prj tuning

He who stops being better stops being good.

LPG – liquid petroleum gas.

Most of us have probably heard about it, let’s see how production cars are converted to it, and what is usually done wrong.

I will talk about the 4th generation LPG system, where extra holes are made in the intake manifold and an injector is added for each cylinder.
Technically the system is pretty simple…

How it works

From a mechanical standpoint we have a high pressure tank, from which the liquid gas goes through an evaporator, that takes it to it’s vaporous form. From the evaporator it is distributed to the injectors via a common rail.
After the injectors are nozzles, which are sized to a certain size, according to calculations. So the same injectors can be used for many different applications and the injected quantity is adjusted by sizing the nozzles.
The evaporator also serves as a pressure regulator for the fuel, and is usually connected to a vacuum line in the inlet manifold.

Electronically most LPG ECU‘s are fairly simple. The cars petrol injectors are re-wired through the LPG unit, and the LPG unit just gets the reading from the main ECU in regards to the injector pulse. After it gets the start of the injector pulse it can correct it to be longer or shorter. It can also switch the gasoline injectors on or off.
The LPG ECU tends to have a gas pressure and temperature sensor and also allows for correction based on LPG temperature, as the density of a vapor fuel is affected quite a bit by temperature.

The way these systems are normally installed, is that they are set up to turn on only after a certain engine temperature is reached (the car only starts on petrol), and then it is calibrated with a narrowband sensor so, that the mixture is roughly stoichiometric in the cruise area. The ignition timing is not touched in most installs, and the car feels down on power. The nozzles are also rarely big enough for the car to work correctly in the entire range.

How it should be done in an ideal world

The best way is to use the additional LPG ECU to only provide correction for fuel temperature and switching settings. The engine management should be done by the already existing ECU by facilitating “map switching” based on whether LPG is used or not. This means that a signal should be taken and fed into the ECU.

A real world example

The car I will talk about is an old Audi V8, with the PT 3.6 liter engine, 2nd generation Motronic management with dual distributors and a 4th generation “Tornado” LPG system.

Map switching

The first thing was to find out how to do map switching in this ECU. This turned out fairly simple.
The ECU is using a SAB80C535 processor, and all it’s data is stored in a DIP28 27C256 chip. Let’s take a look at the pinout, and compare it to a similar chip:


The chips are nearly identical, the only difference is the 1st pin. In a 27C512 it facilitates an extra address line. The 27C256 is a 2^15 = 32kb or 256kbit memory chip and the 27c512 due to an extra address line is a 2^16 = 64kb or 512kbit memory chip.
So here, we can do the map switching purely in hardware. If we ground pin 1, the first half of the chip is read, and if we pull it high (+5V), the second half is read. This can be used to have two complete sets of software inside the ECU and dynamically switch between them without any problems.

The easiest is to take the signal from the LPG evaporator, as we can get a 12V feed from there when it’s on. Inside the ECU we build a simple schematic using a relay to switch the A15 pin:


The last step is to remove the constant +5V connection from Pin 1, luckily in this ECU this was done using a 0 ohm resistor, so no cutting of traces was needed, and finally use a 27C512 chip instead of the 27C256, which can fit two versions of the software.


The Motronic used in this car is pretty simple. It is basically run like two identical four cylinder engines with the same crankshaft.
It is also similar to other M2.X motronics, such as the one in the 2.2T Audi engines, the Opel Calibra, BMW, etc…
Everything is pretty much routine:

  1. Dump all the maps from the rom into an XDF definition.
  2. Find the axis descriptors by looking at the maps (RPM, IAT, ECT, Voltage, LOAD, MAF…)
  3. Dump the maps again, but with the descriptors and conversions in place. Conversions are a little hard on this ECU, because it does not have measuring blocks to disassemble, so you just need to disassemble the measuring blocks on a more modern Motronic, such as one on the Audi V8 4.2 engine or a 2.2T – it’s all the same.
  4. Insert some custom logging routines into the ECU, so you have some sort of logging to go by – copy paste job from a 3B ECU for me.
  5. Find out how the maps are used, and which ones. This ECU has separate part throttle and WOT maps.


The car felt really sluggish on LPG, and we found out why – it was running very lean, 1.15 lambda on LPG. Plus the LPG ECU automatically switched to gasoline if the RPM went over 2800.
Once we turned off the limit, we realized that switching at 2800 RPM was because the LPG ran out at around 3000 rpm.

We needed to sort that out, so we first re-drilled the nozzles to 2.5mm, which meant that the LPG still ran out at around 4500 RPM.
Finally we went for 2.7mm and increase the LPG pressure from 1 bar over manifold pressure to 1.2 bar. At this point we had enough fuel to go to redline, which is 6300 rpm on this motor.

Tuning a vapor fuel such as LPG is very different from tuning petrol. Because the evaporation happens in the evaporator, there is absolutely no point to make the car run rich, as additional fuel does NOT cool the combustion. Best power for LPG is around 0.9 lambda, and going any richer than that is mostly useless.
So on a WOT pull you ideally want it roughly following that.
When speaking of ignition timing – at low loads and low RPM’s, it needs up to 30-50% advance than petrol, and up top it requires less advance. Knock detection gear is recommended when tuning, but it’s really hard to make a naturally aspirated engine with a 10.6:1 compression ratio knock on LPG. Due to the intricacies of LPG, the final timing curve looks pretty interesting, compared to gasoline, with up to 12 degrees more advance in the same areas (gasoline was also returned for 95 RON):



In the cruise area most of the timing map was re-tuned, as well as lambda brought to 1.

Other things that need to be adjusted – LPG needs different warmup settings, the injectors have different characteristics, the dead time is completely different, etc.
All that has to be re-mapped inside the ECU for the car to run well.


  • Going from petrol to LPG and back while driving can not be felt at all. There is no power drop, it is completely unnoticeable.
  • The car can now crank and start perfectly on LPG. This is still true up to a certain temperature limit, but it starts and runs fine at 20C coolant, and probably lower.
  • The performance is now very similar between 95 RON and LPG, the fuel consumption on LPG has noticeably decreased as well.


  1. Greg Smith Said,

    Hi Prj,

    What LPG system was used on the vehicle?

  2. Dmitri Said,

    It was called Tornado.

  3. Sean Said,

    What are the fuel consumption figures now ‘v’s before the conversion, i.e. Petrol ‘v’s LPG?

  4. Dmitri Said,

    I recall a 1-2L drop in LPG consumption per 100km.
    However, this is not too relevant, because the owner decided to swap the 3.6 liter engine for a 4.2L.

    So I will re-tune this car soon.

  5. Tsvetomir Said,

    And what about the 2.2 turbo version of the S6? What are the specifics there?Do you custom maps or use the LPG ECU for tuning?

  6. Dmitri Said,

    I do dual maps in the ECU for this. If there is adequate supply of LPG then the cars are faster on LPG than on petrol as well due to the high octane rating of LPG.

  7. Ceco Said,

    So after tuning on LPG it need 30-50% more advance up to 3000 rpm and after that you decresa the advance on petrol. Right? Can you do that on a Audi C4 2.8 AAH?

  8. Yuriy Said,

    How can I get this???
    I have Audi v8 3.6 with installed lpg and I need advance timing correction for. Can I buy a programmed flash?

Add A Comment