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