Diffuser VS Boat tail
 

how effective would a rear lower diffuser be compared to a boat tail? The Aero advantages of a BT are very tempting but some people may not want to take that large of a step.

Thoughts? Opinions?

A diffuser will get you nowhere near a boat tail will. Moreover, a diffuser without a belly pan is not as effective. On the other hand, a diffuser is not really an involved job to do and install and can cost next to nothing. I made one out of coroplast for the Elantra that coverred everything but a 4 inch hole for the exhaust pipe, but I lacked a belly pan so it was not all that effective.

It looks good though, interested in results after belly pan is added. If it doesn't work it might as well look cool.

I am new on here and I have a 88 crx hf and I drove 210 miles the other day from dallas to this side of austin on exactly 4gal of 87 octane pump gas with 2 side mirrows and a crappy heavy sun roof going 65-75 mph at night windows up and floating the grades, seems to me to get rid of the vortice/vacume efect on a 5ft wide car your car length would have to be 26-28ft. very impractical that is why honda has looked at how to re-
distribute these vortices with sharp wing like protrucions like on the back deck of the insight the boat tail will help but to get it out there where it needs to be you need to start with something like a drop tank and then it will be narrow enough to tapper down at a gradual enough rate to still be manuvarable I am going to shave the mirrows elimnate the sun roof and add rear wheel skirts then look at the front end first sinse the rear would have to bee soo long to make a real difference, I have often thought if I ran across a cheap hf shell about a three wheeled convesion then th boat tail could work and the rear wheel could be a hybrid e drive for in town and traffic jams

It would just clean up the air under the car before it becomes a big wake.

What is the optimum angle for the diffuser?

I'm thinking I could make one that spans the gap from my lower rear cross member to a few inches after my rear bumper. The bumper will most likely have to be cut a few inches up.

Reading with interest as a rear diffuser is one of the next things for my track car (just installed an air dam and most of an undertray).

angle
 

The diffuser cannot have an up-sweep angle greater than 2-1/2 degrees without flow separation.

2.5 degrees steady? could it not curve upwards? ie, if i did make it curve upwards, would flow disconnect at the 2.5* point of the curve?

I don't think you want to curve it upwards. A diffuser is different from the roof surface or top plane of a boat tail in which you are fighting flow separation only. In a diffuser your main boogeyman is flow reversion because you are typically flowing into a negative pressure gradient. This is because assuming you have an effective front air dam and belly pan ahead of the diffuser the air pressure underneath the car is lower than the air pressure in the wake, but we all know because the vehicle is moving relative to the road air will indeed flow against the negative pressure gradient and the diffuser needs to be very shallow to make it happen cleanly.

Take a look at some of the touring race cars, specifically I'm thinking of the Corvette C6R, had a very prominent diffuser. In many of these vehicles I think they're going for downforce since the diffusers typically have a horizontal plane or splitter on the bottom.

I was behind a Prius in line for a tollbooth today and noticed that the lower edges of the rear fenders/bumper cover behind the tires act as quasi-diffusers. It's only missing a few vertical sails and a smooth taper under the car away from being a better diffuser.