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DRY NITROUS TIMING PULL HOW-TO

Disclaimer: Use this info at your own risk. It's for information purposes only. We can not be, or will not be, responsible for any misuse of said information.

THIS IS FOR DRY SYSTEMS ONLY, AND SPECIFICALLY THE LSx/EFI PLATFORM. However, usefull information for wet and dry in the lead paragraph.

Copyright © 2007-2008 Robert Weaver

Why do we need to Pull Timing?

Nitrous increases the speed of the flame front, so you're bringing the advance back to properly time the pressure spike to the retreating piston. Meaning, pre-ignition can occur, and lead to detonation with to much timing (stock and/or added advance). What this is saying is, the nitrous causes the cylinder to fire early, like when it's coming up on the compression stroke, this can do big time damage, and is probably the #1 reason the ring lands go. In addition, you can see this early firing is less than optimal for ultimate power, and thus, not very volumetric efficient. You want it to fire at the period of time when the piston wants to go down for the power stroke, not fighting an up traveling piston (pre-ignition).

Below is a great video, it shows just how fast nitrous causes the combustion process to increase. Watch how the paper towel burns at a normal air supplied rate, and then once it gets to the N2O it just takes off. Thanks to srsnow from the NitrousForum.com site for the video.

http://s108.photobucket.com/albums/n22/srsnow/?action=view¤t=MOV00443.flv
 

Dry Timing Retard

This write up will explain how to pull timing using a tuner program and scanner. It will have no effect on your N/A timing. The data used in this write up was gathered and modified using HP Tuners, however, any tuning program and scanner should be adaptable and be able to get the job done. It has also been successfully been used with LS1Edit.

The Premise

First we need to look at our High-Octane table (see picture #1 below). This is a calculated load placed on motor, and based on (Gms/Cylinder) Gms/Cyl@RPM. So for instance, calculated motor load is .96 Gms/Cyl, and at 4400rpm the look-up table (High Octane table) will select 26° of timing. Remember though, other parameters can pull timing, such as, Intake Air Temperature, Coolant Temperature and a couple others. However, we need not be concerned with these additional timing pulls, as they won't affect what we are doing and may be added (more timing pulled) for safety to timing table, automatically at time of need.

Picture #1

The Nuts and the Bolts of it all

The calculated load, based on the amount of air the Mass Airflow Sensor (MAF) is reading, uses the measurement of Gms/Cly. Now when spraying a Dry Hit, the MAF will now see a higher load to measure and calculate based on a perceived air flow increase. However, it's not really an increase in volume, but rather, an increased density/temperature reading (often referred to tricking, but really its doing, as it should). So now, the PCM through algorithms accesses the higher Gms/Cly, in microseconds, areas not seen in N/A operation or highest demand load N/A, but areas we will modify.

Because of this, we can now change/modify, the part of the table that is seen, or used while spraying the dry hit. This will not effect N/A timing, unless you get into areas that N/A uses and more on that later. This is basis of why this works only on dry hits and not wet hits; the wet hits do not measure the nitrous with the MAF. Now on to the step by step to achieve what we have been discussing, pulling timing in the higher reaches of the table.

Step #1

First, we need to establish the highest load the PCM will calculate under N/A conditions. We need this information so we won't alter any values in the High-Octane look up table that N/A uses. Then we know what to modify above the point that is seen N/A. This allows N/A timing to remain untouched when spraying.

Now we need to do some logging at WOT, using your favorite scanning tool, or the scanner in your tuner program. You will need to log: Throttle Position Angle (TPA), Engine RPM, Main Spark Timing Advance, and Mass Airflow Rate (in Gms/Sec not lbs. per minute). Other than these parameters, you can log anything you like for other uses later, and they will not effect what we need to do, you'll see. Get plenty of data at WOT, at all RPM ranges (I did multiple drag runs and freeway runs). Make sure you save your log data when done logging, as some tuner programs will loose info if you turn car off before saving.

Once your done logging, export your data to an Excel spread sheet for analysis (for those that do not have Excel or do not know how to use it, there is an alternative method which I'll touch on later). You can now arrange your data IE: TPA, Timing, RPM, and MAF Airflow next to each other. Selecting columns not needed in the lettered box, at top, and deleting them do this. We don't need them to confuse things (remember this additional information will still be in the original log and can be accessed later for review). Then sort data, using the sort function at top of page (see pic #2, choose MAF lettered box first then expand to include other rows, that way MAF is the sorting order, the rest will follow). We want descending for the sort choice, this way all the Highest MAF Gm/Sec (LB/Min) data comes to the top. Be sure to expand the selection (choose all columns for sorting) when sorting, all data/columns stay in order this way. You can now delete all data below the 100% WOT TPS, as it is a little less confusing and not needed. You should now have a spread sheet that looks something like this, and you can now see your MAF numbers at there highest flow:

 

Picture #2

Note: The screen print below shows MAF in LB/min, not what we want to record. We want Gm/Sec. However, even though I logged in Gm/Sec, when I export data to Excel, it shows LB/min, a glitch in my system probably (you can use your Unit Conversion calculator in HPT tools to manually change lbs/min to Grams/Sec).

This brings up the optional method, see Picture #2a. Below picture shows what you'll have in Excel, except yours should show Gm/Sec instead of LB/min. You can also see my timing at 14 degrees, from 26/27 above, so this system works, as this capture is an after set-up log just for an example (might be the reason for the lbs/min?).

