RETARDING TIMING ON N20
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This write-up will explain how to tune your PCM to pull timing when you use a dry shot of nitrous. It is fairly simple, if you have the proper tools. All you need is Auto Tap (or a similar data logging program) and LS1 Edit. To perform this, you should have a good working knowledge of Excel, your data logging software and know how to use LS1 Edit. I would like to note up front that this tuning method WILL ONLY WORK WITH A DRY SHOT OF NITROUS!
HOW AND WHY IT WORKS: The timing value that your PCM selects from the high-octane timing table is based mainly on the calculated load (measured in Gms/Cyl) that is being placed on your engine at any given time and RPM. For example, if you are at 4000 RPM and the PCM is calculating 0.56 Gms/Cyl of load, it reads the portion of the table that corresponds with those figures and selects that timing figure. Now remember, I said that it is "mainly" based on load and RPM, but there other factors involved such as intake air temp and coolant temp that may pull a few more degrees away from the base value in the table, depending on conditions. Alright.....because the load being calculated is based on the amount of air that the Mass Airflow Sensor (MAF) is detecting (measured in Gm/Sec), a dry N2O shot will cause a higher load to be measured and calculated, and cause the PCM to access a different section of the timing table; a part that is NOT used during NA operation. Because of this, you can modify the part of the table that is used when you spray and leave the portion of the table that is used during NA operation unaffected. That is why this method won't work for a wet kit; because the N2O isn't being measured by the MAF with at wet kit, so no additional load is calculated for the nitrous and timing is based on the amount of air that the MAF sees. (Sorry wet guys).
1. The first step is to determine the highest amount of load that your PCM calculates under NA conditions. This needs to be done so that you only modify the values in the timing table that are "above" the point that is accessed during NA ops. This will allow your timing to remain unchanged when you aren't spraying.
2. To determine this, you need to do some wide open throttle (WOT) logging with your scan tool (Auto Tap). At a minimum, you need to log Throttle Position Angle (TPA), Engine RPM, Ignition Timing Advance, and Mass Airflow Rate (in Gm/Sec, not Lbs per minute). Beyond that, log whatever you would like. I personally log both O2s and Knock Retard (KR) as well for monitoring purposes, but they will not effect what we are doing. Make sure you get plenty of data with lots of time at WOT. Make sure that your WOT time is spent throughout a wide RPM range. I recommend doing this at the local drag strip to remain safe and legal.
3. Once you are done logging (I would make a few logs), export the data to an excel spreadsheet for analysis. Arrange your data so that your Mass Airflow Rate is in the column next to RPM. The timing and TPA columns can be wherever you prefer them to be. I would delete all of the other rows of data so you don't get confused. Now sort the data (using the data-> sort function in excel). I usually sort based on TPA and have it sorted as descending. That way all of the WOT data will be at the top and you can delete any "non-WOT" data. We won't be using it. Make sure to "expand the selection" when you sort, so that all of the data stays in the proper order. You should now have a spreadsheet that looks something like this.
| TPA | Ignition Timing Advance(°) | MAF Air Flow Rate(gm/s) | Engine Speed(RPM) |
| 99.6 | 28.0 | 248.55 | 5782.3 |
| 99.6 | 27.5 | 44.42 | 5538 |
| 99.6 | 26 | 251.25 | 5221.8 |
| 99.6 | 27.5 | 247.75 | 5870.5 |
| 99.6 | 23.5 | 208.83 | 4890.5 |
| 99.6 | 25.5 | 239.21 | 5238.3 |
| 99.6 | 27 | 235.07 | 5578 |
| 99.6 | 30.5 | 13.13 | 5260.3 |
| 99.6 | 24 | 212.82 | 4602.5 |
| 99.6 | 23.5 | 18.8 | 4543.8 |
| 99.6 | 25 | 200.72 | 4844 |
| 99.6 | 30.5 | 38.89 | 5329.3 |
| 99.6 | 25.5 | 247.43 | 5465.3 |
| 99.6 | 28 | 199.49 | 5670.8 |
| 99.6 | 27.5 | 265.33 | 5437.5 |
| 99.6 | 27.5 | 270.14 | 5744.8 |
| 99.6 | 24 | 202.89 | 4698.3 |
| 99.6 | 24.5 | 231.66 | 5048.5 |
| 99.6 | 27.5 | 173.25 | 3765 |
| 99.6 | 25 | 174.12 | 3995.5 |
| 99.6 | 25 | 194.35 | 4224.5 |
| 99.6 | 24.5 | 204 | 4464.3 |
| 99.6 | 24.5 | 177.34 | 4316.8 |
| 99.6 | 27.5 | 244 | 6215 |
