three clinders on one bank.
Typical for all three bad cylinders:
Broken rings:
http://forums.pelicanparts.com/uploa...1146324861.jpg
Piston
http://forums.pelicanparts.com/uploa...1146325000.jpg
Cylinder
http://forums.pelicanparts.com/uploa...1146325142.jpg
Combustion chamber
http://forums.pelicanparts.com/uploa...1146325449.jpg
Typical for three OK cylinders:
Piston
http://forums.pelicanparts.com/uploa...1146326250.jpg
Combustion Chamberhttp://forums.pelicanparts.com/uploa...1146326120.jpg

My first guess would be excessive clearance in the ring land area, exessive clearance between piston/cylinder and lastly excessively high continueous rpm

Any one of those issues will cause the rings to 'flutter' in the ring groove on the piston cauing breakage like what you have shown.

If the pistons were used and the ring land clearance was on the high side(but acceptable) + the piston clearance was on the high end (but aceptable) and you were really 'winging it' on the track you will still wind up with that problem.

Other factors might include detonation from lean mixture or improper ignition timing or over heating on that side which you did mention.

If ignition or lean condition was an issue the piston/ exhaust valve/combustion chamber would be more telling than what the pics reveal. Pics are looking as if those are in good shape.

Other possible problem would be upon initial install when the piston/rings assembly may have been damaged, but not broken.

This is a very familiar story to me. I had an 87 930 with broken piston rings on 1, 2, & 3. After testing a lot of things I found the fuel injectors on cylinders 1, 2, & 3 parcially plugged. Cylinders 4, 5, & 6 worked great.

After the rebuild and with new injectors on 1, 2, & 3, WOW.

The real question is why would the fuel injectors on one side plug up while the injectors on the opposite side check out just fine??:confused:

Possibly the fuel rail had small debris on inside log thus clogging the injectors on that side only.

Let us assume that the rings were properly installed at the last rebuild. After seeing the ring damage you need to cover all the bases on this rebuild.

If I had an engine with your mild modifications to the 3.2 engine described above, I would put it on an engine dyno so you could check fuel mixture and ignition timing. From the dyno experience I have, Porsche tends to run up to the 100% duty cycle range, which means no more fuel available, thus lean fuel mixture, more heat, more chance for detonation. You could go with a slightly larger injector or do a small increase in fuel pressure, but first you need to know the fuel mixture. If you decide to keep the stock injectors, I would send them to RC Engineering in Calif. and have them rebuilt. They will give you flow data, before and after.
They can also flow check the fuel pump to make sure it flows enough volume.

A dyno would give you an opportunity to check ignition timing at all throttle positions. I have seen Autothority chip with too much timing...thus detonation problems. These were returned for a less aggresive chip.

Another thing to consider with engine mods. would be to go with a colder spark plug heat range...thus giving a safety margin for possible detonation. Of course Premium fuel should be used at all times with a shot of race fuel for track days. A bad load of fuel from the gas station can ruin a chipped engine real fast.

The last thiong would be to consider a air fuel ratio signal modifier if you dyno the engine and can't get the fuel mixture right through-out the rpm range. Split Second makes a real nice control box that works great.

Last of all if the engine is at the track, you want to make sure your oil temp. in within specs. If not an additional cooler may be required.

It is hard to see any detonation issues on the pistons with all the burnt oil deposits, but definitely measure the ring lands and skirts to make sure they are in spec. I would assume you are having the cylinder heads rebuilt too.

Also on reassembly, cam timing needs to be the same side to side.

Spent some quality time with my pistons, cylinders, and some measuring instruments. I measured the cylinders in two directions, at three spots in the bore.
Cylinder out of round range: 0.006 to 0.021mm
Cylinder taper range: 0.013 to 0.033mm
Piston-cylinder clearances are all in the range of 0.03 to 0.06mm
All of the above are well within wear limits, as far as I can tell.
Ring end gap (for the unbroken compression rings and the 2nd and 3rd rings) all are at about 75% of the wear limit
Piston ring groove side clearances for all the 2nd and 3rd rings, and for the three unbroken top rings, are at the new clearances (0.10mm for the top, 0.07 for the 2nd, and about 0.001" for the oil ring).
Piston ring groove side clearance for the three pistons with broken rings vary around each piston, with the best at 0.004" to 0.006", and the worst with one area at 0.009".
Looking at all of this, it appears to me because of the lack of wear on the other three pistons, worn ring grooves were not the causeof the breakage. Instead, probably the pieces of ring caused wear. The engine ran a fair amount this way, I think.
Which leaves me still not knowing what caused the problem in the first place.
It looks to me that the pistons and cylinders are in good shape, other than the three with groove wear.

