Corsa VXR - Grenade?
By John Thorne
The World of car tuning can sometimes be one where facts are warped and twisted in order to suit a particular view point, typically either by an owner who is convinced their modifications are the best or by a supplier looking to promote a particular product or service. It is simple human nature to defend ones decisions and simple business practice to defend ones products, however when it gets to a situation where the bullshit eclipses simple fact then there has to be a point where someone says stop.
When Thorney Motorsport first aquired our own Corsa VXR for development we went through our usual procedure of development for ECU remapping, running the car in, dyno runs, road runs, track testing etc and following our extensive work with theVX220 race cars we then spent a large amount of time investigating analyzing and testing injectors, so as part of all our development now we measure how injectors are performing at all levels of power and changing them as necessary.
The process by which an engine generates power is relatively simple, it simply combines air with fuel and ignites it via a spark within a confined space in order to move a series of pistons to generate drive via a crank – the art is combing the right amount of fuel with the right amount of air and timing the combustion to generate the maximum amount of expansion with the minimum amount of heat.
Air is sucked into the engine via a series of inlets which vary in design and nature, and this airflow varies massively depending on whether the car is moving or not, for this reason simply running a car on a dyno will not replicate conditions on the road, this is especially important if the dyno cell does not have adequate enough airflow via its fan set up, its not just about keeping the engine cool, its about trying to replicate on the road conditions – impossible; but important to try.
Fuel is controlled by the ECU via a fuel pump, a fuel rail and lastly a set of injectors – one per cylinder. To accurately measure a fueling system you need to understand what pressure the fuel pump is generating, the level pressure the fuel rail is maintaining (as fuel pump output pressure will fluctuate depending on accelerator position) and finally you need to know how far open the injectors are – fully open means no more fuel can be sent through.
To generate more power from an engine we adjust the parameters of fuel, air and spark timing to create a bigger explosion in the cylinder, with a forced induction engine such as the Corsa VXR item, this also means the adjustment of boost to control fuel level, it’s a lot more complicated than that of course but this is keeping things simple.
The issue of correct air/fuel mix is the holy grail of
engine tuning, getting it wrong usually means a systemic failure of some kind and there are two ways to
get it wrong. Firstly, if you don’t have enough air in the mixture the engine
will run 'rich' – too much fuel. This can lead to unburnt fuel
removing valuable lubricant from the engine internals known as bore wash. Typically
we err on the side of too rich for all engine tuning as bore wash only occurs
at very high levels of imbalance and the process of using fuel aids engine
cooling which is safer for all engines.
Secondly, you can have too much air in the mixture. This will cause the engine to run lean – too much air/too little fuel. This is the equivalent of garlic to a vampire – it's bad. If too little fuel is used in combustion then the explosion itself essentially is too hot (again, simplified) as the presence of fuel has a cooling effect. Think of it like an explosion on TV, to get the right effect the director wants a nice combination of explosive effect, flame and smoke, too much fuel in the mix and he’ll get a nice flame but the explosion will be less dramatic, too little fuel in the mix and he’ll get a big explosion but the damage will be more severe.
If an engine is running lean then the internal temperatures become too high and damage is the result, typically this is manifested in piston crown damage where it heats up too much and breaks down.
Thorney Motorsport is unique in being the only Vauxhall tuning company to be an authorized Vauxhall outlet, ie this means that we hear of information through other dealers, GM themselves and the grapevine more than some others and during the course of last year we heard of more and more failures of Corsa VXR engines. The damage reported was consistent – number 4 piston failure leading to varying levels of engine failure. Over the last 6 months we personally have inspected 9 Corsa VXR engines with engine failure and on every single one the failure was on the number 4 piston. On every single failure the car had been remapped by a particular tuning company or via one of this company's hand held ECU remapping devices. There was also another engine failure (again on number 4 piston) we inspected that had been remapped by another tuning company but this company used the same software supplier for their ECU remaps, and this was repaired under its own warranty. Had all these failures been handled by the tuning company themselves then we (or anyone else) would have had no knowledge of the issue, however the advice being given to the owners of these cars was to ‘put the car back to standard and take it to a dealer’ and as such these cars were entering the dealership network with ‘unexplained’ failures – all on number 4 piston – in other words the dealership network (and TMS) were handed an issue to repair, this is not a case of trying to score points on a competitor, if the problems had been repaired by the supplier then it has nothing to do with us, Vauxhall or the dealership network.
