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Diffusers and belly pans


rustrocket

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All I can say is I was very disappointed at how the underpan fit on the Bonneville Car.

I was underneath the car last night while changing an oil pan, and was looking at ways to properly attach a bellypan to a 260Z.

 

When I saw Boyd's Alumatub Underpan, I thought "Tha'ts what OURS should have looked like! Surrrre, I'll just go have Marcel make one for me... Riiiight! :D I really like the way the back portion of ZR8ED's pan looks. VEry nice, our pan for Bonneville was not nearly as clean, and was held up by (uh, forgive me) bailing wire...

 

Anyway, I havne't done any Z-Yarn studies, but I have some Dywer Manometers to use for some static pressure stations you can attach to the car at various positions. The pressure that builds up in front of the windshield is quite substantial.

 

For the street, you guys have it made, you can do ALL SORTS of stuff we are PROHIBITED from doing at Bonneville. Like ducting underhood pressure to the Wheel Wells (someone mentioned low pressure there: RIGHTO!)

You wanna increase airflow across the radiator, make some big holes in the fender wells to allow stagnant underhood air to vent to a low pressure area behind the wheels while going down the road. A full bellypan would also make this necessary (or at least some louvres along the spine for letting out exhaust heat!) The bellypan WILL make the floor of the car hot! You need to carefully balance airflow across the undercar components to keep them from raidating heat into the floor and into the car. JohnC mentioned this in his post in July...

 

Aerodynamically, the belly pan and blocking the radiator opening helps with top speed quite a bit! Even more than running that full exhaust...

 

Thought, on the street, a side-exit muffler would be acceptable...

 

I digress.

 

The yarn thing really isn't that difficult to do. Just make sure your paint is appropriately adhered, and DON'T BY YARN IN SKEINS! Use "Latch Hook Yarn" which is ALREADY precut in bundles of like a thousand threads for making carpets.

 

Not that I ever stole latch hook yarn from my mother to yarn up my 62 Microbus after reading some atricles that said your head temperature went up 15 degrees with the windows open... (I am truly diseased, and have been so for a loong time!) I have some 8mm movies on Kodachrome of me driving my bus with about a million Blaze-Hunter's-Orange threads scotch taped to the side! (It was the 70's what can I say? Mom had som bizzare ideas about what colors were acceptable for carpets well into the 80's...)

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Ah... slightly OT here maybe, but after reading thru here and some other threads on bellypans and such, one thing that doesn't really get hit on... what is the most effective way to increase airflow thru the radiator on a 240z? I have the odd problem that my car can run all day in stop and go traffic and the temp sits right in the middle, even on the hottest days, but get on the freeway and get over 60mph, and temps climb 20-30 degrees or more. I figure it HAS to be an airflow problem, as I've addressed everything else. I'm running one of MSA's front air dams, and I do have an FMIC in front of the radiator, though the problem existed before I installed it.

 

If the problem existed before the FMIC was installed, then like John said, stick to the basics. If that doesn't then get a aluminum rad, electrical fan, etc. Course you can just start over and V-mount the IC and RAD....

 

A vented hood like mine also helps get rid of the air built up in the engine bay.

 

Owen

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Guest bastaad525

I have a 280z vented hood... you can see heat escaping thru the vents so I know they're helping at least a bit.

 

By the way I've tried the little trick of popping the hood while driving so air can enter in or exit out that way and help keep the engine bay cool... made NO difference in my coolant temps though.

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I don't think you'd see a difference in the coolant temps, unless your temp gauge was sensititve enough. But I'm positive that just having the vents aids in moving air through your radiator and IC more efficiently, rather than letting the hot air stack up in your engine bay.

Owen

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It would be interesting to see how the 280Z hood vents function at speed. Sems like it would be a underhood pressure vs frontal windshield pressure battle. At certain speeds there might not even be any flow in either direction in or out of the hood vents. I think that for venting at speed, the vents should probably be positioned as far forward on the hood as practical so that they don't get any cowl induction effect from being to close to the windshield.

 

Mike

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you're correct, Z-Tard.

My experiments with some static pressure probes shows the vents on a 280Z hood are about the farthese back you can go without hitting the higher pressure area caused by the diffuser action of the upright windshield at speed.

 

Take a close look at the positioning of the 280ZX Turbo Vents, and porportionally you will see they are actually closer to the back of the engine, relative to the 280Z due to a better windshield angle on the ZX, and consequently the lower pressure at the base of the windshield, and smaller high-pressure area on the ZX.

 

Somewhere long ago I actually drew pressure maps from what I read on my 240 with the static pressure probes attached. I have no idea where they went---that was like 15 years+ ago!

 

I had much less life then, amd far more time on my hands for such foolishness! :D

 

Anyway, my understanding of the 280vents was more for radiant heat removal during slow-speed than for high-sepeed venting. Though the pressure under a hood at speed (say 100mph) is considerable, so I would expect a good stream of hot air being forced out through these vents, and mixing with the slipstream across the hood of the vehicle before being dammed up once again at the windshield.

