Does the reading of the WB O2 sensor ever drift under extreme conditions?

[bobc455]

I am in the process of trying to figure out why I broke a piston.

I run a 455 Buick with the Speedpro and nitrous.

Last weekend, I pulled some fantastic times at the track (on a 150 shot), drove home, and discovered that apparently at some point during the evening I'd cracked a piston. I reviewed all of my runs again on the speedpro (thank God for that thing!) and confirmed that I never ran lean- if anything, I ran too rich (like 10.5:1 A/F ratio).

At least that is what the O2 sensor told me.

My question is if there are ever any circumstances when that sensor will drift causing incorrect readings? The sensor *seems* fine to me driving around, but I wonder if there is something about a full-on nitrous run that can make it drift... very high pressure in the exhaust system? Higher exhaust temperatures at the sensor?

Obviously I am trying to prevent future piston breakage... I know there are a lot of 455's pushing a lot more power on these same pistons, and there was no detected ping/detonation, so I don't think it's likely that I am just creating too much HP. I suppose it's possible that I just had a defective piston, but that seems like such a remote possibility!

Thanks for any insight,

-Bob Cunningham

 

[Lance]

I had some similar concerns when I hurt a couple of pistons last year. You should have plenty of information to try and analyze the problem. If your concerned about the WBO2 drifting, you can check to see if the injector duty cycle was significantly different than previous runs. The same applies to O2 correction. In my case, I ruled it out because the data showed that they were in line with everything previous. So the O2 sensor or fueling was not the problem. In your case, with nitrous, I don't really see a problem with only 150 HP and as a rule of thumb, don't expect any problems when running less that 1HP per inch of nitrous. However, nitrous oxide is a tricky substance and an engine will almost never make the most efficient use of it. It's likely that some will find its way into the exhaust. But with only a 150HP shot, my guess would be that the effect on the O2 is trivial. A cracked piston most likely resulted from detonation. And the causes for that are many. The spark plugs will usually be the best tool for determining detonation. It could just be that you were on the edge all this time and it finally caught up with you. Some of the more hard core nitrous racers would say: "If your not breaking a few parts, your not trying hard enough".

 

[bobc455]

Thats a thing I love about the Speedpro- since everything was recorded, I do have tons of hard data, and not just some vague memory of what I think was happening while I was concentrating on something else. The recordings show what I'd expect for injector DC%, but backed off by 18% (my correction limit at WOT) by the end of the run.

According to the data that I see, whenever I use nitrous, the mixture starts out a smidge rich, then the O2 sensor corrects it, then the motor gets richer and richer over the course of the run. The ECU compensates by leaning back out. In my mind, this could be happening because of the following:

1) There is some sort of problem with the nitrous system where either my nitrous delivery is dropping or my fuel delivery is rising (I have a wet system, controlled w/ seperate FP regulator)

*or*

2) For some reason the O2 sensor drifts and tells the ECU to lean out when it shouldn't be doing that.

Because running lean is when you tend to break things, I want to be sure about #2.

Now I realize that the pressure in the nitrous bottle drops somewhat over the course of the run, but even if I start the run at 1000PSI in the nitrous bottle, it's still over 900 at the end of the run. I don't think that's enough to make the whole motor (air + nitrous) run that rich, so there is something else going on.

The spark plugs actually all look great (except for a broken electrode on a different cylinder), as does everything else in the motor. Right now I'd love to attribute it to a defective piston, but I want to make sure everything else is right before I keep going.

Thanks for any insight!

-Bob Cunningham
bobc@gnttype.org

 

[TurboTR]

re:

Well you would sure start to suspect detonation with a broken plug and a broken piston. Do you run hypers in it? I'd suspect that you shouldn't rely on however you are detecting knock (acoustic ?), especially in the face of the evidence here. What fuel were you running at the track? C16, or? How much timing retard are you applying with the nitrous? How aggressive is your timing set w/o the spray?

TurboTR

 

[bobc455]

Alrighty, from the top:

The plug didn't look like a detonation break, and it was on a different cylinder than the broken piston. Weird enough, the plug on that cylinder looked perfect.

I run forged pistons- I was told that hyperuetectics were no better than stock cast pistons. They are made buy speedpro.

I was running pump gas at the track- that's all I've ever run. Next time up, though, I'll be throwing in a bit of race gas for safety. Also, I'm considering a 2 gallon fuel cell for the nitrous enrichment fuel, that way I can run some 105 for my enrichment.

Without spray, I run 36-37 degrees, and I retard 6 degrees on a 150 shot.

The following changes will apply to my next time at the track:

- spark plugs one step colder
- close gap from 0.035" to 0.027"
- add a bit of race fuel
- back off timing another degree or two

Oh yeah, and I'll be running one different piston

-Bob

 

[64duece]

I thought I'd add...

your judgement needs to include all aspects of the tuneup including plug reading, mph...ect. Those things are just as important as anything recorded by the ECM. Sounds like your well underway in that aspect. Consider getting yourself a Percy's or Computech EGT. I would check that cyl vs other to be sure your tune up stays inline. Also will aids in timing adjustments. You'll need to do some experimenting, but if you see some abnormal temps...you know somethings not right.

 

[quickt]

Originally posted by bobc455
Alrighty, from the top:



Without spray, I run 36-37 degrees, and I retard 6 degrees on a 150 shot.

The following changes will apply to my next time at the track:

- spark plugs one step colder
- close gap from 0.035" to 0.027"
- add a bit of race fuel
- back off timing another degree or two

Oh yeah, and I'll be running one different piston

-Bob

Bob we have had good luck with Nitrous in Steves 72 Skylark 455 and in a Monte SS with a 455. We run a 250 shot on the Monte and a 150 on the Skylark. Both cars have a seperate fuel system with 1 gal fuel cell for the nitrous. We use Race fuel in the Nitrous cell and 94 pump gas in the tank. The timing numbers you have listed are high IMO for a 150 shot. We run 32 on the 150 and pull back to 26-28 and with the 250 32 and pull back to 22-24. Maybe i am wrong but its something to look at. The Monte 455 runs 11.50s on motor and 10.0s on the spray. The 72 is a stock 455 with sealed power forged pistons intake and headers, small cam and 3.55 gears. On motor runs 12.90s on the spray 11.70s on MT protrac tires. We have had problems with the sealed power forged pistons breaking the first ring land down to the comp ring. I dont think those pistons are strong enough for nitrous but we replaced the broken one and have put many more passes on the car. The nitrous is alot of fun If i build another BB Buick for myself it will have the spray no doubt.

Lonnie Diers