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Exhausted by Defender aerodynamics


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Yes I can hear you all laughing - aerodynamics of a house brick, what? :lol:


Anyway, the purpose behind this discussion - each time I open the driver's window, or the vent flaps on my old 110 V8, I am smelling excessive exhaust fumes. Granted the engine is running a little rich on petrol right now, but the fumes shouldn't come in the cab. I also get LPG exhaust fumes coming in when running on that fuel.

My first thought was that there must be a leak in the exhaust system somewhere, but lying underneath the truck with engine running, and sniffing round the engine bay, I cannot hear any typical exhaust leak noises or smell any fumes. I only get the fume ingress when driving, and have the window or vents open.

I am struggling to really believe this line of thought, but is there some anomally about the shape of a Defender that could be sucking exhaust fumes back to the front of the vehicle?

I recall my old soft top series used to suck rain spray into the back if the rear flap was rolled up - I normally felt it on the back of my neck!

The truck has a modern Puma spec exhaust system (a bit quiet, but that's not the concern here), so points rearward under the rear crossmember.

Before I changed the engine, the old exhaust pointed diagonally out at the rear corner, a la 300Tdi style, and I had no issues with cabin fumes.

Any thoughts?

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yep - rear pointing exhausts always did that for me as well.

I hadn't thought about it but my tdi exhaust points out of the corner and the back is clear... so looks like LR put it in the correct place.

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Just to illustrate the current tail pipe:

post-7124-0-25274000-1438271197_thumb.jpg

I am sure a cut part way round the last bend would allow the end to be rotated and welded back on facing the rear corner/sideways. Got to be worth a try....

post-7124-0-62039500-1438271265_thumb.jpg

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I had the exact same on a V8 90. There was some AWDC (IIRC) rule that said the exhaust must point backwards. I modified the exhaust accordingly but found all tHe exhaust was sucked in the back and being a soft top with an often open back that was a problem! I made a little 90 deg elbow which plugged in the exhaust for use on the road and that fixed the problem completely.

Si

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Check the vents at the top of the back of the roof aren't blocked, they need to breath to let fresh air in at the front.

This age of roof was prior to the rear facing roof vents. I'm sure they only featured for two or three years... I never previously understood their purpose.

The sliding rear windows are plenty drafty though.

It does surprise me that the air is drawn all the way forwards when travelling at speed.

Don't Td5/Puma vehicles have this problem? I can't imagine LR could have ignored such a problem.

Si, your experiences are good to hear, thanks.

I think this weekend may see a mod to the tailpipe.

I shall report back.

I thought about your plugin elbow to test the idea, but I wonder how much difference it might make with the exit being behind vs ahead of the rear face of the body, i.e.not accurately representing the final outcome.

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This age of roof was prior to the rear facing roof vents. I'm sure they only featured for two or three years... I never previously understood their purpose.

The sliding rear windows are plenty drafty though.

.

Ah-ha, OK :)

WELL, on my Audi, there are vents in the boot, behind the bumper, with rubber flaps hinged at the top. These are designed so that when the front windows are open the flaps open and allow a through draught, but with little air flow they remain virtually closed, and even act as a one-way valve so exhaust gasses aren't pulled back into the cabin because of the depression that any car produces as it travels through the air.

Side exhaust sounds like it solves the problem though :)

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The aerodynamics of a brick are such that there is a negative pressure generated up the back window/door when moving at speed say about 40mph and above hence the fume would tend to rise up the back door and cling to that area, when you open a front window, the aerodynamics are such that the air billows outward from the windscreen, wings and grille and generates an even greater negative pressure so anything cloning to the back door is drawn inward to the lower pressure area behind the windscreen.

A side exhaust means that the fume will mix more because of turbulence (air from the windscreen wings and grille has stabalised a little and is running down the side of the vehicle) so if anything is drawn up the back door it has mixed to a large degree with the passing airflow.

Try to think about air as water flowing around a boat as its easier to visualise.

An interesting read is over the NACA (the predecessor to NASA) air duct and its development, the actual declassified document regarding its design and testing from the 50's is now available on the net (for the past 15 years since I found it) which tells a lot about airflow and breakaway angles.... Not that it applies to a fender style vehicle though...

There is another document on NACA wing profiles too

Rob

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Think about it:

as the defender moves along it creates a tiny vacuum at the back, cause there is no air feeding into the space where the defender has just left, except from around the side/top and bottom of the defender (and the various panel gaps in the bodywork).

So, ALL the air in the area will go into the gap.....

Unless it is pushed away, like a side exhaust.

So, to avoid gassing yourself: no exhaust pipe into that gap

It's why you see so may trucks with wind deflectors at the back, they're trying to divert air into the vacuum to avoid the drag effect.

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Another interesting thing to look at is a defender with a full hood. When travelling at speed the section above the cab billows out (because of the negative pressure generated by airflow up and over the windscreen). But the rear sections side, roof and back, tend to be sucked inward because of the higher negative pressure from the windscreen. There is also the turbulant effect that the air that billows outward has to return to fill the void.

Hence there should be a bit of thought into exhaust termination positioning as carbon dioxide and monoxide are actually odourless and a classic symptom is a headache, if you smell petrol lpg or oil you're actually lucky as the other bits in higher concentration s aren't smelt.

Main constituent of complete combustion is carbon dioxide. A vehicle running rich can be useful as the petrol or LPG smell could be considered as a tracer smell same as that added to natural gas (which also does not smell) the tracer is called mercaptan or methanethiol.

Rob

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Note that the 90 tailpipe points diagonal and the 110/130 points straight back.

With the right angle of cut I would think you will be able to rotate it without problem, the td5 tailpipes appear to be some kind of stainless so if you have stainless wire or rods use it to stop your welds rusting.

Will.

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