Time for another update as I am re-tuning my engine after I converted the PCM to PFI mode and installed larger 29#/hr injectors since replacing my short block and converting to a hydraulic roller cam setup. I am running Edelbrock's MPFI for TBI trucks and a larger TB from CFM-tech that I bought several years ago. Thought I'd share some useful tips I've learned.
When do I need to adjust TPS and IAC?
I can hear it already, "I thought you said, 'TPS is non-adjustable. Just leave it alone.'". Yup, I did say that and that's true for stock or mildly modified TBI engines. However, once you start upgrading major components like camshafts and cylinder heads, you change the characteristics of the engine. More specifically, you need to recheck your setup for proper operation.
The key is setting the idle set screw to correct the IAC for proper operation, which will require you to remove the plug by drilling a small hole in it to get enough leverage behind it to remove it. It is located on the front of the TB near the throttle linkage. Once the plug is removed, you will see a hex screw behind it. The end of the screw serves as the stop pin for the throttle linkage. In order to adjust the idle screw, you will need the assistance of a handheld scantool or your laptop running TunerPro or DataMaster connected to the PCM via the ALDL port so you know what the IAC counts are. A small change in the idle screw will affect the IAC counts, despite no apparent change on engine operation. This is why you need a scantool to see the results.
At idle and in the "park", you will need to adjust the screw in or out until the IAC counts read between 10-15. The engine should be at operating temp for this and it shouldn't change your idle RPM if done correctly. The IAC itself will be able to open/closed to provide a controlled air leak and therefore provide a smooth idle at the desired RPM setting programmed into the PCM/ECM (via the "Target Idle RPM vs Coolant" table). Then turn off the engine, wait a few secs, and then turn the key on (but not with the engine running). Wait for the fuel pump to finish its cycle and turn the key off. Wait ~30 secs. Turn the key on and restart the engine. This will reset the TPS sensor for the 0% position.
Keep in mind, you have now changed the throttle blade position relative to the TPS sensor. If you turned the idle screw in, the TPS voltage will be higher and it will need to be adjusted back to spec. This is probably why GM installed the idle screw plug on the TB in the first place to prevent dodos who don't know what they're doing from screwing stuff up. You may need to modify your TPS sensor by enlarging the holes so you can rotate it (usually counter-clockwise to lower the sending voltage). Some older or aftermarket TPS sensors may be adjustable BTW, especially if they have elongated oval mounting holes. Mine wasn't adjustable and I had to elongate the holes to allow me to rotate the sensor. Again, this is easy to do and you can verify TPS output with your scantool or a handheld volt meter. Voltage at 0% TPS should be between 0.53-0.57V. Just remember to do the key on-off-on cycle trick to reset the 0% TPS position in the PCM.
I’ve installed larger/smaller tires and/or different rear end gears on my truck. Is there anything I need to change on the chip? How do I correct my speedometer?
You can thank HaulnA$$ for this one. The PCM needs this value to be correct to properly calculate MPH. It has nothing to do with your speedometer, which is controlled independently by the DRAC module. I’ll get to that in a minute.
Speedometer correction: The simplified formula for calculating the ratio for the DRAC is ((Tire Rev's/mile) x Rear end Gear Ratio)/3200 using 40 pulses/rotation. You can get tire revs/mile from the tire manufacturer’s website. Please note that you may come across a website from 614streets about DRAC info. The chart listed has errors in it from 0.707031 down where the numbers have been reversed:
http://614streets.com/drac.html. Another good website for DRAC info is here:
http://www.tbichips.com/drac/. It explains the modifications to the DRAC very well and its table is accurate. Another resource is this PDF doc:
http://pswired.com/misc/pacaudio/drac2.pdf. Good resource, but pay attention to the legend if you use it. The table is accurate but the 1’s and 0’s are reversed!! 0 is pin in place and 1 is pin removed. The pin nomenclature is reversed too: 12345677654321 instead of 76543211234567. Just to make it more complicated with the whole mirror image thing, hey?
I prefer looking at tables to set up the pins, but you may come across an online DRAC calculator:
http://www.philter.com/cgi-bin/drac.pl. Don’t use it!! Using tire size for the calculation is inaccurate. You can use tire circumference to calculate DRAC settings. Just take 63360 (inches/mile) and divide it by your rev/mile number from the tire manufacturer’s website to get the inches/rev (i.e. tire circumference in inches). It says it uses the tables from 614streets, so I question its accuracy. A quick test and it looks like the pin connections are reversed (i.e. “connected” should be “not connected” and vice-versa). Just stay away from it because it might only confuse you more.
To give you an example of the calculation, I will use my setup. I went from the stock tire size (P235/75R15) to a larger size (P265/75R20). As expected, my speedometer was out of spec. My stock DRAC is set up the same way as the pic below and translates to an input ratio of 0.781738.
According to the Nitto website, my tire revs/mile is 682. I take 682*3.42 (rear gear ratio)/3200 = 0.7288875. The closest pin input ratio is 0.731446. The pins (using the 76543211234567 nomenclature listed on the TBIchips website and 1 = pin connected and 0 = not connected) would translate to 1100010//1011100. The table lists the second half pin connections. Rearrange the appropriate pins or use the DIP switches and ta-da, back on track. Unfortunately, there is a wide gap between input ratios in this region of the table, but it is still way more accurate than before!! The best way to test your new setting is to verify speed using one of those GPS nav systems (TomTom, Garmin, etc.).
PCM correction: You will need to change the constant for RPM/VSS ratio. The formula for that is ((Tire Rev's/mile) x Rear end Gear Ratio)/60. This calculates speed accurately for the transmission-related tables. Again, for my truck = 682*3.42/60 = 38.874. Change the constant for this, burn the chip. You’re done.
I'm using TunerProRT and datalogging. How do I tune my VE tables?
Oh man. You're in luck because I’m creating an average BLM/VE table calculator similar to pfk’s AFR calculator…but you’re going to have to wait while I test it. pfk’s thread is here:
http://www.fullsizechevy.com/forums/...t-anymore.html. His files and instructions are listed at post #91. The catch is it uses some of the new functions of Excel 2007, but it does a great job. For the few times I’ve used it with my setup, it corrected things fast after a couple of cycles. This will provide the same functions as the histogram table used in Datamaster. I’ve also added color-coded 3D surface graphs to compare VE tables easily and a function to give you an idea of how many data points are used for each cell population. Smooth as you go in Excel or in TunerPro and perform another datalog. There is also one for the near-idle table as well to finetune that table. It won’t be as accurate as using a WBO2 sensor, but it is another tool for tuning. Definitely better than trying to eyeball it. Stay tuned…pun intended!!