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#4564352 - 11/04/17 09:40 PM exhaust rerouting question
Tornado Red Offline


Registered: 06/01/05
Posts: 1777
Loc: Saint Paul (ex-San Diego)
I have a Mk4 Golf TDI that I use almost entirely as a courier vehicle -- so much so that I have removed the front passenger seat. The rear seats are folded flat and they and the hatch area are covered with 1/2" plywood so I can haul more cargo. A lot of my deliveries are electrical, plumbing, HVAC and other parts and supplies for contractors. Generally it works out very well, because if what needs to be transported is more than my Golf can handle, we have other drivers with pickups, vans, and box trucks. I typically drive about 5000 miles a month.

Often I need to haul bundles of 10' PVC pipe, copper or galvanized conduit, or threaded rod. With the front seat removed, I can get about 9' of these pieces inside the vehicle, with about 1' sticking out the back. I use a bungee cord to hold the hatch down, strapping it to the tow bar.

Fumes sometimes get sucked into the cabin, so I have to crank up the fan and lower the front windows. But with winter coming on, I'd prefer to keep those windows up. So my question is, where should I reroute the exhaust pipe to prevent the exhaust fumes from entering the cabin?

My options seem to be 1) to the side, exiting behind the rear wheel, or 2) extending the pipe several inches straight out the back. But which is more likely to work? and if I extend the pipe, how far?

Other than trial and error, how should I determine which option is most likely to work?
_________________________
'03 Golf TDI, 704k miles: Bosio DLC764, GT1749VB, Malone Tuning, G60/VR6, Bilstein HD, Recaros, Akebonos, Brembos, Pirellis, DG Panzer, Delvac 1 ESM 5W40

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#4564356 - 11/04/17 09:48 PM Re: exhaust rerouting question [Re: Tornado Red]
eljefino Offline


Registered: 06/15/03
Posts: 32635
Loc: ME
Your problem is the air pressure caused by the bubble shape of your car. How air closes around the rear of the car at speed is as important aerodynamically as the front... which is why the "Prius shape" is a thing.

I wonder how hard it would be to modify your hatch so just the glass opens, but the metal frame stays closed. That would be more of a buffer between the interior and the exhaust pipe.

But to answer your question, mess around with the pipe as you're doing. You'll be blasting the heat anyway, on vent. You can get random pipe pieces and clamps for $10 to extend beyond your muffler. They come with a slip fit fitting that'll go over your current exhaust tip. Maybe a good "turn down" will get the smoke into the under-car slip stream instead of the dead eddy it's in now.

The only other idea would be a long tube mounted on the side or roof of your car-- I've seen them for pole vaulters. That would cut into your MPG though.

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#4564379 - 11/04/17 10:23 PM Re: exhaust rerouting question [Re: Tornado Red]
spasm3 Offline


Registered: 05/30/10
Posts: 8651
Loc: North Carolina
eljefino, nailed it. There is a low pressure area at the hatch created by air moving over the top of the car. Mythbusters found the same thing with pickup truck tailgates. More drag with the tailgate down than up.

That low pressure area sucks in the exhaust when the hatch/window is open/down. To get exhaust out of the low pressure area, you would need a long pipe, it would look silly, and probably not legal.

Your option really is roof carry, or keep the front windows open with full blast heat/ac.

EDIT; I wonder what the effect of a rear hitch rack would be on the airflow, one of those might work with an attached solid plexi or plywood floor.





Edited by spasm3 (11/04/17 10:32 PM)
_________________________
13 elantra 68k 5w30 QSUD
03 chevy avalanche76k synpwr 10w30
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#4564381 - 11/04/17 10:27 PM Re: exhaust rerouting question [Re: Tornado Red]
Merkava_4 Offline


Registered: 01/30/07
Posts: 17501
Loc: Clovis, CA
Having the exhaust routed straight up to slightly above the cabin like a semi truck would work the best.

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#4564387 - 11/04/17 10:38 PM Re: exhaust rerouting question [Re: Merkava_4]
spasm3 Offline


Registered: 05/30/10
Posts: 8651
Loc: North Carolina
Originally Posted By: Merkava_4
Having the exhaust routed straight up to slightly above the cabin like a semi truck would work the best.




LOL


Edited by spasm3 (11/04/17 10:39 PM)
_________________________
13 elantra 68k 5w30 QSUD
03 chevy avalanche76k synpwr 10w30
01 saturnsc1 185k synpwr rebuilt
17 mazda cx-5 3900 miles m1 0w20/0w40 mix

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#4564409 - 11/04/17 11:36 PM Re: exhaust rerouting question [Re: spasm3]
Merkava_4 Offline


Registered: 01/30/07
Posts: 17501
Loc: Clovis, CA
LOL! LOL That would work.

