I have a 14cm housing on my 35 and with the little bit of lag it has I hate it when I'm towing. Mark my words you are going to hate the lag that setup will make. I almost want to put a 12cm on but I can live with my current one, anything bigger would be a waste.

When towing with my current setup I am not bothered by the lag... I still don't understand how this will change when I move to a twin turbo setup due to the simple fact that when sizing for compounds you basically size your high pressure turbo to the mass consumption at it's pressure ratio... essentially the same as sizing a single. When sizing the low pressure turbo you calculate using corrected mass flow from the high pressure turbo and multiply by the pressure ratio... This is a bit oversimplified because you need to calculate the ideal temperature rise for the pressure ratio you want on the turbo, and convert the two into density ratio so you can accurately find total mass of air moving through the engine to calculate fuel needs and correctly size the turbos according to your design specifications.

So by the math, this is unfortunately still in L/min because I have not finished my spreadsheet to convert this into kg/min.

At 2250 RPM, assuming a volumetric efficiency of 80% a 5.9L should consume 187L/min of air at 0Psig, 20 Celsius ambient.
At 2250 RPM, with 3.6Psig a 5.9L should consume 209.0L/min of air at 62.7 Celsius at the compressor outlet.
At 2250 RPM, with 8psig a 5.9L should consume 239.5L/min of air at 90.2 Celsius(100C is the boiling point of water at sea level.)

Perhaps this bit of math doesn't apply and I'm still missing part of the equation, but here is what I think about.

If the engine can pull the weight of my truck, empty, at highway speeds with fuel alone and no boost without feeling bogged down, which crusing speed is about 2200ish RPM with my setup and if I tip in, even lightly I quickly start to see positive manifold pressure, it leads me to believe the engine is not struggling or "lugging" to maintain that speed.

If, due to installing a smaller housing my truck sees boost under the same conditions then I am adding load to the engine, in that I must now move more air and compress some of that air adding some additional restriction to the exhaust stroke. If I have to move more mass of air and fight the additional back pressure then I must use more fuel. It takes greater effort during the compression stroke to compress the extra air, but some of these negatives are balanced by the manifold pressure during the intake stoke.

I have read enough threads of people complaining about loosing fuel mileage at highway speeds when going to the smaller housing to make me think about the fact I'm not bothered by a little bit of lag. When I'm empty the Cummins without boost can easily throw my truck around and I usually cannot see more than 20psig before I run out of rpm, until I'm in 4th and 5th, where I'm topping out at 26psig right now.

Towing a load I'm not racing anybody and from my experience driving class 8 trucks, my 5.9L is more than responsive enough in its current form for my towing needs, I just want more total available power without sacrificing empty crusing fuel mileage. As it stands right now my truck is hovering around 9-10L/100Km(can reach the low 13/100 if I roll out too much coal). My car is around 13/100, the LMM duramax I drive is around 14.4L/100 and the bosses 6.4L is ranges from 16-19L/100.

Remember, turbochargers are not free, it takes energy to make boost, just significantly less than a supercharger. It takes back pressure against the turbine housing to produce enough pressure and force to produce positive manifold pressure.

This is the main reason I've been looking at the HX55 as the low pressure turbo, I might even go smaller, in order to increase the responsiveness of the low pressure turbo, so as soon as the high pressure starts to make boost and enough mass is moving the LP kicks in and hard, working together with the HP.

I admit I still must take this from the math, to the reality phase and with everything mechanical there is always art form involved in getting things to work the way you want them to. We'll see what happens, this winter the Cummins is getting yanked and fabrication begins.

Good luck dude, and let us know how you get on. Maybe you'll strike a winner with that setup.

This thread is a good read. I'd like to do a towing compound turbo setup someday. I can only keep 25 psi cool when towing a load up a decent incline. I'd like to increase that to 40 psi. I'm thinking a S300G for the small turbo, but not sure what size large turbo to use.

Keep us up to date on how it works out!

ive been kicking the idea around in my head of using my h1 in a twin setup. my truck is 93 model so im assuming its 18cm exhaust housing i recently bought a gds 60mm comp housing which helped alot but still slow spooling till bought 10psi. so my question is instead of starting all over with another turbo why not put a smaller exhaust housing (12cm) on it and later on build a twin setup. from what i know a hx35 is a 56mm with 12 or 14 cm exhaust side? correct me if im wrong but wouldnt a h1 with a 60mm and 12cm be just as effective or even better? only difference is no wastegate. i dont know much about turbos but seems sensible to me to make decent power in the 400hp range

they will be close. The HX35 has a little better wheel design. Size is not everything, its alot but not everything. since you already have teh GDS upgrade I would look for a 14cm housing cheap to swap on and see how that feels. Your still dealing with the relativly weak shaft on the H1C though. Theyu dont like to be over spun and dont like real high boost (which in that small turbo wont make any power anyway)

thanks RSWORDS. doesnt the the gds housing i have also fit a hx35? or did i dream that up? il prolly buy a 14cm housing someday and just stick with it untill i decide to go a lil crazier with my setup. i was just curious as to how well my h1 with 60mm and a smaller housing would work in a twin setup. thought it might be a cheap way to build a lil more power. if i could find a cheap hx35 could i swap my 60 mm housing over to it? would that be a better and more reliable turbo than upgrading my exhaust housing?

I think the H1C would be Ok in a small towing twins setup, low boost low shaft speed. As far as teh compressor housing I have no clue, call up GDS and let us know what they say! :U:

do you spool pretty quick? what injectors are you running?