Calling All Aerodynamicists - Pressure Differential Query

[Hammer]

Since this forum has members from all walks of life, I hope someone might be able to assist me on this query...this query came as a result of reading the articles of Undertrays, Spoilers, & Bonnet Vents: Part 1, Part 2, & Part 3. Which came from this thread.

Pic 1

My particular interest is on part 3, which focuses on bonnet vents. The bonnet vents described are exhaust vents (as opposed to intake vents).

According to their test results, the best pressure differential over & under the bonnet from their test vehicle (JDM Nissan Maxima), is along the leading edge (front) of the vehicle (see pic 2). Thus the best place to mount the bonnet exhaust vents is just after the radiator (see pic 1). This will give the most efficient removal of hot air from under the bonnet. Their study also showed that placing it at the trailing edge (near the windscreen), actually had the opposite effect. Instead of the vent exhausting hot under bonnet air, it actually sucked in air from outside. Thus increasing pressure under the bonnet, slowing the airflow through the radiator/intercooler, which reduces their efficiency to cool (for more info read the articles – it’s a good read).

Pic 2

Basically, the higher the pressure differential between the outside (+) & under bonnet (-) pressure, the faster air will pass through the radiator/intercooler etc. So keeping the under bonnet pressure low by having an efficient exhaust vents will keep cool outside air passing efficiently through the radiator/intercooler and avoiding pressure lock.

Pic 3

My question to all you aerodynamicists out there...is this pressure differential over the bonnet as described from the article true to all cars with similar profile (Ie. Is this a rule of thumb)? Or does each vehicle differ? And if they do, has anyone tested where the best pressure differential is for the MX-5? Or more to the point, which part of the MX5’s bonnet has the least (or negative) pressure – leading edge or trailing edge? I want to put some exhaust bonnet vents for my 5 but I'd like to make sure they work.

I know I might be asking too much but you never know. People from this forum have always surprised me how knowledgeable they are.

[stefan]

Related to this question, I always thought that the best place to locate a CAI was a low pressure zone (like a forward facing part of the car), not a high pressure one like the base of the windscreen like I notice some MX5 kits are. Of course, maybe this is done because it is a clean convenient source rather than for any small ram-air effect.

If you can find or borrow a copy of "21st Century Performance" by Julian Edgar, it has a good chapter on aerodynamics (as well as on all other aspects of car performance, it's a very good book but out of print now). In fact, I just realised that he writes for autospeed, thought the book seemed familiar.

[greenMachine]

First of all, I am not an aerodynamicist (sp?).

FWIW, my understanding is that this is a fairly universal rule. However, as your caveat suggests, it is dependent upon the profile of the nose of the car. That is, it requires the high-pressure air at the front of the car to make a fairly sharp turn across the front of the bonnet, the sharper the turn (and the higher the airspeed) the greater the pressure drop at the front of the bonnet. I am not sure that this is necessarily a good example, but think Ford GT40 radiator outlets. Also, remember that in most cases, only a proportion of the air in front of the nose of the car passes through the radiator and into the engine bay, and as a result a significant amount of air has to find its way under, to the side, or over the nose of the car.

I am not sure where the idea might come from that an air inlet for combustion air should be anywhere but in a high-pressure zone. Think free supercharging! There may be issues with the length of the inlet tract and throttle response at low speed/low revs, but those issues are quite separate. If the issues create big enough problems, then some compromises will be necessary. Again, think of some of the 5litre Toranas which had big power bulges in the bonnet, stretching back almost to the windscreen, and open at the windscreen end. I remember seeing some discussion about how much extra power this set up provided, I can't remember the figure but IIRC it was well worth having.

Of course, production cars have a whole range of compromises so they are not necessarily good examples to illustrate these points. Better to look at people who are trying to extract the most from their cars, and see what can be learnt there.

[graemebc]

Bonnet vents not only remove hot air, they "can" if properly placed, reduce the pressure wave that builds up on the bulk head that can act as a brake to forward momentum at high speeds. (really high speeds) I don't see your 5 achieving those kinds of speeds. As you correctly wrote they can actually suck air in, if your looking for max cooling then you are looking to get the biggest pressure differential across the radiator you can. Under trays help in two ways, by generating a pressure differential at the back of the radiator and by adding in some "ground effect" by reducing the pressure under the car in comparison to over the car. Again at higher speeds.

If you want to check your cars aero profile cheaply and have a bit of fun, tape some wool strands on the bonnet and go for a drive. My opinion though is get a larger radiator if you are looking to keep the engine cool.


Here's a picture that a LS1 miata driver installed after running some pressure testing on his vehicle.

[bruce]

Right between the webbing;

http://img201.imageshack.us/img201/1891/imgp1879gx5.jpg

[stefan]

Exactly. I looked at doing this to my sillycar to improve airflow across the intercooler and radiator. Air could come in but other than down past the engine, had nowhere to go. The theory I have read indicates that a bonnet vent behind the radiator would greatly improve this, air can travel through and out along the bonnet. Aerodynamics as under the bonnet stops being a high pressure area.

