Twin Turbos - No tail vs Duck vs Turbo - Airflow/Downforce?
 

I'm gathering information for my winter TT project. I will use the info to help locate the intercooler(s) and configure the type of tail, if one is used, or style wing to use.

What would be the difference in these three configurations?
Does a tail really help to direct air into the engine bay or is it needed just for downforce?
Isn't the fan alone enough to pull air into the engine bay?
If a tail is only needed for downforce can you substitute a wing?
What are the affects at speeds from 50-100mph (target)?

The tail creates a high pressure area at the engine grill which does help force more air into the engine compartment. Without the tail the area around the engine grill is a low pressure area which means it's actually wanting to suck air out of the engine compartment. So with the tail you do get more air/cooling. I think I've heard a couple of people on the board report 10-15* difference in average engine temps after adding a tail. (sounds like free HP to me)

I wouldn't think that a wing would be as good for the cooling as a tail. Then if you are looking for intercoolers there's the problem of getting them to fit without the tail. Not easy to do, but has been done.

If you want to know about the aero affects of tails go here
http://forums.pelicanparts.com/showthread.php?s=&threadid=126462&perpage=40&pagen umber=1

http://forums.pelicanparts.com/porsche-911-technical-forum/111538-downforce.html

Thanks for the links!
A wing would be used only for downforce not cooling. What you are saying sounds reasonable, in a wingless car the fan is fighting low pressure for the air it needs to push into the engine.
So I need a wing. Turbo or Duck?
Intercooler placement is a matter of custom fitment. Minimal added wing weight is the focus.

Why twin turbos?

Why not?

Just kidding. I'm gonna post for questions and advice when I get closer to actually starting the work. What I want to do is come as close as possible to eleminating boost lag. From what I have read this can only be done with extremely short exhaust manifolds and twin turbos. I have no intension whatsoever of increasing the horsepower, only reducing lag to its earliest possible threshold. I expect you to be right there with me on this quest. ;)

A ducktail is pretty cool because it reduces lift almost effectively as a Carrera tail, but actually reduces drag, unlike the Carrera/Turbo/etc. wings.

Plus, bragging rights for intercooler fitment under a ducktail are substantial. ;)

Sounds like fun. I'll be going less the gt2 tail on the street.

I've been thinking about using the duck tail for some time. The idea is to make a very shallow full bay intercooler that would fit flush with a grill cut in the duck. With most of the tail cut away it should weigh very little. The GT2 unit I currently run is very functional but also very heavy.

What do the fiberglass vs metal ducks weigh?
Are metal ones even available?

It looks like the fused metal/glass ones are still made by Gary Kempton/Made By Hand, but they're about quadruple the cost of a glass one.

My guess is the weight difference would be a wash.

Here's a link to a Rennlist discussion about fitting an intercooler under a duck.

A twin turbo size intercooler that would fit under a
"Duck" would be costly to fab.
I went with a M.A. Shaw GT-1 one piece lid/wing, you
get down force and greater cooling.
http://forums.pelicanparts.com/uploa...1101144629.jpg
http://forums.pelicanparts.com/uploa...1101144728.jpg

Thanks for the links Jack, I'll take a look.
Amac - the tail I have now is the same as the one on your car. It functions perfectly but is very heavy. I'm looking to reduce weight up high in the rear end of the car. The side scoops are functional but have to be a drag hog.

I am putting together a TT 3.6 for my 930 now. Fitting the proper intercooler under the rear decklid is very challenging in high horsepower situations. You can go with a smaller tail and a smaller custom intercooler...but why go through the effort to go twin turbo and leave a bunch of horsepower on the table with an undersized intercooler? I am modifying two 996 intercoolers...and even at that, I am going to a DP style 930 wing to make sure that I have enough room for the plumbing.
I can't speak to the effectiveness of a ductail, but I know how my car squats with the turbo tail at 130+
Rick
'78 930

Mine is a unique situation. I'm NOT building a high HP motor.
Goal is 400HP tops with as much torque and as little lag possible.
The tail needs a diet. I'm looking for alternatives.

Are you planning to keep CIS?

In case you are, I would forget about TT and go for quality single turbo with good headers.

The main problem is that both turbochargers will have to draw air trough single CIS-cone. It can be solved, yes, but piping will be a maze. Then it all has to go back into single plenum trough double-throated intercooler. Lots' of work and $$$'s.

Second, there are few cheap small turbos that will fit the bill and that will survive w/o watercooling. You can try to adopt some KKK K24's or similar but i don't believe you'll win that much in response.

