1. Car runs with no knock at all off the spray.
2. tank has race gas in it.
3. PCM will retard all the timing it can on the juice.

Any ideas why only when on N2O that it pulls timing? No one seems to want to touch this one on the other sections.
Skip

 

How much nitrous are you throwing at it? Are you sure the nitrous system is getting plenty of fuel? Could be lean detonation.

 

i would hav e to agree with pete on the questions he is posing. what is the shot, and are you getting enough fuel. also if your ignition is not functioning properly that could also cause detonation, but the fact that the car operates fine in N/A mode doesn't really support that. how is your system setup?

 

Is that on a dry shot? If it is, perhaps it is freezing the MAF & causing an improper reading, which could cause the lean condition, and hence detnation. I would increase the fuel & see if it goes down, or move the nozle back further, if it is a dry.

 

Car is a 95 Z A4 w/2.73's and 51000 miles.

1. It is a dry set-up, w/nozzle right at bend in the elbow, opening in nozzle pointed in the right direction.
2. It's a 75 shot.
3. Fuel pressure gage jumps to 80psi when nitrous kicks in. I know O2's don't mean much but I have seen them as high as 1000 when spraying (I have PCMcomm logs to look at)
4. It still drops my ET by almost a second anyway

Thanks for the help. I've posted this question numerous times. It's almost as hard to get answers on this one as it is to find out how to tune the tranny! Keep 'em comin'
Skip
On a side note... when I flash a new file I take out the PCM fuse to reset the PCM. I think it resets the BLM's and INT's to 128? I wonder if that might be causing the knock. (Although it still runs fine w/o the N2O

 

Skip,

I don't believe that you need to pull the PCM fuse with version 1.9 and later of LT-1 Edit. But ideally you want your BLM's & INT's to be at 128 or just a tad lower so I don't see how that would be causing the knock. What do your MAP readings do when you turn on the spray? The reason I'm asking is there is a "burst knock" table in LT-1 Edit that causes the PCM to pull timing if it think the load has changed too fast. I used to get some knock every time I shifted, but tuned it out using this table. Do you launch off the line on the spray or hit it later on?

How fast does the PCM put the timing back in?

 

is it possible that with the dry shot that one of the cylinders is running lean? have you checked your plugs after a run on the bottle?

 

LT1 obd1 cars have a timing correction table that adds or retards timing baised on incoming air temp devation.
The DRY spray will get picked up by the temp sensor in the elbow.
Cool temps will allow for advance then that throws the car into knock and the attack rate pulls the timing back out...silly stuff.
I just limit the advance table To "ZERO" across the rpm range on a nitrous LT1!
...thats the HotRodHawk tech tip of the week

Jeff

 

Very interesting Jeff, I never thought about that.

 

Suddenly I remember why I love carbs and distributors so much!

 

1. Map readings stay at 97-100 all the time even when spraying.
2. MAF readingsclimb from about 130 to 200 when nitrous hits and back down when shifting and climb again. (N2O on at 3000 off at 5400)
3. IAT is at 130*F when launching and drops like you'd expect as speed increases.
4. I spray at 3000, not off the line.
5. I haven't checked the plugs but I don't think they are bad. Not 100% sure. Off the spray it pulls like you'd expect.

I have logs if anyone would like to read them!

Skip
I used to love carbs and HEI's too but I love 27mpg and low 14's better