 

This may sound funny, but on the old Buicks and etc at Bonneville in the 50's there was a common tactic of cutting 12" diameter holes in the firewall, and ducting that firewall area through the interior of the vehicle, to the rear windshield area using stovepipe! In one instance related to me by Dave Bearden, their vehicle showed no appreciable increase in speed from one year to the next after putting in a big hemi engine, of about 2X the HP from the previous year, sticking around 190mph. After doing some "yarn studies" and watching the car at speed, Dave went into Wendover to a hardware store, got some stovepipe, and did the above modification. The speed went from 190 to 237, and they got their record.

This story also involves an engine fire, and the pipes burning through towards the end of the return run... but that's for another topic.

 

Since then, that specific modification (termed in the SCTA Rule Book as an 'air vent' or 'pressure vent') has been specifically disallowed in all classes. Now you have to figure out how to accomplish it without using the easy piping. It becomes increasingly difficult to cheat when you have guys out there who have literally been doing it for 50+ years! 8)

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  • 3 weeks later...

Hey Guys, man this was a BAAAAD thread to come across while your at work!!!

 

A couple things you need to address here.

 

Aerodynamics are NOT an intuitive science!! so what looks good won't necessarily work well!!! There a too many interdependent variables and NOTHING IS FREE!!!! We are merely trying to minimize the losses created by a body moving through the viscous fluid known as "air" and harness some of its energy to our benefit if possible

 

So first of all, some clarifications:

 

1. The concept of two air molecules starting at the same point traveling around opposite sides of a smooth body and meeting again at the back of the body is known as the "Kuta Condition." This is used in inviscid flow theory along with Bernoulli's methods as a way of "characterizing" the function and flow of air that we cannot explain directly. The other concepts mentioned pertain to "downwash" which is a more Newtonian concept of redirecting the air that is used in conjunction with inviscid flow theory to estimate aerodynamic loads. Because we are not able to measure EXACTLY how air functions all the time, we deal in generalities that allow us to approximate what is going on...it is a very artistic science as a result.....

 

2. Given the concepts of #1, an automobile is inherently lifting off the ground due to it's shape being like a wing. The top of the car is longer and smoother causing the air to move faster over it than under the bottom of it. Add to this all of the obstructions and heat transfer from the engine to the air passing under the car which further increases its pressure, thus further contributing to lifting the car as well as increasing the drag resistance on the car.

 

OK so if you can minimize the obstructions to the air passing under the car and better yet, speed up the air in the process, you reduce both the drag and the lift acting on the car......

 

So think of the underside of the car and the ground below it as a wind tunnel.....the air running under the car goes from a large area in front of the car to a small area under the car which (if it doesn't contact any obstructions) will speed the air up and lower its static pressure. This is the function of a smooth belly pan under the car, but nothing is free, so now the air under the car is moving much faster than the air around the car and specifically behind the car so if this fast air runs into the slower air, you "choke" the air flow which will greatly reduce the effectiveness of your belly pan. You must then find a way to slow the fast moving air down back towards the rear of the car increasing its pressure back to the rest of the air around the car without effecting the air behind it still moving through the "neck" of the tunnel under the car. Hence the need for the diffuser which allows the air to smoothly expand and slow down to the speed of the air behind the car, which if done correctly can even help to "suck" the air out the back of the car.....

 

The Diffuser DOES NOT create downforce, it increases the effectiveness of belly pan....

 

This is the baseline to start with.....a continuous smooth surface that runs from a large space at the front of the car, necks down smoothly to a small space at the middle of the car, and expands smoothly to the back of the car. If you add skirts to the sides of the car, you box in the tunnel, further increasing it’s efficiency….

 

The arrangement that ZR8TED has is beautiful, and I dig it, but it is more of a rear wing than a diffuser, thus he will get rear down force benefits, but not see the drag reductions thus will need more power!!! :)

 

This explanation is the benefit of an aerospace engineering degree...

 

The detriment....action movies seem really stupid and borring to me!! :D

 

Cheers,

 

Ian

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I think that for venting at speed' date=' the vents should probably be positioned as far forward on the hood as practical so that they don't get any cowl induction effect from being to close to the windshield.

 

Mike[/quote']

 

This is exactly the reason why my vents start at the radiator core area (actually a little bit behind) and then move back towards the windshield. In order to create effective "suction" from the engine bay, I added lips on the topside of the hood in the hopes of generating low pressure areas.

 

Too bad I don't have a wind tunnel to play with, might have to do some dangerous testing with smoke and a video camera at speed... :D

 

Ian, great post on clarifying the true nature of the diffuser! This thread is a keeper.

Owen

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ZROSSA,

 

Aero improved Z for the street...

