Nissan 240SX aerodynamic modification

[beatr911]

Those 240s are great driving cars. Many happy memories with ours.

Off topic a little but consider the stock air dam from the SE model and/or starting the undertray at the front, going all the way back.

+1 on cleaning up what's there to start. Mirrors, antenna, rear wiper delete if you have it, smooth front wheel covers, rear skirts. You'll probably do these with a rear cone anyway.

The big, inefficient, low geared truck based motor is really working against you no matter what you do aerodynamically. The chassis/body on these is a great starting point for a RWD car however.

[Midnight Drifter]

If you're serious about fuel economy, it's time to start looking into a JDM CA18DET engine and restoring it to optimal mechanical condition (and perhaps changing over to a similar-sized-to-stock but more modern turbocharger and adjusting the wastegate/reducing boost to 87 octane-friendly capabilities - BONUS POINTS, intercooler delete makes for a bit of an instant warm air intake kit) and getting rid of the KA24DE. It will fight you tooth and nail for eficciency and the CA18DET from the Silvia Q's (or even CA18DE from the Silvia J's if you'd rather not play with turbochargers) is a perfect bolt-up to your current transmission.

Plus, it's JDM tyte. /sarcasm

No, seriously. CA18 if you know what's good for your fuel economy.

[Sven7]

Originally Posted by freebeard

Wielding the notorious Template, this is what most people would give you. Match the ground plane and peak of the roofline.
correct

Again I would listen to Sven7 on how big a bustle you might care to drag around.

Of course we are assuming in that photo that the rear window has attached flow. Even if it does, it may be a bit strained (pulled down too quickly but not lost), in which case it would behoove you to build a tail from the top of the rear window down, following the template's curvature.

To build the correct angle on the boat tail from the trunk lid back only, I am told to mark the vertical line on the template that is nearest the bumper (here, in the middle of the template's rear wheel) and simply slide the image down, maintaining its angles.

Getting the right angle will be easier if you get a straight side blueprint, as so:



Edit, ok I did your homework for you

The diffuser bit is taken from Hucho's book. Compared to a horizontal belly pan, the 3* upward tilt should decrease your drag by about 0.02 Cd.

You may find the departure angle to be too conservative, but fear not, as a 15* angle will only increase your drag by 0.01 Cd compared to a horizontal pan.



Keep in mind if it's too long for your tastes you can chop it off at any point without changing the curvature. This will decrease your aero gains, though.

[Sven7]

For comparison,

I run a 15* angle on the bottom (not departure angle- just on the bodywork) and have not had any problems, even off-roading.

The rear plate holding the lights and license plate is 36x7". The entire boat tail extends the rear end by 41". I have no problems parking in standard spots or parallel parking, since it's shorter than a new Suburban, with a comparatively shorter wheelbase to boot.

Straight side view before it was painted. The stock bumper ends about where the vertical piece of tape is.


Progresssss by Tyler Linner, on Flickr

The rear ends up looking like this:

Rear by Tyler Linner, on Flickr

[freebeard]

Sven7's side view actually matches to The Template underbody better than the photo. Don't lower it, just add wheel spats front and rear. I'm sorry, I still didn't fix that bad cropping behind the front wheel

I like my method better because we are looking at 2D side views, which show what happening on the car's centerline. Away from the centerline the air is moving in the direction of the fender top not the backlight.

The inset is to show that it looks like Sven7 moved the Template down, but also forward along the rear departure angle.

Anyway, somewhere in this ball-park. Mix and match as necessary.

How about sunken '59 Cadillac taillights around where the stock bumper ends? They would be further apart, so the turn signals would read better at a distance in the dark.

[Sven7]

First of all, DO lower it. Everything should be lowered.

Originally Posted by freebeard

The inset is to show that it looks like Sven7 moved the Template down, but also forward along the rear departure angle.

Second, I did NOT move the template forward. I moved it straight down, maintaining the correct angles. It looks further forward because I trimmed it with the bottom of the ghosted template (otherwise the ground plane would be wrong). Theoretically, my blueprint should be literally that- a direct guide from which to build the physical boat tail.

Scaling the template down to the height of the deck will give you incorrect angles. Scaling the template down in most circumstances will give you incorrect angles. (Yes, I recall aerohead marking your Beetle as an exception)

I prefer to use a flat blueprint instead of a photo because a flat image superimposed on perspective tends to distort angles, leading viewers to believe it will look much different than how it actually would.

Reference

(I hope I don't sound like a dick here... just trying to clear things up)

[freebeard]

As long as we don't scare off raylit20; I'm OK with this. I see now why your proposal followed the underbody line.

I'm a big fan of redwood bender board. I'd put a stake at the point that these overlays agree on and lay a flexible board on the top of the rear fender and pull it to the height that the overlays also pretty much agree on, and use that. The transition from the car fender to the boat-tail wouldn't have that little break angle to match the line of the backlight.

[raylit20]

Originally Posted by beatr911

Off topic a little but consider the stock air dam from the SE model and/or starting the undertray at the front, going all the way back.

+1 on cleaning up what's there to start. Mirrors, antenna, rear wiper delete if you have it, smooth front wheel covers, rear skirts. You'll probably do these with a rear cone anyway.

The big, inefficient, low geared truck based motor is really working against you no matter what you do aerodynamically. The chassis/body on these is a great starting point for a RWD car however.

I intend to run partial undertrays. I am also looking to install a cowl for the wipers along with the mirror delete and tire spats. Taller LRR tires will come later as my current tires only have ~200 miles on them. I'm thinking about going with 205/65/15's, possibly 195/65's.

Also, my car is powered by an SR20DET with some minor mods. Wet curb weight of the car with a full fuel tank is approximately 2250lbs. I did my research before buying the swap and found that the SR is a far more economical motor for my purposes than both the KA and the CA.

I have been looking into the Q45's 3.54 final drive gearing. With the light weight of the car and incredible amount of power I have at low revs I believe I can considerably lower the rpm's at cruise without any lugging.

A PCM based coolant heat retention system and an engine blanket are both on the radar as well.

My design goal is 35-40 mpg in the city, with whatever is necessary to achieve this.

[freebeard]

So how many inches of boat-tail are you planning? Sven7 is at 41"; I'm planning on 36". Even 18" will give a benefit.

Instead of a mirror delete; how about two mirrors farther forward.


This moves the mirror out of the airflow around the A-pillar. Have you seen the pictures of people running 5 mirrors in the interior. I've only got one, but it's 2 1/2' wide. An example:http://ecomodder.com/...#post208199

[aerohead]

If you wanted to 'push' the contour of the roofline to a 'radical' but 'scientifically arrived at' curvature,you could superimpose Volkswagen's latest L1 roof over the 240 SX.
This contour is from Buchheim's wind tunnel work and goes as steep as 23-degrees maximum downslope.It's 1-degree 'faster' than the 'Template' but maintains the gradual buildup of downslope.
The important thing is to respect the gentle curvature leading up to the 23 -degree tangent angle.
Anything steeper than this will be what Hucho refers to as a pseudo-Jaray form,very prone to flow separation and longitudinal vorticity.
I suspect that the slope on the Nissan is just a bit too fast.
This goes back to the argument that,cars which 'look' streamlined aren't necessarily streamlined.