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Aerodynamics on a 245 for 200mph standing mile?

Lankku

Lankku

Active member
Joined
May 23, 2015
Location
Finland
Almost 2 years ago I drove 188.75mph/302km/h standing mile in quite good conditions with my -90 245 "T6". Because that was surprisingly easy, I have since thought of getting closer to 200mph.

The pass was done with 1.8bar boost around max 700hp. And with that tuning it had a decently large torque curve, peaking 840Nm. Adding 50-100hp and same amount of torque isn't a problem and according to my calculations current gearing allows ~203mph @7600rpm.

I've lowered the car and is in the picture below as low as I'm willing to go. Now it still has some room for suspension travel. I adjust the ride height a little higher for street or track use. I made a front lip under the stock spoiler and taped the gaps(I stripped some of the tape before taking the picture). Just adding the lip made a huge difference on the road on a very windy day by reducing the amount of air getting under the car.

69.jpg


Somewhere around 270-280km/h the front started to feel a little light but it could be just a feel due to the speed. Anyway now I'm looking for more aero mods which would be good for reducing drag and possibly improving stability. Even minor ones. Removing wipers and removing or folding/removing mirrors are on the list. Window list was removed when I replaced the window. Wheels with flat surface are an option, Polaris just looks too good :-P But where to go/what to do after that?

For starters I've thought of making a lip that goes a bit lower and doing a flat paneling to the bottom. I did already start at the front:
25.jpg


I've seen some cars with a rear wing which extends the roofline downwards. Sort of like a rear window does on a hatchback. Could this be used? Of course it would have to be easily removable. Adding a diffusor doesn't seem as good option for reducing drag as does preventing air from getting under the car but does it? Flat bottom and a even a mild diffusor could/will remove air better from engine bay and improve cooling?

Testing possibilities are naturally limited to one day events here and there. Don't want to make completely uncertain changes that could have negative effect on handling. Would also like to have results instead of just testing :) And this isn't even a racecar so the more it looks stock on the road, the better.
 
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This is the most "TB" thread in the history of "TB", except that you're legit. Awesome.

You're on the right track with a diffuser under the front of the car. You obviously have a minor opportunity to make the car more aerodynamic, but evacuating the under hood air (or keeping it out to begin with) could very well be your best bet.
 
Go look at my post regarding spoilers in the do spoilers work thread.

The 240 is NOT an aero car. You would definitely want to add an extended roof line wing and a rear diffuser undertray to help reduce the amount of wake turbulence being caused by the square back end. The air is basically coming off the car cleanly, but will curl and try and "suck" the car backwards. You need to trick the air into thinking there is a fairing on the back end, or put a fairing on the back end. (There's a reason why aircraft (even cargo ones) don't have square back ends.

There is an exact video that will show this on a 245 believe it or not.
https://www.youtube.com/watch?v=19sE4o4Gqp0

hqdefault.jpg


The red and green air mixing is bad. It's wake turbulence.

You need to extend the upper and lower sections so that when the air curls... it's far enough away from the tailgate to generate lift (which is drag to your desired direction of travel).

Remove rear bumper. Vent your front undertray to help dissipate the frontal area drag (you're sucking tons of air in through the grill, AND under the car... it needs some outlets or you'll do more harm than good. Add diffuser at rear of car. Add roofline wing at rear of car. Test. If the car won't do it... consider a vortex generator (the rake of the front windscreen will deflect air over the roofline preventing it from sticking to the roof line... as such, finding a position for the spoiler to be effective could be challenging. That said, finding a position for the VG to be effective could also be challenging.

Anyhow... aerodynamics. How do they work?
 
Lankku - I think I'm the only hard-core Land Speed racer on this forum so I DO know a tad about this.

I have no clue what rules you guys are running against there but I'll add a generic warning to be sure you know what limits the class you're running puts on you, especially as far as aero improvements go. I run an '85 Chevy S-10 pickup (that's the Chevy mini-truck) with a Mercedes OM617 3.0L 5cylinder Diesel. I run in the East Coast Timing Association meets and we use a rulebook very close to the SCTA book so those are pretty generally accepted rules. We have racers from all over the globe at Bonneville (I volunteer in Impound at Speed Week so I see a lot of cars) and international guys seem to accept our rules, too. Now if you're just running for speed and don't care about records (what we call "Time Only" or TO) then it doesn't matter.

The reason for all that is to remind you to stay in class if that's your intent - it's easy to make changes that put you in a class where you're no longer competitive. For example, the belly pan you showed would put your car in "Modified" or perhaps some other class and you would find yourself running against some pretty big dogs.

Again, no clue what your sanctioning body requires but there are likely additional safety requirements as you get faster - at 175 and above we would require a full roll cage, parachute, full containment seat, fire system and other stuff as well.