| 99.6 | 27.5 | 247.75 | 5832 |
| 99.6 | 27.5 | 260.66 | 5396.3 |
| 99.6 | 37.5 | 10.49 | 5201.8 |
| 99.6 | 24 | 213.62 | 4636.5 |
| 99.6 | 24 | 231.92 | 4966.8 |
| 99.6 | 26 | 251.25 | 5321.8 |
| 99.6 | 27.5 | 271.51 | 6066.5 |
| 99.6 | 33.5 | 12.59 | 5058.8 |
| 99.6 | 27.5 | 251.73 | 5592.8 |
| 99.6 | 26.5 | 252.25 | 5308.8 |
| 99.6 | 27.5 | 271.51 | 5914.5 |
| 99.6 | 23.5 | 224.98 | 4913 |
| 99.6 | 25.5 | 251.73 | 5284.3 |
| 99.6 | 24.5 | 188.27 | 4325.8 |
| 99.6 | 24 | 215.87 | 4692.5 |
| 99.6 | 24.5 | 239.21 | 5058.8 |
| 99.6 | 26.5 | 255.48 | 5412 |
| 99.6 | 27.5 | 243.62 | 5749 |
| 99.6 | 27.5 | 14.38 | 4646 |
| 99.6 | 28 | 162 | 3645 |
| 99.6 | 26.5 | 175.71 | 3855.3 |
| 99.6 | 25 | 190.89 | 4040.8 |
| 99.6 | 24.5 | 196.78 | 4233 |
| 99.6 | 25.5 | 247.43 | 5641 |
| 99.6 | 27.5 | 55.72 | 5758 |
| 99.6 | 27.5 | 243.43 | 5798 |
| 99.6 | 27.5 | 260.66 | 5517.5 |
| 99.6 | 27.5 | 252.85 | 5989 |
| 99.6 | 23.5 | 211.96 | 4874 |
| 99.6 | 25.5 | 227.99 | 5225.3 |
| 99.6 | 30.5 | 18.1 | 4000.8 |
4. Ok, now its time to calculate your load, the same way your PCM does. Remember, I said before that timing is base off of load in Gms/Cyl and RPM. What we have logged is airflow in Gm/Sec. We must now convert our figures into Gms/Cyl. This is done with a simple math equation that can be performed in your spread sheet. Gms/Cyl = Gm/Sec * 15/RPM. Lets take my very first row of data for example. The formula would be would be 248.55 * 15 / 5783.2 = 0.64 Gms/Cyl. I now have a figure that I can use to reference on my timing table in Edit. I simply find 0.64 on the left of the table and 5600 RPM (round down to the closest value for both RPM and Gms/Cyl) on the top of the table and I see that the come together at 28 degrees advance. I can now verify this by looking at my log and seeing that I, infact, logged 28 degrees at that data point. Pretty simple, right?
5. Now perform the same function on all of your data sets. You can easily enter the formula into excel and have them all calculated in no time. The point of all of this is to find the highest (numerically highest) amount of load that your PCM ever calculates while driving at WOT. I would do this with a few data logs, just for maximum confidence. This may vary from car to car, depending on different MAF calibrations and a whole slew of other variables, so I recommend that you log YOUR OWN data and see what YOUR highest amount of load is. Don't use your buddies data for reference. My car happens to see a mximum of 0.69 Gms/Cyl at any given RPM (remember that RPM is a factor).
6. Now that you know your maximum NA load value, it is very easy to determine the "highest" point of the timing table that you use NA. Now, simply change the timing values in all of the cells above (physically below on the table) that point. These are the values that will be accessed when you spray. It is advisable to leave yourself a gap between your uppermost NA access point and your lower most N2) access point. That way you will be SURE to never access the retarded portion of the table while running NA. I don't recommend changing the values below the RPM at which you start spraying. For example, I spray starting at 3000 RPM, so I changed all of the values from 2800 RPM (I rounded down) and up, which correspond with values of 0.72 Gms/Cyl and numerically higher. (Basically the bottom right quadrant of the table. What you change the values to is up to you. I personally changed all of mine to 24 degrees, and have had pretty good luck there. How much you retard is really dependant on how large of a shot you are using.
7. Once you have modded your timing table, upload you new tuning, fill your bottle and head back to the track. Do some more logging and you should be pleasantly surprise. Your timing will remain normal during NA runs and will jump right back when you spray. As I said before, you may see some variances in timing, depending on ambient air temperature and coolant temperature. Also, you may occasionally get some odd MAF readings that will cause your load to be calculated incorrectly, and your timing may temporarily jump to a different value. This happens to me both NA and while spraying and has nothing to do with the changes you made to your timing table.
Well, that's. All in all, a pretty simple mod that will allow you to run a healthy amount of timing NA and a very healthy amount of N2O without detonation. HAPPY SPRAYING!