rear underbelly fins idea/exhaust prob fix

[ZR8ED]

I mentioned in one of these forums about fumes getting in the car.. naturally right? its a z. hehe

 

well I replaced all my door/hatch seals.. and the car doesn't leak fumes with the windows up.. windows down is another story..

 

I've heard many times how turbulent the air flow is behind the car.. well i've been thinking of how to smooth it out..

 

I have no idea if this will work or not, I'm copying race cars ie LeMans cars with their belly fins at the back of the car under it.. it directs and smooths air flow from under the back of the car I assume, and I also assume it helps with high speed stability.

 

I'm hoping that it might smooth the air flow out at the back of the car thus allow the exhaust to get away from the car.. It is the aerodynamics of the car that is causing this.. it has to be. ??????

 

Anyways here is a pic of a cardboard mock up I did today.. I plan on using it as a template for some 1/8" plate aluminum.

 

Nothing better to do today I guess. at worst case, it doesn't work.. looks cool though.. it will be removable though..

 

[280Zone]

I am thinking that looks pretty sharp. Have you replaced you tail light gaskets?

[Guest Anonymous]

Keep us posted on your results.

[zero]

if you could make a whole tray under there that was smooth and curved upward you could make some downforce

[blueovalz]

My undertray is only an aesthetic touch to the rear of the car. With all the mufflers and fuel cell cluttering up the bottom of the rear valance panel, I felt compelled to clean it up a bit. Anyway, my understanding of the fences at the rear of the LeMans cars is to contain the diffuser at the rear of the undertray (or venturi) that extends the entire length of the bottom of the car. Without these fences, the carefully controlled increasing cross section of the diffuser would not allow the decrease in pressure at the rear and bottom, and hence, a loss of downforce would ensue (at this time any knowledgeable person on these devices can chime in). The turbulent region behind the Z (especially with a rear spoiler) will most likely still be there. With that said, the fences on your Z really do add a nice touch to it.

[johnc]

What you folks are talking about is a rear diffuser. They are an effective means of creating rear downforce (typically from 50 to 200 lbs) but the design is critical.

 

The surface needs to be flat and the upslope angle needs to be as close to 7 degrees as possible without going over (measured at the dynamic ride height of the vehicle at your projected design speed). Over 7 degrees and the laminar flow separates and you just create turbulance and drag. Under 7 degrees and you're not getting the full effect.

 

The diffuser also needs to be as long as possible (the 7 degree upslope section) and extend behind the rear bumper to get a clean flow break. A small rear spoiler on the rear hatch will help flow as long as you don't get turbulence affecting the flow exit.

 

The diffuser also needs to be as wide as possible and have a clean transition from the flat section under the car (sealing the rear diff area) to the upslope section. You will need to add a diff cooler to keep the temps within reason if the car will be raced. Flattening the bottoms of the control arms with sheetmetal, aluminum, or CF will help at the transition area.

 

Coincidentally, the custom ATL fuel cell in my 240 has a 7 degree upslope on the bottom of the can...

[Guest SpudZ]

John,

At what speeds does it create downforce ? In other words minimum

speed. I am also interested in where the 7 degrees of angle came from.

It is very interesting to know how the math comes into play on aero

dynamics and downforce. MY SO is a math instructor at the local High

School and I have been trying to get her to use a car and how important

math is in one of her classes.

 

BTW .. when you ever going to get that darn car on the track ?

I checked Cal Club results and you are driving the wifes Miata

[blueovalz]

That's a hard question to answer. A lot of it depends on the ride height of the car. In general, the lower the car (or tub, undertray, pods, etc) the less angle needed for the same amount of lower pressure created. Double the ride height, and you may be able to increase the diffuser angle by 50%. Slats or vanes in the diffuser can also help in keeping the air flow smooth with even larger separation angles. Trial and error, which is why all the big teams use the wind tunnels.

[ZR8ED]

Better grip through downforce is not the intention. The car already handles wicked enough for a street machine.

