Rabbid ROM – Boost Control and AFR

* What the problem is with the factory ECU
* What results from the problems
* What Rabbid ROM solves
* How Rabbid ROM solves it
* Proof, and how to check if you’ve been conned into a sensor that won’t work.

The Bosch ME7 ECU is a long standing beast in the automotive world used by many brands of cars and it’s what powers the Astra H VXR, Corsa D VXR etc and similar engined cars.

It’s design in late 90’s early 2000’s never really expected road cars to run more than about 1 bar of boost if that. So when the designers made it, a boost control range of 1.5 bar boost (or from now on 2.5 bar absolute pressure) seemed like plenty.

First of all, why is there a limit in the first place? Well this down to computer memory, and scaling vs resolution. There are two types of pressure values in the factory 16 bit values, and 8 bit values.

In basic computer terms an 8 bit value can count up to 0 to 255 and a 16 bit value can count from 0 up to 65535. You can scale these numbers to change the range, but scaling up will lower the resolutions or larger steps between numbers and scaling down will increase the resolution or smaller steps between numbers.

Great 65535 range so that’s 65535mBar that’s like 64 bar boost right? Expect no unfortunately not, to get more precision these ranges are scaled down by factors to allow you to have smaller steps between values, so rather than count in 1’s you could factor it down by 0.1, so you could count in steps of 0.1, but then your maximum value would be 6553.5.

Here is where the problems begin, all pressure calculation in the ME7 when in 8 bit are factored by 10, that means with 0-255 it can count 0 to 2550 mBar of pressure but in steps of 10mbar. This is used by low priority tasks such as displaying to diagnostics, hence in Tech 2 / Opcom we see boost in KPA

The 16 bit values are unfortunately scaled by a factor of 0.0390625. That means that while we can measure boost in fractions of 0.04mBar (which is way OTT) we can only count to 2560mBar with 0 to 65535.

This means target boost, actual boost, estimated intake manifold pressure, etc all cannot count above this. This means once you cross this line, the car does not know the boost is there. It cannot control it in closed loop anymore.

Secondly, the fuelling while it does come from engine load calculated from the MAF, it also goes through some mathematics which converts it into an estimated intake manifold pressure. This also cannot count above 2560mBar which means you start to lean out once you hit higher boosts.

Tuners for years have been fitting 3 / 3.5 / 4 bar MAP sensors to the cars, makes sense, and it’s logical right? More boost scale in a sensor that fits?

For sure you can properly scale the sensor in so the voltage output from it get’s scaled to the correct pressure reading, but what all of them have failed to understand is the internal ECU mathematics and memory limits, once you cross the 2560mBar line, all values would be scaled down and pegged at 2560mBar.

I say this bluntly – on all mapping by anyone, before Rabbid ROM, fitting an uprated MAP sensor was pointless hence we didn’t bother. If you were told you needed one, you were either conned, or someone didn’t understand the ECU in detail.

In a Closed Loop boost control system, the car has target pressure, a rough starting point for the boost control solenoid then checks to see how close it is and fine-tunes your boost until what it measures is what it wants. This means you can demand different boost levels whatever you want, such as less in low gears etc.

However this only works, if your control system can read and request the levels you want, it also requires some headroom to catch overboosts. This does not happen once you hit or get close to the 2560mBar boost limit.

To work around this, tuners have boost in Open Loop modes, this means you physically limit the boost control solenoid output to roughly the boost you want. However, if it goes higher or lower, you just don’t care nor do anything about it.

The problem with this is if the weather changes, altitude changes or you change to different gears, engine loads such as climbing hills, the actual boost you get will be different, also being unable to max the boost control during spool to scramble the turbo, you generate lag on spool and gear change.

By using Rabbid ROM you can also properly control boost up to 4 bar as if it were a factory car running 1 bar.

As mentioned before fuelling also suffers from this, as MAF readings are converting into an estimated manifold pressure, which also cannot count above 2560mBar. This also means you lean out and therefore corrections are applied to the injectors so that when you hit the limit you squirt some extra fuel in.

But if you think about the problems with boost levels changing you’ll also realise this causes major issues with fuelling if boost changes.

If you underboost you could run overly rich, even splutter or misfire, if you overboost you could lean out. There is a fine balance for the “fuel correction” method to get a car to run good, and while we’ve done a good job in the past, we’ve always said it’s a bit of a bodge to run high power cars.

Rabbid ROM let’s the ECU calculate it’s estimated intake manifold pressure up to 5 Bar absolute pressure (4 Bar Boost) This mean’s that MAF’s as large as you can reliably make will properly fuel your car as if it were a stock car.

So how do we actually solve it in Rabbid ROM?

Well simply it’s a case of scaling, all mathematics in the ECU program code have been reworked with alternative scaling, this allows the 8 Bit values to now count to 5110mBar (511KPA) and the 16 Bit values to now count to 5120mBar, hopefully more than you’ll ever need.

But scaling alone does not work, there are many modules in the software which convert from load, or air flow into pressure, and back again etc, there are comparisons with barometric pressures and ratio calculations which all had to be recoded and reprogrammed in order to make this work.

The attached graph shows a car boosting outside of the normal range with both factory software, the car peaks to 1.8 bar of boost then we’ve dropped down into 1.4 bar to show the sensor maxing and coming back into range.

To do this test we locked the boost control solenoid to 60% duty to ensure boost was the same on all runs. We’ve logged with Rabbid Logger and then we’ve also logged with Opcom to show you how to test your own car.

The yellow line shows boost pressure with Rabbid ROM, the grey line shows the boost pressure with a normal map and you can see it flat lines at around 2560mBar. The ECU is at this point leaning out, and would be unable to stop the boost going out of range if it were in control.

But let’s not just believe me or any tuner, let’s teach you how to prove it yourself.

Opcom is a clone of Tech 2 and commonly used to diagnose cars by enthusiasts. Here we’re showing you how to see if your 3 Bar MAP sensor is actually working or not.

If you start the car and look at boost control it would show approximately 100KPA of absolute air pressure, if this is the case your sensor is correctly scaled in, if you see around 85KPA or 112KPA you’re running the wrong sensor for your remap and your tuner has already failed the first step.

If you then record a run at full boost you’ll see that it will never count higher than 255KPA (1.5 bar boost) even if you have a boost gauge showing more.

When you run Rabbid ROM, Opcom doesn’t know about the scaling that’s happened so all values you record and see on the data screens will be exactly half of the real life values.

When your car is at idle you should see 50KPA, once you’re on boost you should then see half of the absolute pressure your car makes, let’s say you run 1.8 bar of boost, that’s 280KPA you should see 140KPA on the screen.

If you do not see halved values like this, your 3 Bar MAP sensor is doing nothing for you, if you were told you needed it, someone was just trying to make money for nothing, or they do not understand your ECU.

If they claim they can do the same as Rabbid ROM, then values should be halved and if they’re not, you’ve been lied to again.😉

To this day, we’re the only Opel/Vauxhall tuner to have achieved such modifications to the factory ECU, so if you’re looking for a big power car, there is only one logical choice for mapping – Rabbid ROM.