Aero question on road car

[enkidu]

Hi guys


I thought I’d come on here and ask this question because you’re all knowledgeable and I pretty much have a read through this site every couple of days.

Anyway with a road car (capable of 160mph) well it runs out of gearing but gets there pretty quick. Would a plate which covers the bottom of the engine make it more efficient? Its basically a protection plate but it covers the whole bottom of the engine, its originally designed as a sump protection for police cars lol.

[Tom]

It probably would do, if it were a flat plate which doesn't flex too much. It should smooth underody airflow as there won't be a big barrier of turbulence as the air gets sucked up under the engine. I might be wrong but I think thats the very basics. To get an accurate idea of how much better it is you'd really need to do some CFD testing.

[enkidu]

Yes its pretty strong wont flex and flay but has some holes in it etc....

You'd still get the flow of air which goes through the rad and round the engine coming out but it would be channeled at the back of the engine bay by the bulkhead area.

[sasquatch]

creating a smooth under body for your car will decrease the drag on your vehicle. If it is just under the engine compartment and not well positioned, I doubt you would notice an improvement. In the past I have done CFD simulations of a car with the complete engine bay modeled and compared to a simplified smooth underbody. For such a drastic change, there was a minimal change in drag (less than 5% from memory but is a guess). I just remember thinking how little difference it makes.

One thing you must be careful of is how it affects the cooling of the engine. This might be more of a danger to over heating your engine. Sealing behind the engine bay would restrict the mass flow rate through the radiator. On modern cars, the packing is very tight and often the radiator is close to the limit.

In theory yes if done very well, but the added weight and increased engine temperature would probably cancel it out.

[Chubbs]

is it flat? because if u have it at a slight angle it could act like a diffuser and maybe create a bit of downforce, or if angled the other way then u might get unwanted lift.

but then again it could make almost no difference, duno what it actually is like...

[enkidu]

creating a smooth under body for your car will decrease the drag on your vehicle. If it is just under the engine compartment and not well positioned, I doubt you would notice an improvement. In the past I have done CFD simulations of a car with the complete engine bay modeled and compared to a simplified smooth underbody. For such a drastic change, there was a minimal change in drag (less than 5% from memory but is a guess). I just remember thinking how little difference it makes.

One thing you must be careful of is how it affects the cooling of the engine. This might be more of a danger to over heating your engine. Sealing behind the engine bay would restrict the mass flow rate through the radiator. On modern cars, the packing is very tight and often the radiator is close to the limit.

In theory yes if done very well, but the added weight and increased engine temperature would probably cancel it out.

Thats great I wont bother then.. cheers

[mx_tifoso]

Out of curiosity, to which car were you planning on adding this to

[enkidu]

Its only a hot hatch but its been pretty highly tuned over 300 bhp with quaife ATB diff etc....

[Ted68]

it runs out of gearing but gets there pretty quick. .

If you are running out of gears, and getting there quick as you say, aero isn't your problem, gears are. If you want to go faster, you'll need to change your top two gears in the trans or the ring & pinion.

r(t)
-------- =S
g(168)

r=rpm
t=loaded tire radius
g=final driven gear ratio
S=speed in mph (US)

Hope this helps.

Ted

[Belatti]

it runs out of gearing but gets there pretty quick. .

If you are running out of gears, and getting there quick as you say, aero isn't your problem, gears are. If you want to go faster, you'll need to change your top two gears in the trans or the ring & pinion.
...

Put bigger wheels, if you are a tunner then you will like them

[majicmeow]

Like has been said, you are mechanically limited by the gears. Nothing you do to the aero efficiency of the car can change your maximum top speed.

Changing the overall diameter of your wheels/tires will effectivly change your gear ratio and overcome this to a degree...

[Spencifer_Murphy]

Remember to have your speedo adjusted if you do that to your wheels though - or you'll get nabbed doing 70 in a 60 zone because your speedo dial reads 60 still lol!

[enkidu]

Guys I don't want anymore top speed I just want it to be more aero efficient

[SZ]

It probably would do, if it were a flat plate which doesn't flex too much. It should smooth underody airflow as there won't be a big barrier of turbulence as the air gets sucked up under the engine. I might be wrong but I think thats the very basics. To get an accurate idea of how much better it is you'd really need to do some CFD testing.

(whilst i agree that if you're running out of gearing very quickly you should have a good look at your gearing), what's the fascination with CFD? think! there are much easier ways to work out if you've got fundamental problems with your road car.