Anyone want to build a downpipe?

[mike85gn]

The "up" downpipe idea is cool with our tech inspectors here, he thought it would be funny to hear nothing but turbo when lined up against a monster V8 (they like seeing the TR's at the track). And as a side note, the largest gain I have felt since I started modifying the car was definately the ATR 2½" DP. I have a larger turbo and will do some elbow porting and general cleaning up, should only work in my favor. I think this is a good, budget consious mod with an excellent potential for power over stock. I am pleased with it, and I think you could go pretty fast before needing more. Maybe Action Fab could make some more of thiers (I heard bad things, though)? Or we could convince Jay or ATR to make us a 2 3/4" DP, using the 2½" and elbow as a basis?

 

[TType85]

Just to add some fuel to the fire....

I'm finding the ATR 2.5" DP with a ported stock elbow will not control boost on my car. I had it set for 20psi and it was 20psi in 1st, 20 in 2nd, and it crept to 24 in 3rd.

A TH style DP with a bigger puck may work but i'm going to go external wastegate. Like Drew I have no heaterbox so space isnt a big consideration.

BTW the car wouldnt ET this last weekend but it did run 12.6@114.6 with the above boost conditions at 4200ft elevation, hitting the rev limiter going through the traps in 3rd...

 

[REDS HOT AIR]

Re: Just to add some fuel to the fire....

Originally posted by TType85
I'm finding the ATR 2.5" DP with a ported stock elbow will not control boost on my car. I had it set for 20psi and it was 20psi in 1st, 20 in 2nd, and it crept to 24 in 3rd.

mine has that feature too i kinda liked it leaned it out real good on top end.....MPH

 

[hotair85t-type]

Remember, it isn't really the size, but the shape of the bends. The worst thing you can do to moving air is turn it 90° foward, then 90° down (like the hot air dp). I think 2.5" with a large bend will flow better than a 3" that has tight bends.

 

[mike85gn]

I though the gasses would flow the same regarless of bend radius, as long as the piping was the same diameter and the bends were mandrel bent?

 

[dave84gn]

It takes more force to push a fluid through a bent pipe than non-bent pipe. Changing the direction of the fluid flow requires more force, thus less free-flowing.

Ever try drinking something with one of those kids straws that are bent about 50 times? Gota suck noticably harder to get the drink through

Dave

 

[cool 84]

So then everyone would agree that the the stock elbow is a problem. Exhaust comes out of the turbo and makes an immediate small diameter 90 degree turn.

Two friends of mine made their own carbed setup on from diesel turbos on a 305?? Monte Carlo. There's no wastegate, no idea of what boost it's running, and no hood with the downpipe going straight up where the hood would normally be. The car was bought for $50, completely gutted except for the driver's seat and went 14.0 on a peg leg with 215 tires and 108mph. Everytime it makes a pass people bet on if it's going to blow or not.

 

[mike85gn]

But gasses aren't fluid, they're gasses, so they have much less mass, and it's a lot less effort to make a gas change direction than a fluid. The real problem here is the elbow of the turbo, cause that's where you run into odd surface shapes and contours. The 2½" mandrel bent piping should be able to suport us for a good ways, but eventually, we'll need to do away with that elbow. The ATR piece only really has like 2 sharper bends in it, and the fit is as good as we're gonna get. I really want to know if we can build a 2 3/4" DP that will eliminate the stock elbow? Can we modify the ATR piece to use a THDP elbow?

 

[dave84gn]

Yea when I first heard my physics prof talking about fluids I thought he only meant liquids...

[dictionary.com]
flu·id (fld)
n.
A continuous, amorphous substance whose molecules move freely past one another and that has the tendency to assume the shape of its container; a liquid or gas.

But you're right, at the same speed, it takes more force to change the direction of a liquid than a gas (more mass). Of course, the faster the gas is moving (more momentum) the more force required to change its direction.