Problem now is what to do with them.
Aside from cost issues, it seems a pity to replace these P&C's for the sake of three worn ring grooves.
-Replace only three pistons, possibly with used ones?
-look into widening the grooves and installing alternative rings?
-???
As far as the root cause, it seems like it is most likely an air leak or injectors. It is possible I got some bad gas during a trip out of Canada ( I use Sunoco 94 here, but it is hard to find sometimes in the US) but that should have affected all cylinders, I think.
Someone told me today that they had seen this occur on early 3.2's - can anyone confirm that there might have been some sort of chronic problem?
The rest of the motor is in nice shape - no rocker wear, valves and guides are in great shape (other than one torn valve stem seal, which I might have done during disassembly). There are very cool titanium valve spring caps.

I bought a used set of pistons with wider than stock top rings. I don't know who does this kind of work though. I'd call some shops and ask if they can do it. EBS told me that they might be able to put together a set of non standard rings. I didn't persue it as I had another set of pistons available. Probably worth a shot since you have Nickasil cylinders.

-Andy

I did some checking and Total Seal can make me a set of top rings 2mm thick with moly coating that they recommend for Nikasil cylinders. The guy at Total Seal came across like this was nothing out of the ordinary.
I'd need a set of Goetz rings for the 2nd and 3rd rings because Total Seal doesn't offer a 3rd ring with a finish they recommend for Nikasil.
It doesn't seem to me that widening the groove by 0.5mm will critically weaken the piston.
There might be a negative effect from the increased cylinder wall force - up by 1/3 with the wider ring.
An alternative is a new set of JE pistons, but the existing Mahle pistons fit nicely in the cylinders and I have trouble believeing that a new set of pistons will match the bores as well.
So - why not install the thicker rings?

I see no problem with the wider rings, and I would keep the current piston / cylinder combinations instead of buying new pistons.

But please get those fuel injectors tested.

Thinking more about the cause of the problem, with some spirited input about modified chips on Rennlist:

I understand the implications of modified chips. .

Considering the following points:
-the car ran well for several years
-it had a steady diet of Sunoco 94
-this problem started about two years ago, about the time I attended a DE event at Road America (no SSunoco 94 available)

Does the following make any sense:
-the chip is running advance close to the edge - but was OK with Sunoco 94
-using lower octane gas increased the detonation potential
-only the left bank was affected due to either a slightly lean condition caused by an air leak through the brake vacuum line (possibly a small leak, if things were running near the edge) or due to some small assymetry in the cooling, leading to the left bank running a bit hotter.

On the thicker ring question - obviously making the ring heavier is not a good thing. However, I'd be intersted in hearing from anyone who has experience (good or bad) with this approach.

I would replace the pistons and new Goetz and voila, you're there.

Regards,

Bob D.

Once you get through the piston and ring issues, I hope you follow through on the cause.

I believe I covered most of the subject in my previous posts.
Maybe it was bad gas....you should start there and make sure what is in the car is good. If you are running on the track with this setup, you might think about a combination of premium pump gas and unleaded race fuel for a safety margin. Some chips are on the edge...but the edge will bite you.

Question....What is your oil temp. at the track?????

Again...if you can either engine or chassis dyno this new engine with air/fuel monitoring and ability to check ignition timing at various areas and throttle positions it is money well spent.

Ask Steve @ Rennsport Systems......he does chips and not only knows what works, but has a vast knowledge base. Besides that, you'll not find many people as helpful and honest as Steve.

Carry on!!

Well, several weeks later, the car is back togethere.
I didn't find any noticeable causes of the three broken rings.

The injectors were flow tested before cleaning; two showed a slight leak but theree was no evidence of clogging.

