Jump to content
HybridZ
Andrew Bayley

Can a rear spoiler reduce exhaust fumes?

Recommended Posts

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

Share this post


Link to post
Share on other sites
Guest Fast Frog

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.

Share this post


Link to post
Share on other sites
Guest Anonymous

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

Share this post


Link to post
Share on other sites

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

Share this post


Link to post
Share on other sites

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!

Share this post


Link to post
Share on other sites

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 tongue.gif .

Go ahead Michael, try to teach me this stuff again, please. wink.gif.

 

------------------

Pete Paraska - 73 540Z - Marathon Z Project

[email protected]

Pete's V8 Datsun 240Z Pages

Share this post


Link to post
Share on other sites

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

drax2.gif

Share this post


Link to post
Share on other sites

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 Speedâ€, 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.

Share this post


Link to post
Share on other sites

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

[email protected]

Pete's V8 Datsun 240Z Pages

Share this post


Link to post
Share on other sites

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.

Share this post


Link to post
Share on other sites

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. smile.gif

 

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...

Share this post


Link to post
Share on other sites
Guest zfan

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

Share this post


Link to post
Share on other sites

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!

Share this post


Link to post
Share on other sites
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? :)

Share this post


Link to post
Share on other sites

Fumes?? I got em too unfortunatly...

BRE Rear deck and unknown lower spoiler. My hatch does not fit perfect. I don think it matters the spoilers under 60 mph, the fumes would be there anyways. Getting everything sealed the best you can is about the only option really. Venting the cabin too. The Z has sexy lines of Art but not built in wind tunnels. A fair mount of vertical frontal area with low pressure over the rear hatch glass I read once. Who knows??

The serious race cars had spoilers. Bet Paul Newmann didn't breath fumes.

 

The BRE spoiler was not easy to fit right!! At all! A complete joke. It took Considerable mods to get it to fit. Mine was the 2 mounting bolts at each side of the hatch as decribed above. The mounts sucked and were unuseable. Deck shape and gaps to the body not even close. If you go with this one be ready to do some body work. Out of the box I was dissapointed with the so called fit. All it does is gives you something to work with. Far from bolt on. Maybe others had better luck but just my 2 cents worth.

 

One a side note:

3M VHB tape (yes tape!) available these days would have no problems at all holding this part down. With proper surface area say 60% of the BRE outside footprint you would never be able to get it off the hatch even if you wanted too. 3M also provides a solvent to remove the tape and it is clear coat safe. The stuff is awesome.

No, I didn't tape down the spolier. Made 5 inside blind mounts and drilled the hatch. If I were to do it over again... I would have no problem or lack of confidence taping that sucker right on.

 

rear1.jpg

Share this post


Link to post
Share on other sites
Would a "Pantera" style hatch help or hurt the fumes situation?

 

 

Not really, but I don't have any weatherstripping around the doors right now. The fumes are so thick you can almost see it. My Z used to have a triple carbed L28 and i thought that changing the engine would help. Now my '73 is sr20det powered, and the fumes are as bad as ever. I'm going to sell my hatch and install all the proper weatherstripping soon. At speed on the freeway it's like tear gas up in the cabin. I hope changing the hatch will help. I hope to get one of those beta motorsports carbon fiber hatches.

Share this post


Link to post
Share on other sites

there are also other 'spots' on our z cars where those nasty gasses can enter the cabin. i'd check the following; grommet for fuel filler neck, grommet for fuel pump/sender wiring, grommet for license plate wiring, grommets for vapor tank lines, rubber plug in spare tire well, hatch plugs [side and/or underside of hatch] and finally, the hatch seals themselves. additionally, check hatch fitment [ie-closing tightly enough]. once that's complete, i'd also check the length of the pipe that extends from your muffler. it should be at least to the trailing edge of the bumper, if not a couple of inches beyond. i've done all of the above and get almost no fumes in my z at all.

Share this post


Link to post
Share on other sites

Venture... If I read Michael's post correctly, yes, a Pantera treatmenjt should help:

"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)."

The Pantera has the flying butresses of an El Camino, but covers the bed. Should still work (in my none too humble opinion) as the shape is more brick like, and no longer has the absolutely horrible hatch angle.

 

 

.

Share this post


Link to post
Share on other sites

×