Exhaust lengths

[Marcusu]

Hi all,
I’ve never really solved the exhaust fumes blowing back into the cabin issue in my 2+2.
I’m wondering if the length of the exhaust pipe could contribute to this problem. I have the twin exit pipes and they stop about an inch and a half inboard from the rear bumper.
I would be interested to know if this is the same with other cars, or should they protrude further out than the bumper?
Cheers,
Marcus.

[bitsobrits]

The problem is one of aerodynamics, not exhaust length. There is an interesting video (link below) which talks about airflow, using a scale models of an OTS and FHC in a wind tunnel. Worth a watch, especially from about 14 minutes on. At about minute 18 you can see the way that airflow goes forward somewhat from the tail of the car, especially at lower speeds.

[Simonpfhc]

Really interesting video, thanks for posting

[Marcusu]

Thanks for the replies gents.
It sounds it’s just down to the design of the car and aerodynamics, especially with the airflow over the 2+2 drawing exhaust gases forward.
Funny I never noticed this issue with the MGB GT’s I’ve owned.
I’m considering selling the car as I want to enjoy it with my family but don’t want my daughter sitting in the back seat where the fumes seem to pool after coming in through the windows.
Cheers,
Marcus

[lowact]

Marcus, I see you have been trying to fix this for a few years now. Imo the fumes are not coming in the windows. With windows open the pressure in the cabin is less, exhaust gas is drawn in from the rear via, e.g., rear door seal not tight, missing bungs, empty screw-holes, perished light fitting rubber seals, etc. I.e. with the windows open air is leaving the cabin via the windows, being replaced by fumes sucked in, through such nefarious openings, from behind the car. Do the quarter vents give you opportunity to change this; if you open them in such a way that air is directed into the cabin so that, even with the windows open, the cabin is “pressurized” to more than the local low-pressure area behind the car?

Spare a thought for us ots jockey’s, who can’t do this. With ots the cockpit is at even lower pressure and there are openings across the haunches from the boot into the cockpit; means that all the time you are driving along, the low-pressure area behind the windscreen is manically trying to suck the contents of the boot into the cockpit …

Here’s a post by Tim Ward who tried everything to resolve this issue, including extending the exhausts by a foot. Spoiler alert; no joy:
viewtopic.php?f=6&t=17362&hilit=exhaust+fumes#p164475

And here’s (yet another) study of etype (ots) aerodynamics, this one using CFD (computational fluid dynamics)

[Marcusu]

Thanks Colin,
I have suspected leaks from the rear of the car and the rear door seal. To test this I sat in the rear seat with a CO detector while a mate of mine drove the car. I stretched back and pointed the detector around various areas at the rear of the boot including the rear door seal and didn’t notice a spike in CO levels with the windows open or closed.
Driving with the rear quarter lights didn’t make a difference.
There are a few holes in the chrome panel at the rear where previous numberplates were attached. I wouldn’t think that they would be drilled through into the body, but it maybe worth sealing them and seeing if it makes a difference?
I could spend a lot of time and money chasing holes and leaks and sealing them but it doesn’t look like it’s going to make much of a difference anyway based on your reply.
Cheers, Marcus

[Marcusu]

Fumes Update:
After spending a good sum of money fixing oil leaks and other issues on the engine to stop fumes coming from the front of the car, my fumes saga continues.

I’ve been doing some experimenting and the only way that I have been able to get 0ppm CO in the cabin with the windows down is by extending the tailpipes out way beyond the rear bumper. At least that rules out the fumes coming from under the bonnet.

Now, it’s not really acceptable to have a jet tailpipe extending out the back of the car, so I have been experimenting taping around the rear light seals, the reversing light cable apertures, the two bungs under the rear valance and any other holes I can find around the rear valance.
I’ve also tried taping around the boot seal and over the grille on the tailgate.
All with no result, I’m still getting around 20ppm in the cabin both with windows down, and windows down and rear quarter-lights open.

If anyone can suggest any other holes in the back of the car I could try sealing up, that would be great.
Cheers,
Marcus

[Craig Balzer]

I realize you have a 2+2 and mine is an 72 OTS, but these photos from the rear, inside of my boot may give you some hints to research.

Bottom of the boot floor under the spare

I don't know the function of these 4x holes but there they are.

They are NOT covered when the boot is finished (at least in an OTS)

/
Here is a view from inside the boot looking directly rearward . . .

and the same view from outside the car (note the direct path from the rear license plate lamp assembly mounting are and the boot

. . . which provides three possible sources to explore (from the top down):

X- the reverse lamp assembly leads to two possible leak points
o - the mounting points for the lid (or maybe the hatch) latching point
o - two opening for wiring harnesses that are/were sealed with a grommet

X - the six mounting points for the US-style rear license plate (may be = those used for a European plate) (ignore the small rust hole)

X - two small holes along the body seam - I have no idea what their function/purpose is/was unless those are rivet holes to attach the rear plinth

[Marcusu]

Thanks Craig for posting these photos.
They are a good guide to help me understand what to look for in the boot area of my car.
Hopefully sealing them up will help with the fumes issue.
Cheers,
Marcus

[John37]

The rear 2 of the 4 holes in the boot floor are for the S3 rear exhaust mount. There's a stiffened plate spot welded above them. My floor had rusted betweenthese 2 pieces. I welded a new piece in and made it extendinto the stiffen to make the exhaustbolt fit better.
The small holes along the bottom of the vertical rear panel are for the pop rivets holding the stainless rear trim panel. There are another 2 below each rear light on the S3.
The void above the number plate light is part of the boot space. It's open at each end into the boot.
Other openings include:
Fuel tank vent pipe
Fuel filler area drain hose
2 holes in the fuel tank sump area, one for the fuel tank sump, the other takes a grommet for drainage. The fuel tank sump is sealed by a sponge ring.
2 grommets in the floor just forward of the rear panel
There's a few holes in the rear panel for number plate screws. I replaced the complete rear panel with 3mm steel instead of the normal 1mm as that's where the towbar mounts.
There's also some small holes foe the reverse lights and wires.
On the 2+2, the Tailgate seal is critical. If in doubt, replace it. Mine was glued to the tailgate. There's also the rear vent in the tailgate on the S3. That should have a rubber flap over it to act as a non return valve stopping exhaust getting in.
Hope this helps