Vehicle speed effect in IAT

[kmh001]

Mate I hope that didn't sound condescending, I'm just putting everything in for completeness so it can be understood.

I wish I could find a diagram to explain it. Now I have to draw a really bad one myself...



The air flowing over the structure (represented by the blue line) has to take a few short cuts where there are flow impediments. These short cuts leave stagnant area's (like the yellow area) where there is little or no flow so the static pressure is higher.

[2XS]

So what does that mean?

Good for air pickup point?

Bad for hot air removal?

[kmh001]

Probably yes to both questions. But I'll do some tuft testing like you did and find out. I know of some US owners who have removed the plastic at the rear of the bonnet and encountered visibility problems during rain. So we'll see.

[2XS]

I was considering cutting a vent into the bonnet behind the radiator (and maybe one further back above the exhaust manifold) to allow hot air to escape. Do you think this would work well, or just a little bit at low speed? (using a non-MPS bonnet of course)

Do you know much about legality? I've believe that exposed holes are illegal in bonnets, other than factory fitted (eg. Evo and new MPS etc.)

Similar to this type of style:

[mpsgarage]

You will have to use a MPS bonnet as a 3 will not fit.

[ted]

Ah, I forgot about that bit Mal. I had problems with windscreen fogging up on the MX5 so for certain events (ie Targa Tas) it would go back down at the back. But in a 1600cc car, there were not that many times that we were up in the 190-200 range, however whenever we were the front end did get a bit floaty. Its also interesting to see std car wipers trying to work at stupid high speeds, they go up awfully quick but come back awfully painfully! That would suggest the high pressure area. However, if designed correctly (and I think the base of the MPS windscreen is actually hidden slightly) the airflow over the windscreen creating a high pressure are should also create a low pressure area just under or behind the airflow at the base so you could try pulling heat out there. And then work out how to deal with foggy windscreens in the cold! Or just get a shrouded air filter (I tried to stick a BMC CDA in mine but couldn't make it fit).

On the subject of bonnet vents, a top vent will work if you know that the air will go upwards. As long as the air is coming through the nose and under bumper only, it should be OK so people with FMIC and running them in the order of oil cooler, intercooler, radiator with a bit of shrouding at the back of that channeling up to a rearwards pointing cutout would probably find it working well (ala WRCars). However, for those who still use the top mount draw in setup, I think it will try and exit out the back where the turbo/dump pipe etc are.

Another thing to see how it goes (and will apply more to FMIC shod cars), I was told once that the std radiator thermo fan is effective upto roughly 140km/h. Anything above that, the airflow pressure gets a bit too high but the volume of cooler air should keep things in check. But keep sticking things in front of it, and prolong'd high speed may see overheating etc occur.

Now I know why I like keeping my cars relatively stock now!

[rd415]

Mate I hope that didn't sound condescending, I'm just putting everything in for completeness so it can be understood.

I wish I could find a diagram to explain it. Now I have to draw a really bad one myself...



The air flowing over the structure (represented by the blue line) has to take a few short cuts where there are flow impediments. These short cuts leave stagnant area's (like the yellow area) where there is little or no flow so the static pressure is higher.

Agree to a point but doesn't the curved windsreen to the side edges cause that stagnant air to be dispersed.

Pity there isn't a wind tunnel to test all the theories. I know when Mark Skaife was dominant his car was sent to the UK for wind tunnel testing at Tom Walkinshaws Arrows F1 wind tunnel.

Last year the Kelly boys used Woomera to high speed test for drag coefficient etc and got fined for an illegal test day.

Without a proper wind tunnel there are too many variables to be accurate - one can only try some mods but even then there is no controlled environment.

[2XS]

You will have to use a MPS bonnet as a 3 will not fit.

Why won't a std 3 bonnet fit?

[shakespeare]

Why won't a std 3 bonnet fit?

The standard 3 and 6 front guards are quite different from the MPS guards (raised due to bonnet being raised) therefore I'm assuming the bonnet is also different.

[mpsgarage]

1; different guards
2; different bumper
3; different lights
4; there's about eight other different parts as well

[bd581]

Can't see the guards being taller, wider yes, but not taller, the only reason i see why a standard 3 bonnet wont fit it the TMIC, hence the bonnet bump. If you have a FMIC, you shouldn't have a problem.

