Motronic Turbo DIY Street Tuning
As I’m sure there are plenty of ways to dial in a Motronic turbo set up, I will be documenting the adjustments I made in order to get the car where I wanted it to be. My goal for this was simply to get Air fuel ratios in a safe place and preventing the motor from running lean under boost. There are definitely other ways to do this, using different parts, different tuning capabilities, with different goals but considering I couldn’t find a collected write up with pictures, I figured I’d start one. If you have some input that is productive to the Motronic tuning case feel free to chip in (“go MS” is not productive). I do recommend going to a dyno to tune, but this is a basic street tune.
My PSI: 10 psi.
Vehicle: 1986 325es
Motor: 1989 m20b25
Info Pertaining to Tune:
SSSquid m20 turbo chip
93 octane tune
30# Bosch injectors
M30 “027” AFM
BEGI 2025 Rising Rate Fuel Pressure Regulator
.60/.63 T3 Turbo Internal Wastegate (8psi)
2.5” downpipe with stock o2 sensor and wide band o2 sensor bungs
AEM Wide Band o2 sensor and gauge
3.0 Bar Fuel Pressure Regulator
255lph Fuel pump
Two Fuel pressure gauges (Jegs 100psi liquid filled)
Tools Needed:
Flat head screw driver
A 3/16 allen key
A 5mm allen key (AFM)
A 9/16 wrench
Knife/razor
I don’t want to go too into the build itself as that has been covered plenty of times but I figured I’d just include the aspects of the tune. I am assuming that the build is completed and that everything has been installed correctly and is in good working order, you’re using the same fuel management units as me (BEGI 2025), you have two (100psi) fuel pressure gauges (one located on the fuel line going into the fuel rail, and one located on the line between the stock Fuel pressure Regulator and the BEGI unit, and finally that the motor can start up.
20160325_133959
I highly recommend you locate the AFM in front of the turbo, called draw through, and not after the turbo (on charge piping, called blow through). I cannot attest whether these steps are the same for a blow through set up.
9755
You are going to want a fuel pressure gauge on the line going into your fuel rail to make sure your fuel pressure is around 43psi
9761
and a second gauge on the fuel line coming from the stock fuel pressure regulator and going to the “IN” port of the BEGI Rising Rate Fuel Pressure Regulator. (picture shown later)
This is to see what pressure you are setting the BEGI to. The BEGI will restrict fuel from leaving the Fuel rail, in turn raising pressure, by monitoring vacuum and also by the manual changes you make to both its top mounted screw, and needle valve screw.
http://www.bellengineering.net/templ...structions.pdf is the link to the BEGI 2025 Instruction PDF. You should set the BEGI top screw to about half way its length as a good reference to start.
1st: Remove o2 sensor's input
Unplug the stock o2 sensor, as we want the engine to run without adjustments from the computer via o2 sensor.
2nd: Accessing and assessing AFM
Use the knife to remove the top cover of your M30 AFM, I removed mine just enough that the cover opens like a door and doesn’t come completely off. This will give you access to the teeth, inner spring, and carbon track on the inside of the AFM. At this point check to make sure your carbon track is not worn down. I recommend making a mark with a silver marker as a reference to where your AFM teeth are stock, this will help you remember how far you’ve moved the teeth and in what direction and also to reset it if you want to start over.
9756
3rd: Checking Stock Fuel Pressure Before Adjusting
Start the motor and watch the wide band o2 sensor gauge to see where the motor is idling at. We are aiming for as close to 14.7 as we can get. Check the fuel pressure going into the fuel rail to make sure that it is at 43psi (may be a little less if it’s a worn FPR) If the fuel pressure is good then we’re ready to adjust the AFM
4th: Making Adjustments to the AFM
With a direction in mind (to try and bring the A/F ratio down is to make it Richer, and to bring it up is to make it more Lean) Take your flat head screw driver and place it between the teeth in the AFM and this small metal part like this
9760 
9759
Use the screwdriver in a twisting motion to rotate the tooth wheel either clockwise or counter clockwise in order to tighten the spring and make A/F richer, or vice versa. Adjust by one tooth at a time and look at the A/F Ratio. Make adjustments until the ratio is as close to our goal as possible (14.7) making sure that with each rotation of the tooth wheel, the metal tabs fully engage in the teeth to prevent it from spinning on its own.
5th: Making small Adjustments to AFM (Maybe)
I have read of making smaller adjustments to the AFM through the bypass screw but have had no actual luck with seeing any changes from it. Each tooth on the AFM wheel will be a decent size jump in the A/F ratio. If you feel you need to make finer adjustments try the bypass screw located here:
9754
The bypass screw is a 5mm allen key.
