Peter Zimmermann

You would think that a case "exam" would be part and parcel of any "rebuild." Also, setting the deck height is crucial. If the clearance (quench zone) between the top of the piston and combustion chamber is not correct, and equal on all six cylinders, you will have the type of problems you are experiencing. If the problems are confined to only one side, that might be due to past machining issues that were not discovered or dealt with appropriately. My shop went to great lengths to be sure that the deck height on both sides of the engine were equal, and the cylinders were shimmed as necessary to obtain .060 - .065" clearance. In all performance applications the engine/heads were cc'd to determine compression - no guesswork allowed.

If your mechanic isn't intimately familiar with...

http://en.wikipedia.org/wiki/Compression_ratio

...it's possible that he has missed something. For example, if the quench zone is to small, aka tight, it's possible for a hot spot to develop on the top of the piston, caused by the inability of the combustion flame to cover the entire piston. Failure of the piston can be the result.

This might help a little...

pmason

Peter,
Good to meet you at RA. Yes seen you parked on the grass going up to 6 with the safety crew there by the time I came by, I thought might of been a fire or something, thought I seen some smoke at least your ok, hope you enjoyed RA.
Phil

KILRWAIL

Phil - Thanks - I had a great weekend with all you fine folks. I was happy to finally get to RA, but it might have been too much for my car.

Steve Weiner-Rennsport Systems

My pleasure, Sir.

JMHO, but 35 degrees is far too much for 91 octane fuel, especially in hot weather. I would take 5 degrees out of that unless you step up to race gas.

Your targeted AFR's should be around 12.7-12.8:1 at WOT. Max HP occurs at 13.2:1, however thats only attainable with water-cooled engines that have strict control of cylinder head temps. Air-cooled motors are a whole 'nuther thing altogether and their operating conditions are quite different,....

Mr. Zimmerman mentioned something VERY important; deck height. Too much or too little deck height increases the propensity to detonate and this is a critical value. All cylinders must be exactly the same. I shoot for .040"; a little less in race engines as the more squish room you leave at TDC around the perimeter, the more air-fuel is there to spontaneously ignite nearing TDC. Naturally, the piston dome design and CR play a role here.

Your jetting may be too lean, but I'd need ALL the details to offer anything constructive: main jets, air correctors, emulsion tubes, main venturi and idle jet sizes.

Remember, every engine's configuration and optimal jetting is slightly different and taking another's recommendations brings some caveats, as you can now clearly see. I'd strongly recommend taking your car (or better yet, engine) to a dyno and do your jetting under controlled conditions so you do not have an "encore performance".

KILRWAIL

Steve - Thanks again. The PMO's are 46 mm, venturis 38 mm, the main jets 145 and the air correctors 190. I don't know the idle jets or emulsion tubes, but they weren't changed from PMO's settings. I was using JE pistons with a 9.5:1 CR. I am seriously concerned about your suggestion that the AFR should have been richer, as this might explain the previous engine failures using the original CIS system, which was also tuned to 90% lambda at 6000 rpm.

Brian A.

If you're guys has already rebuilt this engine twice I would suggest writing the next check to a new engine builder.

whalebird

+1. Would I be out of line suggesting that you ship that motor out to Steve W? We would all probably agree that the cost of building one of these engines correctly is not cheap. The cost of freight would be offset this time next year based on your historical data.
A well built and tuned 911 can do a decade of track events with proper upkeep and driving.

Steve Weiner-Rennsport Systems

Hi Peter,

Thats damned lean jetting for 38mm venturies, Sir.........damned lean.

Certainly the root of your issues.

If I may say,.......ignore Lambda values here and tune for the AFR's that I provided in a previous post.

Get some 160 mains and 180 air correctors to start.

KILRWAIL

Steve - Ironically, PMO supplied 160 mains and 180 air correctors. An interesting conversation will ensue...

Peter Zimmermann

SW, what is your view regarding the problem only on the 1, 2, 3 side? This is what got me thinking about deck height & compression...

KILRWAIL

Pete - FYI the compression on 4, 5 & 6 was down to 155 from 170, so it may also be affected; just not first.

rusnak

I must say, this is an interesting thread.

Lmbda-AFR
0.686 10.08
0.696 10.23
0.706 10.38
0.716 10.53
0.727 10.69
0.739 10.86
0.750 11.03
0.762 11.20
0.774 11.38
0.787 11.57
0.800 11.76
0.814 11.96
0.828 12.17
0.842 12.38
0.857 12.60
0.873 12.83
0.889 13.07
0.905 13.31
0.923 13.57
0.941 13.84
0.960 14.11
0.980 14.40
1.037 15.25
1.078 15.84
1.121 16.48
1.169 17.18
1.220 17.93
1.276 18.76
1.337 19.66
1.405 20.66

KILRWAIL

I shouldn't have muddied the waters by mentioning lambda. I was treating it as 14.7. The Innovate gauge is calibrated in AFR. I was maxing out at 13.5 the day the engine failed, when the ambient was in the fifties Fahrenheit. Otherwise it had been peaking at about 13.2 at 6000 rpm all summer. Sorry for the confusion.

rusnak

You can graph, or chart the table if you need, but basically your LM-1 is saying that you were too lean at wot. Sw says you can't run that afr w/o liquid cooling or controlling the head temp.

whalebird

Me to. I really enjoy these threads.
I am certain SW and Peter are dead nuts on, and without empirical data, we can only discuss possible variables. Even the best motor is not going to last long with that tuning on the track. Peter raises a good point. The scenario I posted above(2.8/webers)was a left bank failure as well. Not to draw a direct parallel, but it's got me thinking.
Cam timing, maybe linkage, we all can speak about JE pistons(but not now), so I'm with Ivangene on the forensics in this case. I would love to see how this thing goes back together, as compared to how it comes apart(deck height, machine work etc.)
Unless I missed it, what kind of exhaust is in play here? is it a 1in, or 2in?