Detonation in a 3.2SS
 

I have a 3.2SS that I rebuilt about 18 months ago (9.5:1 JE’s, CIS, 964 cam, SSI’s M&K, single plug) for a 75% track car. I noticed a puff of smoke on the downshift at last weekend’s DE at the Glen, plus it wasn’t pulling that hard….I attributed that to the heat 90F ambient temp. After further inspection I found the following :

Cyl #3 85psi compression, all others 115psi.

Pulled the engine and found that the compression ring on #3 was in about 10 pcs and it has significant detonation on one side, see below

http://forums.pelicanparts.com/uploa...1283389367.jpg


Piston two shows a little detonation (compression intact). Piston one shows no detonation. Haven’t pulled the other side yet.

Other comments:

1. I’m measuring the AFR on piston bank 1/2/3 via data acquisition, it measures 12.5 to 13.5 WOT
2. All new injectors 18 months ago
3. Timing set at 33 degrees total advance
4. I did measure the deck ht and cc’d the heads during the last rebuild and they were correct (I have to dig out my notes to get the actual)
5. Pistons were 9.5:1 JE’s (never measured, but that’s what I ordered)
6. Never heard it pinging (exhaust is pretty though)
7. About 5000 miles on engine (driving to and from track)
8. I have a carrera cooler….temps rose to just above 9 o clock on my SC gauge this weekend

What went wrong?

Any comments appreciated.


Cheers, Stu

What fuel (brand and octane rating) were you using at the time? And during your street driving?

Mark

Street driving 93 octance from Sunoco
At the Glen last weekend 93 octane from Valero

Stu,

A few thoughts,.....

1) I've seen people melt their motors using AFR data from some Engine Management and/or DA systems. Personally, I'm more comfortable about using a standalone AFR instrument for tuning and then EM or DA for monitoring, provided they agree.

2) Excellent. I'm assuming they are the right ones for your particular configuration.

3) Timing sounds close, however its usually part-throttle timing and AFR's that wind up killing these things.

4) I'd revisit deck height since a little too much promotes detonation than too little.

5) With all respect to JE, I've found plenty of those things that were NOT made correctly and people had more compression than they thought (or asked for). NEVER assume, always measure. No matter what I order, I check each and every one for compression height and measure CR once the short block is assembled. No exceptions.

6) If you rely on your ears as the defacto "knock sensors" you will buy a lot of parts. These things can detonate into total destruction without hearing a thing, depending on your engine configuration.

Cylinder head temps is what killed the engine, not oil temps. That said, high oil temps reflect high cylinder head temps and running at 235 deg F is too hot. IMHO, you need to maintain oil temps between 185 and 195 for maximum durability and performance.

Hope this helps,

There's nothing wrong with logging data with EMS, however, one needs to be calibrated on regular basis, especially on a track car.

13.5:1 is too lean IMO for air-cooled 911 engine that is driven hard and has such high oil temps (which, as Steve said, equals high CHT's).

What is even worse than detonation, is pre-ignition which occurs in condition exactly like you had (high temps). It does not make a single sound and it kills engines fast. There's a way of logging potential pre-ignition, but when driving hard, you won't have time to react. It has to be made sure before that it won't happen (using appropriate plugs and keeping temps under control.

Thanks for all your comments:

Here's my plan:

A new set of 9.5:1 JE's
Measure the dome to ensure they are actually 9.5:1
Remove and test the internal and external thermostats
Duct air to the front cooler, drill holes and fabricate
Back off ignition timing a few degrees
Measure deck heights on all cylinders.


Since I rather this not happen again, I still have a few questions:

1) How do I reliable measure exhaust temps (I have a G2X data acquisition system to store the data on)
2) I believe the distributor is from an 83 not a 79, I removed the vacuum retard and only run the vacuum advance...would that make a difference
3) Should i get the dizzy re-curved? If so, who could do this for me?
4) What AFR WOT should be safe? The only way I can change this is by mechanically adjusting the WUR (its a 79...no O2 sensor feedback)

Thanks, Stu

1) You need to monitor cylinder head temps as thats more indicative of load. Between this and AFR's, you should be OK.

