Modify roadcar for 300+ km/h - aero ideas

[Flummo]

A friend of mine is planning to get a car just to have fun with, a project with these goals:

*Looking similar to the standard car
*Good handling
*300+ km/h top speed
*4 wheel drive or rear wheel drive
*Not costing an arm and a leg...

Any suggestions for what car to get? Low drag and low weight is desired. Preferably costing equal to $4000/2500€ or less. If the engine is suitable for tuning that would be good too, ofcourse.

Now, for the aero:
I suppose the original aerodynamics can make many cars unstable long before 300km/h, so what I've been aiming for is modifications that keeps it stable without increasing drag.

As I said before, the car should look pretty much like the standard car, and that would rule out any big visible aero modifications like wings. However, I think a flat floor, side skirts, small rear venturi and probably a modified/changed front of the car should be possible, along with lowering it.
In general, how much can that kind of modifications affect aerodynamic drag on the "average" car? And how much downforce (or less lift anyway) can you expect?

[Ciro Pabón]

Wow, 300 kph on 2500 euros...

At this side of the pond, if you had 4000-5000 euros you could exchange them for 8000 dollars and maybe you could get a Mitsubishi DSM or Chrysler DSM, or a Mustang, Fox-body for drag racing. They're bargains for the power they develop. I think that no matter the aero you get, it's going to be hard to hit 300 kph if you don't buy some power.

Perhaps a bike.

Now, in Europe, where powerful cars have engines that seem pencil-sharpeners and cockpits are the size of a walnut, there are other cars that are cheap for the speed they give you, but I don't think you'll get 300 kph unless you spend several years or more money. I have no clues on the price of used, mid-sized cars in Europe: that could be another option. Within budget, a seriously enhanced kit car, like the Lotus 7, will give you only 200 kph... A second-hand Caterham CSR or similar could be something to consider: it won't be that fast, but you'll kick ass in any track.

[Flummo]

The budget is for purchasing the car, at least as much money will be available for modifying it. (You know just as well as I do, that no matter what budget you start with you end up spending much more.)

Don't look at me when it comes to the 300km/h requirement, I would be perfectly happy with 200 if the handling was good enough. Maybe he will realize that too in time, but until then 300+ it is!

[Giblet]

My 2 cents.

4WD might be hard cuz the motor will need to spin all 4 wheels all the way up to 300, even if it biased 90/10. I think you want as little resistance between the drive train and the wheels as you can get.

This being said, I think super light pizza cutter wheels with carbon shafts on an FWD car will give the least mechanical resistance between the engine and wheels. RWD also has the driveshaft's momentum to keep spinning.

I would start with a very light car that is easy to make lighter with some extra room in the engine bay.

A Honda CRX comes to mind. Maybe an Insight. Not just cuz I love them and have owned a couple (CRXs, not Inights!!), but I think it could work.

The CRX-HF was a shaved down higher fuel economy version of the car. It had no rear wiper, lessening wind resistance and weight for example. Starting with this chassis, drilling as many holes in the unibody steel that can be safely drilled, swapping in a high reving turbo charged moto over the ~60bhp stocker. A really big laggy turbo with all its masssive power at the top end, where you are going to need it, then maybe a nitrous shot at the top end to convince the air to actually get out of the way.

Gears are going to be a problem in any car, as you will likely need to look at race tranny manufactureres to get adjustable gear ratios.

Aero wise, I think you will need to flatten out the undercarriage, and completely smooth the whole body out. THe forced induction should allow minimizing of the grill, and chopping down the roof to drastically reduce frontal area, your biggest enemy. Covering the wheel wells as well, which the insight already has in the rear.

The insight had one of the lowest C.D.'s of any production vehicle.

[mike]

i suggest.. this
probly not the 300km/hr bit but definately in your budget and surely handles good and looks great

[youtube]http://www.youtube.com/watch?v=tzwTq7fq ... log/browse[/youtube]

[pipex]

mmm, i think that your budget is a bit tight for 300kph... but lets try. I can think that for that money you could buy an old Corvette (fourth gen). Then i would try to build that engine in order to turbocharge it. Due to the size of the engine, a twin turbocharger system would be needed. In function of the turbocharger pressure you are going to run you may need to lower the compression ratio, use stronger internals, etc... This could take a lot of money to get to run correctly in parts alone. After getting the required power, you need to modify the gearing as stated by Giblet. Next you need to tune the suspension and brakes, essential for safety. I think of upgraded discs and calipers. Above all, i think that the suspension and brakes are of extreme importance because of safety at that speeds. A rollcage and upgraded seats and safety belts may be added for enhanced safety.
As an advice, if your friend is going to do it, please tell him to do it the right way considering safety to protect himself and others.

just my 2 cents...

[Flummo]

I live in Europe, Sweden to be precise, and Corvettes don't come cheap here. I searched the biggest adsite now, and there were only four Corvettes available below (equal to) 7500€. The cheapest one at about 4800€, and the description said "driveable" - hardly encouraging...

About doing it the right way, that is exactly what I am trying to make sure he does. Although it maybe doesn't sound like that in everything I write, I am trying to find good and safe ways to realize his ideas - and to tell you the truth, they are sometimes high up in the clouds... What I write here is a mix of what he wants and what I think is suitable and/or reasonable. I ask these questions partly to find good solutions, and partly to have good arguments about the things that just aren't going to be possible even if he wants it.


The Youtube movie sure was about a cheap car, but you know how hard it would be to pull that off.

