The way I understand it, at the terminal velocity of a car, most of the engine output is used in overcoming air resistance, which increases as the square of the speed, until total resistance (i.e. air and mechanical friction) equals maximum horse power and the car can no longer accelerate any further.

Most owners' emphasis seems to be geared towards tweaking for maximum horsepower, it must be reaching the point of dinishing return. The aerodynamics (drag coefficient) of the Scoob is something that the manufacturer is not too keen to publicise and I believe this is where better returns can be obtained. Ignoring mechanical friction for the time being, a 10% reduction in air resistance may be equivalent to a 10% increase in power output and this is without any risk to the longevity of the engine unlike tuning modifications.

As a simple example, for those who have got a FMIC, maybe changing the bonnet to that of the Impreza Sports without the airscoop will improve aerodynamics a few percentage points, as air need not be diverted into the engine bay and can flow smoothly over the top. Most F1 racing teams are spending much time in their wind tunnel besides the engine test bench and even cover up their latest aerodynamic tweaks in the pits to avoid spying eyes. Yet I have not come across any threads on the BBS discussing effects of say varying ride heights or front and rear spoiler designs on speed performance.

Just some rambling thoughts of an armchair tuner.

The problem here I think is that that has negligible effects until after 100+ mph.

I would imagine most Scoob owners are after better accelrattion than improved top speed.

Aero drag is basically Frontal Area (A) * Drag Coefficient (Cd)

Aero power is Cd*A. Cd is most affected by flow over rear window / brake away from the rear of the car. This is most affected by the angle of the rear window / boot shape.

So what is the Drag coefficient then, or is it so bad it’s never been publicly released?

Well, the CD of the MY01 WRX is quoted as .33 which isn't too bad for a car of its' length and particularly considering the losses caused by the 4WD system (all the extra drive stuff under the car is aerodynamically a liability).

Perhaps of some interest is this "report" posted by Simon Lines (Prodrive export manager for the WRX - apologies Simon if this isn't your corrct title) on Irritable Club when providing details of the UK300 bodykit.

Wind Tunnel Report

Subaru Impreza WRX
MIRA Full size wind tunnel
16 OCTOBER 2000

Testing new model Impreza in the full size MIRA wind tunnel

The car was set-up weighted to represent an average driver and passenger, with all interior vents closed and the tunnel boundary layer fence installed to give the most informative lift / down-force results. The installation of the fence tends to give slightly higher drag figures than a flat floor tunnel (Approx. 3%) but much more accurate lift results.

The test results were recorded both as coefficients and actual forces (loads) at a road speed of 100mph (160kmh)

Aside from experienced test or race drivers most will not be able to feel a difference in lift of around 10 / 15 lbs. however changes of 20 lbs. will be noticed.

The standard WRX has a drag power requirement of 60.5 bhp at 100 mph, front lift is 59.2 lbs. rear 75 lbs. These are average lift figures for a four-door saloon of conventional shape. (Runs 1 +2)

The vehicle was then tested at an angle to the tunnel airflow, 7deg and 15 deg representing typical motorway side winds. At 15 deg front and rear lift doubles the balance remains essentially the same though.

The addition of the low WRX wing has no effect on drag but reduces rear lift by 19.3 lbs. and adds 12.2 lbs. of lift to the front. Adding the new Prodrive front lower molding reduces the drag power requirement to 57.7 bhp and reduces front lift to 5.6 lbs. but rear lift increases to 51.3 lbs.

The Prodrive rear wing was fitted in place of the WRX low wing (initially set at 3-deg incidence). The drag requirement was 58.7 bhp. Front lift 20.8 lbs. rear 21.8 lbs. This is a very good aerodynamic balance.

The WRX was then lowered by 20mm front a rear to represent sports suspension. Drag bhp required for 100 mph was reduced to 54.5 front lift remained the same and rear lift reduced to 14.3 lbs.

CONCLUSION: -

The effect of the Prodrive aerodynamic kit is to reduce drag bhp at 100 mph by 6.5 bhp at the same time as lowering the overall lift from 59.2/75lbs (front/rear) to 20.8/14.3 (front/rear) a total reduction of 99.1lbs

NOTE: -

It is strongly recommended that the front and rear parts be used in conjunction with each other. Fitting the rear wing alone will give rear downforce of 10 lbs. and front lift of 75 lbs. at 100 mph, not a safe condition.