I have a total newbie question:

What is the turbo elbow??? Is it the bend the downpipe has to take right after it exits the turbo? or is it the bend the exhaust gas takes *while in the turbo* after the turbine?

Dave

 

[mike85gn]

Yeah, the elbow is that big pile of poopie that bolts on to the exhaust side of the turbo. Spent gasses pass through the exhaust impeller then enter the elbow to make a 90º bend into the downpipe.

 

[TType85]

I don't think i'm dealing with a restrictive downpipe here. We had the same boost creep problem with my friends car with a TA-61 and a Kirban 3" DP. We solved it going to a TH DP AND enlarging the hole in the exhaust housing. I feel that the puck on the ported elbow I have is too small and can't bypass enough gasses to control boost. I am going to go to a RaceGate with a 1-5/8 dia hole. That should control it fine. In perspective I was running better than I thought this weekend. 92mph in the 1/8 at 4200ft elevation leaving EXTREMLY soft (car rolled out 20ft before the boost came up)

 

[JDEstill]

Some misc facts to chew on...

A. The boost creep problem: If your new downpipe suddenly gives you a boost creep problem, good news - it means it is flowing a lot better than the old one! Think about these facts:
1. The flow through the wastegate depends on the pressure difference between the upstream and downstream sides of the turbine. The more of a pressure difference the more you can flow through the wastegate hole.
2. The turbine is extracting power from the exhaust to turn the compressor wheel. The amount of power it extracts is a function of the pressure ratio across the wheel. Ya know on the compressor map how one axis is pressure ratio? Outlet pressure divided by inlet pressure? Same thing with turbine wheels, there is a pressure ratio across it. This means if you lower the pressure downstream by 1 psi, you lower it upstream by MORE than 1 psi! You will still have about the same pressure ratio however.
3. If your downpipe is flowing better, then you decreased the pressure at the turbine outlet, and you also decreased the turbine inlet by more than that, so you have a lower pressure difference across the turbine.
4. With a lower pressure difference, the wastegate hole can't flow as much - see item 1. So you have to port the hole out to get the flow up.

B. Someone asked if the radius of the bend mattered for flow. Answer: yes. A long radius elbow does flow better than a short radius elbow, even if both are mandrel bent.

C. An external gate should help. If you aren't routing the wastegate gases back into the elbow, then the flow through the elbow is less than what it is using the stock wastegate. With less flow through the elbow it presents less restriction, with all kinds of good things happening from that. The better the flow characteristics of the elbow (ie TH style vs. stock), the less of an advantage the external gate will be, because the restriction penalty of putting the wastegated gases there gets less and less.

John Estill

 

[dave84gn]

Cool explanation JDE.

So installing just an external waste gate will improve performance... never even thought about that

Newbie question:

How does an external wastegate work? I know you can get ones that hook up to the crossover. So the idea is to reduce the amount of air that flows through the crossover, right? So to reduce it, you have to have some exhaust gas exit the crossover pipe so it never reaches the turbo, correct? So where does this exhaust go? Directly out into the air, or does it somehow route through the exhaust?

Dave

 

[JDEstill]

Hey Dave -


How does an external wastegate work? I know you can get ones that hook up to the crossover. So the idea is to reduce the amount of air that flows through the crossover, right? So to reduce it, you have to have some exhaust gas exit the crossover pipe so it never reaches the turbo, correct? So where does this exhaust go? Directly out into the air, or does it somehow route through the exhaust?


The stock wastegate opens up and lets exhaust gas go directly from the turbine inlet to the elbow on the outlet without going past the turbine wheel. This limits the speed of the turbine wheel, which in turn limits the boost level. A pressure signal from the compressor side opens and closes the wastegate; higher pressure opens it further, lets more exhaust bypass the turbine wheel, which keeps the boost pressure in check.

