Tuning for Boost

[SSTJ]

For those of you who have supercharged or turbocharged your TJ, what aspects of the PCM's programming need to be re-tuned? I know the devil is in the details of how to do the tuning. I'm just looking for a list of what would be tuned.

So other than Injector Pulse Width (to increase fuel delivery, to keep air-fuel-ratio from getting too lean) and other than ignition timing (typically delayed under boost), what else needs to be tuned? I realize that those two alone monitor a number of sensors, but just wondering what else to add to the list.

Updates: (Specific to my 97 with JTEC PCM)

• MAP Sensor:
  • New MAP sensor that can detect boost, of course.
  • And then, recalibrate the voltage of the new MAP sensor.
  • On a Viper JTEC (maybe TJ?), range is 0.45V (high vacuum) to 4.8V (low vacuum).

• Barometric Pressure
  • With key-on and engine off (no vacuum), MAP determines barometric pressure.

• Pulse Width:
  • MAP is the most significant variable for pulse width.
  • With boost, pulse width must be increased, to keep air-fuel ratio from getting too lean.
  • This may require larger injectors or a change to fuel pressure regulation, depending upon boost level.

• Ignition Timing:
  • Typically delayed under boost.
  • Keep in mind that this may need to be adjusted even in pre-boost conditions, just due to the installation of the supercharger or turbocharger having an effect on stock volumetric efficiency.

• Idle Air Control:
  • IAC position is determined by monitoring (among others) the MAP sensor.
  • In the other direction, PCM uses IAC to reach desired MAP during deceleration (to prevent stalling).

• Other:
  • For very high levels of boost, you may need to adjust "ignition coil dwell", to ensure a sufficiently strong spark?

 

[Obi-Wan]

A big thing is if you have a manual or AT. The ECM can and will calculate the input and output speed of the AT (slip) and pull fuel / timing between shifts or under certain loads. Some slip is allowed, but is constantly being monitored by the ECM. I can't confirm if the 32RH or 42RLE do this as my experience with this is with tuning GM transmissions.

 

[Steel City 06]

I seem to recall the first step involves installing a 2 bar MAP sensor. Presumably you’d have to remap the MAP voltage to pressure values to that sensor.

Then you’d have to add rows into any table referencing MAP or PRatio to account for the higher allowable pressures. I don’t think you can actually add more rows than stock, but you can decrease the resolution on the rows to give you space to add the new information.

If you’re tuning an 05-06, the NGC3 has also been used in OEM turbocharged applications. I’d take a look at some Dodge Neon SRT4 tune repository files to see how it was implemented by OEM.

That said, I don’t know if all versions of the NGC3 would operate the wastegate, or if you’d need a standalone controller.

 

[SSTJ]

I seem to recall the first step involves installing a 2 bar MAP sensor. Presumably you’d have to remap the MAP voltage to pressure values to that sensor.

Then you’d have to add rows into any table referencing MAP or PRatio to account for the higher allowable pressures. I don’t think you can actually add more rows than stock, but you can decrease the resolution on the rows to give you space to add the new information.

Yes, agreed on having to re-calibrate the MAP sensor voltage. I learned a little bit about that here.

And yes, then one would have to re-tune any tables that monitor MAP. But other than pulse width or ignition timing mentioned above, what are those that should be on the list?

 

[SSTJ]

A big thing is if you have a manual or AT. The ECM can and will calculate the input and output speed of the AT (slip) and pull fuel / timing between shifts or under certain loads. Some slip is allowed, but is constantly being monitored by the ECM. I can't confirm if the 32RH or 42RLE do this as my experience with this is with tuning GM transmissions.

Thanks. But if I follow you correctly, this just boils down to the fact that 'load' (MAP) plays a part in fuel management. But pulse width (for AFR) and ignition timing are already on the list above. Am I missing something else that should be added?

 

[Steel City 06]

Addition of anything into the intake tube will generally change the VE characteristics somewhat even before reaching boost. So you may have to edit these values slightly. Assuming you’re running in closed loop most of the time, this should be fairly straightforward to log and edit just by watching the fuel trims.

If you decide to tune for higher octane, this will affect the timing tables below boost as well. Higher octane fuel is more resistant to detonation in part because it is more resistant to ignition. So a small amount of advance might be needed in some cases below the boost threshold. If you choose not to run an aftercooler (I highly recommend one), you will need to aggressively pull timing in boost.

For turbocharging, you can also tune some of the fueling characteristics to decrease turbo lag. For example, getting a portion of unburnt or partially burnt fuel into the exhaust can keep the turbocharger spinning faster during shifts because it will ignite in the exhaust and “pop”, increasing the exhaust pressure as a result. Personally I wouldn’t do this since it shortens the life of the components and burns extra fuel, but a lot of ricer tuners do this for faster throttle response.

