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.

Rear diffuser
 

A rear diffuser can reduce drag when it has a small area ratio (equals small angle). It works the same way as a boattail, increasing base pressure. However, it is not effective if the underbody is smooth. See: "The Aerodynamic Performance of Automotive Underbody Diffusers" SAE 980030 and "Selecting Automotive Diffusers to Maximise Underbody Downforce" SAE 2000-01-0345.

curve
 

Sorry MazdaMatt for late response! In Hucho's work,he cites R&D done with diffusers,and the research spells out that if the diffuser upsweep angle exceeds 2.5-degrees,that you'll experience flow separation,losing any static regain benefit of the diffuser.---------- MechEngVT and Kevin Cooper may be more current than Hucho,as Kevin is or has been doing work at Canada's National research tunnel I think.Virginia Tech may be doing some current stuff too.I'm not current with SAE so maybe use Hucho as a rule of thumb,with an expectation that diffuser knowledge is at a higher state-of-the-art.We ought to see if SAE is on-line now,and if we can purchase papers as could be done in the past.They used to cost $3.50 per paper( a great bargain!) and SAE needs to be supported for all the good they do.

Now that's of much interest to me. I would honestly have thought otherwise. Why is this so? Could you please elaborate a bit for those of us who do not have free access to the mentionned SAE papers.

Hi All,

The Basjoos boat tail is designed so the point is above the center line. This is creating a upward motion of the air from underneath, and around the lower half of the car. This is good for stability, and pulling stagnant air along the sides to fill in the rear flow.

A diffuser is limited in the amount of air it can direct, as the air into it is only from beneath the car. The BT can pull air from aside the car upward and backward too.

Diffuser
 

I would like to make a correction to my post on diffusers. I stated that the diffuser is not effective if the underbody is smooth. It should read unless the underbody is smooth. The diffuser can be viewed as a rather weak pump that does not work well if the flow path has high drag due to roughness typical of car underbodies. You will note that all race car underbodies are very smooth.

Also, on the topic of diffusers, if the diffuser angle is small a small drag reduction can occur. Certainly a boattail on the top and sides is better, but the diffuser is out of sight and can add to the boattail. The same comment applies to needing a smooth underbody to get the most from the lower panael of a boattail, which is effectively a diffuser.