Hier zitten de winterbandjes ook al onder de DD. De Porsche blijft op zomersloffen staan. Als het echt te gek wordt rijd ik wel een weekje niet.

Andy Stodart, oud werknemer van Jaguar:


I can understand people's potential scepticism just reading the title of this thread. Could there really be a cheap and simple modification that improves both performance and fuel economy? If it was that simple why didn't Jaguar do it?
Well the reasons are quite complex, but I will try to explain them.

When I worked at Jaguar between 1988 and 1995 it was std policy to map the ignition timing 3 degrees retarded from borderline knock on a mid-limit compression ratio engine, at all speeds and load where the engine was knock limited. The ignition timing to achieve borderline knock was determined by running the engine steady state on a dynamometer at each speed and load until the combustion chamber temperatures were fully stabilised. This is not a condition that tends to happen in normal driving in the UK where it is difficult to operate the engine at wide open throttle for more than a few seconds at a time, during which time the temperatures due not reach their high stabilised values.

Consequently the lower temperatures that occur during normal driving mean that the mapped ignition timing is significantly retarded from the level at which knock would occur.

A couple of years ago I designed a revised crank sensor bracket to take advantage of this situation. It was initially designed for the AJ16 engine and moved the position of the crank sensor with respect to the engine. As a result the signal to the engine ECU which should indicate when each piston is at TDC is generated 5degrees early.

Consequently each ignition event generated by the ECU occurs 5degrees earlier than the mapped value. It's a bit like advancing a distributor in the days when they contained contact breaker (unlike the one on an AJ6 engine). The extra ignition advance increases the performance at all speeds and load where the engine was deemed to be knock limited when tested on the dynamometer. I initially implemented my bracket on the AJ16 engine as the engine management system on this engine also has a knock sensing system, which I helped to develop.

This meant that in extreme circumstances should the extra 5 degrees cause any knock, then the system would simply retard the ignition timing out of knock. This bracket proved very successful and I have now sold over 120 of them.

About a year ago I was asked to produce a couple of brackets for some AJ6 engines which were fitted to race cars running on super unleaded fuel. At the customer's request, the bracket was designed with an extra 7 degrees of ignition advance. The two engines fitted with these brackets went on to complete a season's racing and the cars they powered were quite successful in their class in terms of performance. This was quite a severe test of an AJ6 engine to run at this level of ignition advance.

I was recently asked by another Jaguar owner to design a bracket for AJ6 engines. Having produced a batch of these brackets which provides an extra 5 degrees of advance I thought I would offer these to other owners as well.

It takes under an hour to replace the std bracket with one of my brackets using nothing more than std tools. In fact, if like me you have had a lot of pratice, you can do it in 15mins on an AJ16SC engine which is the most complicated installation due to the neeed to remove the SC drive belt tensioner.





The AJ16 engine was fitted to all 6 cylinder X300s and 6 cylinder XJS models from 95.25MY onwards. I worked at Jaguar during the early 1990s on the development of the Lucas/Sagem GEMS 6 engine management system (EMS) fitted to these engines. My experience of this system has enabled me to develop a simple, low cost, modification to these engines which improves both performance and economy.

During the process of developing any engine it is necessary to determine what value of ignition timing to use at every speed and load at which the engine would operate. The values of ignition timing are stored inside the EMS software in a table or “map”. The axes of this table are engine speed and engine load. The later is expressed as the air flow entering the engine every two revs, as calculated based on the signal from the air meter. The process of determining the value to put in these tables is called mapping. It requires the engine to operate at each combination of speed and load for many minutes at a time. The table consists of 16 columns for speed and 16 columns for load, giving 256 test points in all.

In normal driving, owners will never be able to maintain a constant engine speed at wide open throttle, because the car would accelerate to either the rev limiter or the maximum allowable vehicle speed. However, these kinds of conditions are achieved during the engine development process by connecting the engine to a power absorbing electrical generator called a dynamometer. It is important to be able to maintain these steady conditions for engine development purposes, but it is not always representative of real world driving. When on the dynamometer, the engine combustion chambers and piston temperatures stabilise at higher levels that they would achieve during an acceleration in the car. At high loads, the ignition timing values are set sufficiently retarded so as not to cause knock even on engines built with the highest possible production compression ratio. The higher temperatures achieved during development testing result in the setting of lower levels of ignition advance than are possible, when the engine is operating in the car. At high loads, being able to increase the ignition timing will increase engine torque across the speed range and at lighter loads it will improve fuel economy. Many after-market companies take advantage of the difference between the mapped ignition timing in the EMS and what is suitable in normal road driving, to offer the service to “chip” the engine ECU. However, Jaguars have always been sold in small volumes compared with other brands and so have not really attracted the attention of this part of the auto industry.

One of the new features of the GEMS 6 EMS was knock control. Two accelerometers are mounted on the intake side of the block where they can detect the vibrations that are generated if knock/detonation/pinking occurs in the combustion chamber. Using the signals from the crank sensor and the camshaft position sensor (so named because it rotates at cam speed, not because of where it is mounted), the EMS can determine which cylinder is knocking. It then retards the ignition timing of that cylinder until knock stops. It then re-advances the ignition timing back to the normal value. If knock is detected again, then the cycle of retarding and re-advancing starts again. The presence of the knock control system means that the ignition timing can be advanced beyond the values in the map, without the risk of causing damage to the engine through knock. I personally developed this system and know that it is extremely effective at detecting and preventing knock.

I have produced a new design of crank sensor mounting bracket which moves the position of the signal generated by the sensor with respect to the cylinder TDC events. This is the equivalent of turning the distributor on an old engine which had contact breakers. My design of bracket effectively advances the ignition timing of the engine by 5 degrees at all engine operating conditions. This improves both performance and economy. Below is a table of some in-gear acceleration times that I have measured before and after fitting the bracket to a manual XJR6.

Fuel economy of the engine also improves when the ignition timing is advanced. The level of improvement will depend on how the car is used, but in my experience of over 3,500miles of driving since fitting the bracket, I have measured a 1-2mpg benefit.

Replacing the std bracket with the new design is very straightforward and can be undetaken by anyone with a normal tool kit in under a couple of hours.

This revised bracket has proved quite poular with owners of Jaguars fitted with all versions of the AJ16 engine. The bracket has been fitted to 3.2L, 4.0L and supercharged 4.0L versions of the engine with both manual and automatic transmissions in x300 and XJS. All the owners who have contacted me since fitting the bracket have reported improvements in performane and fuel economy.

It is not necessary to change from running the engine on regular unleaded fuel, but there may be some additional performance benefit from using super unleaded, if the engine has a top limit compression ratio or has been modified in other respects to increase performance.

Just as an aside, there is no performance benefit in using super unleaded fuel in a std AJ16 engine, which is not fitted with this bracket or other modifications intended to increase performance.


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Ouwe baardmans is hier net langsgeweest. Economie is weer goed gestimuleerd
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