ACE1252

Just wanted to pass on some timing and knock hunting experience.

In doing data logging for my injector issues, I noticed that I was having 2-4 degrees worth of knock retard in my logs. I went back as far as 2008ish when I installed the CC503 cam and it was there as well.

Present at WOT and when doing light cruise.

After some research, I decided to drop down to a colder range plug to see if it would stop what the knock sensor was picking up. I choose to go with Autolite 104 plugs gapped @ 0.040. According to this thread(good stuff on plugs).....

https://www.camaroz28.com/forums/n2o...-chart-431689/

....this plug is 1/2 range colder than the stock plugs(I had AC Delco stockers gapped @ 0.050).

Low and behold, my knock went from 2-4 degrees to 0.11- 0.38 degrees at WOT and cruise. They totally solved the problem. So the stock plug was getting hot enough to detonate the mixture. Great to solve that problem, however...it created another one.

This last outing at the track....I could not break out of the 8.50's. Also, I could not get into the 87mph range. Was mostly stuck in the low to mid 86mph range. Weather was good, so I don't think that was to blame.

I decided to do a log of my last couple of 1/8th mile runs to see if something was going on.....I figured that curing the knock would get me more power....certainly not less.

My log did show a change. The ignition timing with the knock @ WOT was in the 36-37 range. With my knock cured, the PCM was pushing the timing into the 39-41 range. I think that is where my HP went. Curing the knock has caused the timing to advance too much. Or at least, that is what I think is going on.

I have pulled 2 degrees out of the timing from 80kpa to 100kpa in the 3600rpm band all the way to 7000rpm. Then smoothed the transitions to the adjacent columns a degree or so.

Lord willing, we'll see what happens this Friday when I take it back to the track.

ACE1252

Can't say that my adjustments helped it any. It ran about the same as last time.

Best 1/8 ET: 8.507@86.293
Best 1/8 MPH: 8.619@86.661

Just really bothers me that the car has lost almost 2 mph trap speed and a little ET from when I first got this combo completely together. Not sure if maybe my springs are now giving up on the top end....or maybe my 16 year old fuel pump is having issues keeping up. Maybe I'm on crack for trying to find this lost speed, but I was proud of that 8.3/8.4@88+. Would be nice to get it back.....but I guess that's racing for ya.

On the bright side, absolutely zero knock. I'll bump up the timing one more degree to give me 38(saw mostly 36/37 tonight).

Edit: Did some fuel pressure testing. Looks to be dropping on WOT runs. Drops to 40psi....not terrible, but could be where my HP went.

chief455

Interesting that the spark plug heat range could effect logged knock.
I am currently logging here and there, seeing some knock retard and will keep this in mind.
On the Mondo file share site one member said the LT4 knock module cured light retard he saw while logging.
I appreciate when anyone shares information based on facts

sherwinZ28

Are you still using OBD-2, if so what are you using to log with. I created a thread to see if there was anything wrong with my logs since I started tuning the motor. I am using the JET DST tuner and obd-2.com software for logging. I can't seem to make the correct correlation between BLM's (OBD1) and Long term Fuel Trim (OBD2). What should I be aiming for? Can you post up a log file so I can see the knock?

Thanks,
Sherwin

96capricemgr

Now I have no idea what I was running for timing back when the car was cam only but I know these days I am only running very low 30s. Maybe try dialing it back a few more degrees just to see what happens. Worst case it slows a little but no harm is done.

Injuneer

The BLM is referenced to 128 being zero correction. Anything above 128 means the BLM is adding fuel. Less than 128 means the BLM is subtracting fuel. The long term fuel trim (LTFT) in OBD-II uses the same basis. It just expresses the correction as a positive or negative percent.

For example, divide the BLM by 128 and subtract 1 from the result. What is left represents the decimal expression of the percent correction..... + means the BLM is above 128. - means the BLM is below 128.

If your BLM is 141 and you divide by 128 you get 1.10 then subtract 1 and you are left wirh .10 ...., that means the LTFT is +10%. Ideally you want a 128 BLM or a 0% LTFT. Both the same thing. But don' t just jump into something like changing the MAF calibration to get all Cells back to 128 (0%). Figure out if there is a problem causing the corrections that are outside the range of -5% to +5%.

sherwinZ28

THANK YOU!!!! That clears up the confusion for me! I am seeing between 0% and 25% LTFT but I have a feeling its due to a leaking exhaust before the O2 somewhere, have to find where. Sorry for hijacking thread.

ByByC5

Eric,

What is your A/F ratio at WOT.
Did it show leaner with the drop in fuel pressure?

I do think valve springs get weaker with usage, so that may have something to do with it too?

Keith