In the previous blog post we talked about the tuning possibilities on the market for the 987 platform and concluded that if your target BHP is above a certain level and you want the best BHP per £ spent you will be looking at forced induction. In simple terms, forced induction refers to air being forced into your engine at pressure, more air means you can burn more fuel thus make a bigger bang and more power. There are two common ways of achieving this, Supercharging and Turbocharging.
Supercharging - This is where a mechanically driven compressor (air pump) is fitted, typically driven by a belt run from the crank of the engine. There are a few different types of supercharger but the principle is the same for all. The nature of a supercharger being driven from the crank means that there is a link between engine speed and supercharger speed which means the way that the boost builds is quite linearly which can make the car drive quite nicely, there is normally less lag (time to build boost) with a supercharger than with a turbocharger. A supercharger requires power to turn it, this is referred to as a parasitic draw. Adjusting the boost pressure has to be done by changing the drive pulley ratio (sizes) to speed up or slow down the supercharger.
Turbocharger - A turbocharger uses energy that is normally wasted down the exhaust pipe to turn a shaft (think of that like a wheel from a water mill), on the other end of that shaft is a fan that pushes cold air into the engine. A turbocharger can make peak boost much lower in the rev range than a supercharger then hold a pressure all the way to the max engine speed, this will result in power coming in very hard at a low RPM. There are advantages and disadvantages of both systems but ask yourself what modern vehicle manufacturers use most and that will give a good idea as to what is the best route in terms of efficiency, cost and reliability.
When deciding which route to take for an aftermarket conversion there are lots of considerations, the primary ones are:
Packaging - which system best fits within the existing vehicle platform?
Complexity - which system would be best for a kit based DIY installation?
Cost - which system is most likely to yield the best performance/cost?
Reliability - which system will give the best reliability/cost?
Performance - which system will give the best performance? (Performance is not just peak power, it is how the vehicle will perform in all aspects of driving)
Based on all of the above we concluded that a turbo would be our route of choice here at MRN and here is some of the reasoning behind that decision:
Packaging - A supercharger ideally needs to be mounted at the crank pulley end of the engine, there is very little space in the mid engine 987 to allow for this.
Complexity - With a turbo conversion there is exhaust removal/replacement and intake mods. The nature of the exhaust design in the 987 means that the original (or aftermarket manifolds (headers)) can be retained and the majority of the components fit where the OEM exhaust was.
Cost - There is not a vast difference between the two types
Reliability - Again, not a huge difference between the two but the belt does get a hard life on a supercharger and replacing this is not particularly easy on a mid engine car
Performance - we see how OEMs have utilised turbocharging to great affect and we can draw upon those advantages. With modern components and a well designed system we can almost irradiate lag and allow flexibility with varying boost pressures using a turbo system.
A final point of note which comes under the packaging, complexity and performance headings is that our conversion will be a twin turbo arrangement. The reasons for this are as follows:
Component sizing - the nature of this conversion being a large CC low boost application means that sizing a turbo and wastegate is problematic. There is not an "off the shelf" single turbo that will do the job sufficiently. If we look at the engine as two halves you will get a 1.7ltr engine and (half of) the target BHP is 225BHP, we are spoiled for choice in that region for turbos and most of those have appropriately sized internal wastegates. Also, a single turbo will take longer to spool due to the inertia of the larger rotational components resulting in more lag.
System optimisation for efficiency - With single turbo setups we are yet to see one with equal length exhausts, this can lead to an imbalance between the cylinder banks which is not ideal for efficiency.
In the next blog post we will be looking at out kit layout in further detail so feel free to subscribe for notification.