Well, I'm sure there are quite a few people who'd agree with you, as most people just settle for what's out there.

But if you have ANY experience with BB turbos you know that there's simply no comparison to the response of a dual BB turbo like the Garretts to a journal-bearing turbo like we're using now with all these B-W/Schwitzer based turbos, at least in an apples-to-apples test of similarly sized turbos. This allows you to either run a much larger turbo with the same amount of lag, or run a similarly sized turbo that has a lot less lag. That's right, you could actually run a slightly bigger turbo and have LESS lag. Not bad, eh?

A supercharger isn't an option for a couple reasons. The biggest one is inefficiency. They are TERRIBLY inefficient, usually around 40% compared to a turbo that's 65%-78%. So they tend to make more heat than boost. They also take up a ton of space and it's likely you'll have little to no room under the hood. Finally, a blower is always on and sucking power from the engine. It doesn't matter whether you need the power or not, the supercharger is trying to make the boost and is KILLING your efficiency and MPG. Finally, at high RPM that blower becomes a huge restriction, because the engine cannot receive air faster than the blower will deliver it.

If a supercharger was all that good a setup, I'm sure PDR would have actually developed a kit for sale, instead of just toying with that blower they had a few years back. It's novel, but just not practical nor cost-effective. It's overpriced and underperforms.

The wastegate thing is something I haven't really addressed at this point. I suspect that the BB turbo is a LOT less restrictive at higher RPM than a journal bearing turbo. The viscous drag on the turbine shaft at high rpm is considerable. Because of this, the turbo takes more energy to drive as the RPMs increase. Thus, the turbine side gets progressively more restrictive, even before the flow in the housing itself goes sonic.

With a much lower degree of oil-caused viscous drag on the shaft, the BB should be able to speed up its shaft speed and relieve some of the drive pressure that would tend to build up.

Now, I'm NOT saying that you can really get away with not wastegating the setup. What I'm saying is that a BB set of twins would operate with less severe consequences if a WG was not used.

For example, the ability of the BB shaft to turn faster means that the same size turbine wheel can support more overall flow (and hence, deliver more turbine shaft power). Imagine that you installed progressively larger and larger turbine housings on a stock HX35 turbo. You go past 16cm, 18cm, even the gigantic 21cm housings for the old H1C. You get to the point where no matter how big a housing you install, you cannot make the turbine flow more that it already is. This is because the WHEEL ITSELF is limiting flow, and the shaft can only spin so fast before it grenades. But if this was a ball bearing turbo, the point where the wheel itself become the restriction is delayed to a higher shaft RPM, and you can run even larger turbine housings if you want.

I'm not sure any of that matters or is even correct, but it's my analysis.

As to the question of "why bother", well I guess that just comes down to personality. I've always been predisposed to try a new or different way of doing things--even if the cost/benefit isn't necessarily favorable. It's just me.

 

Since I typed in Justins message I have to put something here.

 

ph4tty, Would a 64 be big enough with the hx35? How do I get a hold of Idaho CTD? Any ideas on the cost for the plumbing?

 

Hmm. whats the equivalent of a phat shaft 62 in a garrett turbo? size and hp wise?

 

A bit smaller than the GT3788. It flows about 67lbs/min and the GT3788 is a bit over 70lbs/min.

 

Hohn if you can pair the right superchager it wont take up to much space at all. I got a compressor off a mercedes engine from my bud in Germany. It has an electric clutch ativation and can produce 31psi max. I'm thinking if I can use the compressor to quick spool the lager turbo could reduce lag and smoke (this is all in theory) and porvide quick spool up for the large turbo (or primary) Only problem i can see is the electronic side of the house. Look at maybe running the compressor from idle to 1800-2000rpm's then shut it off and the primary will be GTG or have a blow off in the exhaust to keep the charger at running at whatever preset pressure that works best to reduce heat. Only time will tell. I've also agree with on the temps. Was thinking (twins)of running the air coming out off the pirmary into the intercooler then back to the secondary and run water cooling through the piping to the air-intake. Still friguring out which way I want to tackle it. Your write up has help me greatly. Got the old brain flowing again. I know it's a long shot but the compressor to turbo idea I'm liking the most because has not been ventured with much. Just got to figure out how to use the compressor boost while the big trbo draws outside air at the same time???? Back to the drawing board. Again, great write up!