Now onto my experience of vehicle vs. speed. Remember that the resistance to increasing speed goes up at the square of wheel speed. You and I have similar issues in that we're trying to push a brick down the track and the faster we go the harder it is to push that brick. What I've seen at the track is that it takes between 900 and 1,000 HP to push an un-aerodynamic car down the track to 200 MPH. My OM617 will NEVER have enough HP to make that happen and I'll be thrilled with 150 some day. Sounds like you COULD get there.

EDIT: Keith Turk is a friend and he owns the Hot Rod Magazine Camaro that you often see in the HRM videos. He's also a freak for wind tunnel testing. He says that the one hard and fast rule for aerodynamics is that they are NOT intuitive. Things that work on everybody elses' car may not work on yours and to REALLY find out you need to take it to the tunnel. We're lucky here in the US to have access to the A2 Tunnel near Charlotte, NC and it isn't horribly expensive though out of my budget. No idea if there is anything like that in your part of the world.

BEST of LUCK!

Dan
 
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Also very cool!

I have no clue about to post a pic or I'd post up my truck, Mutt. My pics are not hosted - don't need to be on the other forums I use.

Dan
 
EivlEvo said:
Go look at my post regarding spoilers in the do spoilers work thread.

The 240 is NOT an aero car.
Click to expand...
Did read your post but didn't consider it to cover this topic that well.

Noo really? :rofl: And it doesn't have to be. Just trying make it even slightly better.

One thing I know for sure is that rear bumper and other body parts are staying. Slight modifications can be done to them but thats about it. It's just a personal preference. So everything has to be done accordingly. Even when using less than optimal modifications it is better than original?

Roof line wing with adjustable angle does seem one of the easiest things to do. It might be best to do round shaped to join better with roof line. How smooth does the transition need to be for air to follow? Rear diffusor would have to be quite mild or then perhaps ridiculously long? Because of the rear bumper. How do they interfere with each other if they aren't lined up correctly? Just not a right type of airflow but still perhaps better than stock?

Reducing airflow through the front could help also to the overpressure in the engine bay? And also reduce the need to vent. But it has to be tested what is the best ratio for least pressure and enough for cooling. Of course this doesn't mean that it would improve anything if air under the car is trying to go up in to the engine bay. Though don't know how much air really tries to get there from underside the car. I don't think it's even close to what it is as stock when compared to using the extra front lip. Just estimating from the reduced feel of lift at the front in high speeds and slightly smaller feel of resistance on road speeds. To be sure it would have to be measured with a manometer(?).

Some testing might be possible to be done on a track nearby but even there on the longest straight I think I can achieve only around 250-260km/h before braking. Anyone can drive there all day with reasonable cost, but what is enough to show what will really work on higher speeds?


Dan Stokes:
These events are not races. They are open for anyone and depending on the event for just about any vehicle. You'll have to sign a form for taking responsibility if something goes wrong. Not much more rules than that you don't bring in a broken turd. It could be a broken turd afterwards though...

A couple more years back I drove with the same 245 end speed of 251km/h/156,87mph. At that point it was only lowered(less?) and around 450hp/650Nm with holset HX40.


Don't know if I got my writing according to my thoughts. Not too easy in English. ;-)
 
I remember reading about someones attempts to improve the airflow at the back of their wagon some time ago. Of course I cannot remember who it was or what their motives for the improvement were, but made a note of their results.
They took as their inspiration an articulated lorry tractor cab which often have extensions to the side & roof of the cab. They did some experimenting with sizes & angles & found the most efficient shape to be an extension to the roof 20 cm long, angled down at 12 degrees. The sides were just supporting triangles.
 
Lankku said:
Did read your post but didn't consider it to cover this topic that well.

Noo really? :rofl: And it doesn't have to be. Just trying make it even slightly better.

One thing I know for sure is that rear bumper and other body parts are staying. Slight modifications can be done to them but thats about it. It's just a personal preference. So everything has to be done accordingly. Even when using less than optimal modifications it is better than original?

Roof line wing with adjustable angle does seem one of the easiest things to do. It might be best to do round shaped to join better with roof line. How smooth does the transition need to be for air to follow? Rear diffusor would have to be quite mild or then perhaps ridiculously long? Because of the rear bumper. How do they interfere with each other if they aren't lined up correctly? Just not a right type of airflow but still perhaps better than stock?

Reducing airflow through the front could help also to the overpressure in the engine bay? And also reduce the need to vent. But it has to be tested what is the best ratio for least pressure and enough for cooling. Of course this doesn't mean that it would improve anything if air under the car is trying to go up in to the engine bay. Though don't know how much air really tries to get there from underside the car. I don't think it's even close to what it is as stock when compared to using the extra front lip. Just estimating from the reduced feel of lift at the front in high speeds and slightly smaller feel of resistance on road speeds. To be sure it would have to be measured with a manometer(?).

