Corsa VXR Injectors

There is a lot of information and mis-information on the internet regarding Corsa VXR injectors. We thought we would give an insight into the situation and some  facts. When mapping Corsa VXR, we can add injectors to Corsa VXR when mapping at any stage.

The Corsa VXR injector is a 12 point multi hole injector. If these injectors are in good flowing condition the engine is capable of running at Stage 3 which for us is power normally around the mid 230’s. Bosch’s flow rate for the injector also suggest this with its cc’s per minute rating. However there is a proviso……

As the cars/engines have got older this multi hole injector can start to block up, right down to the point where by when mapping at even Stage 1 we have seen injectors not capable of the supplying enough fuel, hence the injectors would read correctly as maxed out.

As most know we do not just flash maps on the car, we map all the cars on the dyno and optimise timing, fuelling & boost etc to the car. We use Can 2000 the GM data logger and programming tool. With this you see the injectors running not as a percentage but in real time terms i.e.  read in milliseconds.  Milliseconds are the time it takes for the injector to close, charge and open to fuel again. When the injector is maxed it runs out of time to carry out this operation so has to close before enough fuel is delivered. This is referred to as an injector running out of injection duty cycle or more commonly being “maxed out”. When the injector max’s the value locks as it has not got enough time to  deliver the fuel required before it needs to close to charge for the next opening. So if it was maxing at 5000 rpm it would show 24 milliseconds from 5000 rpm through to the limiter. If it was doing this at 5500 rpm it would show 21.81 milliseconds right through to the limiter and so on as a rev related sliding scale. If we see this on a car then the injectors are removed, cleaned or replaced with bigger injectors after all they are listed on the web site and we do sell them so why would we not use them?……

We have on a couple of occasions when customers’ funds would not allow, put the cars back to standard or reduced the rev limiter to a threshold that is within the duty cycle, until the customer can deal with the issue by means of an injector clean/change.

As regards to fuel pressure changes, the industry standard for injector measure is taken at 3 Bar. From this you can see what the injector will approximately flow for given power. So if you take an injector that is 395cc at 3 bar and fit it to a Corsa which runs a standard OE pressure of 3.3 bar this injector has a true flow rate in the Corsa of 415cc. In the ECU there is a factor that is changed to recalculate what injector size the car has got and also the static fuel pressure, this then re calculates the base fuelling curve value to the new injector size and pressure before you fine tune it in with fuel trims and target air fuel ratio etc.

Now if all the cars we mapped were race cars we would fit the biggest injectors we could as emissions are really never an issue with race cars even if they do run token cats, nor is part throttle fuelling, town driveability and fuel economy. These are however important for the bulk of our road driving customers.  Therefore if someone is close to the top end of an injector range let’s say an injector flowing at 415cc at 3.3  then we can change the fuel pressure to 4.5 bar, re correct the base map and now the injector will flow 485cc for more range and margin.  The alternative is an expensive injector change to the next size up which in the case of the Corsa would be 650cc at 3 bar or 682cc at the Corsa’s 3.3 bar. Nothing wrong in doing that, but it could be a bit over kill. I will explain; a smaller injector working at a high pressure verses a large injector working at low pressure will always yield better atomisation therefore better part throttle/transient fuelling which means in simple terms better economy. VW’s TFSi engine for example.

A perfect case of this was a 350 bhp Astra VXR running a correctly calculated 550cc injector returning on cruise 28mpg, however when fitted with a Astra VXR injector running at 4 bar corrected in the map the car’s economy went up to 34 mpg; in simple turns the bigger injector does not atomise the fuel as efficiently at slow duty cycle/engine speeds.

I hope this just gives in insight into our approach as we do data log,  but it’s not just a sweeping case of the injectors not  being capable at  any given stage.  We simply take each car as we find it given it’s condition and mileage etc and go from there.

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