slate blue

Now this post is meant to be helpful and not condesending (I know I just spelt that wrong) I can probably gives some worthwhile tips here, when you weld stainless, that word is more of a trade name than a description BTW, you need to be careful not to burn it. If the metal takes on a greyish/black appearance if it is burnt.

Also after stainless is welded it needs to be cleaned of oxides otherwise corrosion will occur and a much greater rate than it would otherwise. Pickling paste/acid is used, N.B it is very dangerous, do not come into direct contact with it. It is designed to remove the iron oxide only. This acid has been the bane of stainless welders/fabricators. A couple of Australians have invented a electrolisis method which is not dangerous.

A stainless weld should look like a rainbow, however when welding an exhaust full penetration is needed, So more heat, the best way to get full penetration this is back purging, or is it? I also use Solar Flux, it is a great American product and you paint this on the inside of the pipe 1/4 inch from the edge being welded. It stops oxidation on the inside of the pipe and it aids peno.

If you have ever seen a rusty stainless railing, now you know what has happened to cause it. If you have a stainless wire buff, that can suffice if used straight after welding. Stainless with a tig flows quite well, inconel is a different matter, I will have had a years tuition/practice before attempting that but it is really light and apparently great for exhausts.

Greg

Carl Fausett

Inconel is great - but very expensive and I dont think I have ever seen it commercially available in mandrel bent tube.

One issue here is cost - and the readily available supply of mandrel bent stainless helped our choice for 0.45 wall stainless.

This car of ours goes through so many changes, that we know the exhaust system will last only until we change headers or motor, etc. In this case, we have the 16v motor in it until late summer, then we will be installing the 32v motor. So we knew we were buuilding an exhasut for only 6 months or so.

Still - the point is weight reduction, and thats why I posted it.

Carl Fausett

Yes, grinding of the welds was mentioned in the text. And there is a grinder laying next to the pipes in three of the photos.

Yes, welding 0.45 (thin) wall stainless was a challenge without burning it nor burning through. The low amperage needed to prevent burn-thorough of the thin material also yielded higher bead heights than I wanted. Hence the grinding to knock the excess down.

My welds are not as pretty as a pro welder would have made, but good enough for a race car and an exhaust that I only have to have last 6 months or so.

But the point of this thread and this post is weight savings. For those building 928 track cars, hopefully these little suggestions give them some ideas where they can look for weight to take off.

Fabio421

Burns will supply mandrel bent inconel but it is very expensive.

http://www.burnsstainless.com

slate blue

By Fabio421
I rang them about this, they have no stock and just buy it in from another supplier, I do deal with Burns and there stuff is nice and they are helpful however you will do far better dealing with the people who make the pipe themselves.

www.goodfabs.com these are the guys that have helped me out, surprisingly helpful, will make you any pipe with any degree angle if you can believe that. They gave away some of the their tricks. The inconel exhaust I am making including the Tri-Y headers will weigh around 50 pounds if the full length version is used, if the side pipe version was used it would be lighter.

Greg

Carl Fausett

Good info. I have always thought the 928 was ripe for side pipes.

Carl Fausett

Finishing the roof placement post... A couple of pages ago I mentioned we were replacing the steel roof with aluminum. I said the steel roof weighed 7 pounds.

Well, we got more than that. I failed to mention that, once the skin was off, the integral roll bar halo hoop that Porsche had placed in there was ripe for some lightening, and we picked up another 1.5 pounds there, for a total of 9 pounds out.

Here are the final pictures.

Of course - this one is not for the faint of heart nor was it easy to do. Look at the shape of the aluminum panel after stretching and some panel beating while it waits ready to be installed.

I had my doubts.... but it did finally come in well, and after adding the 1.5 pounds of aluminum back onto the car, we saved 7.5 pounds very high up.

Jim M.

