Attention: TDR Forum Junkies 
I just upgraded my BD twins to an MPI set. While I was reasonably happy with the BD's, there were some things that really ticked me off at the same time. Namely, I wasn't happy with being stuck at a very hot 575 HP with my mods and I wasn't happy with the level of service on the forums from BD over the past year or so in the sense of asking for specifications and maps and in return getting the run-around with stick-figure cartoon maps and non-technical people trying to answer technical questions. -Rant off-
Aside from this, I was pleased with the super fast spool of the BD set coupled with both turbos always being "on-boost" so I wanted to retain these features. The key to this was keeping the BD Super B in the next set of twins versus using a stock turbo. The Super B is a very robust turbo:
-It's basically identical to an HTBG turbo (S300 based).
-Similar to a stock 03-04 turbo in size on the compressor (57 mm), but has the extended tip compressor to move a hair more air.
-14 cm exhaust housing to reduce back-pressure, but has an aggressive turbine wheel to promote fast spool.
-Dual wastegate ports for quick dumping of exhaust pressure.
Super B turbine:
/webdata/photopost/data/500/medium/Super_B_turbine.jpg#ad
So the logical choice for me was to contact Nathan at MPI. He's obviously well-recommended on all the various forums and he only lives about 4. 5 hrs drive from me. Anyhow, we started talking about what I wanted, which was:
-fast spool at my 5,000 feet plus driving elevation
-easily breaking 600 HP with lower EGT
-better boost control
-fast response during towing
-more off the shelf parts as opposed to the custom BD stuff
I was initially thinking of adding a K31 as a primary turbo because it was cheap, it was wastegated, and it was proven with numerous documented installations highlighted on the various forums. However, Nathan immediately suggested a Garrett GT4202 with 1. 15 a/r housing due to:
-moving slightly more air than a K31 (96 lb/min versus 90 lb/min), but less than a S400
-slightly tighter exhaust housing than a K31, but having a better flowing turbine wheel
-wastegated like a K31 for the ultimate in boost control
-the GT4202 would spool a hair slower than a K31, but noticeably faster than a S400
-more choice in a/r exhaust housings (1. 01, 1. 15, 1. 28, 1. 44) than a K31 should I decide to upgrade, especially if I changed my main driving elevation.
GT4202 map:
#ad
BD Primary map:
Uuuhhh... couldn't find one anywhere.
How does this compare to the BD primary?
Since BD doesn't give out any info, when I removed the BD twins I took them apart and measured the specs and compared them to what Garrett has published (quite willingly I might add) on their website:
-BD primary compressor ~65 mm inducer/~96 mm exducer
-GT4202 compressor 74 mm inducer/102 mm exducer
-BD primary turbine ~80 mm inducer/~73 mm exducer
-GT4202 turbine 82 mm
From these measurements it's easy to see why the BD primary limits power output with its 65 mm compressor and tight exhaust side. At the same time, it's easy to see why they are marketed best as towing twins:
1) drives very responsive as low as 390 HP in my experience
2) supports non-continuous power spurts with quickly climbing water temps in my experience up to ~550-575 HP from air delivered from a ~65 mm compressor
3) fast spool from the Super B and a tight primary exhaust.
Probably where BD makes up for the 65 mm wheel is to get it to spin faster to get more of a “bite” of air in a unit of time. However, this would be countered with the tight exhaust that would increase drive pressure as well as the faster spinning compressor heating up the air more than a bigger wheel that moves slower.
BD primary compressor:
/webdata/photopost/data/500/medium/BD_Primary_Compressor-1.jpg#ad
BD primary turbine:
/webdata/photopost/data/500/medium/BD_Primary_Turbine-1.jpg#ad
Huge GT4202 wastegate:
/webdata/photopost/data/500/medium/GT4202_wastegate.jpg#ad
GT4202 turbine:
/webdata/photopost/data/500/medium/GT4202_turbine.jpg#ad
Now enough of the BD's! The installation of the Super B/GT4202 combo was a relative peace of cake since I had twins already installed and it was matter of just out with the old and in with the new. I will say that the MPI's are easier and less-frustrating to install because of a little bit less pieces (less clamps and boots to blow apart), everything's well labeled, use of threaded parts and less nuts, fastener size was more consistent, and pieces were designed around the fastener so I didn't have to constantly change tools. I could tell that a lot of thought went into them because Nathan probably encountered these same issues when making the custom sets on his truck so making it less of a PITA for him = less of a PITA for everyone else.