Here is the actual log in HPtuners (below) that will be exported to Excel; however, because of my glitch, I was able to get info from this log. You can use the cursor to find your highest load area as the picture shows below, along with rpm and TPA. Use a few different logs (I used a couple drag runs and a couple freeway runs) to make sure you get a good idea of max load the car sees N/A. I made a hand written list of about 5 samples from each log. Now we have the 2nd method to gather the needed max load in Gm/Sec.

Picture #2a

So with second method we can still get and use info from log. My cursor has one shift spot on a drag run highlighted. The Gm/Sec at this high load spot was 414.92 gm/sec. The TPA was 100%, RPM was 6562 (shift point) and timing (Advance) was 26°. You can go all the way through log and gather all 100% TPA high load areas and jot down by hand for later use. We can clearly see that the information can be gathered this way, without the use of Excel, though Excel is much easier and will supply much more info.

Step #2

We now want to calculate our load, just as the PCM does. Again, the timing is based off of load in Gms/Cyl vs RPM. What we have logged is airflow in Gm/Sec. Now we need to convert our gathered Gm/Sec data into Gms/Cyl. We do this with a basic math formula, which can be performed in your spreadsheet, though I did mine by hand. Here is the equation: (Gm/Sec x 15) / RPM = Gms/Cyl. Lets use my first example in table/Picture #3 below, acquired from the log in Picture #2a. The formula will be filled like this (414.92 x 15) / 6562 = .95 Gms/Cyl.

We now have a figure to reference on our "Timing Table" (see Picture #1). Just find .95 Gms/Cyl on the left of the table, and 6500rpm (round RPM and Gms/Cyl down to next value) on the top of the table. We see the junction is 26° of advance (.92 and 6200 rounded down). You can see the advance in the log above (Picture 2a) at it's data point.

Do this math to all of the collected data sets. Once again, this can be done in Excel, or manually by hand. You'll end up with the same information either way. The point of all this is to establish the highest load the PCM will calculate at WOT N/A. Every car will be different, so we need data specific to each vehicle. My 408 sees about .96Gms/Cyl maximum load N/A at WOT, but yours will differ.

Picture (Table) #3

  • (414.92 x 15) / 6562 = .95Gms/Cyl @ 26* Timing
  • (407.96 x 15) / 6702 = .91Gms/Cyl @ 26* Timing
  • (415.90 x 15) / 6490 = .96Gms/Cyl @ 26* Timing
  • (416.96 x 15) / 6664 = .94Gms/Cyl @ 26* Timing

Step #3

We now know our maximum N/A load values, and we can clearly see the highest calculated point in Gms/Cyl on the timing table (Picture #4) that we will see N/A. So now we simply change the timing values in all the cells above that point (physically below on the table). Important note: I chose to overlap my timing retard one step into N/A territory, a safety issue for my big shots, it's up to you, but smaller no real need. These new values inserted, is the Advance you'll see while spraying. Don't change the values below the rpm you start spraying. For example, (Picture #4) I start spraying currently at 3400rpm, so rounding down to the next RPM available would be 3200rpm, and all RPM ranges higher than this start point, and corresponding to our max N/A Gms/Cyl which mine is .96Gms/Cyl and higher. This will basically be the right bottom quadrant of the table. What you change your values to is up to you. I changed mine to 18° for 200/250rwhp shots (Picture #4), and 14° for 300rwhp shot, and you can see how I really overlapped into N/A territory (Picture #4a). So, retard is dependent on what size shot we run.

Picture #4

 

Picture #4a

 

Step #4

Once you have modded the timing table, upload your new tune. I have a couple tunes per size of shot, used on any given day. It only takes a minute to change tunes.

Now, do some more logging on the spray at the drag strip, or a dyno. I found it very difficult to spray a 300rwhp shot on the street. On the way to the strip/dyno, log some N/A WOT pulls. This way you can see that N/A timing is not effected, and spraying, the timing is being pulled. You may or may not need to do some fine tuning, dependent on ambient air temp and/or coolant temp, I never had to fine tune. Occasionally, you will get some random oddball MAF readings that cause load to be calculated incorrectly, and timing temporarily jump to a different value. Don't worry, the count in this cell will be one or two (cell count can be checked in your Histogram), meaning it doesn't matter. This has nothing to do with our timing modifications.

Below pictures shows my actual logs/Histograms running 18° tune (Picture #5, 5a) and 14° tune (Picture #6, 6a) respectively. You can clearly see maximum timing and the Gms/Cyl. The Gms/Cyl on the spray is mostly all in the 1.xx Gms/Cyl area, so not to close to N/A timing. This system works great; good luck with your new tune. You now have a safe and effective way to pull timing, wet hits need not apply.

Picture #5

Picture #5a  Here you can see timing dropping at 3200rpm, and .96Gm/Cyl, perfect, on a 200rwhp shot. Now, overlapping into N/A timing one Gms/Cyl level, was my choice, to make sure pull is started before spray gets to motor.

Picture #6

Picture #6a  Here you can see my more conservative 300rwhp tune dropping timing at 3200rpm, if load was there timing would be pulled at .92Gms/Cyl, as it is at 3600rpm. I chose to drop into N/A timing two rows for safety on the big hit. Again, just what the nitrous doctor ordered for dry users.