TC

 

TexasCowboy- if this is the compressor from the 2.3l gasser you'll have to limit it's usage to about only 1300 rpm of the cummins and will have to use pulleys that are speeding it up to it's peak throughput at this rpm. Consider a 2.3l gasser running 14:1 at 6k rpm vs a 5.9l diesel running 25:1 at X rpm. 78% more air than the gasser for the same displacement and speed. Then it's roughly 2.56 times the displacement. - This means that you need roughly 4.55 times the air per revolution for the CTD. This means that for the Mercedes' 6000 rpm you get 1314 rpm from the cummins with the same air flow. I think this isn't gonna work out too good

AlpineRAM

 

Normally a htb3 or s400 is used with a hx35. Here's Nathan's contact info:
http://www.maximizedperformanceinc.com/

 

I would put in a bigger compressor wheel into the S300. The 66mm impeller size might be still to small. Go with a bigger exhaust housing and turbine. BD uses the S300 in there set-up. A cheaper turbo is the S400 or BTH3B. I just bought a re-built 3B for $600 bones. Yes Yes its old school, not the best but it works and works well with your stock turbo. The hot down tube and air tubes are available from various shops. For about the same cost you have a built twins. The last time I priced a S300 turbo, they where around $1500. I just had my HX-40 re-built at a local shop and its dumping oil into the compressor outlet, basically into my intercooler. They had the end float to tight and it tore my new bearing seal out on start-up. Its been fun. Building twins from nothing is alot of work. You have to be good at fabricating. I would I had to it again I would use stainless for the hot down tube. Get it jet coated (power coating) and wrap it with insulation. It get hot and will melt your wires on your firewall. Been there, done that. Its also very close to your a/c. Lots to think about. Same goes for the position of your oil supply and drain lines to/from the turbo(s). Even the PDR kit isn't done perfectly.

 

The GT3788R's exhaust outlet is small (3") this is a restriction that will cause drive pressures to go through the roof. The GT3788R is a nice turbo for a 400 RWHP truck but I don't think it would work well without a different exhaust housing......

I work a ton with FI, I will ask them this week what options would be available and what drive pressures would be with this setup.

 

Alpine I was told this is off a v8 engine with rpm range to 6100 and can provide max of 33psi boost pressure. It can still work. Just have to experiment with different size pulley's. Like I said before. just need it to spool down low the get the big one rolling. If this doesn't work I'll look else where at differ automotive applications. But however I think I can get it to work just going to take a LOT of R&R.

 

This is what has me not likeing the 3788 all that much. Andy (Hammer) has me thinking that the housing is super restrictive and just won't move the air needed to keep drive pressures in check. The "elbow" that Garrett includes is a disaster for flow.

The other side to that is that the poorer flow of the 3788's turbine shouldn't matter as much in a twins application as it does if it was a single. Turbine on the primary reduces the delta P across the turbine on the secondary, hence it's proportionally less restrictive.

Moreover, the 72.5mm wheel on the GT37 should flow decently if the outlet/elbow is opened up. The "made for Cummins" version of the GT3788R doesn't seem to have enough outlet flow on the hot side, though Hammer said ditching the adapter elbow helped flow a bunch and dropped over 200º EGT.

That leaves the 4088R, which is probably a little too far in the other direction, with the 77mm wheel.

I think the GT3788R could perform well as a secondary, BUT you'd want to make sure that the cold pipe maintained the outlet diameter all the way to the primary's turbine. The idea here is that the 3788's outlet should never be nearly as restrictive as the 4202's inlet, though that depends on which housing and inlet your are using on your 4202 (it appears that you can get both T4 and "large diesel" inlets on the housings). I'd go with the larger inlet of the "Large diesel" flange, myself.

I'm learning a lot from all this hypothetical exercising, and I appreciate those of you who have chimed in.

jh

 

this is an awesome thread!!!!

great posts hohn...I need to sit down now and reread it to soak it all in

ben

 

awsome post i will say that i love my garret stage 3 turbo its way over worked so i am working on use it in a set of twin with one of the bigger garetts

i also want to say that hohn is right we as a diesel comunity need to start to look at the high performace turbos look at all the drag cars and other high performace cars out there and see how many of them are running bb turbos
i bet it wont be long and garett has a kit for sale

anyway great post hohn

 

IN defense of those who disagree somewhat with my advocacy of BB turbos, I'd like to point out that we do have somewhat unique demands of a turbo compared to the racers.

They bolt on a turbo and expect it to last a whole season. Ocassionally, they get more than one season out of a turbo. (eureka!)

We bolt on a turbo and expect it to last. Indefinitely. Period. We don't warm to the idea of yearly BB rebuilds!

On balance, though, I think this is not that big of a factor. The stress on a turbocharger in a drag car is many orders of magnitude higher than what a turbo would see in a BB twins application on a DD truck.

Garrett says their turbos are tested to 2000 start/stop cycles. Cold starts are the toughest on a BB turbo, because there's no oil in the turbo. But 2K cold starts is over 5 years on a daily driver. I feel that a minimum of 5 years is a reasonable life span for a BB turbo, in light of the superior performance they deliver in return.

Heck, I've just now had my truck 5 years and I'd be delighted if a BB turbo lasted at least this long, but I know others have more stringent requirements and aren't as easily pleased.

jh