Airflow over the rear trunk?

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
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This is funny since it would take a lot less time to not latch my rear trunk than to tape a bunch of yarn on it! The rear trunk lid is seriously heavy for such a small panel so there has to be a lot of lift to overcome the weight of that thing.

I noticed yesterday that since the lower portion of the rear trunk near where the jack sits is rusted out, that opening could be a source of ducted air to the carbs.
 
So Carl, you are going to justify the rust on your car as airflow for the carbs? Maybe some ducting also from the driver side floor pan? And that crack in the dash is for enhancing windhsield defrost?
 
Hey, you take opportunities as they come to you! The rusted bodywork around my windshield allows for fresh air at face level. The mounting holes in the body from removing the bumpers allow for additional brake cooling and exhaust extraction from the rear trunk, also possibly causing small vortices that keep the airflow against the car surface. Rust holes in the floor, everyone knows, allows you to monitor the condition of the street unless you are stupid enough to weld in patch panels.
 
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 used to do this quite a bit so much so I eventually rigged an idiot light on the dash to warn me if the key were on and the lid unlatched. At speeds of 40 or more it would raise up, at anything above 50 it would be VERY open and eventually a full 45 degrees and it was heavy so yes, there's A LOT of air moving up through the engine bay it would seem.
 
I'm just guessing but I imagine the engine lid lifting is a combination of: 1) a pressure gradient between the engine bay below and the air above, and 2) air vortex / swirl patterns circling around and moving back against the open edge of the lid (something like this, sorry for my crappy art-work).
fiat-bertone-x1-9-orange-stripes-monkey-crisis-on-mars.jpg
 
motoTrooper said:
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.

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Great link. Very interesting and worth the time to go through his various videos.
 
jvandyke said:
I used to do this quite a bit so much so I eventually rigged an idiot light on the dash to warn me if the key were on and the lid unlatched. At speeds of 40 or more it would raise up, at anything above 50 it would be VERY open and eventually a full 45 degrees and it was heavy so yes, there's A LOT of air moving up through the engine bay it would seem.
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We might be on to something here! With limiter straps to keep the rear trunk lid from opening too much, we could have a fresh air supply that varies with speed! Except, of course, there is probably more air rushing out the trunk than trying to get in to feed the carbs. What is needed is a wireless pressure sensor to let us know what is really happening in there. You DCNF guys have it easy, you know the carbs are sucking in the hottest air in the engine bay (just above the exhaust manifold).
 
Very interesting discussion here, just thought I would add my 2 cents worth. When I fitted my DCOE Webers I wanted to get cool air supply to them in the rear trunk, I was aware of the low pressure area above the engine cover, (lifting when not latched) so I did some wool tuft testing and found that along the side should work ok. Though tending to flow upwards slightly just above the wheel arch it was as far forward as I could make the duct and still have room inside the body work. It feeds straight into a pod filter then a plenum chamber.
P7120004.JPG
intake duct.jpg
 
I think we have pretty much concluded that getting fresh air into the rear trunk, for DCOEs or any other reason (oil cooler?) via the trunk lid won't work. Side access like yours or bottom access is the way to go although I have no data on air pressure at the bottom of the jack storage well (mine is rusted out and totally open).
 
Hey @carl !

I actually went through this when I had my Noble and my 3rd gen MR2. In both cases I discovered that the manufacture didn't really make an effort to force airflow either up from the bottom or down from the top of the car. In both cases I ended up forcing air up from the bottom because I always had negative air pressure across most of the back deck lid due to the back window. Getting air to come up from the bottom was not hard, I just stuck a small flap under the car on the Noble and cut a hole in the belly pan for the MR2. I never thought about doing this on the X1/9 but it would make sense and would probably be super effective. Unlike the Noble and my MR2, there isn't anything super hot along the backside of the firewall on the X1/9, which should make this very effective.

If you want, we can try some cardboard mock ups and use a leaf blower under the car to see how much air gets caught by it.... I am around for a couple of weeks...
 
Never thought of a leaf blower under the car, great idea. I'm currently finishing up some panels to close off the trunk from the engine bay but would be ready soon to try this. I'll provide the car, heated garage and gas powered leaf blower and you can buy lunch.
 
IMG_0582[1].JPG
Here's a pic of the rear trunk. I have the lower panel covering my exhaust and a mock-up panel closing off the engine bay. Filtration is by two huge K&N oval filters and the jack cavity on the right is open to the floor courtesy of rust.
 
There is a reason the factory made the periscope intake to get some ram air from above the roof.

Just because the air is roiling over the engine compartment and rear trunk area doesn’t mean it is not fresh, cool air, it is just air that is not laminar in its flow over the car.

The carbs will work just fine while the car is sitting in place with no movement of air over the car, they will work equally well with the unsettled but still fast moving air above the engine compartment and trunk. Just make sure the trunk is sealed off from the hot side of the engine compartment and ensure a supply of outside air into the trunk, from the sides is seems the best possible choice.

What you want is cool, unheated air to get to the intake. Getting ram effect would be nice but is unlikely without extreme measures. Ensure there is an adequate air inlet into the trunk whose surface area is equal to or greater than the area of the four intakes. Use a large air filter and not one of those ridiculous cones to maximize flow and you should be set.
 
Just because the air is roiling over the engine compartment and rear trunk area doesn’t mean it is not fresh, cool air, it is just air that is not laminar in its flow over the car.
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A valid scientific observation and conclusion. You're one of my peoples.

Thought I would share another one of those projects (on this topic) I never got to. While wool tufts or similar are useful for visualizing flow and magnitude (the Wooly Scale?), I had a more quantitative project in mind. I have a few of these similar sensors



https://www.ebay.com/itm/0-5-4-5V-1...hash=item51f11cc71d:m:mY2vwGiJsxUgx5-hGa56Eng

They would allow for precisely mapping out points of lower and higher pressure gradients.​
 

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Great discussion here. Another of those things I need to address on mine - ensuring the coolest intake air possible.

The EFI air filter snokel points straight down to the space between the back of the engine and the bulkhead. It would seem that that had very practical reasoning behind it's placement. Seemed odd to me when I saw it for the first time..

Looking forward to seeing how you resolve it, and to the results of the leaf blower test - might need to think about adding some ducting to mine to feed the filter :D
 
Could always add a 12v leaf blower to the engine compartment to get fresh air into the compartment with an intake under the car...:p
 
There was a certain gentleman from Texas who used an auxiliary motor in his Can Am racer (during the glory days of that series) but in his case it was to create downforce.
 
I experimented with adding a large extraction fan to the engine cover. Without a doubt it removed heat soak and dropped temps in the bay. Pulling air from underneath works. Couldn’t live with it though

X19_0658.jpg


The other issue with ducting air from underneath in a direct manner would be water volume - driving the car in wet weather will saturate your filters and/or potentially hydro lock the motor if enough is pushed through. Light mist would act as a nice water-injection though :D

On one of my intake setups on my old Volvo I tried pulling air from air ducts in the front spoiler & the filter water-logged if there was water accumulation on the roadway, or in heavy rain. Had to revert to an inlet higher up.
 
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