2) Your distributor needs to be in excellent condition. I would have it rebuilt (as needed) and recurved, once you sort out your AFR issues. Barry Hershon is the man for this job.

CIS systems have always been tough to tune since you cannot make individual mixture adjustments for idle, part throttle and WOT like other fuel delivery systems. You must make sure that your WOT AFR is around 12.6 to contain cylinder head temps. Anything at or over 13:1 is too lean for WOT operation, especially in hot weather.

Stu,

What heat range spark plug are you using?

Im using NKG BPR6ES plugs.

Below is a plot of a 3rd gear pull on the front straight at last weekend's DE.

http://forums.pelicanparts.com/uploa...1283477859.jpg


I am running a little lean...do you think this caused the detonation?

Thank, Stu

I think your AFR is not stable enough, shouldn't have such lean spikes/rich dives.

Also, IMO NGK heat range 6 plugs are too hot on track driven 911 engine, 7 is more appropriate.

Stu,

I would have liked to see some smoothing on that graph to average it out better and a matching plot to show throttle position, but overall, its much too lean (except for 5300 RPM).

6's are too hot for track use: a 7 is better.

What did the plugs look like? Where was the timing mark and any specs on the porcelain?

Stu,

We have seen detonation problems like this in the past. For sure I would limit your total ign. timing to 29 degrees, and check this at high rpm...5,000 plus. I would also say the NGK 6 plugs are much too hot for track use. I would use a NGK 8# which is a Bosch W4. If these stay clean you could even go to a NGK 9 or Bosch W3CS.

Thanks for the advice, to answer a couple of questions: apart from piston #3, which was black, the plugs were light grey in color. Timing was set at 33 deg.
Stu

Detonation leaves tell-tales on the spark plug porcelains if you know what to look for and a strong magnifier to see it.

One sees tiny balls of aluminum embedded and on the surface of the insulators. Further, one can see some blistering of the coating, as well.

Coloration is mainly a reflection of the additives in the fuel: the color ring position shows mixture strength. This is the time to mention that it takes a lot of experience to accurately read spark plugs and thats why AFR meters are so nice.

33 degrees may be too much for your fuel, AFR, and cylinder head temps. Scotty's suggestions are really VERY good. :) :)

Exact some issue and circumstances. Took the engine appart and #2 JE piston, also 9.5 / CR, was missing the top ring and a chunk by the exhaust valve notch. I suspect detonation as my car runs hot on the track and my distributor was having timing issues. Mine was also low milage.

Question now is to find a replacment piston or buy 6 new ones

Chris

73 911 E

Chris,

I was able to buy single 98mm JE piston from EBS. I'm getting the distributor rebuilt and recurved by Barry Hershon.

Stu

Chris,

If your engine was detonating hard enough to break a piston, I would closely inspect ALL of the rod bearings as the upper shells really get hammered under those conditions.

Terminal detonation? That's the bogey-man for tweaked turbos as well...

Took my single-plugged 930 with a newly-fitted J&S Safeguard to the dyno Tuesday. Still needs a little more dialing in (other issues) and it's set quite conservatively right now, but the initial results were very encouraging - everything I'd hoped for from the unit, which seems to work very well.

No affiliation (other than being a satisfied customer):

J&S Electronics SafeGuard Indivdual Cylinder Knock Control

Way I figured it before purchase, if it did nothing more than protect me from a tank of sub-standard gas on a hot day, it just paid for itself (at least) ten times over right there.

I've had JE pistons in a big block blower application that came out unscratched during a detonation episode that broke the squish on an iron rectangular port GM head. It looked as though someone hit the head with a ball peen hammer. The head also lifted burning the gasket and making a hell of an oil fire in the lifter valley right before the whole thing exploded. 93 mph GPS in a 30' Scarab Panther....then BOOM! It was loud, smokey and not so much fun but the JEs did not have a mark.

Lindy