I'm trying to plan the airflow in my engine bay:

- airflow to stop the exhaust cooking the starter motor
- cold air intake

My car has a mid engine'd layout, and I assume negative pressure over the rear deck, (hopefully) positive pressure into the side ducts, and 'who knows' pressure underneath.

I want to confirm this my measuring the air pressure at these points at, say, 30, 60, 90mph.

The only locost way I know of doing this is using a home made manometer - a length of thin hose full of liquid, mark the levels at either end, put one end in the cabin, then observe whether it rises or falls when the car gets up to speed. This will tell me the pressure differential between the cabin and the measurement point. It seems messy, difficult to calibrate, and prone to error to me - is there a better way?

Cheers

Gary

ideally you want a sensitive digital manomoter. Do you know any plumbers/heating engineers?

Their boiler analysers usually have such a thing on them, then just run hoses to the appropriate places and read off the pressures in millibar on the meter.

I have a super sensitive digi manometer in the garage, never used it, you're welcome to borrow it, probably be a couple of quid to post either way - I'll dig it, and the spec out tonight (memory allowing )

alternatively maplin sell what they call an airflow meter iirc im sure i saw one in the catalogue when i was leafing through deciding what to do for my physics a level sensor coursework

Seems as though digital manometers are cheaper than I thought

manometer

I think I'll buy one of these.

Any tips on how best to use it - should I use the inside of the cabin as the reference point - my concern with this would be that as it's a hard top, I'd have to leave the window open a crack to route the pipes, and so the amount the window is opened might affect the repeatability?

Thanks for the offer Kissy, I like the idea of having my own now though, so that I can take my time, and then do some playing around with the nosecone and rad airflow too...

Gary

No worries, make sure it has the right range - what's in the advert may be a bit misleading. You need something that is millibar sensitive - check also the resolution. HTH.

The £55 unit goes down to 3 milibar accuracy - do you think this'd be suitable, or does it need to be more sensitive than that?

What you really want is a Magnehelic gauge which is a precision pressure gauge. They can be a little pricey but ebay maybe your friend.

Autospeed have done several articles on DIY aerodynamics and reducing induction restriction and they usually use one of these gauges.


Autospeed website

The other simple measure is to stick lots of short pieces of wool over the body work. Get a friend to follow with a camcorder and watch for laminar or turbulent flow

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I've never tried measuring air pressure in a moving airflow, but I think it might be messed up by the Bernoulli effect
http://en.wikipedia.org/wiki/Bernoulli%27s_principle

Anyone have any experience of measuring pressure arround a moving car?

this might give you some ideas
http://www.komar.org/faq/manometer/

If youre handy with CFD software you may be able to use ANSYS, but it is a LOT OF WORK on something that complicated, both in terms of geometry and heat flow.

Either that or use some turbo-tape!

Ed.

We do this type of thing every so often to correlate with work done either in the wind tunnel or using CFD or both.

Making meaningful and reliable pressure measurements whilst on the move is not straightforward and, for the purposes of your two aims, not really necessary.

The easiest way to check on cooling airflow is using a vane anemometer like: http://cgi.ebay.co.uk/Anemometer-AIRFLOW-INSTRUMENTATION-DVA-30VT_W0QQitemZ7612904002QQcategoryZ77985QQssPageNameZWDVWQQrdZ1QQcmdZViewItem

These will give you airspeed and (usually) a +/- indication to show which way it's flowing through the probe head. This is more useful than you think because it is fairly common for air to be coming out where someone's intended it to go in...

If you do go for pressure measurements then it would be worth doing some preliminary runs with wool tufts taped to the body nearby to give you an indication of flow direction in the vicinity of where you're going to put the probe. Whether the probe is at 90 degrees to the flow or straight on will make a very big difference to what you're measuring. Anything netween those two extremes and you're in a very grey area.

For the reference pressure at the other side of the manometer you can use a probe on a boom a long way away from the car (problematic) or fit it to a nicely flat, vertical, forward-facing surface at the front of the car - like a number plate. This will be the highest pressure anywhere on the car but by an amount directly related to road speed, and hence calculable. All your measurements will be negative but it's an easy bit of maths to work backwards to see what was actually a positive pressure and what was negative.

Don't worry about doing the extra 90mph run though. Except for around a wing/diffuser the flow pattern will be virtually identical at both 60 and 90. What works for one will surely work for the other.

Good luck

How about a MAP sensor as found in MegaSquirt and most other engine management systems.
Should give 0-5V reading which you can display on voltmeter.