Measured aero pressures on Insight

[JulianEdgar]

Originally Posted by MeteorGray

Quote: "That view is of the back of the Jaguar, not the front!"

Ooops, it's my assbackwardness coming out again.

I guess I'm still surprised there would be high pressure on the lid of the trunk as well, rather than some lift.

Jaguar showed higher pressures on the trunk lid in the CFD image, but except for just ahead of the trunk lid kick-up (spoiler), I measured on the road slightly negative pressures. This is what I would have expected.

[JulianEdgar]

Originally Posted by JulianEdgar

Yes it would - do you have a price?

I try to avoid using instruments that are too expensive for normal modifiers. The low cost digital manometer I have has a highest resolution of 0.1 inch of water - no good.

Edit - this looks good https://www.amazon.com.au/Perfect-Pr...22441494&psc=1

Second edit - just bought it.

The PerfectPrime AR1890 digital manometer has arrived. The instrument feels fairly cheap, doesn't come in any packaging and doesn't have an instruction manual (it is available here). But using the meter is self explanatory. It has a switchable backlight. Needs 9V battery (not supplied).

I compared its reading with a Magnehelic gauge - they match within about 10 per cent.

The meter needs to be switched to differential mode - its default is non-differential.

The meter also switches itself off after a while (25 min - but this can be disabled).

I have also now done multiple readings on the road. Set to inches of water, the display shows 0.00. The last decimal place is not really very usable because the on-road readings dance around a bit. So it might be showing 0.37, 0.45, 0.42, updating twice a second. So in order that you can select a mental average, you need to look at the meter longer than for a Magnehelic gauge (that shows a steadier reading). I suppose you could add some damping volumes to settle the digital manometer reading. You can select an averaging function but then you'd have to hold that speed for a reasonably long time (I am used to just getting to the speed and then noting the reading).

But the digital manometer is really cheap, can work over a wider range that a single Magnehelic gauge, and gather the data you want.

US$35 from Amazon for the meter and US$6 for the pitot tube static reference. Add a bit for some plastic hose and you're good to go.

Edit: I should add that the stagnation pressure on the Honda at 80 km/h was a measured 294Pa with the dig gauge.

Edit: I found the instruction book, that was included - along with a fitting and some short lengths of hose. (They were in the pouch the instrument can be stored in.)

[cr45]

As the last decimal place is dancing around you could try measuring in cm of H20. The readout will only go to 1 decimal place - a resolution of 1mm of H2O. This resolution is approx 4 times courser than 0.01" resolution in inch H20 mode but should provide a more stable reading.

Just 1 problem though, this represents a resolution of 10 Pa which is not going to be accurate enough for a lot of locations on a car particularly at the rear.

It looks like damping or the average function will have to be investigated.

[cr45]

Your meter looks identical to mine bar the color and the name.

That "dif" button stands for difference and not differential. In fact the manometer is always in differential mode.

The "dif" mode is used where you wish to find the difference between 2 readings.

An example would be if you hit the "diff" button at 80 km/hr then increased your speed to 100 km/hr then the reading on the manometer would be the difference in pressure for the 2 speeds.

So it is possibly best not to hit the diff button at start of test but rather if you do not have a zero reading whilst stationary then hitting the "hold" button for 2 seconds will zero the gauge.

[JulianEdgar]

I added two pressure dampers to the sensing hoses. These are often used on the emissions plumbing of older cars.

If selecting them at a car dismantlers, be careful that they're not one-way valves (also often used on the plumbing). Check them by sucking on them (and if you inhale fuel vapor - that's your issue). The dampers will allow air to be just sucked through them in both orientations. Conversely, the one-way valves will hold a vacuum when you stick your tongue over one port after inhaling.



The use of the dampers dropped the stagnation pressure variation (the largest value, and so the greatest variation of the pressures I measured yesterday) to about 0.03 inches of water, making it much easier to get a reading.

[Vman455]

I've been using the same digital manometer for my testing. Rather than dampers on the hoses, I use the recording function on the device: one press turns on recording, and three more presses of the same button displays the average pressure recorded over the interval. After the car is up to test speed and I start recording, it typically displays a stable number--but since all the roads here are on a one-mile grid I usually have 40-45 seconds of recording time just in case it was on the verge of ticking up or down, which happens occasionally.