A couple of points here;

• I've used the reduction in drag figures caused by lowering the car to persaude my wife that I need to fit lowering springs to the car to save fuel ;)
• The frontal lift of the WRX with std wing but no front lip is fairly evil at 71.4 lb, and this is quoted as doubling when tested at an angle to simulate a cross wind. Given the test was a 15 degree angle this represents a crosswind of only 27 mph which isn't that high.
• Interesting to note that D T-S has fitted a front lip (Subaru sports kit, not Prodrive) to his MY01s to counter the effect of the Scooby Sport wing! Perhaps he or Pete Croney may wish to comment on the effect of the wing without the front lip. I accept that their cars are significantly lowered, but the lift must still be there.

Duncan :)

PS Completely ignored the question - I haven't been over 120 mph yet (on a private road, honest ;)), hence top speed is less important than acceleration. Hence, to me improving acceleration by increasing power is more important than improving top speed by reducing drag. Also, I like as much grip as possible, hence the desire to reduce lift.

PPS Don't think that it is only the aerodynamic losses that are signficant - the drag from the tyres and transmission are very significant. On rolling roads people are typically loosing 60 - 70 BHP at max revs.
[Edited due to oddities in the quote command]

[Edited by BugEyed - 2/13/2002 1:09:30 PM]

Duncan, thanks for the very useful bit of info. on the tests. A Cd of 0.33, while nothing to shout about nowadays, is not to shabby a figure, considering that when the Audi 100 CD was first introduced with a figure of around 0.30, it was a new record then for a production car.

Correct me if I am wrong, but if wind resistance does increase proportional to the square of the speed, then an increase in speed of say from 100 to 125 mph (+25%) will produce a 56% increase in drag, which can be quite substantial. I agree that most owners will tune for increased hp and thus acceleration, but aerodynamic improvements, like air, is free, other than the initial capital cost, as there is no tradeoff in terms of fuel consumption or engine longevity. This is like engine oil/additives advertising claims of increased hp from reducing engine friction, only that we are improving air friction. I remember seeing on TV the controversial new swim suit designs (an adaptation of the skin of the shark) that can reduce drag sufficiently to provide a winning edge to the wearer. Maybe we will have dimples on our Scoob body panels like that on a golf ball if some tuner come out with a claim of up to 10% faster acceleration times. BTW, the Wax Wizard supposedly has a teflon formulation for one of his product, and teflon is the "most slippery substance" around, so that should be good for another 20 bhp when applied on your Scoob, LOL.

James, the way I understand it, drag coefficient is the ratio of the drag force of an object to that of a theoretical flat surface of the same frontal area, so if the drag increases with airspeed, then the absolute drag will also vary proportionally (to the square of the speed, I believe), until at a high enough speed, it is the constraining factor to the road speed of the car.

[Edited by lokokkee - 2/13/2002 6:16:40 PM]

the drag increases with the square of speed.

Yupp, reducing the drag of the car is a worthwhile modification if done correctly. As you say, it will give back a benefit in terms of improved fuel consumption as well as potentially an increase in top speed. However, there are a few downsides .....

• you need to keep the car practical and stylish (well, maybe not in the case of the MY01 ;) ). An example of the former would be door handles - removing them would make the car more aerodynamic, but less practical ...
• you need to keep the car stable in cross winds - often removing a rear wing will reduce drag, but increase rear lift with a consequence of less stability.
• there is often a trade between form and function. Narrow wheels and tyres give less drag, but ....

As I said in a previous post, some changes give an improvement both in drag and performance (such as lowering the car).

Duncan :)

Craig, I think even for 'slow' speed sports like swimming, cycling, skiing and skating, a lot of efforts have been spent on improving the aerodynamics of the competitor, witness the new swimsuits, helmets, ride positions etc. that are the results of wind tunnel testing. I suppose since water is a lot denser than air, any improvement in streamlining will have a much larger effect on the swimmer in spite of the much lower speed, but cycling is all aerodynamics nowadays, there is only so much you can do to bulk up your leg muscles (engine) and lung capacity (air intake), and we are talking about maybe 30 mph.

Slightly off topic but reducing the weight will give you better acceleration, so are there any other easy ways to reduce weight?

In my old 405Mi16 we always kept the fuel tank 1/2 full and you really noticed the difference but its not that easy to do with a thirsty scoob so the ones I know are....

remove space saver
remove crap from boot, carpets etc
remove mats
remove underbonnet insulation?

any more or would I be wasting my time?
Dan

Dan, anything that reduces the weight of the car will improve the power/weight ratio, which will effectively increase the performance in the same proportion. If you own an UK Turbo, you can swap for aluminum bonnet and boot cover from the STI, lighter wheel type/size, carbon fibre bits and pieces (e.g. racing seat), but because of the low volume demand and high tech of the latter, these things will probably cost an arm and a leg. PC of ScoobySport, how about a carbon fibre bonnet?