Installed:
/webdata/photopost/data/500/medium/Progress_MPI-1.jpg#ad
/webdata/photopost/data/500/medium/installed_MPI-1.jpg#ad
/webdata/photopost/data/500/medium/installed_MPI-3.jpg#ad
The other things I did during this installation were:
-upgraded to 5” exhaust to match the GT4202 outlet
-replaced exhaust manifold bolts with studs, which makes for a quick installation in terms of hanging the gaskets, manifold, and turbo easier
As far as performance goes?
-58 psi total @ 1100* EGT at previous BD twins max dyno settings = 575 HP. I still have more wastegate tuning to do because the primary is putting out 25 psi at this point and I want to get it towards where it is wastegated at 30 psi.
-The BD twins would be at ~48 psi total ~1350* EGT in comparison.
-Spool is ridiculously freakin' fast! I would say faster than when I had the BD's because now the Super B can now pull through a more free flowing primary turbo. The tires now light up way easier too.
-underhood bling? Pictures speak for themselves.
What would I recommend to others?
If you want a custom set, you should:
1) Send Nathan the turbo you will use on top or just buy new from him if you can afford it. I cheaped out because I got the Super B for a very good price. Anyway, we found that every manufacturer machines differing amounts of material off the turbine housing: some machine the minimum required whereas others go to the other extreme. This measurement affects how everything else is located on the twin set.
2) Clarify what style of exhaust housing on the secondary you are running. For the 3rd gens, Nathan likes the 3rd gen style flange since it has more meat and area to seal exhaust. The Super B has an HX40 style so he had to get flanges made which added to the time it took to get this set made.
3) Let him know which brand of intercooler you are running. For example, I am running the Banks IC. Its inlet sits a lot more into the engine compartment then say a stock intercooler. Fortunately, for the rest of you that have a Banks IC and want MPI, Nathan has all the measurements required!
Aside from this, I was pleased with the super fast spool of the BD set coupled with both turbos always being "on-boost" so I wanted to retain these features. The key to this was keeping the BD Super B in the next set of twins versus using a stock turbo. The Super B is a very robust turbo:
-It's basically identical to an HTBG turbo (S300 based).
-Similar to a stock 03-04 turbo in size on the compressor (57 mm), but has the extended tip compressor to move a hair more air.
-14 cm exhaust housing to reduce back-pressure, but has an aggressive turbine wheel to promote fast spool.
-Dual wastegate ports for quick dumping of exhaust pressure.
Super B turbine:
/webdata/photopost/data/500/medium/Super_B_turbine.jpg#ad
So the logical choice for me was to contact Nathan at MPI. He's obviously well-recommended on all the various forums and he only lives about 4. 5 hrs drive from me. Anyhow, we started talking about what I wanted, which was:
-fast spool at my 5,000 feet plus driving elevation
-easily breaking 600 HP with lower EGT
-better boost control
-fast response during towing
-more off the shelf parts as opposed to the custom BD stuff
I was initially thinking of adding a K31 as a primary turbo because it was cheap, it was wastegated, and it was proven with numerous documented installations highlighted on the various forums. However, Nathan immediately suggested a Garrett GT4202 with 1. 15 a/r housing due to:
-moving slightly more air than a K31 (96 lb/min versus 90 lb/min), but less than a S400
-slightly tighter exhaust housing than a K31, but having a better flowing turbine wheel
-wastegated like a K31 for the ultimate in boost control
-the GT4202 would spool a hair slower than a K31, but noticeably faster than a S400
-more choice in a/r exhaust housings (1. 01, 1. 15, 1. 28, 1. 44) than a K31 should I decide to upgrade, especially if I changed my main driving elevation.
GT4202 map:
#adBD Primary map:
Uuuhhh... couldn't find one anywhere.