Hang on though we can hear you say, why are you so sure that it is the mapping of the cars that is causing the problem? The answers:
1. All the cars inspected had number 4 piston crown failure
2. The damage is consistent with excess heat from the piston running too lean
3. Nine cars were running this particular company’s software and the 10th software from the same supplier
4. There
have been no reported failures in the
5. Our evidence shows that the injectors are exceeding their maximum flow rate which would explain the cause of the damage.
This list generates a couple of questions. Firstly how do we know whose map is on the car? Well in the cases we have actually seen the owner told us, secondly we have software available that can interrogate a cars ECU and even when the map has been ‘removed’ it leaves enough of a trail that we can identify what map was on the car. The second question of course is how do we know the injectors are maxxing out?
As already explained the available fuel to an engine is controlled by the fuel pump, the fuel rail the injector and ultimately the ECU, the ECU controls the parameters of how much power is required via boost level. Now fortunately enough another company has been kind enough to demonstrate that the standard pump and rail is able to generate enough pressure for the standard car and even higher power levels, there is a video on youtube (for now):
This video shows the fuel pressure being maintained on a dyno run. However using the available fuel pressure as a statement that the engine is getting enough fuel into the cylinders whilst relevant is not complete enough, at the end of the day the size of the hole in the injector is constant, it doesn’t matter how much pressure you throw at it it will only be able to flow a maximum amount of fluid and that maximum is determined ultimately by its size, this is calculated by the Bernoulli equation which if you are interested is this:
The available pressure to the injector is only one thing it
needs to flow enough fuel, to actually deliver the fuel needed it needs to be
large enough to flow it, period.
So, we can measure an injector, we can ascertain that there
is enough fuel pressure how do we measure the point at which the injector
maxxes out? An injector is an electronic valve, once energized an electromagnet
moves a plunger that opens a valve that allows fuel to pass through, this
opening and closing of the valve occurs many times per second and this number
of times is controlled by an electronic pulse known as pulse rate, so an
injector is a physical valve opening and closing determined by an electronic
pulse controlled by the cars ECU. The simple way to measure such a device is
with an oscilloscope. An oscilloscope is a device that displays a graph of an
electronic signal, they are used by a myriad of industries and have been for
decades importantly they can measure with utmost accuracy the moving parts of
an electronic circuit – an injector.
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Now watch this video, it shows a Corsa VXR running a stage 1 map (peak power approx 215-220bhp) in the foreground you will see an oscilloscope which shows the electronic output from the injector. At 4900rpm the electronic signal changes, the injector is fully open and cannot inject any more fuel, from this point the cylinders are demanding more fuel but there is no more to flow, this means that the cylinder is progressively running leaner and leaner as more air is sucked into the chamber, increasing heat and leading to damage. All of this is controlled by the ECU map, the higher the power generated the further down the rev range the injectors reach their maximum, standard cars (194bhp) reach their maximum at 5600rpm (approx), 230bhp equates to 4900rpm which is as low as we are prepared to take them, any higher and the rpm level is simply too low to be safe. This explains the advice given from this particular tuner that full power should not be used for more than 5 seconds or that some people have problems earlier than others; if you use full throttle more you are likely to run the engine leaner for longer.
In our opinion running a car where the injectors cannot flow any more fuel is not wise, it is acceptable to have this limit at the upper levels of the cars RPM as at these levels the ability to run the car at maximum RPM for any degree of time is limited but to have this limit any lower than 4900rpm is not something we would advocate, to run the car at power over 240bhp would mean the injectors are maxxing out a good deal lower in the RPM range than 4900rpm. For this reason we have developed larger injector maps for our own upgrades, these allow for 250bhp+ power but injector levels well within their maximum.
We are strong advocates of owners knowing as much as they can about their car and how to modify it, however to go to the extent of writing up in such detail is not something we would anticipate undertaking (I can think of better things to do with my Saturday nights if I’m honest). However we were faced with multiple cars with serious engine failure which was beginning to give the Corsa VXR a poor reputation and that’s not good for any owner. Had the matter been dealt with internally by the supplier then none of this need to have come out but if the stated process for repair was to try to con the dealership network, the customer and anyone else then they only have themselves to blame, not us.
The Corsa VXR is an excellent engine to tune but as with all upgrades its knowing what can be done is paramount.