 

As far as and undertray goes, I already have a front one that goes underneath the entire front end of the car.. all the way back to the oil pan. It works good, and does not affect handling in a negative way. (I can't confirm that it improved it though.. way too many changes at once)

 

This is more of an exhaust fix for fumes entering the car. I've done everything possible (I think) and still have fumes come in with the windows down. It just happens to look "racey" hehe

 

It was not hard to build, so I will probably do it just for something to do. I still run a stock fuel tank, so until I go fuel cell, I probably won't bother with a rear tray. The tank hangs so low, it would be difficult to put in a proper diffuser.

 

Anyone else have any luck solving the exhaust in the cockpit? Like I said above.. with the windows up it is no problem.

 

I'll keep you guy posted.

[ZR8ED]

Here is the pic of my front bellypan.

 

 

I guess I should update some of these pics to take into account of the new paint job. hehehe

 

but you get the idea.

[ZROSSA]

Howdy,

 

Have a look around here. http://www.mulsannescorner.com/

 

lots of good stuff on downforce.

 

7 degrees is the acepted angle that the air wont seperate from. There are exeptions. Look at the back of any current F1 car and there is more than 7 degrees. I dont know the math behind it though.

 

There really is a lot more to do then just puting one of these on the back. You have got to take the whole car into it. The car needs to be very low. Like 2 inchs of the ground. The underside of the car need to be as smooth as yo can get it. You would really need a belly pan from the front to the back. You also need side skirts. Not like the ones you see for looks. These need to be shaped so that they create a vortex on the side of the car that prevents air getting under the car.

 

The only cars I know of that use this tech. on the streets ate the Ferrari 360 and the Maclaren F1. You will still need to be going pretty fast to notice the diff. I would guess at least 80 mph, more likely 100 + .

 

Mabye have a look around some viper sights. The race ones have a pretty mean one hanging out the back. They must do something.

 

Cheers

 

Douglas

[blueovalz]

Last but not least is the fact that if you depend on the ground effects for your grip and handling pleasure (street car), look out for the dip in the curve or typical uneven street roads surface. 200lbs of down force in the middle of a sweeping curve can turn to 0lbs in a split second, and with no where to go but your neighbors yard and tree. This is the biggest reason I don't even want to produce a full fledged "true" ground effects package (not to mention how hard it would be with the Z under side).

[1 tuff z]

this may be a small item, but i found that the seal around my license plate light wiring [that goes thru the tail panel] was gone. i'd check yours-it could b a source. i also find no probs with windows up. my problem is with only 1 window down. if i crack open the other-usually passenger, the prob goes away.

 

hope this helps...

[johnc]

As Douglas implied and I will expand on...

 

Aerodynamic modifications need to looked at as a whole car system if you are not just making changes for cosmetic reasons (nothing wrong with that.) You can achieve a lot with multiple aerodynamic modifications but be aware there's not "one thing" that gives you hundreds of pounds of balanced downforce. Its more an accumulation of many 20, 50, 75 pound changes that lets you realize significant downforce and the realted increase in grip.

 

Using the vehicle's underbody for aero imposes a lot of ride height and suspension design limitations. Specific modifications to side sills and rocker panels will allow a car with 5" of ride height to see significant downforce with a diffuser, front splitter, turning vanes, etc. The volume, velocity, and control of the air under the car is what's important. Actually, the most important thing to control is not the actual ride height, but fore/aft pitch - witness Mercedes problems at Le Mans a couple years ago.

 

Given extensive enough modifications aero can work well at low speeds. A Mod autocross cars are perfect examples of vehicles that can generate over 2gs of lateral grip at speeds under 60 mph. Most of that comes from gigantic wings though.

[DAW]

Maybe you could emulate what the carmakers/engineers do when designing these things and afix little streamers across the rear surfaces of the car and have someone videotape your car at highway speed to see what your changes do to the backdrafts/airflow, etc.