I couldn't find any air leaks.

I wonder if the heater air being extracted from the left side might create a bit lower flow and thereefore higher temps on the left side. If so, then a possible cause might have been a combination of advanced timing, low octane gas, and high air temperatures, all of which i encountered at a DE event.

I know that without the ducting connected, theree is a fairly significant air bypass, which I cannot imagine being particularly good. I am thinking that

The car runs great. I did a cold leakdown test after the initial break-in, and got numbers in the 1-2% range, which I think is pretty good considering the car was dead cold. FYI - I installed new rings in Scothbrite'd Nikasil cylinders. the leakdown was after a 20 minute fast idle, followed by an oil change, and then a warm-up and about 15 minutes of hard on the throttle, hard lift-off - more or less as per Wayne's book.

I'd appreciate any opinions about the air idea.

FWIW - a few things I learned from this rebuild:
-if you are showing your 15 year old daughter how to install and torque rocker arm pins, there is a chance one will be missed and will come out of position and create a huge oil leak, but fortunately no othere consequences
-if you let one of the chain ramps slide into the crankcase, it is possible to get it out with 2-3 hours of work, a lot of creativity with lights, mirrors, and bent pieces of wire. Suggestion - go to extreme lengths to prevent it from happening.
-I'd definitely do it again.

Bob:

Broken rings are the result of detonation. The ensuing shock waves breaks rings, distorts ring lands, and hammers rod bearings.

Detonation is caused by many things that include:

1) Too much ignition timing

2) Insufficient octane

3) Lean fuel mixtures

4) Too much deck height

5) Excessive cylinder head temps


RSScotty offered some excellent advice and I'd wholeheartedly concur with his comments about some chips having excessive mid-range timing. I've seen quite a number of these engines with broken rings resulting from a chip that should have required race gas.

I would strongly suggest that you either use the stock chip or one of Steve Wong's excellent products that he custom-tailors for each application.

Have you ever seen anything that would suggest that the left cylinder bank runs a but hotter because of the heater air being bled off the left side?

Not to the extent that would trigger detonation.

I have glanced at this thread on and off over the past few months, and although I hate say it, I believe your broken rings are probably a result of too much ignition advance in the chip. Most of Autothority's MAF programming is outdated and designed for 93 octane on a cool day, and would ping severely with 91. Couple that with the lack of an air temp sensor in the MAF sensor, and you have a formula for predetonation in hot ambient conditions. The stock air flow meter has a NTC sensor, which Motronic uses to measure intake air temps, and dials down the overall ignition timing as the temps rise, reducing the propensity to predetonate. The reduction in a stock chip is 4.2 degrees retard at 150 degrees F. The Autothority MAF system eliminates the air temp sensor, and instead places a user adjustable potentiometer in its place to fool the temp sensor input for fine tuning the idle mixture. Since the value is fixed, the system cannot retard ignition at high temps.

A lean condition could also be a possibility. MAF sensor contamination from K&N filter oil and dirt will desensitize the hotwire elements to reading a lower airflow than actual. The sensor should be periodically removed, and carefully cleaned with a Q-tip and brake cleaner. Sonoco 94 can also not be the same fuel as it once was. A high percentage of alcohol used to increase the octane rating also reduces the density of the fuel and leans out the air fuel ratio.

The best solution to prevent such reoccurence would be dyno your car after assembling the motor and obtain an air fuel ratio log with which to use to retune your chip program for maximum power and tuned to the grade of fuel you will be using.

Interesting analysis, Steve W.
I agree that it is an end result of the factors you mention, although I am a bit baffled that one bank was affected.
Your comments about Sunoco 94 are thought-provoking, because it ties into why (perhaps) this problem did not emerge for several years, even though the use of the car has not really changed.
For my info, how many dyno runs are necessary to provide an adequately repeatable data set for chip reprogramming?
And while the MAF happens to be installed, I have all the stock intake system; are the benefits from the MAF (mainly, I think, less intake restriction) worth the hassle? It would be easy to change back.
I HATE the K&N cone filter - I just haven't found an alternative that fits - if anyone knows of a paper element cone type filter I'd love to hear about it. I carefully clean the hot wire from time to time.