Some testing might be possible to be done on a track nearby but even there on the longest straight I think I can achieve only around 250-260km/h before braking. Anyone can drive there all day with reasonable cost, but what is enough to show what will really work on higher speeds?


Dan Stokes:
These events are not races. They are open for anyone and depending on the event for just about any vehicle. You'll have to sign a form for taking responsibility if something goes wrong. Not much more rules than that you don't bring in a broken turd. It could be a broken turd afterwards though...

A couple more years back I drove with the same 245 end speed of 251km/h/156,87mph. At that point it was only lowered(less?) and around 450hp/650Nm with holset HX40.


Don't know if I got my writing according to my thoughts. Not too easy in English. ;-)
Click to expand...

The reason I suggest removing the rear bumper is because you can see it clearly interferes with the rear airflow. But your argument is valid if that's your plan.

Regarding the wing and diffuser... what your goal here is to control the wake turbulence. As the car is stock, (in that video) the red and green fluid making the mixing motion at the back of the car will actually "pull" the car back. That's because as that air flows off the top or bottom of the car, it turns upwards to fill the low pressure area you've created behind the car. But BECAUSE this air is now flowing up or down in that mixing fashion, it's generating lift across the tailgate (which is what's pulling the car back). If you add a wing, or a diffuser... what you're effectively doing is moving the roofline or underside of the car aft. This negative pressure area will still be created, but since the burbling airflow is moved off of the tailgate, you'll reduce drag significantly. The same principle applies to many things, but most specifically... winglets on an aircraft.

Your wagon tailgate would be the top of the wing in this example, the winglet would be the spoiler you're intending to add, and the underside of the wing would be the front of your car. So basically, rotate this image 90* and you'll get the idea.

1-vortex.jpg


For your purpose, I'd suggest something like the Stagea type wing... because of the speeds, maybe longer by a couple cm.

IMG_0451_0tz0_4.jpg


The point I was making about the front of the car. The undertray needs to be vented to prevent the engine bay from acting like a scoop. Again... this is why the front of the Evo is built the way it is. Air is forced in through the large front airdam, through the IC/Rad. The hood vent has a front lip on it that creates a negative pressure area, and the air is drawn up over the turbo and out the hood vent over the roof of the car.
 
K9 240 said:
They did some experimenting with sizes & angles & found the most efficient shape to be an extension to the roof 20 cm long, angled down at 12 degrees. The sides were just supporting triangles.
Click to expand...
This is exactly the type of roof line extension that I was thinking.

speedn_j said:
Can the air going under the car be diverted to flow to the sides of the car instead out the back?
Click to expand...
It could but is it any better than back? Then that air swirls around the back corners(?).

EivlEvo said:
The reason I suggest removing the rear bumper is because you can see it clearly interferes with the rear airflow. But your argument is valid if that's your plan.

For your purpose, I'd suggest something like the Stagea type wing... because of the speeds, maybe longer by a couple cm.

IMG_0451_0tz0_4.jpg


The point I was making about the front of the car. The undertray needs to be vented to prevent the engine bay from acting like a scoop. Again... this is why the front of the Evo is built the way it is. Air is forced in through the large front airdam, through the IC/Rad. The hood vent has a front lip on it that creates a negative pressure area, and the air is drawn up over the turbo and out the hood vent over the roof of the car.
Click to expand...
Don't want to make this post longer with the whole quote so I'm shortening it a bit.

I can always build the beginning of the diffusor in under the car and then add the rest of it when needed. This way it still could improve airflow on the road even without the removable part. Bottom of the bumper frame is also a fitting place for attaching points.

Does the wing have to be upwards like that Stagea wing? Even when it moves the roofline away from the car, wouldn't it be more beneficial to have it pointed slightly downwards as I said before and K9 240 said to be done in some research? This way the area for air to swirl after the wing would be smaller than with a wing pointed upwards? If it is any significance at that point.

On an Evo venting throught the hood also removes heat from the exhaust side. If done similarly on a car with engine longtitudally the exhaust side is out of that airflow and possibly heating up more? Of course the heat from the cooling wouldn't be going through engine bay anymore. Aand cutting the hood is not first on the priority list. I'd do it some place less noticeable but there isn't many that could be as good. And there is that thing if something goes wrong, I don't want possible smoke or fluids coming out of there over the car and on to windshield.

Engine bay scooping air is a regular thing in these bricks especially if you don't have a undertray at all. It takes huge amounts of air in and can be seen in higher speed as hood trying to lift. Adding even an stock undertray and that extra lip I've got have reduced that to minimal. So now all is left is "just" to improve it even more.

I had a fiberglass hood on a 142 and in higher speeds it looked like it was going to take off. Just by adding a front lip it calmed down.
 
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