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Learning how to build Lotus Seven replicas...together!
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PostPosted: April 5, 2018, 1:54 am 
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TRX's Top Radiator Outlet thread prompted a question regarding how to best manage airflow out the other end of the car.

Given a traditional Seven body-style, a full length floor (including a diffuser) closing off the underside, wrap-around boot bodywork, a lid on the top of the boot and side openings inside the rear fenders to accommodate rear axle and suspension movement, what happens to the airflow moving from the firewall through the tunnel. Is anything needed to get air out of the back of the car, like cutouts or louvers in the boot back panel? Do the rear axle cutouts suck air in or provide an exit path for the tunnel air?

Seems there should be a low pressure area behind the car that could be put to use.

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PostPosted: April 5, 2018, 7:24 am 
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Tom,
I agreed with your assessment when designing the diffuser for my car and chose to leave a gap between the top of the diffuser and the bottom of the car behind the diff.

Ron


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PostPosted: April 5, 2018, 11:49 am 
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seattletom wrote:
Is anything needed to get air out of the back of the car, like cutouts or louvers in the boot back panel?
If not necessary, I would at least call it beneficial. Cutouts or louvers would work, or a gap between the diffuser and boot...Which is what I typically envision. Another reason for ensuring the air has a place to flow out from the rear, is that you'll probably also be best served to have a constant supply of air flowing around the heat-generating differential. With an LS3, if you have plans to do more than cruising, you may even consider ducting some fresh air into the tunnel (or boot) for this reason as well.


seattletom wrote:
Do the rear axle cutouts suck air in or provide an exit path for the tunnel air?
Probably a little of both, as the area between the front of the fender and the front of the tire should be low pressure, while the area between the rear of the tire and the rear of the fender should be high pressure. Regardless, going through the wheel wells is probably not the easiest way for the air to escape. I'd definitely try to provide an intentional path of least resistance for the air, like the vents/louvers/gap you're already thinking of.

seattletom wrote:
Seems there should be a low pressure area behind the car that could be put to use.
Absolutely there is. :cheers:

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Last edited by Driven5 on April 5, 2018, 12:38 pm, edited 2 times in total.

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PostPosted: April 5, 2018, 12:19 pm 
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Any way we can pull in our resident aero guru Jack McCornack on this thread? And maybe on the prior Radiator thread?

(He's a member of the EAA's Hall of Fame)

https://www.youtube.com/watch?v=-kb7Tm5tDwI

And in James Bond movies..

http://www.dropzone.com/forum/Skydiving ... ps_P295418

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PostPosted: April 5, 2018, 2:07 pm 
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Hi Tom-
I'd vote for louvers or even open areas with wire mesh in the back panel. Ventilate that thang! Maybe tall rectangles on either side of the (center) license plate area? Seems to me there would be quite a high pressure in the rear given the amount of air that might come thru the tunnel or around the rear suspension openings, arrive at the rear bodywork and have nowhere to go.

It's kind of apples-vs-oranges, since the Slotus is track only and not really a Se7en body style at all, but I went for lotsa ventilation on the rear of mine... :mrgreen:
Attachment:
Slotus Rear A-vs-A 2017.jpg
Slotus Rear A-vs-A 2017.jpg [ 97.79 KiB | Viewed 1442 times ]
Just ignore that derelict wino that wandered into the shot.

:cheers:
JDK

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PostPosted: April 5, 2018, 2:12 pm 
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You just wanted to show off your rear end.

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PostPosted: April 5, 2018, 3:22 pm 
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Driven5 wrote:
You just wanted to show off your rear end.
Ain't the first time I been accused of that... :mrgreen:

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PostPosted: April 5, 2018, 5:56 pm 
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My vote is for cutouts with wire mesh. Right now the diffuser build has been pushed out as a future upgrade. When I do close off that portion of the rear floor I'll still have positive air flow through the entire chassis. The cutouts I put in the bonnet are right in front of the engine face and right in front of the fire wall. Seems pretty effective a getting hot air out of the engine compartment. Although there is still a ton of hot air that funnels through the chassis as evident by a warm transmission tunnel. Note that my floor covers 100% of the bottom of my chassis, trans/driveshaft included. This will hopefully help make the diffuser actually functional instead of just looking good.
Attachment:
rear hitch.jpg
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PostPosted: April 6, 2018, 12:46 am 
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Beautiful car, jaf. I like the way you did the rear cutouts. Exceptionally clean. Do you normally run with a boot cover? I see what looks like snaps.

JD, thanks for introducing us to the business end of the Slotus. Once you get the rebuilt engine in, that's all the other folks at the track will see. :thmbsup:

I plan on running hoodless, so venting up front won't be a problem. But that will probably put more air down the transmission tunnel and I don't want the boot to act like a parachute.

Pictures of rear venting solutions are really helpful. Please post your solution. :cheers:

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PostPosted: April 6, 2018, 1:15 am 
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Just a note: Wire mesh blocks air like you wouldn't believe. At speed it is effectively solid.

I'd intended to cut some large triangles out of the rear panel to let the air out of the rear panel. Figured it would cool the trans/rear if nothing else.

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PostPosted: April 6, 2018, 8:59 am 
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I could be wrong :ack: :( :oops:
I can not believe there is that much air passing thru the tunnel area. I hardly have enough space to fit a wrench on the side of the trans case, let alone room to fill it. Probably max clearance of an inch down to zero. Air is going to take the path of lease resistance. Most of the air exits thru the bottom of the engine bay or hood. My bet is that 90% of the rear air manager problem is the air passing under the flat bottom and is caught and dammed up behind the rear body panel, if you are not using a diffuser. My guess is that the guys who installed a diffuser did not see any real change in engine cooling air flow after closing in the rear area. DaveW


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PostPosted: April 6, 2018, 2:17 pm 
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C10CoryM wrote:
Just a note: Wire mesh blocks air like you wouldn't believe. At speed it is effectively solid.

I'd intended to cut some large triangles out of the rear panel to let the air out of the rear panel. Figured it would cool the trans/rear if nothing else.



What he said! Just try to pull a landscaper trailer with the wire mesh ramp sticking up in the air. I have to take the ramp off if I'm towing any distance at all or the wind is high.

But the little amount of air coming out of the car I wouldn't think would build up that kind of "bow wave" and limit flow greatly.

It's the bow waves from each piece of mesh that build up at speed and finally join up in one impenetrable wall.

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PostPosted: April 6, 2018, 3:45 pm 
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Start with 30-50% reduction in opening, dependent upon the screen design. Then add that "wave" which reduces the "effective opening" even more.

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PostPosted: April 6, 2018, 5:08 pm 
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seattletom wrote:
Beautiful car, jaf. I like the way you did the rear cutouts. Exceptionally clean. Do you normally run with a boot cover? I see what looks like snaps.


Yes I have a boot cover. Look at my build for details/pics. It is not water/wind tight so I have that going for me. I'll have to see how much it "puffs up" as speed.

rx7locost wrote:
Start with 30-50% reduction in opening, dependent upon the screen design. Then add that "wave" which reduces the "effective opening" even more.


Nothing comes for free. If you shop wisely most places list the % open area for screen types. I picked the one at the bottom: https://www.customcargrills.com/Hexagon-Grill-Mesh.asp

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PostPosted: April 6, 2018, 5:54 pm 
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rx7locost wrote:
Start with 30-50% reduction in opening, dependent upon the screen design. Then add that "wave" which reduces the "effective opening" even more.

Well, true, unless you're using the screen to block stones from hitting the radiator. Have to draw the line somewhere on flow versus protection.

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