Underbody engine bay air scoop

15103 Views 74 Replies 24 Participants Last post by D.R., Jan 2, 2013

SlySimon

Discussion Starter · Sep 24, 2010

Still mulling over the idea of changing the side scoops but starting to wonder whether I'd be better off fabricating an Underbody engine bay air scoop.
Has anyone tried this mod? Does it make a difference to engine bay temperature or handling?
My major thinking is two-fold:
I am better at fabricating than finishing off and it wouldn't matter what it looks like as it'll be under the car.
I would imagine that I can force more air through the engine bay than using side scoops.
Any thoughts?

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Petevick

· #2 · Sep 24, 2010

what would happen if you drove thru deep water :dunno: it would possibly get ripped off causing other damage.

COPPERKNOB

· #3 · Sep 24, 2010

Petevick said:
what would happen if you drove thru deep water :dunno: it would possibly get ripped off causing other damage.

The engine would be nice and clean though :sofa:

SlySimon

Discussion Starter · #4 · Sep 24, 2010

Petevick said:
what would happen if you drove thru deep water :dunno: it would possibly get ripped off causing other damage.

Good point, though I don't intend to take her swimming and if I did I could just take it slow.
I don't think the scoop would have to be very deep. Probably wouldn't need sides on either, so plenty of room for water to escape at a slow pace.

philmgf

· #5 · Sep 24, 2010

probably get ripped off with speed humps too as the cars arent that high when pumped up.

alan1234

· #6 · Sep 24, 2010

what about some sort of extraction scoup for the boot vents that drags air out thus more will flow in.

g7nbp

· #7 · Sep 24, 2010

Ive looked at an underbody scoop a few times but still not come up with anything specific yet.

At the moment I just have a flex pipe that pokes slightly below the bottom of the engine bay (cable ties) and the other end is cable tied near to my K&N which is open rather than closed airbox. The side of my K&N nearest the pipe gets dirty far quicker than the rest, so there is obviously some air movement. Possibly over winter I may make up a bit more complex arrangement for the underbody end

By the way, water is more of an issue than you think. I used to have an induction kit quite low in the front valance of a previous car and several times in the past just driving in motorway spray its become apparent that water was finding its way into the system big style lots of coughs and splutters - the airbox and cone were absolutely sodden when I checked.

If using a closed system you need to make sure its very much uphill from the intake and ideally have a small exit pipe in a low point to allow the water to drain off.

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g7nbp

· #8 · Sep 24, 2010

..... Ive just had a quick look on ebay: http://desc.shop.ebay.co.uk/Vehicle...ref=1&_dmpt=UK_CarsParts_Vehicles_CarParts_SM

There are a few on there that could possibly be adapted for an underbody situ

Edit: http://cgi.ebay.co.uk/SMART-CAR-AIR...arts_Vehicles_CarParts_SM&hash=item2a0a821210 looks interesting - bit expensive but has nice big flange to fix via a plate and is low enough profile to not foul on bumps

g7nbp

· #9 · Sep 24, 2010

... or better still one of these poking out of the side

http://cgi.ebay.co.uk/Lotus-Elise-2...arts_Vehicles_CarParts_SM&hash=item3ef87b663e

SlySimon

Discussion Starter · #10 · Sep 24, 2010

Interesting. I wasn't thinking about induction so much as forcing a little more air through the engine bay. Rather like the MR2 underbody scoop, which is basically just a bit of curved sheet metal that hangs slightly lower than the underside of the car, but not lower than the underbody pipes etc.

g7nbp

· #11 · Sep 24, 2010

I see, yes a simpe deflector would probably be useful, though Im not quite sure where I could mount it to make it practical. I would be wary of making too many changes to airflow under the car, its surprising how much that can change handling at speed.

As commented a few posts back a mod to the top of the boot is probably a better option - the airflow over the body and boot has taken a longer path than the airflow under the body so will be at lower pressure, a simple bigger vent cutout on the top of the boot would probably suck much more air up through the engine bay just through the pressure differential - though your shopping is going to get wet when it rains

SlySimon

Discussion Starter · #12 · Sep 24, 2010

g7nbp said:
I see, yes a simpe deflector would probably be useful, though Im not quite sure where I could mount it to make it practical. I would be wary of making too many changes to airflow under the car, its surprising how much that can change handling at speed.

