Aerodynamics

Numerous people have told me that the reasons why GT6's get so hot under bonnet is down to poor air flow and that what I need is bonnet vents, while this may be true I don't want to hack holes is my bonnet without understanding what is going on.

I have fell into the trap of believing the more air I get into the engine bay the better things will be, I am now starting to doubt this. It originally suited me to have maximum clearance at the front of my bonnet as it allowed me to get my radiator in without fouling the bonnet. The down side of this is that at high speed the bonnet can be seen to lift and starts to move around - this suggests to me that I have a high pressure area under bonnet which in turn leads to poor airflow through the engine bay. After reading several articles I have decided to try and understand what is going on aerodynamically. I plan to take numerous pressure measurements around the front of the vehicle / engine bay / under chassis in a bid to come up with some potential changes.


My aim is to have a high pressure in front of radiator, slightly lower in engine bay area and identify even lower pressure areas under the vehicle so that I can use these areas to draw the air from the engine bay, I will also try and determine if bonnet vents will actually improve the situation.


Over the weekend I have moved my radiator back and pulled the bonnet down at the front as far as adjustment would allow, after a quick run this morning this has stopped the bonnet lifting at speed with no adverse effects to the cooling performance, if anything it actually ran a little cooler than normal.


Next thing is to start taking pressure measurements, I have also made an undertray that I intend to try.

If you have any idea's, comments or theories please let me know.........

Cold Air Intake

One of the down sides of fitting a larger radiator is that it restrict where you can get a good source of cold air for the intake. I had made do with a KN on the end of my plenum, only problem is that when idling and driving slowly the air intake temperatures climb quite rapidly, the air temperature sensor fitted in the plenum lets the ECU know whats going on which in turn backs off the ignition timing to prevent detonation, this can have a major effect on performance.

Following numerous cardboard templates I have come up with something that seems to work quite well, I have fabricated an air box with filter that sits below the radiator.



This is fed cold air via a scoop that fits in front of the radiator



The air exits the air box via a 60mm outlet fitted to the rear and connected to the plenum via trunking.

I don't expect a performance increase from any forced air effect but it should help overcome some of the restrictions of the filter and trunking. Having driven the car with the old and new intakes there is no noticeable difference in performance when travelling at speed.

Idle and low speed intake temperature's have drastically reduced, the next job is to move the air intake sensor into the air box as it does pick up a fair bit of heat soak in its current position - Its currently fitted on the underside of the plenum.

Cooling Mods

Although the cooling system worked ok prior to my rebuild, I have tried to improve the set up to try and ensure I don't end up with trapped air in the system. I have basically added pipework from the header tank to the downstream side of the EWP. Originally this pipe went back into the head.

I have also moved the bleed from the top of the radiator to the thermostat housing.

This hose is fitted with a 4mm restrictor, the idea being that as the pump draws coolant, from the radiator and header tank, it creates a lower pressure in the header tank, this in turn should draw any air in the thermostat housing back into the header tank.

Made up a couple of pipes that fix to the rocker cover studs to keep it all looking tidy. Unfortunately not very easy to see in this picture.

The heater feed pipe (originally the heater return pipe into the back of the original water pump) is up stream of the EWP, with the heater valve closed all coolant flow is directed through the engine, with the valve open, some coolant flow will go through the heater and back to the header tank. All seems to work ok, just experimenting with water levels and pressure caps. Started with a 20 Psi cap but have now got this down to 13 Psi.