For high levels of boost, you’ll probably want to run open loop so that you can enrich the mixture a bit to help prevent detonation. It might be possible to run stoich/closed loop in low levels of boost if you have your timing, aftercooling, and octane figured out.

You’ll also probably need bigger injectors and to make the changes for those accordingly.

 

[SSTJ]

Addition of anything into the intake tube will generally change the VE characteristics somewhat even before reaching boost. So you may have to edit these values slightly. Assuming you’re running in closed loop most of the time, this should be fairly straightforward to log and edit just by watching the fuel trims.

That's a good point. But again, in terms of 'what' to tune, what would be impacted by the non-boost changes to volumetric efficiency? Anything other than pulse rate (for air-fuel ratio) and timing?

If you decide to tune for higher octane, this will affect the timing tables below boost as well. Higher octane fuel is more resistant to detonation in part because it is more resistant to ignition. So a small amount of advance might be needed in some cases below the boost threshold. If you choose not to run an aftercooler (I highly recommend one), you will need to aggressively pull timing in boost.

Great point. So again, timing is on the list, but with this additional factor.

For turbocharging, you can also tune some of the fueling characteristics to decrease turbo lag. For example, getting a portion of unburnt or partially burnt fuel into the exhaust can keep the turbocharger spinning faster during shifts because it will ignite in the exhaust and “pop”, increasing the exhaust pressure as a result. Personally I wouldn’t do this since it shortens the life of the components and burns extra fuel, but a lot of ricer tuners do this for faster throttle response.

Interesting!

For high levels of boost, you’ll probably want to run open loop so that you can enrich the mixture a bit to help prevent detonation. It might be possible to run stoich/closed loop in low levels of boost if you have your timing, aftercooling, and octane figured out.

You’ll also probably need bigger injectors and to make the changes for those accordingly.

Noted. Thanks again.

 

[SSTJ]

I am eager to hear from @Jezza if he's willing.

 

[SSTJ]

I've updated the original post with what I've learned so far.

 

[Jezza]

I am eager to hear from @Jezza if he's willing.

Lots to unpack here. It really depends on what you are trying to do along with what PCM and operating system you are using. NA is reasonably easy on the TJ as there is not much that can or needs to be changed. Fuel and ignition timing being the big ones. With boost it's a bit more complicated especially with the NGC.

 

[SSTJ]

Lots to unpack here. It really depends on what you are trying to do along with what PCM and operating system you are using. NA is reasonably easy on the TJ as there is not much that can or needs to be changed. Fuel and ignition timing being the big ones. With boost it's a bit more complicated especially with the NGC.

Thanks. I'm mainly interested in the older JTECs. And my TJ is a 97 2.5 with AX-5.

For this thread I don't intend to understand all the "how to DIY". Just want to get a clear list of what else would need to be addressed if moderate boost were added to an otherwise stock engine. What I know so far is in the bottom of the original post.

 

[SkylinesSuck]

I think you are not going to be able to get your answer from some posts in this forum unfortunately. It's a very complicated topic that takes time and a practice to learn. I know just enough from helping my buddy tune big HP turbo cars to know I don't know near enough. I can fumble my way through a few things though.

I suggest going in to the forum for whatever platform you are using (HP tuners, right?) and starting in with the newb FAQs and tutorials. Not all of it will apply to our specific platform, but it will still give you an idea of how things work in general and enough of a base to tinker with your N/A tune. Once you get a handle on tweaking N/A stuff, you can start looking in to what you'd need to do to rescale maps for a sensor that can see boost and get it running well n/a on that sensor. By then you will likely have a good idea of at least how things work and the theory behind it.

 

[SSTJ]

I think you are not going to be able to get your answer from some posts in this forum unfortunately. It's a very complicated topic that takes time and a practice to learn. I know just enough from helping my buddy tune big HP turbo cars to know I don't know near enough. I can fumble my way through a few things though.

I suggest going in to the forum for whatever platform you are using (HP tuners, right?) and starting in with the newb FAQs and tutorials. Not all of it will apply to our specific platform, but it will still give you an idea of how things work in general and enough of a base to tinker with your N/A tune. Once you get a handle on tweaking N/A stuff, you can start looking in to what you'd need to do to rescale maps for a sensor that can see boost and get it running well n/a on that sensor. By then you will likely have a good idea of at least how things work and the theory behind it.

Thanks, that's good advice.

And for the purposes of this thread (more 'what' than 'how'), I've made some updates to the original post with some info from your JTEC PDF.

 

[SkylinesSuck]

I like the idea of connecting JTEC specific info. You'll need that at some point after you get a good handle of how the particular tuning platform you are using works and are familiar with it.