Posted 03 July 2007 - 11:03 AM
Had a brief chat with Alex_Os at Pro Alloy and he came up with the following regarding charge coolers (in between eating pies)......
Wow….. some interesting comments!
Some may recall that the last time I posted on here about charge coolers it all got a little bit waspy. However, I will happily post some more info, not for any commercial gain but to maybe answer some of the questions relating to the reasoning behind the various aspects of any good charge cooler system design.
As mentioned earlier, generally a conventional air to air intercooler could be considered as a more efficient method of cooling charge air than a water cooled intercooler (charge cooler). However, for any cooler, be it a water radiator or intercooler to operate efficiently it requires good air flow. This is where most mid or rear engine cars suffer.
We have looked at, and worked on numerous different options for cooling solutions for various mid engine cars and have found with both the VXT and various turbo charged Elise conversions that its very difficult to design a suitably sized air to air intercooler which will see enough air flow to cool anything producing much more than 250bhp without major bodywork modifications. For this reason a charge cooler would be required.
One of the key disadvantages of a charge cooler is heat-soak and there inability to dissipate heat once hot, this is due to there relatively high thermal mass. In order to combat this, the charge cooler system would require a reasonably large water volume, a good sized pre-rad and an effective way of circulating the coolant around the circuit. The smaller the water volume the faster the system will become saturated. A good example of a system which has been designed correctly would be an air charge temperature reading which increases as expected whilst the car is driven full throttle on maximum boost along a straight, then, during braking and slowing for a corner, cornering on partial throttle and then exiting the corner the charge air temps should reduce back down to a consistent level, then when back on full throttle along the next straight you should see peak temps not exceeding those from the previous full throttle run. Obviously, in a race environment the first couple of laps will see the ACT’s gradually increase until the system reaches its stabilised temperature. However, the key as to whether the system is performing adequately is what temperature the system becomes stabilised.
PRE-RAD:
One very important factor which directly affects the stabilised temperature, is the pre-rad.
The airflow into the mouth of the VX is not only critical for pre-rad effectiveness but also for the water radiator efficiency. This was highlighted by Vauxhall discovering that the original position for the front number plate was very slightly too high and recommended that moving the number plate down would have a big effect on air flow to the radiator.
Although installing the pre-rad in the mouth of the VX means its right at the front of the front, we felt that there simply wasn’t sufficient space to install a pre-rad of adequate dimensions, so in order to produce the optimum solution the front clam would need to be removed. With any cooling solution the surface area exposed to airflow is the key factor. A small cooler which is thick in section will generally be quite inefficient as the back part of the cooler will see negligible airflow and will be warm as it’s rejected from the front part of the cooler.
Installing anything in front of the water radiator could potentially affect the water temps. However, over the years we have found the best way to combat this is to design a package which mounts both units closely together ensuring that the air is forced from one unit to the other. The air flow will always take the path of least resistance, so if a unit is mounted in free space the air will simply flow around the cooler making it quite inefficient. On applications such as the VX this can also dramatically disturb the air flow to the water radiator by causing the air to spin and turbulate before it turns within the crash structure to flow into the water radiator. We have seen this on some un-ducted front mounted intercooler installations, where the intercooler has been mounted with a large space between it and the water radiator, this results in both units working inefficiently. By fitting the pre-rad to the main radiator all the airflow entering the front of the car has to go through both units. In order to do this the original water radiator fan has to be removed but is replaced by two fans fixed to the pre-rad. These aren’t required to help with the charge cooler temperatures, they are simply there to operate in the same manor as the original fan set-up and operate from the original water temperature switch (although they can be switched independently also).
CHARGE COOLER:
Although charge air temperatures are a key factor when tuning any turbo charged engine, there are certain other important factors to consider.
A charge cooling package which is capable of generating very low ACT’s may not necessarily provide the best tuning solution. Another important design criteria is the ability of the system to flow the charge air. If a system becomes restrictive to charge air flow the turbo will be required to work harder to flow the same levels of charge air, this in turn generates more heat, which is what you are trying to reduce.
The standard intercooler is of marginal effectiveness. On my STANDARD vx, during ROAD testing we recorded peak inlet temperatures of 59°c with 20°c ambient temperatures. For this reason we totally remove the intercooler. JT is certainly correct in suggesting that the intercooler will help to reduce the charge air temps; however we felt that there wasn’t enough of a gain to warrant the potential restriction to air flow that this would add. The charge cooler unit has been designed to ensure that the air can flow smoothly with the inlet and outlet tanks shaped to help the charge air pass through with as little turbulence as possible. These considerations will help to ensure the car feels responsive and not laggy and holds boost at higher rpm.
Another very important consideration is to ensure that which ever kit you opt to buy, that the air temperature sensor is mounted after the charge cooler. Although this would appear obvious, we have seen some kits which utilise the original intercooler with a charge cooler installed on the outlet side, but still leave the air temperature sensor in the intercooler. The result of this is that the ECU will read the charge air temperatures at the outlet of the intercooler before it has been cooled by the charge cooler, this results in the ECU backing the power off as its not sensing the true charge air temperatures and believes that the temps are actually far higher.
There are many cooling options available for the VXT, all of which have some positive and some negative issues. However, our kit has been specifically designed for cars running at least stage 4 and has been well proven on our own 296bhp car. We believe this has been designed with as little compromise in performance as possible ensuring that the kit performs properly in all aspects.
The install on the front is also where we differ, the VX has a horizontally mounted front rad with the front having a deflector plate designed to direct air though and up through the rad, this is the reason we mounted the front prerad so high with a thick but narrow core, it gets perfect direct airflow but doesn't interupt the airflow to the main rad at all. If you mount a prerad directly under or over the main rad you are placing it directly in the main rad airflow which has to compromise the ability of both rads to do their job. This is probably the reason why you need to use fans, fans should only be used where you need to suck air through a rad where airflow is compromised, the very design suggests this occures.