Front wheel drive might be most efficient, but not an option. Not good for acceleration from low speed, not very good for high power, not suitable for fun on the icy roads or frozen lakes in winter... Basically, not suitable for a car that is going to be used as a toy.
Lowering the roof, covering wheel wells and such will not be done. As I said, the car is supposed to look similar to the standard car, a sleeper if you will, not something that everybody stares at when you drive through town. Minor visible modifications, such as a wing, skirts or whatever a 18-20 year old may do to his car, fine, but nothing that screams "Ferarri killing monster" or "this car is nowhere near road legal, pull me over and start writing the tickets".

[SZ]

Your friend has a lot of reading yet to do.

(Assuming you're not gearing limited) power=0.5*air density*(velocity^3)*Cd*frontal area (use SI units; W, kg/m^3, m/s, m^2). This is if gearing doesn't get him first. Weight/mass only really affects your rolling resistance, which unless there's something wrong with your running gear isn't a significant factor at those speeds.

Aside from underbody details, almost anything you put on a car for stability is going to give a noticeable drag penalty, which explains why 300km/h cars are either super streamlined for *just* that purpose, or hugely powerful, or have a very well developed/researched combination of aerodynamics and power delivery.

Cooling a powerful engine requires either decent sized or finely tuned apertures, the former increases drag.

Suspension needs to be setup such that the car is well damped at target speed (e.g. ride not handling) or your aerodynamics won't work effectively.

Let alone the safety implications of getting a car safely from his target speed.

It's a huge challenge not to be sneezed at.

Your friend should aim to familiarize himself with his local engineering/technical library if he's even half serious.

[mike]

well what i meant was it a hard job you are trying to do
not trying to make fun of you..
does the 2.5k euro include the labour??
to achieve 300km/hr you should be aleast looking at 400-500hp and the best way for u to achieve that is probly to get MR2

stock it come with mid-engine rear wheel drive
its light 1.1tons take out all the useless bits probly about 950kgs its got a 2l turbo engine, if u get one that is later than 1993 is comes with 245hp
the car is also smooth aerodynamically
all you than have to do is buy some after market parts with the spare cash and the car will have about 350 to 450 hp at the engine.
the engine is 3sgte it is in use in GT500 in Japan with plenty of parts
the gearbox is S54

the car cost about $4000 aud 2000euro if u can import car straight to Sweden from japan

[Flummo]

Yes, no doubt alot of reading to do!

I've used that formula, air density 1,2 (according to wikipedia) and area/Cd numbers found in "Race car aerodynamics" (Katz) to do some calculating. It seems that both Ford Sierra and Mercedes 190 (used those because they are common here, have low Cd and are cheap) should require around 310hp to reach 300kph. That should be power at the wheels, so adding losses in the transmission and some margin I'd say that around 400hp should be enough to reach target speed. Correct?

Ford Sierra: 0,5*1,2*(83,3^3)*0,34*1,94= 229kW= 311hp

Getting the gearing right is another problem, but plans for that has to wait until the car has been selected.


The 2500€ is to purchase the car, and another 2500-5000€ will probably be planned for all modifications, labour and parts.

The MR2 seems interesting, but a bit on the pricey side. They cost from 5000€ here, and importing one from Japan will most likely cost about the same.

[Giblet]

I'm no FWD fanboy, cuz I love RWD, but in a car with a wheelbase as short as a CRX, you can have more fun then you think.

Reason I say this:

When the wheelbase is that short, you can powerslide quite easily in "the snap it loose and hammer it straight through" style. That is, the closer the front wheels are to the back, the more it behaves like an AWD drive car tha is sliding, and being 'pulled' out of slide through the corner. But the car I talk about getting to 300km/h wouldn't have enough low end to be a fun at lower speeds due to its small displacement. That is, the handling characteristics I enjoy in a standard CRX wouldn't be at all helpful for a 300km/h version.

As for RWD, I wonder if a Fiero GT would be a good start. Old, crappy, but cheap and fixable, a turbo/super-chargable V6, and a very small frontal area with the engine in the back. Maybe old MR2 would be similar.

Reagardless of forumalas I don't know, frontal area is going to be your biggest enemy, then power, then maybe tires?

[G-Rock]

You are going to kill yourself. A cheap car will need many basic safety items first such as new wheel bearings before consistantly being able to handle those speeds and the aero parts (air dams, hood, fenders) will need to be strengthened as well.

[Scotracer]

Sorry to be the voice of doom but I do not condone those sorts of speeds from a car that costs that much. Why do you think big German saloons are limited to 155mph/250kph? It means they don't have to run 300kph brakes, 300kph tyres, 300kph wheel bearings etc etc.

I have a friend who has spent 10x that amount on a Mitsubishi Evo VI and it has a top speed of around 165/170mph (280kph) and has around 400BHP.

I just can't see it happening on that budget.

[tomislavp4]

Here is a good formula...
p=d*s^3+14.65*s^2
p is the power in watts -15% for the drivetrain losses
d is drag example: 0.30
you are looking for s which is the top speed in m/s

For a car with 0.28Cd you need 400hp (assuming 15% power lost in the tranny)to reach 82m/s or 295km/h

Get the Cd of all the cars on your mind (if you can find it) and start from there, that´s what I would do

Lycka till!

[SZ]

Watch where you get your aerodynamics data from, most manufacturers will understate their Cd through a number of totally legal avenues.

An interesting consideration you'll face is that many roadcars do have low drag, but also generate lift, particularly around the rear axle. At your target speed that's a problem, though wings etc add induced drag and the like, and will require more power to go quickly. So then, like most, it's an iterative design problem.

Seriously, when your friend's done studying Cd/A data tell him/her to go and get him/herself a second hand, de-restricted Hayabusa for the money. They'll hopefully (likely) --- themselves getting anywhere near top speed, sell the bike and dedicate their time/money to more reasonable pursuits.