Can a rear spoiler reduce exhaust fumes?
Posted 26 February 2001 - 05:54 AM
Posted 26 February 2001 - 08:08 AM
Posted 26 February 2001 - 08:45 AM
I've never had an exhaust fume problem from the rear of the 383 Z, though from time to time, I've had exhaust smells from my other 3 Zs eventho my hatch weatherstripping is brand new.
Posted 26 February 2001 - 09:11 AM
Posted 26 February 2001 - 10:58 AM
Posted 26 February 2001 - 11:52 AM
Posted 26 February 2001 - 05:36 PM
The way he explained the spoiler to me (let's see if I get this correct) was that it helps create a stationary vortex just in front of the spoiler, and this causes the flow to separate higher up the hatch, which shortens the streamline path over the car, and reduces the lift. I don't know if it does much to kill the effect of the Kamm (sp?) tail of the Z, but it might cause the flow to detach enough that it won't just hang behind the car as much.
That's my shot at "winging" this topic .
Go ahead Michael, try to teach me this stuff again, please. .
Pete Paraska - 73 540Z - Marathon Z Project
Pete's V8 Datsun 240Z Pages
Posted 27 February 2001 - 06:17 AM
Since I'm an idiot and left all my M.S.A catalogs at my Dad's place, could someone please give me a ballpark figure on how much they run. Also, I can't imagine the installation being all to difficult... maybe someone could also reassure me on that.
Posted 27 February 2001 - 10:30 AM
One has 2 mounting bolts at each side of the hatch, drilled through. Hard to explain, but think of it as 2 bolts per side through the hatch surface, as far away from the centerline of the hatch as possible.
The other way isn't as nice! It depends on the spoiler, but mine had a fibreglass webbing with holes drilled in it, between the front and back of the spoiler. I used a magnet to get the bolts into place in the webbing, and then put them through holes drilled in the hatch surface. (about 1/2 way from the centerline to the edge of the hatch, 1 on each side) Then I removed the inside cover from the hatch (the leather covered interior panel) and put the nuts on there.
Both methods are very solid, but the first one is definately more user friendly. I'm not sure which of the spoilers I've seen was from MSA and which weren't.
Just remember if you get the one with the webbing, a magnet can save you HOURS and HOURS of frustration, trying to move a bolt into position from a very small place.
Posted 01 March 2001 - 09:17 PM
Sadly, the 240-60-80Z has one of the worse shapes in terms of aerodynamics of cars in recent memory. It has small cross sectional area, which greatly helps. But apples-to-apples, it had more drag and un-downforce (lift) than cars like the Chevy Chevelle. Basically, the reason is that for good aero performance, the carâ€™s body should be smooth and swoopy in some places, but sharp and abrupt in others. Generally, the smooth part should be up front â€“ the grill area, the headlights, the hood/windshield juncture. The sharp and abrupt part should be in back â€“ the trunk, rear of the cabin, etc. The Z is swoopy and smooth in precisely the wrong places, and sharp and abrupt â€“ again! â€“ in precisely the wrong places. Itâ€™s almost as if the Zâ€™s designers worked long and hard to deliberately do a bad job on the carâ€™s aerodynamics. And to add to the injustice â€“ cars like the VW Rabbit and Ford Pinto had far superior aerodynamics!
Second, all spoilers, whether highly-cranked NASCAR wings or rice-boy trunk jobs or the flat panels behind NHRA pro-stockers â€“ are all intended to do primarily one thing: increase downforce. Downforce â€“ or negative lift â€“ can be explained by several alternative and equivalent ways. One way is: higher pressure above a body than below it will cause downforce. Another: pushing air up will cause (by conservation of momentum) downforce on the body doing the pushing. The main difference between a NASCAR spoiler and a F-1 wing (which passenger cars sometimes try to emulate) is that the former is designed to act as an accessory to the carâ€™s body, while the latter is designed to act like an upside-down wing in isolation. And that design choice makes sense: NASCARS are big and cause a lot of air disturbance, while F-1 cars are small and narrow. Also, all spoilers cause induced drag (drag due to lift). But some also cause additional drag, while others actually reduce drag! A downsized NASCAR-style spoiler will actually reduce drag for most street cars, especially when coupled with a nose airdam. The long, essentially horizontal â€œspoilerâ€ of NHRA pro-stock cars is based on the idea that the fully-separated flow behind the car and underneath the spoiler is low pressure, while the partially-separated flow on top of the spoiler is of higher pressure. It probably has very little impact on the drag.
Third, the exhaust smell plaguing the Z is indeed caused by a low pressure above the hatch lid, which leads to entrainment of the exhaust. Any gap in the hatch seal will allow a leak. Opening a window makes this worse.
Fourth, and most important: because flow over a car is largely separated and unsteady, with strong ground effects thrown in, car aerodynamics are much, much more complex than airplane aerodynamics. The reason that there are so many competing designs, so much mystery and so much empiricism from the mom-and-pop race track to the Daytona 500, is NOT that car guys are ignorant and airplane guys are knowledgeable, but that cars are so much harder to figure out. Most of the improvements in recent years have been through trial and error. Of course, some things really are obvious; these include things like reducing gaps between body panels.
So, what to do about the Z? Well, precisely because the stock shape is so bad, this is a fortuitous case where just about any hack job will actually be an improvement. Cheezy bolt-on parts that you find on E-Bay for $20 actually can help â€“ whereas the analogous parts for a Honda Civic only screw things up.