As commented a few posts back a mod to the top of the boot is probably a better option - the airflow over the body and boot has taken a longer path than the airflow under the body so will be at lower pressure, a simple bigger vent cutout on the top of the boot would probably suck much more air up through the engine bay just through the pressure differential - though your shopping is going to get wet when it rains

I see what you are saying. I also agree with you. I'm just not sure I could bring myself to cut into the bodywork.

pascal

· #13 · Sep 24, 2010

Forcing more air from under the car to the engine bay isn't a good idea, as this air needs to get out of the engine bay somewhere again. The F/TF was bild in such a way, that when driving, cold air is getting into the engine bay through the vents in the rear bonnet and leaving the engine bay at the bottom. This is done by the air pressure difference above and under the car.
To get the hot air out of the engine bay, you need to have bigger vents on top and create a bigger air pressure difference under the car.

supasoaker

· #14 · Sep 24, 2010

pascal said:
Forcing more air from under the car to the engine bay isn't a good idea, as this air needs to get out of the engine bay somewhere again. The F/TF was bild in such a way, that when driving, cold air is getting into the engine bay through the vents in the rear bonnet and leaving the engine bay at the bottom. This is done by the air pressure difference above and under the car.
To get the hot air out of the engine bay, you need to have bigger vents on top and create a bigger air pressure difference under the car.

Can you confirm where you got this info from?

I was of the impression that the vacuum created behind the rear screen would draw air up through the engine bay?

I have tought of an underpan vent but it might not be such a good idea - not only speed bumps but also water.................

SlySimon

Discussion Starter · #15 · Sep 24, 2010

supasoaker said:
Can you confirm where you got this info from?

I was of the impression that the vacuum created behind the rear screen would draw air up through the engine bay?

I have tought of an underpan vent but it might not be such a good idea - not only speed bumps but also water.................

I agree with Supersoaker. The air under the car will be a higher pressure than that on top, hence causing lift, much the way an aircraft wing works.

Tim.

· #16 · Sep 24, 2010

I've also been toying with this idea in my head for a few years without coming to any satisfactory plan.

I am quite sure that any airflow enters the underside of the engine and exits through the boot vents. Apart from any consideration of pressure differentials, look at the engine bay fan. It is set up to draw air in not extract it.

I'll share a few of my thoughts - its up to everyone to consider if they are valid. I haven't really done any flow tests or sums :-

1) Unless you are going to drive at speed through a foot of water I don't believe flooding the engine bay is issue other than it's impact on necessary strength of the deflector. There's potentially a lot of spray from the rear (and maybe front) wheels so be prepared for your engine to get dirty as droplets of water get shot up the scoop.

2) Air needs to be deflected up both the front and the rear of the engine so two smaller plates are likely needed. The front one(s) shouldn't screen the rear ones.

3) Don't screen the bottom of the sump from a decent airflow. I am sure a lot of heat from the oil gets dissipated from it's cooling fins. (otherwise MGR wouldn't have put them there in the first place)

4) Channelling any airflow over the exhaust manifold may result in a less than positive effect.

5) The biggie is how close you dare go to the ground you go with the deflectors in terms of physical damage and what happens when they get ripped off.

:dunno:

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pascal

· #17 · Sep 24, 2010

Hot air only exits through the vents when the car is standing. During driving cold air is sucked in.
Testing done by a member of the MGCC-L. Some small nylon wire attached to the engine bay bonnet around the vents. Their movements were filmed during driving, and you can see that they are sucked in when the car is in movement. This was done with the roof down and without the hood cover.
*
When the car is parked and the engine is off, then the engine bay comes on when the temperatures in the engine bay are too high. The fan sucks air out of the engine bay and not in. Can easily be tested with a thin sheet of paper which you can hold in front of the side vent when the fan is working.

SlySimon

Discussion Starter · #18 · Sep 24, 2010

pascal said:
Hot air only exits through the vents when the car is standing. During driving cold air is sucked in.
Testing done by a member of the MGCC-L. Some small nylon wire attached to the engine bay bonnet around the vents. Their movements were filmed during driving, and you can see that they are sucked in when the car is in movement. This was done with the roof down and without the hood cover.
*
When the car is parked and the engine is off, then the engine bay comes on when the temperatures in the engine bay are too high. The fan sucks air out of the engine bay and not in. Can easily be tested with a thin sheet of paper which you can hold in front of the side vent when the fan is working.

Hmmmm...... I think I can feel some testing coming on.

I have a feeling things will be very different with the roof up or hard top on.

Tim.

· #19 · Sep 24, 2010 (Edited)

pascal said:
The fan sucks air out of the engine bay and not in.

I'm pretty sure this is not right. Can't prove it at the moment though

Why is it ducted on the inside? not normal for an extract fan.

SlySimon said:
Hmmmm...... I think I can feel some testing coming on.

I have a feeling things will be very different with the roof up or hard top on.

Yes quite likely but I would still expect at least some depression at the boot vents. Although prepared to be wrong about this

Tim.

· #20 · Sep 24, 2010

Was sure I remembered an old thread on the fan direction and here it is - dating back to 2005. Its not 100% convincing but the general consensus was that the fan draws air into the engine bay. http://forums.mg-rover.org/showthread.php?t=88453.

See Perry's test in post #30

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