6th: Allow o2 Sensor to make adjustments
With the A/F ratio dialed in to 14.7 at idle you can plug the stock o2 sensor back in and let the sensor warm up. The computer may make small corrections richer or leaner but it should stay around the 14.7 mark.
7th: Setting BEGI Fuel Pressure at Atmosphere
Now that we have A/F adjusted for idle and non-boosted driving, we need to adjust the BEGI to for when we get into positive pressure. This is done while the car is at idle. Put the 3/16 allen key into the top screw and loosen the locking nut holding the screw in place with a 9/16 wrench.
9757
The fuel pressure gauge between the BEGI and stock FPR will be lower than 43psi but that’s okay as this gauge will be reading the fuel pressure leaving the fuel rail and being restricted by the BEGI, at this point (vacuum) the engine is running on just the 3 bar FPR.
9763
you need to remove the vacuum line going into the BEGI and plug the line with your finger. The fuel pressure gauge should have increased and held that increase.
9758
9764
Removing the vacuum line and exposing the BEGI’s vacuum port to atmosphere the BEGI is seeing 0psi or the point in which your engines vacuum pressure turns to positive pressure or boost. This is also the pressure we are going to use as our measurement while adjusting, also be mindful of your injector's capabilities and don't turn the pressure up so high that it may ruin your injectors.
Now this is a matter of comfort, I decided that boosting in the mid 11 A/F ratio is a safe area and therefore my goal. If you feel you want to run leaner or richer than this then this is where you do it. With the vacuum line off the BEGI and plugged, either tighten (clockwise = richer) or loosen (counter clockwise = leaner) the top set screw and watch the fuel pressure gauge.
As you tighten it the pressure should increase, at the absolute least I would set this pressure to 45psi, so that it essentially picks up where the stock FPR lets off. Though I have mine set to 62psi and think realistically 50 psi would be a safer starting point than 45psi.
8th: Setting BEGI fuel pressure vs Boost pressure Rate of Gain
The next step is to adjust the needle valve on the side of the BEGI. In all honestly I set mine to its tightest setting and have left it there. This according to the BEGI PDF is to control the rate of gain under boost.
9th: Test Drive Car with NO Boost
Let the car warm up and make sure the A/F ratio is good, and the fuel pressures are spot on before driving. Then drive the car. Before hitting boost you should not be running lean or rich as the car should be driving on just the stock FPR (granted it will go lean for a second when you shift, and if you are in gear rolling and not using the throttle, don't worry)
Though it’s not 100% necessary, it is helpful to drive the car with the hood off and the gauges lifted up in the engine bay so you can see the fuel pressure while driving. I did this with a massive Fuel tester gauge I had in a kit, but setting the fuel pressure before-hand will most likely be spot on and not cause any trouble, just be mindful of your gauges.
10th: Test Drive car WITH Boost
When you feel everything is running right off of boost, it’s time to find a safe straight road to test it on boost. Be mindful of the road and your gauges. As you start building pressure you should see the A/F Ratio getting lower (richer) and settle at a point throughout building boost. If this isn't the case and you notice you're getting leaner under boost you will need to make adjustments under the hood to the BEGI, either with the needle valve or the top mount screw.
just remember that according to BEGI: The top mount screw is adjusting fuel pressure at the atmospheric pressure point (when your boost gauge reads 0) and the needle valve adjusts the rate of gain of fuel pressure vs boost pressure.
Always make sure you are not running too rich or too lean while under boost and to tighten the locking bolt on the top BEGI screw to prevent it from moving around. After initial lower speed adjustments are made it’s a good idea to also go on the highway and boost at higher gears to make sure that you are not leaning out while in extended periods of boost (as opposed to the few short seconds during lower gears) especially in higher RPMs.
Conclusion:
Hopefully everything is running smooth and this has helped someone out who’s looking to street tune and boost on analog. As it’s made clear by the limited, laughably simple instructions listed, tuning software like Mega Squirt is empirically better in order to tune and manage your engine. That being said, please don’t liter this page with non Motronic tuning recommendations, there are PLENTY of MS threads already.
Also as I mentioned at the beginning of this, these steps are how I was able to adjust my turbo m20 and is mostly based on trial and error and small bits of information I’ve found around. If you have some first-hand knowledge and experience in Motronic “tuning” (use tuning loosely since this is mostly just manual adjustments) that you feel will be helpful to the thread as a whole, or corrections to anything I’ve said, please post it below and I’ll make changes.
