I'm a self proclaimed petrol head, and in the past I've driven big engine, small car type stuff. Nothing like stuffing a 3.5l v6 in a Corolla😆.

 

But since there's a speed limit on my comute, I'm wanting to be as efficient as possible.

Car is a 2012 170bhp vRS estate.

Previous owner apparently got 1000kms (620mi) on a single tank.

 

Is anyone able to help me with drag coefficient, and other aero calculations that may give me an excuse to get some aero pieces, mainly a front splitter.

 

Quick research shows that Octy 2 has 0.31 drag coefficient, is this correct?

And how does one calculate this? I've read stuff online, but, data is garbage in, garbage out... So can't exactly understand it all.

 

Thanks in advance.

6 minutes ago, HESH said:

Car is a 2012 170bhp vRS estate.

Sorry, should clarify it's a 2.0l TDI.

1 hour ago, HESH said:

I've read stuff online

In which case you should realise that you need either a wind tunnel model or a Computational Fluid Dynamics model of the car you want to modify. It is particularly important to get the ground clearance correct, and indeed the rake of the car correct.

You don't need an excuse. Get one or do not. Can't imagine the aero improvement would be a lot, so the cost would take some time to recoup. Not getting one would allow you to plough the money into fuel instead.

Won't make much difference on a low speed commute. Improving drag only really works at higher speeds as it is cube law of drag against speed.

 

I get my best mpg on roads with traffic at 50mph. I do that by anticipation, leaving a longer gap to the car in front, lifting early on the gas pedal, coasting to slow down (which cuts off the fuel supply entirely) and only braking at slow speed. Typically get 60-65 mpg out of town as long as I am not stationary for too long.

So 620 miles per tank is believable on a long run at modest speed.

Edited 1 November, 20214 yr by pikpilot

On 02/11/2021 at 03:25, pikpilot said:

I get my best mpg on roads with traffic at 50mph. I do that by anticipation, leaving a longer gap to the car in front, lifting early on the gas pedal, coasting to slow down (which cuts off the fuel supply entirely) and only braking at slow speed. Typically get 60-65 mpg out of town as long as I am not stationary for too long.

So 620 miles per tank is believable on a long run at modest speed.

Thanks pikpilot, tried it over the last few days and it definitely helps. Thanks 👍.

On 01/11/2021 at 23:38, KenONeill said:

In which case you should realise that you need either a wind tunnel model or a Computational Fluid Dynamics model of the car you want to modify. It is particularly important to get the ground clearance correct, and indeed the rake of the car correct.

Ok thanks, will do some more research into it. 🧐

On 02/11/2021 at 01:55, Me-109 said:

You don't need an excuse. Get one or do not. Can't imagine the aero improvement would be a lot, so the cost would take some time to recoup. Not getting one would allow you to plough the money into fuel instead.

😆 That's the spirit!

On 01/11/2021 at 15:25, pikpilot said:

. Improving drag only really works at higher speeds as it is cube law of drag against speed.

 

 

Squared not to the power of three.

 

It is linear at lower velocities, I do not know what the transition point is, it would be very usefull to know if anyone does.

 

Editted, when checking I found that the power requirement and hence fuel consumption does indeed rise by the power of three with speed even though drag raises with the power of two. It is because "power is the rate of doing work" so even with the slipperiest car in the world drive at twice the speed the aero resistance is 4 times and you will use 8 times as much fuel.

 

Incredibly wasefull when you consider that most of the energy of the fuel disappears as heat in any case.

Edited 5 November, 20214 yr by J.R.

Keep in mind that most aftermarket aero parts are designed primarily for downforce rather than drag. Now, it isn't impossible that something does both, particularly if it reduces airflow under the car, but it is difficult (otherwise it would probably come from the factory like that, unless it was  too vulnerable to damage, in which case it will be too vulnerable to damage for general use).

 

And all this depends on the aftermarket suppler doing a competent job with the aero, and my impression is that they only really care about appearance, so they may not have done anything about  testing aero at all.

 

By the way, all the 'drag rises as this power of speed' only really work before flow becomes turbulent', so at lower speeds, which may not be the speeds where aero is really relevant.

 

In any case, in spite of all the reservations, the cost of the part is always likely to be higher than any potential saving, and given the possibility that it may make fuel economy worse, it really doesn't  seem to be a good prospect. 

You started right with an estate with the longer roof.

Add very little extra crap.

Just add lightness so strip the car including wipers.

Narrow wheels / tyres, underbody smoothed and always drive where you have a tail wind or it is downhill. 

If not draft a vehicle uphill that might help. 

 

 

 

 

 

 

 

Edited 5 November, 20214 yr by roottoot