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We can offer a lot of options from a simple re-map, to larger airflow meter, different plenum/induction, or even larger engine (4.6, 4.8, 5.0 or even 5.3 Litre). These upgrades don’t have to be all applied in one go, which sometimes helps with the budget.
The 4.0 Litre Discovery actually had a smaller Mass Air Flow (MAF) meter fitted than the later 4.6 version, or indeed the 4.0 and 4.6 P38 Range Rovers. In my humble opinion, it was a pretty blatant attempt to limit the power output of the Discovery when compared to the 4.0 Litre P38 Range Rover. Coupled with the lean fuelling and retarded ignition mapping, it was a very effective method of hobbling the Discovery to suit marketing needs.
The Bosch ECM can be reprogrammed to suit a larger engine, but there is another very worthwhile modification you may wish to consider at the same time. You can add the larger Mass Air Flow (MAF) meter from the P38 Range Rover, which was also used on the 4.6 versions of the Disco II in North American Specification (NAS).
Adding this is a good idea, which should add around 10 BHP. Fortunately this is very easy to accommodate when reprogramming the Motronic system, but will not work properly otherwise. You do need to replace the Air Filter box lid with one from a P38 Range Rover to accommodate the larger MAF, but apparently the trunk from the MAF to the throttle can be stretched on with the help of a little heat.
Using a larger MAF will cause the system to see less air flow than the engine is actually consuming, so it will never add enough fuel. On top of this, there will probably be too much ignition advance at high loads for the same reason. This is why it is a great time to incorporate this modification, as all these things can be accounted for in one go
Motronic actually performs a very sophisticated measurement of the exact quantity of air the engine is consuming, which is the basis of all the fuelling and ignition calculations. In order to do this it needs to know the exact characteristics of the MAF meter, which are hard-programmed into the software.
Whilst the Motronic system will run a 4.6 engine even with a 4.0 Litre tune and standard MAF, you certainly won’t be getting the best out of it. As your motor has Oxygen sensors in the exhaust, the fuelling runs in closed-loop which gives it the ability to trim the fuelling to suit the engine. Of course it will only be using the preset factory targets for this function. The ignition has knock detection to prevent detonation, which will also trim back any dangerous levels of ignition advance.
A few years ago when nobody could do anything with these systems, the stark choice with a capacity upgrade was to drop it in the hole and see what happened. As you can see from the last paragraph, Motronic will have a good shot at making the best of it. Indeed there are hundreds of these vehicles running around with 4.6 engines operating on 4.0 Litre tunes.
The fact that they work has lead to the general opinion that the conversion works, which is evidently true. However I will explain why you can get a very big improvement by reprogramming the ECM to suit the new larger engine.
As a bit of background, there are over 150 versions of the software for the Disco and it is important that you have the correct one. Earlier systems such as GEMS had software that would cope with pretty well any configuration of the vehicle, with just a few software switches. Although there are four types of Motronic ECM, the software is unique to each possible configuration of the vehicle.
There are some solutions for this one, although none of them are ideal. Sending the original ECM puts you off the road for a day or two at least, even if using a courier such as UPS. Anyway - that is all the negative stuff. There are considerable benefits to be had if you can take the hassle.
The ECU mapping will need revision to accommodate the larger capacity, and here are the reasons why you would get a bad result from a 4.6 engine on a 4.0 map. The engine's fuel and ignition requirements are determined by engine speed and load. Engine load is determined by measuring the true mass of air that the engine is consuming at any particular speed, using the "Hot Film" Mass Air Flow (MAF) meter.
Mass airflow is proportional to the torque that the engine is producing (for this type of engine), and hence it is closely related to the engine capacity. This is also heavily influenced by atmospheric conditions such as barometric pressure, temperature, etc.
Therefore it is vital that the software is programmed with the correct maximum mass air consumption of the engine, known as airflow meter scaling. This is vital for good driveability and fuel economy especially on large capacity engines, and allows correct control of part throttle fuelling. Note that airflow meter scaling determines when the top of the fuel map is reached. When using an engine of larger capacity than the software was designed for, the top of the map will be reached too soon. This leads to over-fuelling at part throttle, and under-fuelling at full throttle.
A re-profiled Fuel Map which provides good fuel economy on part throttle, and allows engine to rev more freely (standard profile reduces fuelling heavily after 4000RPM). This produces quicker, more progressive throttle response and better mid range torque.
Although the ECU can work around large errors in the fuelling where Oxygen sensors are present where the map is significantly incorrect, when it reaches the limits of adjustment it will upset the entire fuelling learning process.
When re-calibrating the Motronic ECM for a larger capacity engine, there are more than 38 maps and variables that need re-scaling for both engine load and speed. This level of detail is one of the reasons why the Tornado upgrades are not cheap, although we do believe that the quality is unmatched.
The following information refers to either upgrading a 4.0 or 4.6 vehicle in the same capacity, but the gains are much larger when going from a 4.0 to 4.6. If you want the technical details of why this is so then I would be very happy to provide them.
Please note that the programming can be biased towards economy or performance or somewhere in between, according to your preference. The performance version doesn't really offer any significant improvement in economy over the standard tune, but it gives the car a very seductive mid-range punch that you can't help using! Personally I use the happy medium version, which I am very satisfied with.
The Bosch engine is mapped in such a way that it produces maximum power when it hits the Rev Limiter, which is set at 5300 RPM. At this point the mixture is rather rich, but the ignition timing is fine for good power production. Therefore the peak power will not improve more than 2-4 BHP with any upgrade, unless the Rev Limiter is moved up to say 6000 RPM. Although I can do this and the power is still increasing when it hits the Rev Limiter, it would be something of a “smoke and mirrors” or Snake Oil improvement. Power increases here are largely academic, since the vehicle will change gear long before this limit is reached.
However the dominant engine characteristic is actually a huge hole in the torque curve, centred around 2500 RPM. This is filled in very well with the upgrade, delivering an extra 10-15 lb/ft of torque and much faster throttle response. Economy improvement is 10-15%.
When driving a standard vehicle it actually pulls away from a standing start quite well initially, but loses interest once it gets to 3000 RPM. Personally I find this trait especially annoying, and it doesn’t even sound like a V8 either! All the action happens on the first or last quarter of the throttle pedal movement, and the bit in the middle doesn’t make a lot of difference. It is also a real nuisance to my clients who tow, since it often means the gearbox will kick down two gears when they only wanted a bit of extra effort (on a gradient for example).
The real pain with the upgrade is that I have to have the ECU here at Tornado HQ for therapy, which of course means that you would be without use of your vehicle for a few days at best. This is necessary because each individual ECU has a unique identity, which is combined with the vehicle identity to produce the software image.
Once we have extracted the software from your ECU, we can clone it in future if you should ever lose or damage your original. However, if it gets lost on the way up then it would be necessary to have a new one installed and programmed using TestBook or RovaCom – expensive hassle you don’t need!