Jump to content


Photo

Can a rear spoiler reduce exhaust fumes?


  • Please log in to reply
78 replies to this topic

#1 Andrew Bayley

Andrew Bayley

    Member

  • Members
  • PipPipPipPip
  • 358 posts

Posted 26 February 2001 - 05:54 AM

Just wondering if anyone had some before and after comparisons of exhaust fumes with a rear spoiler. My 74 is really bad and I'm starting to think that many of my summer headaches are related to my stinky Z car. I've already went nuts with a tube a silicon sealer in the hatch area, but it still stinks. Would a modest spoiler help move the gases away from the car at all?

-Andy

#2 John Scott

John Scott

    Member

  • Members
  • PipPipPipPipPipPip
  • 1115 posts
  • LocationNorthern Colorado

Posted 26 February 2001 - 08:08 AM

This has been addressed a couple of times..about a yer ago. Was it Mark(?) who said the rear spoiler did in fact reduce the fumes from entering the rear? Must be a common problem. I get sick from the stuff too. My hatch could use some tightening.
JS

#3 Guest_Fast Frog_*

Guest_Fast Frog_*
  • Guests

Posted 26 February 2001 - 08:45 AM

Most definately!! I've got a standard wing from MSA. It reduces the eddying effect of allowing turbulant air mixed with exhaust gases from moving back up the rear of the car while driving at speed.

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.

#4 Guest_Anonymous_*

Guest_Anonymous_*
  • Guests

Posted 26 February 2001 - 09:11 AM

Any thoughts on spoiler shape? Does it need to be the bre type spoiler, or does a wing or whailtale do the same? Reason I ask is I've been toying with the idea of a wing off one of the later cars (one that looks like a potiential candidate is the pontiac grand am or grand prix can't remember which) and I'm wondering if that'll help. Is it just upsetting the flow back there to break things up and keep the exhaust out? Curious.

Lone

#5 blueovalz

blueovalz

    HybridZ Supporter

  • Donating Members
  • PipPipPipPipPipPipPip
  • 3291 posts
  • LocationLittle Rock, Arkansas

Posted 26 February 2001 - 10:58 AM

Intuition (plus observations of rain and oil smoke behind various air handling devices) tells me that a wing would work in this sense, but not a spoiler (i.e NASCAR). The wing is designed (help me out here if I'm getting too far off base here) to force the air up (thus pushing the car down), and keeping drag to a minimum (considering the application), whereas a spoiler is designed to create a high pressure center over the back of the car to help eliminate (or push) the low pressure over the top of the car. But the result is a big low pressure area behind the car, and a "rolling" vortex of air (axis parallel with axles) behind the car, thus allowing the exhaust gasses to pass back into the car. Anybody else??? Because I've seen some good air research out there before.

Terry

#6 DavyZ

DavyZ

    Super Moderator

  • Super Moderators
  • 5366 posts
  • LocationCharlotte, NC

Posted 26 February 2001 - 11:52 AM

Terry, this sounds correct, but I don't want a wing! Shoot...may have to suck MTBE fumes with a spoiler...shoot, shoot, shoot. Oh, well, I'm not even there yet, so I'll see what everyone says first! Man, I love this site!

#7 pparaska

pparaska

    Retired Admin

  • Donating Members
  • PipPipPipPipPipPipPipPip
  • 5057 posts
  • LocationGlenelg, MD, USA

Posted 26 February 2001 - 05:36 PM

Michael (Ol), our resident aerodynamicist, will hopefully chime in soon.

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 Posted Image .
Go ahead Michael, try to teach me this stuff again, please. Posted Image.

------------------
Pete Paraska - 73 540Z - Marathon Z Project
pparaska@home.com
Pete's V8 Datsun 240Z Pages

#8 Andrew Bayley

Andrew Bayley

    Member

  • Members
  • PipPipPipPip
  • 358 posts

Posted 27 February 2001 - 06:17 AM

Ya know, that BRE style wing is growing on me...

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.

Thanks
-Andy

#9 Drax240z

Drax240z

    Stuper Modifier

  • Super Moderators
  • 3623 posts
  • LocationVictoria, BC

Posted 27 February 2001 - 10:30 AM

I've seen 2 different types of installs on the BRE spoilers. Not to worry, both are fairly easy!

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.

------------------
Richard Lewis
Posted Image

#10 Michael

Michael

    Member

  • Members
  • PipPipPipPipPip
  • 680 posts
  • LocationSW Ohio

Posted 01 March 2001 - 09:17 PM

Since Pete invited a response, I shall oblige. :-) It is rather awkward to attempt to “instruct” anyone, but I will try to share some thoughts I had about the Z’s aerodynamics. A proper explanation really requires a book. One such recent book is “Race Car Aerodynamics: Designing for Speed”, by Joseph Katz (Robert Bentley Publishers, 1995). And that is not an especially good book – but it is a good intro.