 

It's surprising to see some streamers just lying in dead air at 70mph. DAW

[Michael]

The 7-degree thing is true. It’s an empirical fact, and not the result of fancy calculation. Separation of the boundary layer is a response to an adverse pressure gradient. If you do the math, trying to solve for an attached flow, you would find that for sufficiently high adverse pressure gradient, the attached-flow solution is no longer possible. Basically, it’s like trying to drive uphill – if the slope gets too high, eventually you lose traction and roll back downhill. It so happens that 7 degrees corresponds to a particular pressure gradient, which is the threshold at which a conventional diffuser can no longer maintain attached flow. By the way, that’s for turbulent boundary layers. Laminar boundary layers (not practical for street cars) are even MORE sensitive to adverse pressure gradients. Think of it this way: the turbulent boundary layer is like an off-road 4x4 truck with studded tires trying to go uphill, while a laminar boundary layer is like an economy car with narrow, hard-compound tires. The economy car experiences less friction, which means that IF it can make it uphill, it will have less drag. The 4x4 truck has much higher drag, but it can make it up a much steeper hill.

 

The reason that some diffuser designs can exceed 7 degree upslope without separation is “spanwise relief”, A.K.A. 3-D effects. The pressure gradient is locally relieved to keep the flow attached. How is this done? Good question.... It definitely goes on a case-by-case basis. I don’t have a good answer for that one.

[Guest gprix1]

Hey ZR8ED,

What materials did you use for your belly pan and spoiler lip?

[Bob_H]

Thought on your exhaust problem.

The shape of the exhaust tips play a role in this, and I don't know if you have covered this before. The idea is to make it at 90 degree cut,(or angle it down) and if you were not fashion sensible, extend it just to the end of the bumper, or just past. With the pic you posted, your exhaust is pointed right up into the turbulent dead area behind the car. So any leaks in the rear will allow it to infiltrate the car. I have also been told, that at lower speeds,(less than 60-80), there is back flow up the sides,(above the fender, below/at the roof), which comes back to the corner of the window, and is the source of much of the "exhaust smell". It is not so much back flow as it is turbulent air not smoothly flowing past the windshield. Pointing your tips down might make enough of a difference to eliminate your problem. It is an easy thing to change and try. The traditional 45 degree cut on the tip is just as bad as angling it up into that dead spot. I can't explain enough about the crazy flow behind the Z at the speeds we encounter the "smell" or about 30-70. I think your exhaust tips are a major contributing factor to the fumes problem.

Think of it this way, the air leaving the bottom of the car,(talking close to the ground) has more energy than the air closer to the the bottom of the car. If you dump the exhaust fumes into the air with more energy, it is more likely to contiune on with that air and go behind the car. If you dump it into that area with low energy, it has a tendancy to stay where it is, right behind the car.

Give it a try before you go nuts with difusers and undertrays. I think you will be pleasantly surprised. I have worked with several Z's and had them change the exhaust from a 45 degree cut to a 90 degree or an angle down, and it usually eliminates most, usually all of the fumes with the window down.

-Bob

[Guest Anonymous]

Ok, so who wants to put streamers on the back of they're car and video it with a chase car? Give everyone a idea of whats going on. We had someone start to do that with a different area of the body using a manometer(?) , but he, well he isn't here anymore. At any rate, who's willing to give it a try?

 

Regards,

 

Lone

[Phantom]

Back to what started this all. ZR8ED's 280Z stinks of exhaust fumes, like nearly every other gen one Z out there. The fact is, they didn't when they were new or they would not have sold. So what has changed (failed)? Need to address hatch seals, tail light seals, and emmission control hose seals. My 280 stunk in its original stock configuration and later with the MSA Aero II kit on it too. I really think it's seals, not so much aerodynamics. The totally stock 83ZX I drive right now doesn't have the problem at all. So what is the difference between it and the '77Z? Are the aerodynamics all that much better or does it have a better seal design? Any ZX drivers out there with the "Stinky Cabin Syndrome"?