spark knock under spray
 

In datamaster, when I get on the spray I see between 3-5 degrees of spark knock retard. What could be causing this? When I make a WOT pass off the spray the knock isn't present. I have ngk tr6's, jetted for 100 shot, and the 02's readings don't seem to be too much different than running n/a so I don't think the jetting is too far off. I only sprayed it once like this, and the car seemed to fall on it's face for a quick second after the spray started and then it started to pick up again (probably as the computer gave back timing). If the o2's read close to the same in datamaster, could my jetting be off? If there is knock present, wouldn't that mean the system is probably too lean? Should I just pull 5 degrees of timing and see what happens? Am I right in thinking that there should be NO knock retard present at any time?

Car is a 94 Z with a very lightly built 355 lt1 and plate kit, 94 octane fuel.

Re: spark knock under spray
 

Were you pulling any timing for the spray? Try running your tank low, then putt 100 octane (or the highest you can find with no lead) in the tank, and make another pass. If the KR goes away, it was real knock. FWIW, there are a few different rules for spray; some say pull 1 degree for every 50 hp, some say 2 for every fifty. I haven't seen confirmation on the dyno, but i know that pulling a few degrees for a shot won't hurt your times, and may save your motor.

Good luck man, let us know what's up.

Ian

Re: spark knock under spray
 

What RPM are you spraying at? When you fill the cylinder with nitrous, the combustion speed increases and the combustion temperature increases. That causes knock, and that's the reason it is commonly recommended that you retard timing when you use nitrous. Who did the tune on your 355? How aggressive is the timing? You need to ask your programmer so you know how much timing you need to pull when you spray.

The lower RPM you spray at, the bigger the problem. You probably shouldn't be spraying below 3,000rpm. What is your window switch set at?

My personal experience with "plate" systems on the LT1 is that they don't distribute the fuel very evenly among the cylinders, and that could also cause knock, if you have just one or two cylinders that are running leaner than the rest. In that case you may need to richen the whole nitrous system up a bit to account for the leanest cylinder. How do the cylinders compare when you examine all 8 plugs after you spray?

You say that the O2's are the same when you run NA or when you spray the nitrous. Another indication that you're going to get knock. The stoichiometry of air/nitrous is a much richer mixture than air/fuel stoichiometry. When you spray, you want to see as richer mixture/higher O2 mV's. And the stock, narrow-band O2 sensors are really useless for the kind of A/F ratios you want.

Re: spark knock under spray
 

I have the tunercat program now, but there are so many spark tables I am not sure which ones to pull the timing from. I had heard of people having no problems running stock timing with this shot, so I figured I would try it once. The nitrous engages at 3100 rpm.

I didn't get a chance to look at the plugs that day, which I know is really important. After I figure out how to back the timing off a little bit I will make another run and pull the plugs. If when I pull the plugs the front cylinders indicate lean, it's probably better to go with a single nozzle setup, right?? Where is a good place to plumb this into the intake elbow and have it aim right? Anyone have pics of this setup?

Would a wideband gauge help me see the actual a/f better?

Re: spark knock under spray
 

http://www.harrisspeedworks.com/gall...s.php?album=27

Thats some pictures of my single nozzle kit from HSW. I replaced the stock intake elbow with a trick flow one and added an airfoil to the throttle body since i took those pictures.

Re: spark knock under spray
 

You don't want to make nitrous timing changes in the NA program.... why reduce NA power? You leave your NA timing at the point that produces the best power, then pull the required timing for nitrous using an external box.

Re: spark knock under spray
 

Very good point made. IF you are looking into pulling spark for nitrous look into a box such as the Mallory 685

Re: spark knock under spray
 

So should I ditch my accel 300+ setup, or can I make something work with it? It doesn't retard timing (bought it before I thought of nitrous). It makes sense not to change the programming, I won't touch that.

Would a single nozzle distrubute the mix better to each cylinder?

As far as the whole system goes, would a edelbrock air/fuel ratio monitor (summit edl-6593) help me monitor my a/f during spray, or do you get what you pay for? I don't really want to shell out the cash for the INNOVATE wideband setup.

Thanks for the help!

Re: spark knock under spray
 

I am pretty sure the Edelbrock O2 meter is a narrow band. If price is an issue check out this wideband unit that Dynotune sells. http://www.dynotunenitrous.com/store...?idproduct=150