Under bonnet air build up

[lotuselanman]

Gidday,
I have made moves to complete my +2 body interior restoration.
Zetec on Hyabusa Injection, larger valves, custom cams T9 etc.
Several years ago at a Sprint meeting, I pulled the Bonnet release at around 80 MPH on the slow down lap
and was surprised how far it lifted probably 10 to 12 inches, enough that I had to look around the Bonnet.
Now if there is that much pressure build up at 80 MPH what is it like at 100 or so ??
Has this ever been a topic.? I have a couple of ideas re exiting air build up but would like to hear more polite, realistic comment from others.
Les.

[cal44]

I reckon if it bugs you enough, you can install vents into the front wheel wells or vent out through the fenders out to the side. As tight as the front area is I wouldn't think much air could force it's way in.

It could be possible the hood was lifting as air passed over it. Like wings on an airplane gets lift. Just a thought.

mike

[Elanman99]

Les

What you need is a wind tunnel to test this theory, but as Mike said the bonnet would get lifted by the low pressure above it. I imagine that at any steady speed it will have a natural position, maybe as you go faster it gets pushed down a bit, maybe not!

As long as you can see over the top of it you are OK (until it flies off!)

Ian

[Gordon Sauer]

I have seen a +2 with two about 3 inch cut outs at the back of the engine compartment directly below the intake vents. When I was cutting out my Plenum chamber for air conditioning heater evaporator I thought about making a way for engine air to pass Through such vents and then up through the exterior intake vents by making a fiberglass passageway on the interior side of the firewall for that purpose but decided against it. But that may have been one person's solution to venting the engine compartment Gordon Sauer

[Andy8421]

Flat out on the long straight at Goodwood, the bonnet on my S3 would bow upward alarmingly at the sides with the engine compartment air pressure. I have the regulation Lotus two holes cut into the left hand side inner wing, so my analysis was that at high speed the front grill aperture was too large and was allowing more air through the radiator than could be adequately exhausted around the engine.
From a racing standpoint, as long as there was sufficient air through the rad for cooling, the best solution would be to blank off part of the grill aperture as this should reduce drag - although I never got round to doing it. As the air intake for the carb plenum chamber is in front of the rad and in this high pressure area, there may be some minor 'ram charge' benefit to be had for engine performance. I did have plans to measure the excess pressure, but like grill blanking, I never got around to it.

[lotuselanman]

Gidday,
Interesting replies re the Bonnet, I am convinced the is exiting the air accumulated under the bonnet
Gordon Sauer's reply interested me, any photos Gordon ?
Having said that what is the best way to get it out ?
Interesting topic.
Les.

[Gordon Sauer]

Attached are three pictures showing under bonnet air evacuation options-the one I referred to earlier in this thread, but I just have a picture of one of the two vents, wherein there are two round vents at the back of the engine bay that looks like they could exhaust out the grill vent that otherwise goes into the passenger compartment, by doing some fiberglass duct work; and another picture on three vents on the passenger side and then another picture of holes with mesh screens over them that were made at the top of both of the front wheel wells as part of the Millerton New York air-conditioning installations. Gordon Sauer

[john.p.clegg]

..and a piccie of the vent holes in the inner wing ( n/s ) on my Plus2

which should reduce the bonnet pressure and aid airflow near the exhaust manifold,or at least that was the idea.

John

[elansprint71]

Flat out on the long straight at Goodwood, the bonnet on my S3 would bow upward alarmingly at the sides with the engine compartment air pressure. I have the regulation Lotus two holes cut into the left hand side inner wing, so my analysis was that at high speed the front grill aperture was too large and was allowing more air through the radiator than could be adequately exhausted around the engine.
From a racing standpoint, as long as there was sufficient air through the rad for cooling, the best solution would be to blank off part of the grill aperture as this should reduce drag - although I never got round to doing it. As the air intake for the carb plenum chamber is in front of the rad and in this high pressure area, there may be some minor 'ram charge' benefit to be had for engine performance. I did have plans to measure the excess pressure, but like grill blanking, I never got around to it.

Check out what Costin did to the front aperture in the Elite. What goes around comes around.

[65ginetta]

Another possible modification to reduce the under bonnet airflow build up is to vent the rear of the bonnet. I have not actually made the modification but I do believe it would reduce under bonnet lift and not have major visible or aesthetic impact.
At the rear of the bonnet panel is a radius section to prevent a foul condition to the front scuttle when the bonnet is opened. It would be simple to bore a series of 1"- 1.5" diameter holes in the radius section. I attach a picture of the standard panel and may make the modification.

[el-saturn]

..................at 200 to 220kmh on a italian autostrada (following a stupid testarossa made in maranello 10 years ago) i had no problems: you want proper bonnet fasteners HOWEVER [url]PLUS[/url] my bonnet is also made of proper fibers (kevlar and carbon) - i reckon i could do 300 kmh with the current parts if i had more power - i have to rely on 160 street legal hp - revs up to 8 thou thanks to the proper ENGLISH technology!!!!

sandy 4982 PS : i used to work in and with northhampton profesionells: john thompson and southgate racing -------- pies at the pub fer lunch and beer PLUS fer dinner ---------------------------------------------- just luv it

[prezoom]

A vent hole with screen in each inner fender well, will dump air into a low pressure area.

[Matt Elan]

As I see it there is a pressure difference between the top of the bonnet and the bottom of the engine bay due to the Bernoulli effect - the air running over the bonnet top is moving faster than the air under the car (as it has to move up and over the body) causing lift - hence the bonnet will lift when the catches are pulled. I'm pretty sure Lotus had a good grip on the aerodynamics of the engine bay......

[elansprint71]

iirc (it's a very long time since I studied aerodynamics at college) Bernoulli's work referred to incompressible flows, i.e. liquids and became less valid to anything other than very simple cross-sections when applied to gas flow.

In the case of a car we have the flow over the top of the bodywork which can be considered "simple" whereas the flow through the radiator aperture and under the car is rather more complex.

One thing which non of us have mentioned so far is is that not all the air going in through the front aperture has to escape via openings elsewhere in the engine bay- the engine is gulping in rather large quantities, which get pumped out of the exhaust pipe!

[Matt Elan]

iirc (it's a very long time since I studied aerodynamics at college) Bernoulli's work referred to incompressible flows, i.e. liquids and became less valid to anything other than very simple cross-sections when applied to gas flow

Hence my reference to Bernoulli's Effect which relates to gasses and fluids rather than Bernoulli's Principle which relates to fluid dynamics...........