Airflow over the rear trunk?

carl

carl

True Classic
Has anyone done an analysis of airflow over the rear trunk lid or even the rear panel? Now that I have the DCOEs mounted, I would like a fresh air source for the carbs. The ends of the carbs pretty much line up with the crossmember so intake air should really be coming from the rear trunk. Thought about putting a mild scoop in the rear trunk lid but don't want to do that if there is a negative pressure and it will just have hot air coming up and out the scoop rather than a positive pressure allowing air into the trunk. I guess I can do the old yarn taped to the body trick but flow with no opening in the trunk lid could change if there was an opening, I guess it depends on what the pressure is inside the trunk which for my car is not sealed up.

I'm looking for actual evidence, not theoretical.
 
Thanks Jim. I figured air flow would get funky just behind the rear window but not sure how far back that effect extended. Pretty certain the rear (taillight) panel would be a high pressure zone....124 owners are always complaining of exhaust fumes getting into the cabin via bad taillight or trunk seals.
 
Hey Carl, some of the best videos I've seen which highlight the aerodynamics of cars are Gray's Garage on Youtube. He doesn't test an X1/9 or for me a Scorpion model but the variations of the MR2 models he tests are indicative of what our cars experience. In the video I link to (at the 14 minute mark) he tests a first gen MR2. Hope it helps even if it's not the same car.

 
I think the MR2 is close enough in design to make fairly direct air flow comparisons. Very cool.
 
I have lexon for the rear widow on my X and it will fold down. With the rear window open and side windows up at around 60 mph I can feel air coming in. With the side windows down at 60 the wind come in strongly from behind and rear window will slam shut. I have no more info other than that so you will have to draw your own conclusion.
 
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carl said:
Has anyone done an analysis of airflow over the rear trunk lid or even the rear panel? Now that I have the DCOEs mounted, I would like a fresh air source for the carbs. The ends of the carbs pretty much line up with the crossmember so intake air should really be coming from the rear trunk. Thought about putting a mild scoop in the rear trunk lid but don't want to do that if there is a negative pressure and it will just have hot air coming up and out the scoop rather than a positive pressure allowing air into the trunk. I guess I can do the old yarn taped to the body trick but flow with no opening in the trunk lid could change if there was an opening, I guess it depends on what the pressure is inside the trunk which for my car is not sealed up.

I'm looking for actual evidence, not theoretical.
Click to expand...
Try driving down a dirt road and see where all the dust ends up.......all over your engine and engine bay on the X. The front end is not bad but anything past the rear window is covered in dust.
 
From what I have gathered, the engine lid and rear trunk have stagnant air flow, at least compared to the normal air flow over the rest of the car. The flip side of course is whether there is positive or negative air pressure on the inner side of my rear trunk lid. The rear trunk area is completely open to the engine bay as the separation panel is removed. If I could find a louvered panel to weld in to the trunk lid, that would be a nice custom touch....or do the Abarth thing and prop up the trailing edge of the trunk lid about four inches.....all kinds of silly ideas.
 
Disclaimer: I probably have no idea what I'm talking about, but this topic has come up a few times in the past on the MR2 forums.

Flying buttresses (like on the X, MKI and MKII MR2, Ferrari 308GTB, etc.) create a vortex above the rear deck-lid. This creates negative pressure and thus rear end lift. My suspicion is that this effect is why flying buttresses are no longer predominantly featured on mid-engine sports cars.

Its very similar to the vortex that is created in the open bed of a pickup-truck. I believe someone over on the MR2 forums did a wind tunnel comparison between a pickup truck and the MKI MR2. The airflow over the rear was very similar.

The MK2 MR2 incorporates ducts below the engine that scoop air up from beneath the car and force it up through the engine bay and out the vents on the decklid. I don't think this is for engine cooling - I think it is to reduce the negative pressure created by the flying buttresses.

To answer the question, if you're looking to install an intake vent on the trunk lid or engine lid, I'd be skeptical of its efficiency. Probably better to route the intake to a side scoop or snorkel.
 
IMG_8524.JPG
Dr.Jeff said:
Well you can't say something as interesting as that without showing us pictures and giving us details!
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Not too exciting. In my quest to shave weight I decided to replace the rear window with lighter plastic. Figured why not make it open?

Looked pretty neat at first but the plastic is now scratched from fumbling around opening and closing.

With the cold weather finally coming on I plan to pull my X into the work garage and tear it down and do some inspection and preventive maintenance. From daily use my car is getting a bit frayed around the edges so I plan on polishing the plastic and doing something clever do minimize the scratching.

