did you produce your diffuser?
or was yours a one off for testing only?
how did you determine drag?
yes it's matt.
i made my own diffuser, designed, then produced in a CF, E-glass, epoxy conglomeration (for lack of a better word)
well the original intention was to sell them on either socalevo, or though a vendor there. however, school happened, and the project sits idle.
determining drag is relatively simple. drag is a force. newton says F=ma. i can find the mass of my car (with driver and timer) in kg, because metric is easier to work with. i can also find the acceleration (or rather deceleration of my car due to air friction)
how to do that-pick a target speed/range. i did 40-45, 60-65, 70-75 for my tests and documentation. let's make this simple and say we're going to do it for 40-45.
find a road, straight, flat, no traffic whatsoever (because we'll be speeding, slowing, and they will disturb the air - think about how they draft in nascar) go up to 50, put into neutral, the car will nodoubt start to slow have the timer start at 45mph and time to 40 mph (please dont try to do this yourself, you're supposed to be driving). now that we have a delta v (change in velocity) convert to m/s units. and we have a time. well a velocity per time is an acceleration. and ta-da! we have a mass and an acceleration, we can multiply and get a force (in newtons), convert to lbf (pounds). and we have pounds of drag.
this isn't however the whole story. there is also friction in the bearings of the wheels that is contributing to this drag, and the rolling resistance of the tires which also contributes as well. therfore the total drag is equal to:
A + Bv + Cv^2
because things like rolling resistance (moment of inertia stuff) is constant regardless of speed
things like bearking friction vary with speed
and things like aero drag vary with the square of speed (as senn here: D = 1/2(rho)v^2 * Sref)
so solve for A, B, C. and you could theoretically calculate the approx. drag at any speed. or figure out how much aero drag contributes to total drag.
so you need to test stock vs. diffuser. you need to do it at different speeds (and you can make a graph showing trends) and you need to do it both directions on that street you picked out (to account for any windage or slope), and you need to do it abunch of times which will hopefully average and counteract any error you created.
if you dont like this way, because its inprescise, you can get an accelerometer, and that will calculate your acceleration as well. but i dont have one, so this will have to do. [additionally an accelerometer calculates acceleration the same way we do using F=ma and a=v/t, it just calculates it alot more times (instantaneous) than we can do]
some other considerations. air is more dense when it is colder outside. i did my tests in the summer (90+F) and now that its winter (50-60F) my drag numbers may vary by 3 lbf or so
i had pictures of the whole process uploaded and in a progress thread on socal, but they changed software and all the pictures weren't uploaded anymore, and there were atleast 100 of them. on a not fast internet connection, that becomes boring. i can post some specific pictures if you would like.
***mods, im not actively trying to sell a product, i am using a product i designed and developed to demonstrate the purpose of it***