Here is what happens in a boxy sedan: flow that streams over the windshield then goes over the roof, then â€œseparatesâ€ (no longer follows the bodyâ€™s contour) where the roof ends. That flow could go on behind the car. But it can be argued from the basic equations of fluid mechanics that a fast flow (over the roof) mixing with a slow flow (behind the rear glass) will cause the fast flow to bend toward the slow flow. So, the wake behind the roof bends down. In a well-designed sedan (best example ever: Lexus GS430), the truck lid probably (I say probably because I have NOT seen Lexusâ€™s data, and I doubt that they would be eager to share it with me!) â€œcatchesâ€ the wake as it bends down. That means that thereâ€™s separated flow behind the rear glass and over the trunk, but the wake behind the car is only as high off the ground as the rear of the trunk. Generally, the smaller this â€œeffective areaâ€ of the separated flow behind the car, the lower the overall drag. So paradoxically, the blunt-rear sedan emulates a â€œteardropâ€ shape!
But what about an actual teardrop shape? Well, for the flow to remain attached over the rear portion of the teardrop, the teardrop has to contract very gradually. A truly teardrop-shaped car would be something like 40 feet long! If you guys ever get the chance to look at a wind tunnel, note that wind tunnels are very long and gradually expanding pipes (from the test section to the fan). Thatâ€™s in order to avoid flow separation from the diverging walls; this also follows the teardrop shape. The same idea is used in air conditioning ducts, jet engine nacelles, and all sorts of devices that depend on airfoil not separating from the walls. So, while the Zâ€™s hatch looks gently sloping, from the aero point of view, itâ€™s not.
One idea is to make a 40-foot long teardrop-shaped car, and then cut off the back 20 feet. Then, maybe the flow will not separate until it reaches the very back of the car, which is fairly small in cross-sectional area. This is called the Kamm tail. The Z is NOT a Kamm-type design. NO PRODUCTION CAR HAS EVER SUCCESSFUL FOLLOWED THE KAMM DESIGN! The closest attempt that Iâ€™m aware of is the Saturn EV-1. Ugly looking thing, but it works. Unfortunately, a failed Kamm-type design can be worse than a brick-type design. That is what happened with the Z. Hereâ€™s a paradoxical (but verified!) trick: remove the hatch lid of the Z completely, turning your Z into a mini-El Camino. The exhaust smell will go away (for various reasons) !
Is a BRE-type spoiler superior to a whale-tail spoiler for the Z? Well, to be honest, I really donâ€™t know. My GUESS is that the BRE spoiler will be better, because the whale-tail acts more like a wing, which is better in a smoother flow than that behind a Z hatch. But itâ€™s very apples-to-oranges because the whale-tail has much larger area and sits further back than the BRE spoiler. Look at what the GT-2 Zâ€™s ran â€“ I think it was a NASCAR-style spoiler, an aluminum plate. That is probably best for high-speed stability (makes more downforce and stabilizes the wake).
Finally, back to the original question â€“ will a rear spoiler reduce exhaust fumes in the cabinâ€¦. My opinion is that the spoiler will help, but to a limited extent. The reason is that the spoiler â€“ regardless of its shape â€“ will mount to the hatch, and not to the body underneath the hatch lip. So, the low pressure underneath and behind the spoiler will still be sitting over the hatch gap. The exhaust will be entrained into the region underneath the spoiler, and will still try to get into the crevice between the hatch and the body. In that sense, the spoiler fails. But, with good spoiler, it is likely that the overall flowfield behind the car will be â€œmassagedâ€ such that the overall exhaust entrainment behind the car will be weaker. So if you are thinking about a spoiler to reduce drag or increase downforce, but are also concerned about the exhaust fumes, by all means buy the spoiler. But donâ€™t buy it solely for purposes of controlling the exhaust smell.
Posted 01 March 2001 - 09:34 PM
Somebody, if they have ever seen something as deep and thought provoking posted on a car related web site, chime in.
Michael, that was very impressive and informative! Thanks!
Pete Paraska - 73 540Z - Marathon Z Project
Pete's V8 Datsun 240Z Pages
Posted 01 March 2001 - 10:27 PM
Posted 01 March 2001 - 10:48 PM
Posted 04 March 2001 - 07:36 AM
As far as getting rid of the fumes, have you considered doing a side exit exhaust? Instead of trying to get rid of the aero wake, get the fumes out of the wake instead...
Posted 04 March 2001 - 09:41 AM
pipes out the side, kind of noisy though. Will not be sneaking up on anyone anytime soon. Come to think of it with that pearl purple paint job on my Z, I doubt I would sneak up on anyone anyway.
Posted 18 January 2006 - 11:26 AM
Posted 18 January 2006 - 08:00 PM
Although, the side pipes *do* look really mean!
Posted 18 January 2006 - 09:35 PM
Posted 18 January 2006 - 09:54 PM
The Z is swoopy and smooth in precisely the wrong places, and sharp and abrupt â€“ again! â€“ in precisely the wrong places. Itâ€™s almost as if the Zâ€™s designers worked long and hard to deliberately do a bad job on the carâ€™s aerodynamics.
So, what I'm hearing is that we should be driving them in reverse all the time?
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