i know your setup... thats why i didnt say anything negative about the hx55 but you have the right small turbo.. it shouldnt be laggy with the hx35 and 12cm exhaust housing. if anything id think your setup would be lit really quick (of course depending on which hx55).

i just think an 18cm h1c and a hx55 are not a good combo. hell, what do i know.

 

The H1C and HX35 are almost identical in regards to performance, with the only major factor being the turbine housing volume. With the same sized housing performance would be near the same. A wastegate opens at a certain pressure, so when you are at a cruising speed the smaller, quicker spoiling housing is effecting your fuel consumption. I see maybe 1psig at 100kph and about 3 or 4psig at 130kph. I've read people having 5 to 8 psig at 60mph(~96kph?) with a 12cm housing, which represents a fair amount more fuel being consumed. As I have mentioned before the minimal lag from my 18.5cm3 housing doesn't bother me now, so why would that change in a twin setup where it will initially behave the same as it does now?

 

why does it effect your fuel milage besides boost moving your fuel pin down? add a valet switch or stiffer afc spring.

i mean you tow so you want the power as soon as possible, right?

 

Boost means backpressure, which when cruising means the engine is working harder than it needs to for the given load.

As I've mentioned before the minimal lag doesn't bother me, and once the H1C starts to wake up so will the hx55, maybe a little slower than an hx35/hx55, but will keep egt undercontrol and get me to the power level I want. I find that once I'm in the hills the engine is working hard enough to quickly spool the turbo when I shift and the rest of the time it's just a matter of pulling the hill.

 

Doesn't boost mean resistance in the intake path and drive pressure (back pressure?) mean resistance in the exhaust path?

They arent the same thing.

 

Basically the airflow at say, 2200 rpm, with the smaller area flowing over the turbine has enough pressure to drive the turbine and compressor hard enough to make boost. What this means is that you are using some of your fuel to power the compressor without actually needing the turbos help.

At 2200 rpm, on my truck, my cummins has more than enough power to maintain cursing speed without needing the turbo, so if I put a housing on that is restrictive to exhaust flow at higher volumes the engine must work harder to overcome that restriction and maintain the output needed for the given load. Boost isn't free, it's just cheaper than some of the alternatives.

Smaller volume turbine housings increase responsiveness by offering a smaller area open to the turbine, increasing the pressure of the mass flow, this increase in pressure causes the turbine to spool quicker. When you get into the higher RPM the mass flow is so great that it would over spin the turbo if it wasn't for the wastegate, which starts to open up at a set pressure and not before.

By using a wastegate you're throwing away potential energy in order to achieve faster acceleration but at the cost of top end breathing.

I am not concerned with initial acceleration I want a combination that will allow the engine to breath as easily as possible under light throttle and achieve the best efficiency but be able to use the twins to efficiently produce the airflow needed for higher power.

 

Your gonna want a faster spooling set-up, for performance (I'm not talking speed), spool up (from a stop and from a 55 roll) and better fuel economy. Also by making more torque down low from a faster spooling setup your truck will tow much better.

A turbo by definition does not use up any extra energy to operate, it takes wasted energy (exhaust flow/heat) and turns it into boost to increase power. No boost under load while towing is not only gonna be impossible but for what you want to do it would be the total opposite of what you want. You don't want to be running crazy boost but your gonna need the power to get the mileage. At 0psi your truck will not be making enough power to efficiently move your load. For best fuel mileage try to keep your truck operating close to peak tq. `

 

My truck spends probably 80% of it's time empty cruising at highway speeds. I do need it to be able to tow efficiently when I do pull a load though. If I find the truck bogs out too much I might try a 16cm housing.

 

18cm to 16cm isnt going to be worth your time or money... if it bogs too much with the 18 you might as well jump down to the 12-14cm housings.

 

x2. From everything I've read from you, NavyF350, you might think you have the answers, but JBradley and RSWords are giving you the deal. But as JBradley said, what the hell do I know? Go on and build your twin set up the way you want, and then come back and reread what these guys have written when you find out you're overheating and your mpg's suck. I also am looking to build a set of low-power twins, but I'll be going with either an HX35 or HX40 over an HX55, unless I can find an HE351 for a good price. Why? So I can cover most of the boost range. As RSWords said, there is no such thing as wasted energy from a turbo. It's wasted if you DON'T use what's available to harness.