I used to have a Nissan Pulsar GTi-R, it was a common thing to fit a standard bonnet when you fitted a FMIC.

On that topic, the Pulsar had 2 small rearward facing vents either side of the bonnet, they are sealed with a blanking plate from factory, but was a 5 minute job to remove the plates.
The amount of heat that came out of them was amazing. Your hands would hurt if you held them there long enough.
But you could feel no real difference to the cars performance, heat soak of the TMIC was a problem, you could really feel it, but that was only slow driving, at normal cruising speed it was fine. Oil temps also went down as long as you were moving (above 60km/h) and would raise quickly at slow speed and idle (conferming my belief that turbo timers are a waste of money)

[2XS]

Can't see the guards being taller, wider yes, but not taller, the only reason i see why a standard 3 bonnet wont fit it the TMIC, hence the bonnet bump. If you have a FMIC, you shouldn't have a problem.

Thats what I thought. I have FMIC. I don't care about the bumper.

The lines of the bonnet follow the gaurds. I really don't think the gaurds would be taller.

[shakespeare]

Not sure of the actual height differences on the 3, but on the 6 they are quite substantial, this was something I put together for someone who was looking at an MPS 6 bonnet for a std 6.

[2XS]

[QUOTE=shakespeare;70830]Not sure of the actual height differences on the 3, but on the 6 they are quite substantial, this was something I put together for someone who was looking at an MPS 6 bonnet for a std 6.

I can sort of see the height difference (in the 6 yes, maybe)

The bulge in the MPS 3 bonnet isn't as aggressive as that, is it?

[TD]

Buy a wrecker bonnet to try? Or bring tape measure tomorrow to CT and measure up Adam's or Jay's bonnet to check?

[bd581]

The bulge in the MPS 3 bonnet isn't as aggressive as that, is it?

The 3's bulge is nowhere near the size of the 6's

[shakespeare]

Buy a wrecker bonnet to try? Or bring tape measure tomorrow to CT and measure up Adam's or Jay's bonnet to check?

Sedans are different from the hatch though! At our last cruise we had a SP23 and MPS3 side by side and the top of the guard on the MPS appears higher like the MPS6, the bulge is not as big on the MPS3 but the top of the bonnet/guard in the low pressure area is higher! Intriguing, you're right Dan we need to measure this.

[TD]

Interesting...sounds like experiment time!

Enjoying the thread too, some good intelligent discussion in here.

[kmh001]

This morning I had a look at the air flow characteristics around the car. We took videos of the air flow at 50, 80, 100 kmh and some other speeds. There was virtually no difference in the aerodynamic behaviour or flow direction at any speed, except that as speed increased the low pressure area got lower and the high pressure area got higher, as expected. I would also say that if you are engineering an aerodynamic change for a racing car, you could expect the aerodynamic situation to change significantly over 200 kmh.

The results are exactly as expected. This photo was taken at 110 kmh.



There is very stable low pressure air flowing over the bonnet.

The large step at the rear of the bonnet appears to enhance the high pressure zone at the base of the windscreen. The tufts circled red disappeared into the groove and stayed there the entire time with no apparent turbulence.

The tuft circled green (and a couple of others in that area that can't be seen) curled up and barely moved, suggesting there was very little flow in that area and quite high static pressure.

The tuft circled purple was turbulent, but showed a consistent flow toward the front of the car. I think there was an eddy in the centre of the car curving under itself and then splitting to go left and right.

The bottom row on the windscreen were highly turbulent and showed distinct sideways flow.

The second row up showed normal flow direction but were moderately turbulent.

The third row up (not seen in the pic) showed normal flow direction and were slightly turbulent.

The blue circled tufts show a stable sideways flow which is probably the high pressure zone pouring out into the slip stream.

There's no doubt the high pressure zone is adding to the total aerodynamic drag situation. I'd love to think of a way to harness all that air and feed it to the intake or intercooler.

More to come.

[mpsgarage]

Who said anything about been talker.
The guards are different cuts; E.G. around the light, bottom of door.
If you want to put on a 3 bonnet you will need
1;bonnet
2;guards
3;bumper
4;head light brackets
5;inner guards
6;bumper brackets
7;splash pan

Do I really need to get a complete parts list.
Yes it will fit, but not my its self.