Using a magnehelic gauge

[MTXA]

Pretty soon, I'm going to be experimenting with some aerodynamic mods on my car. Some of the mods will include grill blocking, a front under tray, smaller door mirrors, partial kamm back. I want to collect empirical data supporting whether or not the modifications are a net loss or gain. I'm self teaching myself about aerodynamics as they relate to automobiles, and am still in the early learning stage. I would like to know how, for instance, the under tray effects under hood pressure and temperature change.

To the point, I have a few novice pressure reading questions.

1. I want to keep my test equipment costs low and prefer to only have to purchase one magnahelic gauge. How many inches of water scale would serve both interior and exterior testing? 0 - 1 wg? 0-3 wg? Higher?

2. Are pitot tubes necessary for measuring the air in the slip stream?

3. Is there a range of selection in pitot tubes or does one size fit all?

4. I would assume that when measuring pressure in a slip stream, I would position the hose end of the gauge directly downstream as indicated by tuft testing, correct?

Thanks in advance!

[scianiac]

I'm also interested in doing this type of testing although I'm too cheap so I'll probably just make a water manometer. I found the article "Ultimate DIY Automotive Modification Tool-Kit, Part 3" on autospeed.com (can't like yet) provided a good basis on how to do pressure testing although I'm also still looking for some more information if anyone has some. Like for say are we looking for just a reduction in pressure or pressure over area.

To my (limited) understanding a pilot tube is only used for measuring "dynamic" fluid velocity instead of pressure. I think we are more interested in pressure, as in the negative or positive pressures that generate most of our drag. To measure pressure the tube should be perpendicular to the flow (as seen in the autospeed article). I think the air dynamic pressure would only be used if you want maybe determine airspeed before one of your modifications (IE placed in front of it on the car) to determine how the mod should be made.

P.S. First post, Hi everybody!

[MTXA]

I've found a little more information since this post. I think I'm going to try hunting down a used 0-3 or 0-4wg gauge. One experiment I'm interested in doing is measuring the pressure at the rear of my car and seeing if using a nozzle to direct the air that passes under my car to reduce the lower pressure at my hatch. This is different than using a diffuser in that I'm not looking to shape the lower pressure zone but rather actually break the suction. It's my understanding that pressurizing air has less impact on efficiency than the drag created in the wake. I've already added a front lower tray and want to verify that the pressure under my hood isn't increasing as a result which could cause poor flow through the heat exchangers.

[scianiac]

I was thinking of making small clear side deflectors to better shape the wake. One of my concerns is the spread of pressure, maybe the pressure in the center of the trunk will be the same but if I've increased the pressure (as in less negative pressure) on the sides then the total drag force is reduced. I was thinking maybe hook up several hoses together to average the pressure so I can better measure drag force instead of pressure at one point. I think this would work as long as all the tubes are the same size. I think in that article he used two hoses hooked up to the guage, one on either side of the rad to measure pressure across it and therefore flow.

[MTXA]

What you are describing sounds like a partial kammback. From what I have read, it seems 7 - 10 degrees of turn in is the maximum before the air detaches from the outer edge. It is a well proven way of decreasing the low pressure wake size.

I have been thinking about attaching a dowel to the rear of my car so I can get incremental outward measurements to maybe 4 feet behind the hatch and also make the measurements at gradually faster speeds to see if I can get a good idea of the neg pressure wake zone. Kind of 3D mapping the wake.

My first experiment is going to be trying to infill the wake zone with higher pressure air channeled from under the car. I have not found any literature describing what I want to attempt and from an intuitive standpoint it makes sense to give it a try. Due to the cold weather, right now this project is going to have to wait.