Hope this was helpful, thanks for reading.
As I’m sure there are plenty of ways to dial in a Motronic turbo set up, I will be documenting the adjustments I made in order to get the car where I wanted it to be. My goal for this was simply to get Air fuel ratios in a safe place and preventing the motor from running lean under boost. There are definitely other ways to do this, using different parts, different tuning capabilities, with different goals but considering I couldn’t find a collected write up with pictures, I figured I’d start one. If you have some input that is productive to the Motronic tuning case feel free to chip in (“go MS” is not productive). I do recommend going to a dyno to tune, but this is a basic street tune.
My PSI: 10 psi.
Vehicle: 1986 325es
Motor: 1989 m20b25
Info Pertaining to Tune:
SSSquid m20 turbo chip
93 octane tune
30# Bosch injectors
M30 “027” AFM
BEGI 2025 Rising Rate Fuel Pressure Regulator
.60/.63 T3 Turbo Internal Wastegate (8psi)
2.5” downpipe with stock o2 sensor and wide band o2 sensor bungs
AEM Wide Band o2 sensor and gauge
3.0 Bar Fuel Pressure Regulator
255lph Fuel pump
Two Fuel pressure gauges (Jegs 100psi liquid filled)
Tools Needed:
Flat head screw driver
A 3/16 allen key
A 5mm allen key (AFM)
A 9/16 wrench
Knife/razor
I don’t want to go too into the build itself as that has been covered plenty of times but I figured I’d just include the aspects of the tune. I am assuming that the build is completed and that everything has been installed correctly and is in good working order, you’re using the same fuel management units as me (BEGI 2025), you have two (100psi) fuel pressure gauges (one located on the fuel line going into the fuel rail, and one located on the line between the stock Fuel pressure Regulator and the BEGI unit, and finally that the motor can start up.
20160325_133959I highly recommend you locate the AFM in front of the turbo, called draw through, and not after the turbo (on charge piping, called blow through). I cannot attest whether these steps are the same for a blow through set up.
9755 You are going to want a fuel pressure gauge on the line going into your fuel rail to make sure your fuel pressure is around 43psi
9761and a second gauge on the fuel line coming from the stock fuel pressure regulator and going to the “IN” port of the BEGI Rising Rate Fuel Pressure Regulator. (picture shown later)
This is to see what pressure you are setting the BEGI to. The BEGI will restrict fuel from leaving the Fuel rail, in turn raising pressure, by monitoring vacuum and also by the manual changes you make to both its top mounted screw, and needle valve screw.
http://www.bellengineering.net/templ...structions.pdf is the link to the BEGI 2025 Instruction PDF. You should set the BEGI top screw to about half way its length as a good reference to start.
1st: Remove o2 sensor's input
Unplug the stock o2 sensor, as we want the engine to run without adjustments from the computer via o2 sensor.
2nd: Accessing and assessing AFM
Use the knife to remove the top cover of your M30 AFM, I removed mine just enough that the cover opens like a door and doesn’t come completely off. This will give you access to the teeth, inner spring, and carbon track on the inside of the AFM. At this point check to make sure your carbon track is not worn down. I recommend making a mark with a silver marker as a reference to where your AFM teeth are stock, this will help you remember how far you’ve moved the teeth and in what direction and also to reset it if you want to start over.
97563rd: Checking Stock Fuel Pressure Before Adjusting
Start the motor and watch the wide band o2 sensor gauge to see where the motor is idling at. We are aiming for as close to 14.7 as we can get. Check the fuel pressure going into the fuel rail to make sure that it is at 43psi (may be a little less if it’s a worn FPR) If the fuel pressure is good then we’re ready to adjust the AFM
4th: Making Adjustments to the AFM
With a direction in mind (to try and bring the A/F ratio down is to make it Richer, and to bring it up is to make it more Lean) Take your flat head screw driver and place it between the teeth in the AFM and this small metal part like this
9760 9759Use the screwdriver in a twisting motion to rotate the tooth wheel either clockwise or counter clockwise in order to tighten the spring and make A/F richer, or vice versa. Adjust by one tooth at a time and look at the A/F Ratio. Make adjustments until the ratio is as close to our goal as possible (14.7) making sure that with each rotation of the tooth wheel, the metal tabs fully engage in the teeth to prevent it from spinning on its own.