I did read that to minimise turbulance, you want some straight area after the vent so the flow is smooth by the time it hits the windscreen, which is I suppose why vents towards the front of the bonnet are recommended. That and the pressure area reasons in the first post, and probably legal reasons as well to do with engine fluids on windscreen if it is a road car. Not sure of the exact NCOP rules here.

[Hammer]

As posted from the other thread....does anyone know where I can get a bonnet like this for an NB8B in Australia?

It seems perfect. As you can see, the bonnet exhaust vent is placed at the leading edge, just after the radiator - the spot with the highest pressure differential.

[Matty]

My question to all you aerodynamicists out there...is this pressure differential over the bonnet as described from the article true to all cars with similar profile (Ie. Is this a rule of thumb)? Or does each vehicle differ? And if they do, has anyone tested where the best pressure differential is for the MX-5? Or more to the point, which part of the MX5’s bonnet has the least (or negative) pressure – leading edge or trailing edge? I want to put some exhaust bonnet vents for my 5 but I'd like to make sure they work.

I know I might be asking too much but you never know. People from this forum have always surprised me how knowledgeable they are.

Yes, it's a pretty reliable rule of thumb - most cars are roughly the same shape.

I believe someone on miataforum has done this exact testing -<goes off searching...> ... here you go:
http://forum.miata.net/vb/showthread.php?t=296259

[greenMachine]

As posted from the other thread....does anyone know where I can get a bonnet like this for an NB8B in Australia?

It seems perfect. As you can see, the bonnet exhaust vent is placed at the leading edge, just after the radiator - the spot with the highest pressure differential.

Not really.

Looking at the numbers on the photo at the top of this thread, and placing them over the bonnet you are referring to, the area of max low pressure is well in front of that radiator outlet, and to my eye it looks like the pressure there is pretty close to nominal. Having said that, any low pressure area would help. At least, assuming the low pressure there is actually lower than the low pressure area under the engine bay... if it isn't, what is intended as an exit, may become an inlet

It would be most unwise to install something like this without pressure testing the bonnet (that is, everything from the leading edge of the bumper back to the widscreen), and while you are at it, do the same for the underbody. For example, lowering the air pressure under the car (ie the engine bay area) may be simpler, cheaper and/or more efficient. Not as much ricer impact though, I grant you.

The tufting mentioned above is of limited value - good to find out if your airfoil is stalled say - but doesn't tell you much about pressure.

[Juffa]

Got me one of these this week. I have run some pressure sensing tubes under the bonnet and hope to take some readings of my 5 in the near future, including pressure differential across the intercooler and radiator.

I thought it might be fun to get some readings from NA, NB and NCs, including those that are bog standard and those that are modified... I hope to be calling for volunteers in the next few weeks.


J

[Jeo]

Very interested in the results there Juffa.

I'd happy to help out, if I was in Melbourne.

[Hammer]

Got me one of these this week. I have run some pressure sensing tubes under the bonnet and hope to take some readings of my 5 in the near future, including pressure differential across the intercooler and radiator.

I thought it might be fun to get some readings from NA, NB and NCs, including those that are bog standard and those that are modified... I hope to be calling for volunteers in the next few weeks.


J

Sweet!

If I was in Melbourne, I'd be happy to volunteer.

I too will be keen to know the results. In particular on the NB.

[Juffa]

Ok. First results are in.

I ran a pressure differential test across the intercooler/air cond/radiator. The results came back as zero difference, i.e. the pressure was the same on both sides. Which would indicate that the air is getting into the engine bay, but is not able to exit quick enough. I plan to verify these results again this weekend and then look at what options I have to resolve this.

At the same time I was running the above test I was also monitoring the intake temps. I had bought a cheap indoor/outdoor digital temp gauge from bunnings ($11). I placed the 'outdoor' sensor on the air intake pod, and therefore should be reading the air temp as it enters the intake, before the turbo. I have a permanent temp sensor mounted in the intake system just before the intake manifold. This is used by the ECU to monitor the air temp and make fuel/timing adjustments based on that. I obtained interesting results in that the 'outdoor' sensor on the pod filter was showing the same temp as the intake mounted sensor. This would indicate that the intercooler is not acutally doing much (if anything ), which could be another sympthom of the zero pressure differential mentioned above. I'm going to try and verify the accuracy of the cheap temp gauge on the weekend just to make sure this is not a red herring.

J

[Hammer]

Thanks for the update Juffa. Keep them coming.

I too am more keen to exhaust air out of the engine bay as opposed to ramming more in. Then both the radiator and intercooler can be more efficient as more air passes through them.

[greenMachine]

Hmmm, very interesting, as they say.

I would have thought you needed low pressure on one side to induce the air to flow through the rad/intercooler/codenser..... Same pressure on both sides of a radiator/intercooler suggests to me that the flow of air through these is low to none , which suggests an overheating problem in your future.... This is corroborated by what you have found with your intercooler charge temps.

But this is highly improbable, and warrants further tests which I see you are doing.

Can you calibrate the device you are using, eg with your MAP sensor? Alternatively, see if you can reproduce the results at the beginning of this thread?

This is great work you are doing Juffa, wish I could help out....