Aerodynamic tests and modifications on the MR-2 Spyder

[Autobahnschleicher]

When I did some maintainance on my car, I noticed a point where drag could be reduced:
The cooling system.
My car doesn't have a ducted radiator, wich implies I have more cooling drag than I need to.
As it is mid engined, I have the space in the front to add some ducting and the front bumper does have a nice shape to implement a grille block or intake duct.
Idealy I could take some inspirations from the Porsche 911 (996) and use the cooling air as air curtains around the front wheels.
But I will have to look a bit more closely at how I can route the airflow from the radiator.

I've taken a look at the CDA in the wiki:
Even though my cars CD is not that good at 0.31 (softtop), the CDA of 5.68 (soft top) is not that bad.
A gen 1 Insight has a CDA 5.00, to match it I only need to go from a CD 0.31 to down to 0.273.
I haven't done any measurements yet, but the hardtop alone certainly makes a difference and so does my VG mod.
With a flat undertray, ducted radiator and a fastback, I think I could beat an Insights CDA.
After all, I only need a 12% drag reduction to do so and that's based on a softtop.

[AeroMcAeroFace]

Only concern with flat floor/undertray/diffuser on mid engined and rear engined cars is the gearbox/differential temperatures and exhaust heat. There may be enough cooling already through the rear vents but it is something to think about.

[Autobahnschleicher]

Originally Posted by AeroMcAeroFace

Only concern with flat floor/undertray/diffuser on mid engined and rear engined cars is the gearbox/differential temperatures and exhaust heat. There may be enough cooling already through the rear vents but it is something to think about.

The transaxle shouldn't produce much heat, it's a 5-speed manual running good synthetic oil and the engine only has 100kW.
The exaust is mostly behind a heatshield and airflow in there is from front to back via the sidescoops.
Besides, there are plastic panels under there anyway, but they aren't at all aerodynamic.
They act more lile a parachute, wich is why I didn't mount them again after doing some engine work.

[AeroMcAeroFace]

It wasn't that the transaxle produces heat, more that it could be heated by the exhaust. But if there is high airflow through the engine bay anyway, you should be okay.

[Autobahnschleicher]

Just for any non-EU Spyder drivers reading this:
DON'T reduce the airflow around the oilpan.
The 1ZZ runs its oil notoriously hot, especialy in the Spyder.
This causes serious issues and is part of the reason why they don't last.
Only EU models have an oilcooler.
If yours doesn't have one, I seriously suggest you get an aftermarket one.

[Autobahnschleicher]

When looking from the front I'm questioning how much drag reduction a full grille block would yield.
The air intake on this car is quite massive, I don't need that much cooling and with over 10L of coolant, I likely don't need any cooling at all on many trips.
These side-vents next to the radiator don't seem to do anything, so taping them shut with duct-tape would't have any downsides.
But with such a blunt grille block I'm not quite sure if the stagnation in front of the car might even get worse

Well, only one way to find out...

[aerohead]

In 1986, FIAT ( SAE Paper 860212 ) did a wind tunnel examination of a large-scale model under many different configurations.
This car's cooling system drag was measured @ Cd 0.017. However, with a 100% grille-block, drag was only reduced by Cd 0.003.
By closing off the grille, upper-body drag was increased for this particular vehicle, limiting drag reduction witnessed on different vehicles.
Which brings us back to the argument about case-specific dynamics. There's just no way in advance, that one could anticipate what any given modification might do to the drag.

[AeroMcAeroFace]

Sometimes forcing more air over the bonnet is a bad thing, sometimes forcing more air under the car is a bad thing. sometimes reducing air flowing out the wheels is a bad thing. You may reduce the cooling inlet drag, but lose the drag reduction caused by where the flow exits, or force more air under the car causing more drag there.

Which brings us back to the argument about case-specific dynamics. There's just no way in advance, that one could anticipate what any given modification might do to the drag.

completely agree^ , although you can guess based on past examples, but even then it is not always useful if you overlook a critical difference between your car and the previous example's car.

[Autobahnschleicher]

I just had a look under the car in the front-section.
There is absolutely no ducting in front or behind the radiator, the air just spills on and around it and below the car.
Sadly there is no space to make Porsche 996 style air-curtains as the radiator ends about where the wheels start.
There are also some components in the way.
The small vents in the bumper don't actualy do anything and could as well be taped off (or re-purposed for air-curtains)

Possible modifications to the radiator air routing:
-intake duct and smaller intake cross-section
-exit ducting from the fan-shrouds to the wheels
-fully closed flat undertray from the front bumper to the rear end (black ABS plastic untill engine, aluminium sheet metal near the engine and exaust)

Allthough idealy I would design a whole new bumper, the current one has large flat surfaces at the front, wich are less than ideal.
I haven't yet been able to measure pressures there, but a rounder one would certainly be better.

[freebeard]

Graysgarge used to post on Ecomodder, before he went to work for Lucent. He has a first-gen MR2 project car.

MR2 Radiator Ducting - Gray's Garage at Gray's Garage



It looks like there is nothing but a bumper bar behind the front fascia. Pull air from the sides of the radiator intake into your air curtains.