How does this compare to the BD primary?
Since BD doesn't give out any info, when I removed the BD twins I took them apart and measured the specs and compared them to what Garrett has published (quite willingly I might add) on their website:
-BD primary compressor ~65 mm inducer/~96 mm exducer
-GT4202 compressor 74 mm inducer/102 mm exducer
-BD primary turbine ~80 mm inducer/~73 mm exducer
-GT4202 turbine 82 mm
From these measurements it's easy to see why the BD primary limits power output with its 65 mm compressor and tight exhaust side. At the same time, it's easy to see why they are marketed best as towing twins:
1) drives very responsive as low as 390 HP in my experience
2) supports non-continuous power spurts with quickly climbing water temps in my experience up to ~550-575 HP from air delivered from a ~65 mm compressor
3) fast spool from the Super B and a tight primary exhaust.
Probably where BD makes up for the 65 mm wheel is to get it to spin faster to get more of a “bite” of air in a unit of time. However, this would be countered with the tight exhaust that would increase drive pressure as well as the faster spinning compressor heating up the air more than a bigger wheel that moves slower.
BD primary compressor:
/webdata/photopost/data/500/medium/BD_Primary_Compressor-1.jpg#ad
BD primary turbine:
/webdata/photopost/data/500/medium/BD_Primary_Turbine-1.jpg#ad
Huge GT4202 wastegate:
/webdata/photopost/data/500/medium/GT4202_wastegate.jpg#ad
GT4202 turbine:
/webdata/photopost/data/500/medium/GT4202_turbine.jpg#ad
Now enough of the BD's! The installation of the Super B/GT4202 combo was a relative peace of cake since I had twins already installed and it was matter of just out with the old and in with the new. I will say that the MPI's are easier and less-frustrating to install because of a little bit less pieces (less clamps and boots to blow apart), everything's well labeled, use of threaded parts and less nuts, fastener size was more consistent, and pieces were designed around the fastener so I didn't have to constantly change tools. I could tell that a lot of thought went into them because Nathan probably encountered these same issues when making the custom sets on his truck so making it less of a PITA for him = less of a PITA for everyone else.
Installed:
/webdata/photopost/data/500/medium/Progress_MPI-1.jpg#ad
/webdata/photopost/data/500/medium/installed_MPI-1.jpg#ad
/webdata/photopost/data/500/medium/installed_MPI-3.jpg#ad
The other things I did during this installation were:
-upgraded to 5” exhaust to match the GT4202 outlet
-replaced exhaust manifold bolts with studs, which makes for a quick installation in terms of hanging the gaskets, manifold, and turbo easier
As far as performance goes?
-58 psi total @ 1100* EGT at previous BD twins max dyno settings = 575 HP. I still have more wastegate tuning to do because the primary is putting out 25 psi at this point and I want to get it towards where it is wastegated at 30 psi.
-The BD twins would be at ~48 psi total ~1350* EGT in comparison.
-Spool is ridiculously freakin' fast! I would say faster than when I had the BD's because now the Super B can now pull through a more free flowing primary turbo. The tires now light up way easier too.
-underhood bling? Pictures speak for themselves.
What would I recommend to others?
If you want a custom set, you should:
1) Send Nathan the turbo you will use on top or just buy new from him if you can afford it. I cheaped out because I got the Super B for a very good price. Anyway, we found that every manufacturer machines differing amounts of material off the turbine housing: some machine the minimum required whereas others go to the other extreme. This measurement affects how everything else is located on the twin set.
2) Clarify what style of exhaust housing on the secondary you are running. For the 3rd gens, Nathan likes the 3rd gen style flange since it has more meat and area to seal exhaust. The Super B has an HX40 style so he had to get flanges made which added to the time it took to get this set made.
3) Let him know which brand of intercooler you are running. For example, I am running the Banks IC. Its inlet sits a lot more into the engine compartment then say a stock intercooler. Fortunately, for the rest of you that have a Banks IC and want MPI, Nathan has all the measurements required!
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that must have been hot??? So I wonder if over time will our exhaust manifold interior imperfections get "honed" out?