5th: Making small Adjustments to AFM (Maybe)
I have read of making smaller adjustments to the AFM through the bypass screw but have had no actual luck with seeing any changes from it. Each tooth on the AFM wheel will be a decent size jump in the A/F ratio. If you feel you need to make finer adjustments try the bypass screw located here:
9754 The bypass screw is a 5mm allen key.
6th: Allow o2 Sensor to make adjustments
With the A/F ratio dialed in to 14.7 at idle you can plug the stock o2 sensor back in and let the sensor warm up. The computer may make small corrections richer or leaner but it should stay around the 14.7 mark.
7th: Setting BEGI Fuel Pressure at Atmosphere
Now that we have A/F adjusted for idle and non-boosted driving, we need to adjust the BEGI to for when we get into positive pressure. This is done while the car is at idle. Put the 3/16 allen key into the top screw and loosen the locking nut holding the screw in place with a 9/16 wrench.
9757The fuel pressure gauge between the BEGI and stock FPR will be lower than 43psi but that’s okay as this gauge will be reading the fuel pressure leaving the fuel rail and being restricted by the BEGI, at this point (vacuum) the engine is running on just the 3 bar FPR.
9763you need to remove the vacuum line going into the BEGI and plug the line with your finger. The fuel pressure gauge should have increased and held that increase.
9758 9764Removing the vacuum line and exposing the BEGI’s vacuum port to atmosphere the BEGI is seeing 0psi or the point in which your engines vacuum pressure turns to positive pressure or boost. This is also the pressure we are going to use as our measurement while adjusting, also be mindful of your injector's capabilities and don't turn the pressure up so high that it may ruin your injectors.
Now this is a matter of comfort, I decided that boosting in the mid 11 A/F ratio is a safe area and therefore my goal. If you feel you want to run leaner or richer than this then this is where you do it. With the vacuum line off the BEGI and plugged, either tighten (clockwise = richer) or loosen (counter clockwise = leaner) the top set screw and watch the fuel pressure gauge.
As you tighten it the pressure should increase, at the absolute least I would set this pressure to 45psi, so that it essentially picks up where the stock FPR lets off. Though I have mine set to 62psi and think realistically 50 psi would be a safer starting point than 45psi.
8th: Setting BEGI fuel pressure vs Boost pressure Rate of Gain
The next step is to adjust the needle valve on the side of the BEGI. In all honestly I set mine to its tightest setting and have left it there. This according to the BEGI PDF is to control the rate of gain under boost.
9th: Test Drive Car with NO Boost
Let the car warm up and make sure the A/F ratio is good, and the fuel pressures are spot on before driving. Then drive the car. Before hitting boost you should not be running lean or rich as the car should be driving on just the stock FPR (granted it will go lean for a second when you shift, and if you are in gear rolling and not using the throttle, don't worry)
Though it’s not 100% necessary, it is helpful to drive the car with the hood off and the gauges lifted up in the engine bay so you can see the fuel pressure while driving. I did this with a massive Fuel tester gauge I had in a kit, but setting the fuel pressure before-hand will most likely be spot on and not cause any trouble, just be mindful of your gauges.
10th: Test Drive car WITH Boost
When you feel everything is running right off of boost, it’s time to find a safe straight road to test it on boost. Be mindful of the road and your gauges. As you start building pressure you should see the A/F Ratio getting lower (richer) and settle at a point throughout building boost. If this isn't the case and you notice you're getting leaner under boost you will need to make adjustments under the hood to the BEGI, either with the needle valve or the top mount screw.
just remember that according to BEGI: The top mount screw is adjusting fuel pressure at the atmospheric pressure point (when your boost gauge reads 0) and the needle valve adjusts the rate of gain of fuel pressure vs boost pressure.
Always make sure you are not running too rich or too lean while under boost and to tighten the locking bolt on the top BEGI screw to prevent it from moving around. After initial lower speed adjustments are made it’s a good idea to also go on the highway and boost at higher gears to make sure that you are not leaning out while in extended periods of boost (as opposed to the few short seconds during lower gears) especially in higher RPMs.
Conclusion:
Hopefully everything is running smooth and this has helped someone out who’s looking to street tune and boost on analog. As it’s made clear by the limited, laughably simple instructions listed, tuning software like Mega Squirt is empirically better in order to tune and manage your engine. That being said, please don’t liter this page with non Motronic tuning recommendations, there are PLENTY of MS threads already.
Also as I mentioned at the beginning of this, these steps are how I was able to adjust my turbo m20 and is mostly based on trial and error and small bits of information I’ve found around. If you have some first-hand knowledge and experience in Motronic “tuning” (use tuning loosely since this is mostly just manual adjustments) that you feel will be helpful to the thread as a whole, or corrections to anything I’ve said, please post it below and I’ll make changes.
Hope this was helpful, thanks for reading.
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