Peter Cecil mentioned dirt roads. I have a half mile of dirt before I get to the pavement and do not drive it even at 15mph with the rear window open on real dry days.
Peter Cecil said:
Try driving down a dirt road and see where all the dust ends up.......all over your engine and engine bay on the X. The front end is not bad but anything past the rear window is covered in dust.
Click to expand...

I am constantly blowing dust out of the rear bay while the frunk stays clean for weeks. I think this is so with a lot of cars though. While the trunk of my wife's dodge is sealed the dirt gets thick on the rear deck from that short drive on the dusty road.

Around here it is something you learn to live with..."You know you live in Arkansas if directions to your house says turn onto the dirt road".
 
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carl said:
The flip side of course is whether there is positive or negative air pressure on the inner side of my rear trunk lid
Click to expand...
Clearly I'm no aerodynamicist so this falls into the unwanted category of theory, not evidence...sorry.
It seems apparent there is some sort of negative pressure above the rear lids. As you suggest, the flip side is if there is positive pressure below that to create air flow across the lids. Like "AARP card" said, positive pressure under the lids could be created with a scoop under the car (e.g. MR2). Or how about adding side scoops to either side of the trunk for the same effect? Combined with your idea of a 'louvered panel' in the trunk lid (or on the rear panel), you would get flow through the trunk; with outside air coming in (from the side scoops) and flowing out (through the lid or rear panel), it would be a cooler area to feed your carbs.
We saw one example similar to this in another thread: https://xwebforums.com/forum/index.php?threads/side-vents-ducts.33130/page-2
{Scroll about half way down page 2 to the photo of the red X, posted by "motoTrooper"}
Here is more detail on that example, with trunk mounted carb's intake like yours: fiatlancia.asn.au/originalsite/tech_material/tech_item01.html
 
carl said:
From what I have gathered, the engine lid and rear trunk have stagnant air flow, at least compared to the normal air flow over the rest of the car. The flip side of course is whether there is positive or negative air pressure on the inner side of my rear trunk lid. The rear trunk area is completely open to the engine bay as the separation panel is removed. If I could find a louvered panel to weld in to the trunk lid, that would be a nice custom touch....or do the Abarth thing and prop up the trailing edge of the trunk lid about four inches.....all kinds of silly ideas.
Click to expand...

Or maybe mount the rear license plate out a quarter inch and louver behind it to vent the heat?
 
It would be interesting to see if the vortices around the rear half of the vehicle create a pressure differential between the top of the trunk and the rear panel. If so, add a vent on top of the trunk lid and one in the rear panel to create flow through the trunk.
 
I has thought of putting long rectangle scoops on side of the rear targa b pillar and sculpt them into the vents on either side of the engine cover. or maybe get yhe rear firewall panel(the bolt on one that has the cooling fan bow and duct the lower left intake into the fan box and have air forced into the carb. jes' thinking
 
Dr.Jeff said:
Clearly I'm no aerodynamicist so this falls into the unwanted category of theory, not evidence...sorry.
It seems apparent there is some sort of negative pressure above the rear lids. As you suggest, the flip side is if there is positive pressure below that to create air flow across the lids. Like "AARP card" said, positive pressure under the lids could be created with a scoop under the car (e.g. MR2). Or how about adding side scoops to either side of the trunk for the same effect? Combined with your idea of a 'louvered panel' in the trunk lid (or on the rear panel), you would get flow through the trunk; with outside air coming in (from the side scoops) and flowing out (through the lid or rear panel), it would be a cooler area to feed your carbs.
We saw one example similar to this in another thread: https://xwebforums.com/forum/index.php?threads/side-vents-ducts.33130/page-2
{Scroll about half way down page 2 to the photo of the red X, posted by "motoTrooper"}
Here is more detail on that example, with trunk mounted carb's intake like yours: fiatlancia.asn.au/originalsite/tech_material/tech_item01.html
Click to expand...



Great article.
 
Sounds like it has already been covered, but from my understanding there is definitely a zone of low pressure over the engine bay. Why do I think this? The other day I was driving with my engine cover not properly latched, and I watched the hatch rise up at high speeds. You can probably duplicate this at home
 
abalslev said:
Sounds like it has already been covered, but from my understanding there is definitely a zone of low pressure over the engine bay. Why do I think this? The other day I was driving with my engine cover not properly latched, and I watched the hatch rise up at high speeds. You can probably duplicate this at home
Click to expand...
I forgot about that. I too had that happen. Didn't latch the cover and at around 60mph looked back and the cover was lifting about three inches.
 
Would be interested in the flow changes from adding a 'lip spoiler' to the back of the roof/targa.
 
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