Body/Trim: Aerodynamics...don't laugh I'm serious.

[240ratrod]

Morning,

So the topic of aerodynamics has been covered before I'm sure.
But what tweaks can we do to improve the aero and thus the efficiency of our bricks.
I'm an optimist and always say it's worth trying something before its dismissed, if only for fun!

I'm interested in the front and this theory...the gaping wide open grill is allowing a lot of air to pass in to the engine bay. It's what and where this air does/goes once in there: theoretically the air has no choice but to drop out and under the car.

I've been thinking about venting this air out of the bonnet. Removing this pressurised air effectively would allow the car to move through the oncoming air more efficiently? Also we would be improving engine cooling and reduce under bonnet temperatures.

You need to have a pressure differential. The area at the base of the windscreen is a positive pressure area as I feel air coming in through the vent when moving, with the fan off. One assumes the area to the front of the bonnet is lower as the air is boucing up and over the nose.

So theories aside...do we think there could be any real world benefit to venting the bonnet?
We're aiming to reduce air pressure to improve aero and efficiency and reduce under bonnet temps. Would venting towards the rear of bonnet create a differential enough that the hot engine air is drawn/blown out still?

[Clifford Pope]

For what it's worth, when I had a 240 with LPG conversion I had problems with the ram-air effect from the normal engine air intake just in front of the front bulkhead, to the side of the radiator. When I routed the intake to inside the engine compartment, just behind the radiator, the effect vanished.

I interpreted this as signifying there was increased pressure just in front of the radiator, and lower pressure just behind. Obviously that has to be true or air wouldn't flow through the radiator. But the LPG set-up I had was over-sensitive to too high air pressure immediately in front of the car being forced into the air intake.
Presumably though this effect is intentional with normal petrol operation?

[classicswede]

The air is supposed to go down and out around the gearbox area.

If wanting to keep drag down

Make sure the fornt splash pan is fitted

No roof rack or roof bars

You could look at door mirrors with better air flow

The best front end shape is the flat bonnet with rectangular head lights.

Taping the gaps between the lights and bonnet gaps will help. You could reduce the grill size but in a way that you can adjust for season so more air flow in summer and less in winter. Reduced air flow in winter would help the engine warm up faster (volvo used to sell a grill insert for this)

[geoffo]

And you’ll find the only thing to do is fit a rear spoiler and increase the horsepower by at least 50%

[Nicholas Lewin]

Interesting that an estate has a better CD rating (not much) than a saloon...

I guess the shape is more regular if you are a molecule of air passing over?

[Nicholas Lewin]

And again not joking - I wonder whether a freshly polished car slips through the air better than a car needing a clean? Would that be measureable?

I have a fondness for Landrovers - the sparewheel mounted on the bonnet was a huge improvement to the aerodynamics of the vehicle. And also turned the bonnet into a lethal high powered frisbee in the event of a collision. You can't win them all!

[Clifford Pope]

Originally Posted by Nicholas Lewin

the sparewheel mounted on the bonnet was a huge improvement to the aerodynamics of the vehicle.

There's the way to go then, that answers ratrod's question

The most unlikely things seem to improve aerodynamics. Modern cars have big dents down the doors and look as if someone has kicked in the cills.

[mylittletony]

You could attach some bits of ribbon to the bonnet and go for a drive to see what they do. If they get pressed onto the surface, then the air is high pressure, if they flutter then it's low.

Regarding the estate/saloon Cd - that's why the 850 touring car was an estate when rear wing size was limited.

Other DIY improvements:-

Lower the car, less airflow underneath
Front splitter, as above
Flat faced wheels
Side skirts, help guide the air down the side of the car.
Rear spoiler, to improve the flow off the rear of the car.

Otherwise look at Bonneville/Nascar racers or read some of the hyper-miling forums for inspiration.

There's a big toss-up between cooling, aero and handling

[230ina245]

I remember someone trying to improve the airflow coming off the back of the estate car. I think it was one of these improve your mpg sites. I was interested because I know the airflow must be bad because the the rear screen (estate) gets very dirty, very quickly when the the roads are still moist after rain.
If I remember correctly his interest was aroused by those fibreglass extensions on the back of tractor unit cabs, anyway I wrote down the result he found to be the best after various experimentation.
He found that an "extension" to the rear roof line, 20 cm long worked best when angled downwards at an angle of 12 degrees.
Haven't tried it myself yet to see if my rear window stays clear but you could always add a Gurney strip or vortex generators if you were feeling frisky!
Tim