 

There in lines the conflict!!

 

if you said Z for the track, well that’s different, but for the street, you must have sufficient ground clearance for driveways and speed bumps, not to mention sufficient airflow through the radiator in stop and go traffic.

 

I think for a streetcar, the G-nose setup Tony-D spoke of is the best compromise, just add a vented hood

 

BUT make sure the vents are at least 18 inches or more away from the windshield, otherwise you force more air into the hood due to the hi pressure region in front of the windshield (otherwise known as cowl induction)

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One of these days the fiberglass work on my Z will be finished and you will see that this is exactly what I have done. I may even duct the radiator straight to the bonnet vent so that the air doesnt get confused on the way out. The fiber glass guy has had the thing for almost a year now. Nose cone and rear spoiler are done but still need my new bumper "diffuser" to me made. Good things take time............I guess :roll:

 

Douglas

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Interesting topic.

 

Does anyone have a best guess as to what the aerodynamic affects of the chopped top and laid back windshield will have on Henrik Schiolde's car? (He is the guy from Sweden with his car posted on cardomain).

 

A guy named Matt Weaver has done some interesting studies on laminar flow using human powered vehicles (bicycles with aerodynamic carbon fiber shells). http://www.speed101.com

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That front air dam looks a lot less than 4 inches! There was a thread on air dams and chin splitters too, don't remember if it was in this thread or not...

 

I think what we all need to do is step back and wonder, do we really need a diffuser, etc. on a street car? I'd probably make on for my own street car, but it would never get any use, pity...

Owen

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What about 4 inches of ground clearance. WOuld that be suffucient for created a "tunnel" under the car with this concept that i've designed?

 

Nope. A splitter needs to be very close to the ground. I tried to do a big splitter on the front of my 240Z but I didn't get the ride height down enough and it became a lift device. Really scary in a big sweeping turn at 120+.

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I found out recently that there is a high school, yes a high school, that has a wind tunnel relatively close by... They get tons of money because there is a nuclear power plant there and yeah, you get the picture. But, there might be a chance that I can get my car in there because my cousin has a friend who is a teacher there and they might be able to use my car as a class project or something. Basically, if there is some aerodynamic stuff that you guys want tested out, I could try making it and take the car to the school to see if they will test the aerodynamics of it.

 

I have tons of ideas for different aerodynamic items to put on it and this would be an awesome way to find out how well they work. Hopefully I can pull the right strings and get this done. Any yays or nays?

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Building an entire belly pan might be a little labor intensive for such an undertaking. I'd suggest looking at the airflow characteristics of different types of spoilers: BRE, Whale Tale, Ricer Shopping Cart Handle, etc. Depending on the speed of the air in that thing, I suppose parts could be made from balsa wood or cardboard to reduce cost. A cardboard belly pan would be pretty easy to fab up. Maybe just loan the car to the class for a few days, along with some resaerch material concerning automotive areodynamics, and see what the students come up with. Sometimes a fresh perspective on things can make for some interesting solutions.

 

Mike

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I would think that if one were successful in creating a pan, tray, sheet, etc, under the car that met all the characteristics of "ground effects", that it would require quite a bit of rigidity and a strong means of attachment. Otherwise, higher pressure above the pan, tray, sheet, would force it to separate from the unibody. A very small amount of pressure difference over the broad bottom panel's area would seem to make for easy separation.

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What about 4 inches of ground clearance. WOuld that be suffucient for created a "tunnel" under the car with this concept that i've designed?

 

Nope. A splitter needs to be very close to the ground. I tried to do a big splitter on the front of my 240Z but I didn't get the ride height down enough and it became a lift device. Really scary in a big sweeping turn at 120+.

 

How close is close? Do drifters get and benefit in regards to aerodynamics because those cars are pretty low. Or are you talking like two inches or less, similar to a GT race cars, which are literally sitting on the ground? If it has to be this low more factors come into play. When cornering at 50+ mph you need to be sure your suspension is stiff enough so that you don't rub the ground.

 

Another thing I'm beginning to wonder is whether a front "wing" (fascia) (valance, you get the idea) is more benificial to have the overall shape like this or like this >. If the latter is more beneficial for aerodynamics, back to the drawing board for me. Just the shape those lines make is what I'm referring to.

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I've been watching drifting in Option videos even before it became a "sport" here in the US and I know they go fast, but do they ever go fast enough to benefit from all the ground effects, canards, etc? It's a strange relationship to me...downforce and drifting...In any case, I think the ground clearance of a GT car and a stiff suspension is what you'd want.

 

As for the shape of the front air dam, I've asked about this before and I think John Coffee replied that his design was good (at least on paper) :D

 

Anyone know what sort of shape is beneficial for side skirts? I've seen plain boxy ones as well as ones on the Porsches that have a kind of twist in them, Just looks???

 

Owen

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