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#4564436 - 11/05/17 01:46 AM Re: exhaust rerouting question [Re: Tornado Red]
Srt20 Offline


Registered: 07/14/14
Posts: 494
Loc: Wisconsin
Drive with your windows open. Problem solved.

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#4564440 - 11/05/17 01:14 AM Re: exhaust rerouting question [Re: Tornado Red]
Linctex Offline


Registered: 12/31/16
Posts: 6148
Loc: Waco, TX
Exhaust out the side in FRONT of the rear tire... but a vertical stack like a "deuce and a half" would be best.
_________________________
"The evidence demands a verdict".
(Re:VOA)"it's nearly impossible to actually know the particular additives that are in there at what concentrations."

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#4564444 - 11/05/17 01:46 AM Re: exhaust rerouting question [Re: Tornado Red]
DoubleWasp Offline


Registered: 05/21/12
Posts: 5236
Loc: Fort Lauderdale, FL
You have to get the exhaust out of the boundary layer that surrounds your vehicle at speed. Doesn't matter where you route the exhaust. If it doesn't escape the boundary layer, it's going into the hatch.

Is there a reasonable way to get the exhaust to esxapw the boundary layer? No, there isn't.
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#4564519 - 11/05/17 06:29 AM Re: exhaust rerouting question [Re: Tornado Red]
DoubleWasp Offline


Registered: 05/21/12
Posts: 5236
Loc: Fort Lauderdale, FL
Oops. Double post


Edited by DoubleWasp (11/05/17 06:30 AM)

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#4564520 - 11/05/17 06:30 AM Re: exhaust rerouting question [Re: Tornado Red]
Cujet Online   content


Registered: 02/15/03
Posts: 7153
Loc: Jupiter, Florida
I think the best result would be a small pipe exiting to the side. Using the exhaust velocity to push it away from the car's aerodynamics. Piston powered aircraft use the exhaust's directional blast to prevent CO from entering the cabin. However, I don't think that's a complete solution, as at light throttle, exhaust velocity will be lower. In much the same way that piston powered aircraft on descent (with throttle pulled back) often get fumes in the cabin.
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#4564630 - 11/05/17 08:41 AM Re: exhaust rerouting question [Re: Tornado Red]
Kuato Offline


Registered: 10/23/05
Posts: 7094
Loc: Colorado
I'd go out the side, or the long exhaust above the vehicle.

How about some sort of custom foam fitted to fill the gap?
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Thick vs Thin test: 15k / 43k miles complete

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#4564928 - 11/05/17 01:47 PM Re: exhaust rerouting question [Re: Linctex]
Tornado Red Offline


Registered: 06/01/05
Posts: 1777
Loc: Saint Paul (ex-San Diego)
Originally Posted By: Linctex
Exhaust out the side in FRONT of the rear tire... but a vertical stack like a "deuce and a half" would be best.


So you think in front of the rear tire would be better than behind? I could do that, but I'm interested in hearing your reasons.
_________________________
'03 Golf TDI, 704k miles: Bosio DLC764, GT1749VB, Malone Tuning, G60/VR6, Bilstein HD, Recaros, Akebonos, Brembos, Pirellis, DG Panzer, Delvac 1 ESM 5W40

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#4566005 - 11/06/17 03:23 PM Re: exhaust rerouting question [Re: Tornado Red]
Ducked Online   content


Registered: 10/25/12
Posts: 4358
Loc: Taiwan
You might try a scoop that entrains higher pressure air around the exhaust, say a halved traffic cone, or metal equivalent if the exhaust is too hot to be plastic friendly.

Or do a lateral-thinking body swerve around the problem and rig yourself (not your engine) a schnorkel (and a nose clip) so you'll be able to breathe with the car full of fumes (or water).

No need to provide for passengers since there are no available seats.

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#4568837 - 11/09/17 11:57 AM Re: exhaust rerouting question [Re: Tornado Red]
Linctex Offline


Registered: 12/31/16
Posts: 6148
Loc: Waco, TX
Originally Posted By: Tornado Red
Originally Posted By: Linctex
Exhaust out the side in FRONT of the rear tire... but a vertical stack like a "deuce and a half" would be best.


So you think in front of the rear tire would be better than behind? I could do that, but I'm interested in hearing your reasons.


Turbulent airflow, and the "bow wave" in front of each tire. It's also the easiest way. Make the exit 12"/30 cm in front of the rear tire and make it a good 2-3 inches out past the outside sidewall.
_________________________
"The evidence demands a verdict".
(Re:VOA)"it's nearly impossible to actually know the particular additives that are in there at what concentrations."

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