Carl, I'm not a racer and not fluent in aerodynamics, but what is all that crap attached to the rear of your roof? Porsche designed the 928 using wind tunnels and was careful to keep drag to a minimum. Are you destroying all their good work?

hacker-pschorr

Even Porsche had very little understanding of aerodynamics in the mid 70's when they designed the 928. A drag coefficient of .41 is nothing to brag about by today's standards.
Things got a little better with the S @ .38, S4 @.34 (.35 for the GTS).

By comparison, the 1984-1989 Corvette is .34, 90-96 @ .31, 97+ @ .29

I have no expertise to say if Carl's aero additions are a benefit or not, there is room for improvement over the factory design.
The best way to improve the aero on the 928 is to drive backwards.

From what I've gathered, the little ducts or whatever they are called on the roof, are supposed to clean up the air as it goes over the roof. The Mitsu Evo came from the factory with similar devices on the roof (except they appear to face the other direction):

http://www.seriouswheels.com/pics-20...r-1280x960.jpg

On a side note, it's not all about reducing drag. I cannot begin to imagine how much drag his rear wing is causing.

largecar379

The product you see are AirTabs, from Aeroserve Technology in Ontario Ca.

Originally produced for the trucking industry, they are reported to increase fuel efficiency by 4-6%, improved handling in all conditions (wind or not), increased steering tire life, and increased visibility in wet conditions (reduced tire spray).

Aeroserve states that the AirTabs decrease drag (see above).

Most popular uses for AirTabs in on the trailing edges of tractor cabs/sleepers (to smooth air flow from cab to trailer) and the trailing edges of box (style) trailers.

There's your answer.....


--Russ

Fabio421

Fixed it for ya'.

And I googled it and it seems Russ is correct. http://www.airtab.com/en/Product_App...ons_24/22.html

Don't tell Ryan but I think Russ just posted something about trucking that apply's to 928's.

Carl Fausett

Those are vortex generators, and Russ is right, the particular vortex generator I used is from Air Tabs.

Taking nothing away from Porsche and their solid aerodynamic work in 1972-75 on the 928... they never should have dropped the hatch glass down suddenly as they did at the top.

The sudden and tight radius bends at the top of the hatch glass detaches the air flow at that point, disrupting efficient laminar flow of the air and causing a wake of turbulent air down the hatch and off the *** of the car.

The turbulance hurts 2 ways: it creates drag (you are actually towing that flume of turbulent air behind your car or truck) and it renders the rear deck spoiler just about worthless.

The function of a vortex generator is to give direction and control to that otherwise turbulent air, and it helps wrap the air around that corner, allowing it to stay laminar longer, and reducing its wake.

On the sides of a semi trailer, the vortex generator helps the air wrap around the last edge and reduce the wake behind the trailer so the truck receives an improvement in MPG.

I also use them on my enclosed race car hauler, where we have received a benefit of 2 MPG. Photo below.

On the race car, the same technology reduces my wake behind the 928, and we picked up 4-6 MPH on the long straight at Road America as a result, but more importantly, the rear wing became much more effective as the air flowing over the wing was now laminar and not boiling.

The Air Tab website has more info if you are interested.

Vlocity

Carl,

I just bought a few of these, so this post is very timely.

Would you mind starting an Aero thread, I have some ideas for a front splitter.....

Have you thought about using the airtabs under the car, what I have in mind is a splitter with a large flat surface behind the air dam and 50-70 mm in front of the air dam. The second thought I have on the splitter is the same large area, but with one or two well placed diffusers made as steep and as long as possible. (I would want to make sure that they were not overly steep and cause seperation) The diffusers would reduce the static pressure area in the forward underbody.

Now...back to weight loss. (I'm down 10 pounds in the last 4 weeks) Those 5 mile runs are paying off!!!

Ken

FBIII

Hey Carl, Still haven't gotten rid of the snow?

Black Sea RD

Hi Carl,

Thanks for sharing.

I asked about these on your car months ago in one of your Pike Peak threads since I also looked into these to increase the air onto the rear wing instead of having to raise the wing higher causing more aero drag. Glad to finally hear this information.

Cheers,
Constantine