An external gate does the exact same thing, it just isn't located in the same spot. It isn't "internal" to the turbine housing and elbow, instead it is mounted on the crossover or on the header or on the pipe going into the turbine. It is still keeping exhaust from going through the turbine wheel on its way out. It can discharge to atmosphere or into the exhaust system, whichever you prefer. Straight to atmosphere is a little noisy, some people like the sound, it is rather distinctive. Others fab up a pipe to connect the outlet to the exhaust system.

John

 

[national84]

I just got home last night from a 2300 mile round trip to see Jay and put down some cash on the I/C set up. While I was down there, I went to see Dave at S&S, and picked up a 3" dp that he said would only be tight at the A/C box. I hope this is the case, but of course this will still be going through the stock elbow. I am looking to install an aftermarket WG, so I fully intend to eliminate the ELbow and fab up a Lee Thompson style attachment for the DP. As soon as I get a chance I will post a photo of the DP.

 

[Lee Thompson]

external wastegate

as this post has drifted on to external wastegate, this is why I prefer an ext. WG.
If you look at the inlet hole of a turbo, it tapers down to less than 2 in. past the inlet. This is for a reason,( to create air speed across turbine wheel) once desired boost is reached, the extra exhaust gas is diverted away from wheel to down pipe.
ALL EXHAUST gas out of combustion chamber must pass through this small hole.
With an external gate this extra gas doesn/t have to be forced through this small hole once boost is reached.

 

[turbowrenchhead]

Too much fabrication, money and work to go with an external wastegate. A racegate is 400 dollars alone. So if an external is so much better, then why is the terry houston so popular with the i/c cars. I know of a 10 second i/c gn that uses a terry houston d/p. He has no complaints about his. If someone could make one for our cars they would sell no problem I bet.

 

[JDEstill]

Originally posted by turbowrenchhead
So if an external is so much better, then why is the terry houston so popular with the i/c cars.

Like I said above, the better the downpipe the less of an advantage the external gate is. The more restrictive your elbow and downpipe are, the more of an improvement you get by not putting the wastegated gases through them.

The THDP is so much better than the stock setup that people don't see as much benefit with the external gate, but before the THDP was available people did see some increases with them.

I was sitting on the pot sometime last year, reading an old Car Craft... wait, just found it, Jan '89... and they were reporting on a "late model performance". They were running 8 computer controlled cars, Duttweiler had 2 TRs there (and NOS and Turbo City had TR's there too). Kenny was quoted in the article as saying the external gate on one of his 87's was worth an 8 psi reduction in backpressure, which was worth about 16 hp. I don't think that would be true today if he was running a THDP, but I think it would be true for a hot air with a stock elbow and downpipe.

John

 

[ATEAV8GN]

External WG expenses....

Guys,

I picked my Turbonetics Deltagate external wastegate up off the very trader on this message board for a grand total of $120. It was six months old and had less than a 1000 miles on it... maybe I just got one hell of a lucky deal on that but I dunno. Then as for the fabrication costs of it all... cost me like 15 dollars in materials (stainless flange from ATR, nuts and bolts) and then $55 at a local welding-fab shop to get it all TIG-ed up. Now granted, if you do not want to have it venting externally then your fab/materials cost will increase greatly, but I chose to vent it externally (if it vents then the old lady knows I'm driving too fast from the noise ). So grand total I have about $200 in the whole setup considering shipping and running all over town... and I am damn pleased with it.... most rock steady, reliable, and consistant boost control I have ever experienced

Also as for what John was saying about the THdp and old external wg's... TH's weren't available until relativly recently when compared to the external wg's... for a long time the externals were the only option as for "better than stock"... and also you have to think about the stock appearing issue with an external (were talking IC cars now), unless you mount it on the x-over on an IC car, and ext wg sticks out like a sore thumb when mounted near the turbo/dp....

Just my $.02

Drew H. Carlton
'85 GN, 11.918 @ 111.6, NO IC, NO SPRAY, DAILY DRIVER

 

[Buick From Hell]

Here we go...free breathing, and won't hit the frame!