Chances are, you might end up with similar boost response after lot's of dough being spent on upgrade. Remember, twin turbos also mean twin air-paths, twin shafts spinning, twin oiling systems etc.

If I was you, I would ditch the twin-turbo ideas and go for short thermal-wrapped headers w/o heat, Garrett GT30R BB turbo and maybe some lighter tail.

Remember, turbo-lag isn't only in lower shaft intertia. It also depends on turbocharger size, efficiency, intercooler volume etc.

I have to agree with beepbeep, I think intercooler size has alot to do with lag. I have a monster intercooler and when you get on the boost and switch gears it's ready to go again.
http://forums.pelicanparts.com/uploa...1101181266.jpg
http://forums.pelicanparts.com/uploa...1101181374.jpg

There was a recent article in 'Excellence' about a 930 / 934 that incorporated a ducktail w/ intercooled / turbo engine underneath. Granted ther were mods to the ducktail, but the profile remained intact. An interesting alternative to the "tea tray & larger spoilers.

Jeremy

Ya bummed me out Beep.
I need significantly lowered lag threshold. The damned thing litterally jumps like a toad when boost hits. I have to ease the throttle up to 3000 to avoid the shock.

Here's what I've got:
Stock CIS
Euro Exhaust
Stock Muffler
0.7 bar spring
B&B Intercooler
K27-7200 Turbo
SC Cams

The motor has a TON of off-boost torque, it jut needs to be a less violent transition. The threshold is right at 3000rpm.

That red car looks just like mine!!

http://forums.pelicanparts.com/uploa...1101261803.jpg

Install425,
Wow. A dinner table sized intercooler. That looks even larger than the big Kokeln model. I like it.


RarlyL8,
Seems like you have several options here. Here's a few that pop into my head (demented slightly as I am):

1. Adjust driving style. Always keep the rpms above 3000rpm so full boost is always available. I guess this is only possible above 20mph or so since you are using a 915. A guy I used to see at track events told me that was the way to avoid turbo lag on the track.

2. Ducktail - would look really cool and be even cooler in combination with early hood and RSR bodywork. It'd be a shame to start over on the paint, though. Yours looks too nice already.

3. Twin fender Intercooler plumbing - Two small intercoolers in the rear fenders fed by two small turbos. Extra air ducted to the intercoolers from NACA ducts in the rear turbo fenders.

4. Six E-Rams - One for each cylinder. Sorry. Just kidding.....

5. Intercooler in the engine compartment - Feed it with extra air from hoses coming from NACA duct rear quarter windows.

6. Intercooler for twin turbos located where the muffler used to be. You don't need a muffler since the turbo muffles the sound anyway, right?

7. EFI, megasquirt on one end of the scale and Motec on the other.

8. Sell your motor and build a 3.6 turbo. Or maybe even just a 3.8 NA motor like Charlie Swanson built (3.8 w/ 50mm PMOs). Lots of torque and INSTANT throttle response.

9. Swap me your turbo motor for my 3.2 and put a turbo on it instead. :)

10. You could also go stiffer on the rear shocks or torsion bars to eliminate some turbo squat. If you haven't already, it may add to a more responsive feel upon acceleration.

Everything in the list will cost you more money including the driving style comment(more gas). Sounds like a lot of fun. I look forward to seeing what you do. I'll just live vicariously through your experience while I try to keep one car running and get the other one running for the first time.........

Just do what you want to do. If you enjoy it, then it's worth it. Don't skip out on something you want to try because people tell you it can't be done. When someone tells you it can't be done, it's only because no one's done it yet. I really enjoyed showing up to the track after getting my 8x16 and 9x17 wheels under stock SC flares. Many people told me it would never work, including some pretty experienced guys. With enough time, money and ingenuity anything can happen.

On second thought , you better ignore everything I just wrote because I'm sure I don't know what I'm talking about.

Have fun.

You might want to look at your exhaust configuration and go to a setup that minimizes the lag on your K27. You may also want to investigate a different turbo with a modified impeller. I have an old style B&B setup with a k27and 1 bar spring. The boost starts to kick in gradually with load as early as 2000 rpm and is in full boost by 3,500/4,000 rpm.

I have to agree with the previous twin turbo comment from beep beep. It's alot of money and fabrication. If you look at what Stephen Kaspar (Imagine Auto) is doing with some of the single turbo setups, he is building some very powerful single setups with minimal lag. The lag is really a carefully balanced use of exhaust, electronic boost control, effecient intercooler , and well prepared turbo housing.
Rick
'78 930

Rarly8 Twin turbo 3.6 with your tail....

http://forums.pelicanparts.com/uploa...1101315981.jpg