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 car’s body should be smooth and swoopy in some places, but sharp and abrupt in others. Generally, the smooth part should be up front – the grill area, the headlights, the hood/windshield juncture. The sharp and abrupt part should be in back – the trunk, rear of the cabin, etc. The Z is swoopy and smooth in precisely the wrong places, and sharp and abrupt – again! – in precisely the wrong places. It’s almost as if the Z’s designers worked long and hard to deliberately do a bad job on the car’s aerodynamics. And to add to the injustice – 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 – are all intended to do primarily one thing: increase downforce. Downforce – or negative lift – 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 car’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 “spoiler” 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 – 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 “separates” (no longer follows the body’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 Lexus’s data, and I doubt that they would be eager to share it with me!) “catches” the wake as it bends down. That means that there’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 “effective area” of the separated flow behind the car, the lower the overall drag. So paradoxically, the blunt-rear sedan emulates a “teardrop” 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). That’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 Z’s hatch looks gently sloping, from the aero point of view, it’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 I’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. Here’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 don’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 it’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 Z’s ran – 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 – will a rear spoiler reduce exhaust fumes in the cabin…. My opinion is that the spoiler will help, but to a limited extent. The reason is that the spoiler – regardless of its shape – 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 “massaged” 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 don’t buy it solely for purposes of controlling the exhaust smell.

#11 pparaska

pparaska

    Retired Admin

  • Donating Members
  • PipPipPipPipPipPipPipPip
  • 5057 posts
  • LocationGlenelg, MD, USA

Posted 01 March 2001 - 09:34 PM

WOW!

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
pparaska@home.com
Pete's V8 Datsun 240Z Pages

#12 DavyZ

DavyZ

    Super Moderator

  • Super Moderators
  • 5366 posts
  • LocationCharlotte, NC

Posted 01 March 2001 - 10:27 PM

Michael, thanks for the information. I'll download it when they make a disc big enough...No seriously, I am going to use that info to make a decision. I like the BRE type spoilers better then the wings or whale-tail spoilers. If it doesn't matter a whole lot, I may suck fumes anyway, but hopefully not.

#13 Drax240z

Drax240z

    Stuper Modifier

  • Super Moderators
  • 3623 posts
  • LocationVictoria, BC

Posted 01 March 2001 - 10:48 PM

Great stuff Michael. You know, you've almost convinced me to do my 4th year mechanical engineering studies in aerodynamics/fluid mechanics. Posted Image I love this stuff!

------------------
Richard Lewis
Posted Image

#14 Mike kZ

Mike kZ

    Super Moderator

  • Super Moderators
  • 2521 posts
  • LocationAtlanta GA

Posted 02 March 2001 - 06:25 AM

Michael, what about Motorsports Supra style wing, the one that sits at the top of the hatch like a roof spoiler? What affect does this wing have? downforce? will it help with fumes?

------------------
http://members.tripo...er/mikekz1.html

#15 TimZ

TimZ

    OG Member

  • Members
  • PipPipPipPipPipPipPip
  • 2318 posts
  • LocationDearborn, MI

Posted 04 March 2001 - 07:36 AM

Wow. I normally don't look in this forum that often, just because I wasn't planning on doing any body mods in the near future. Glad I decided to look. Posted Image

Excellent discussion.

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...

#16 Guest_zfan_*

Guest_zfan_*
  • Guests

Posted 04 March 2001 - 09:41 AM

That is what I just did. Exited 2 1/2 inch
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.
Mike

#17 Ed260Z

Ed260Z

    Member

  • Members
  • PipPipPipPipPip
  • 533 posts

Posted 18 January 2006 - 11:26 AM

I thought I might bring this thread back from the dead. It is a very interesting read, and Mike just about wrote a book on one post.

#18 beren

beren

    Member

  • Members
  • PipPip
  • 38 posts

Posted 18 January 2006 - 08:00 PM

Thanks for bumping this, Ed260Z. I hadn't read it, and was wondering the same thing, myself. Can this be a sticky?

Although, the side pipes *do* look really mean!
Posted Image

#19 EZ-E

EZ-E

    Member

  • Members
  • PipPipPipPipPip
  • 530 posts
  • LocationNear: Edinburg, Pennsylvania

Posted 18 January 2006 - 09:35 PM

I myself have never read a more informative post on something such as this, I have learned a lot just form that one post, and I think that this should be kept alive to understand more about the Z characteristics of aerodynamics.

Fantastic post!

#20 beren

beren

    Member

  • Members
  • PipPip
  • 38 posts

Posted 18 January 2006 - 09:54 PM

The Z is swoopy and smooth in precisely the wrong places, and sharp and abrupt – again! – in precisely the wrong places. It’s almost as if the Z’s designers worked long and hard to deliberately do a bad job on the car’s aerodynamics.


So, what I'm hearing is that we should be driving them in reverse all the time? :)
Posted Image




0 user(s) are reading this topic

0 members, 0 guests, 0 anonymous users