Mods to a BD Twin Primary Turbo Discussion.
 

I am posting this to show what can be done, but also to see if I can do more or if I screwed up. Please let me know.
Recently I installed a set of BD Twins. A very nice addition to my truck, spool was instant and it did as claimed, kept my egt's way down no matter what I gave it. In other words, VERY happy. As with anything, it is a compromise, I need drivability and room for fun. The downsides to the set are that the primary is small (good spoolup) probably resulting in high drive pressures (I don't have a guage). This will also limit the ultimate HP of the engine and will increase stress on the head gasket.

One thing that stands true, is that at some point you will be putting studs in, either before or after you blow the head gasket. After in my case. ANother point I noticed is that even with the factory gate set at 50psi, It would creep up to over 60psi (limit of guage is 60).

Since I am waiting for my block and head back from the machine shop, and my family is out of town for the weekend, what a great opportunity for a curious mind. (I said curious, not always the smartest).
First thing I did was open up the compressor discharge (first photo). When I measured mine I initially had 2.00 in^2 of area at the exit. Some Minor grindage put me to 2.54in^2. About a 26% increase. With this, the limiting orifice is now at the tips of the compressor wheel which has an effective area of 2.20 in^2. What this means is that out of the factory the limiting orifice on the turbo was the housing discharge. I then went to the inlet and flared the casting to smooth the air flow entering the turbo. (photo 2 is before photo 3 is after.) I left the rest of it alone, not wanting to screw anything up. I could have ground more on the discharge end but I didn't want to make it too thin and since it was no longer the limiting orifice there wasn't much I feel could be gained. (Besides, my compressor bit the dust)

I then went to the hot end. Which I will discuss in part two.

Part 2. Mods to the Hot End
 

Now that the Compressor was done and ready for polish, I turned my attention (or lack of) to the hot end. Since the turbo set builds boost very well, I felt that to get it to flow better at higher power levels and rpms I should start at the waste gate.

Upon opening up the wastegate housing, I found that the gate is quite restricted in its movement. The cover plate limited the travel to 0.1 inches, which when you calculate the open area was almost equal to the size of the ports. (photo 1 shows this, if you look at the plate you can see where it was hitting it). So I went to pile of old turbos and found an extention plate which gives the chanber more volume, and alows for more travel of the wastegate flappers. (photo 2). To get a better idea of how much this will allow it to open it took photo 3. In this position it makes contact with the cover plate. I also ported out the wastegate ports approximately 0.10" each. This still provided enough seat area to insure that it closes. (i'll put that photo in the next post since I can only do three each)

Final Post on Primary Mods.
 

THought I would give some photos showing more detail of the wastegate mods.

Photo 1 are the ports before grinding
Photo 2 are the ports after grinding
Photo 3 assembled turbo

Looking good!

I like the turbine side mods, what turbo did you get the extension plate off of?

I bought mine for my Super B from HTT.

I never thought of opening up the ports though! How much minimum wastegate flapper overlap is required to still maintain a good seal against drive pressure?

I think the key is making sure there are no burrs. I left about 0.12 overlap and made sure they cwere flat.
I expect with the increased surface area, the gate will open sooner, I am hoping around 40psi boost. I can always set it higher if needed.

JStieger,

I take it that this extension plate mod is a common thing then.

I don't have a super b yet but if I ever upgrade I hope to go straight to twins and will certainly consider BD's as I don't intend to go over 500~550 hp and want more low end performance than anything.

Lots of information, gives me a better idea on how those things work

I don't know if it is a common thing. I read about the extension plate via this and other forums in regards to some other 3rd gen guys experimenting with BD twins so I ended up doing the same to mine. If I would have known it came off another turbo I would have tried that route first. I also recently seen some ads from HTT and II that showed this plate coming on their new turbos. The only thing that sucks about this plate is when you have twins it takes up some precious space, but this is kind of nit-picky!

I actually had BD twins before this Super B/GT4202 set that I had Nathan make recently. I liked the Super B so much that I decided to keep it! Whenever I dynoed with the BD twins I routinely maxed out at 550-575 HP. They were awesome for towing at 425-450 HP.

Wow Scott, that is some good stuff. I would love to do it but I dont have the steadiest hands lol. Let us know how it helps out when it's all back together.

thought I'd bump this up to the top... this weekend, I found the same interference with the wastegate as the original poster. Now it's time to locate an extension plate.

definitely a good read.

thanks for posting Scott[guitar]

Good mods! I would hope that the compressor discharge mod was tapered, like a nozzle would be shaped (think Space Shuttle rocket discharge). That will give you minimum turbulence.

jh

The compressor outlet size is relevant to the parallel diffuser hight and diameter; in the case of the Super B the PD is small so the outlet size is smaller than usual but doesn’t cause back pressure.

The parallel diffuser is the round flat area between the comp wheel and the compressor scroll; it starts the slowing down of air velocity that will eventually be converted to static pressure. this turbo is designed for high pressure aplications and has very good high pressure surge protection, opening up the compressor outlet would reduce compressor outlet velocity and could effect surge and cause off throttle surge or pressure back up.

As for the wastegate opening more this will increase negative delta pressure (exhaust pressure higher than compressor) you should try closing in the wastegate or shutting it off; when you open the wastegate the lower turbo speeds up so the overall pressure may not drop. You may have to reduce fuel slightly until you O-ring the heads.

During testing we controlled any negative Delta P by closing in the wastegate, the results were at 60lbs the pressures were even; and at 70lbs the exhaust was 5lbs higher this is very good.

You must remember the purpose of the wastegate is to control boost pressure not to lower exhaust pressure.