Twins spooling

NPloysa

NPloysa

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Do twins usually spool faster or slower than a stock HX single? A twin-turboed Dodge owner on another site complained the new Ford 6.4 diesel spools slowly due to the twins. I asked him if his twins spool slow in his '04 Dodge.

I know one of the points is to hold higher HP with lower EGTs and lower boost, but what about off-the-line spool?

Nick
 
you have a restriction on the intake side and in the exhaust path of the small turbo. twins will spool later/slower than a single
 
Having never had twins I don't know jack but that doesn't make sense. That restriction as you put it is a smaller turbine housing on the small turbo right? Does the compressor on the big turbo really restrict the intake to the point that it would sufficiently restrict the amount of air coming in that it would reduce the exhaust gas energy? Seem like that if not with the same amount of fuel and expansive exhaust gas energy then begin to spool the small turbo faster?
 
the rotating mass of the compressor wheel and turbine wheel and shaft on the big turbo is pretty high... it definately acts as a restriction in the exhaust path and intake parth, which both hurt spool-up. it's the exhaust restriction that hurts the most I think, but both contribute.

note the difference between a stock 3" exhaust system with cat and muffler vs. a straight 4" system. turbo spools up much nicer.

the turbine housing of the big turbo itself is a restriction, add a heavy turbine wheel/shaft, and you've got even more restriction
 
You have a half answered question.

If you take a turbo of a specific configuration, say a stock hx-35, as a single it will spool faster than as a secondary(top) turbo in a compound setup. The benefit to compounding is you can use a small turbo such as the 35 and add the low pressure turbo and get massive airflow. You will have the flow of a huge turbo but with the spool time close to a 35.

As for the Ford's twins, i don't know exactly what they use but most likely they have a very small secondary turbo and a primary turbo sized a little larger than the 6.0's single. With this setup they would have as much air as the single but with greatly improved spool. With better spool you have less emissions and that is just what everybody wants....right? LOL
 
the benefit of most street twin setups is EGT reduction.

I ran a 35/S400 setup, and my sps62 spools way faster and made more horsepower. EGT's are obviously higher w/ the single, but pure performance is better
 
the restriction on the exhaust side that hurts horsepower...

go with a bigger top turbo, and the power will come up, but the spool will suffer
 
That is where wastegating works its magic. If you just try and throw twins together you will end up with crap but if you take your time to plan it out and understand what needs to be done there will be no restriction. Yes the second turbo will be a "restriction" but from it comes more air so you spend power to make power.
 
I think a twinset using a Garrett St. II or III and S475 with a 1.10 A/R housing would spool like mad and support some nice horsepower, don't know from experience though. I really want to make a set of twins and get it all going lol.
 
Just because you had the divider drilled does not mean that you had enough wastegate flow.
 
not going to disagree with that, but a bunch of 90* turns doesn't exactly make for smooth exhaust flow.

twins are great and all, but they aren't the end all IMO
 
Forrest is right..

twins, more maintainance, mine turned out to be, chasing boost leaks, chasing tightening clamps, and also chasing oil leaks, and so on and so forth is a pain in the ass... It gets pretty darn irritating when u know the truck is running rough due to some problems with the chargers.. Problems i have seen most common are leaks,

As Forrest stated, exhaust isnt smooth flowing either and it takes a lot to get that bigger turbine going, thus the lag in my twins, but once it hits, especially with a load behind it, its a feeling like no other, especially for the loads that i tow, the main reason i have twins is for egt reduction, and for towing, and yes my set spools up rather nicely with a cross drilled, slightly modded stock wastegate.. NOt fast enough for my liking, but if i add more fuel it gets laggier.. Yes i know, i need tuning, its all in the tuning to get these twins lit up to the best of their ability..

I do see, for a big bang hit on hp, a huge single and spray is the way to go.. for towing and more daily driven applications.. and a more steady powerband. twins...

either ways pick your poison...

But aside from that, i have bigger problems right now.. no gears in my nv4500..... update soon..

Rick
 
Rick, I've got a complete, fully splined NV4500 sitting here. fresh rebuild. if it helps you get going sooner, we can work something out on it
 
ugh...a HX35 at a top turbo blows goats!!!

if the set is built correctly, the primary is not a restriction...it just sits there idling away untill you need it...and when you do...ziiiiiiiing she comes right up.

Wastgating is a HUGE issue. Most production twins are somewhat lacking in this regard. Both turbos need gates....

Using a OEM style gate in addition to hot water heater reliefs to control boost, while effective, actually wastes power.
 
i'm considering going twins, a hx35 over a ht3b with a small housing or mayb a s400 cuz i thought i might get better spool up... over a 62 which sounds like is very wrong, reading Forest nearing's post.
The htb2 62/12 on the auto just seems very slow to spool up on our 98! it hits hard at 17 psi, and at around 2000 rpm...makes 43 psi max. we live at over 6000 feet elevation.

i thought i had read from "Got diesel" from cumminsforum, that his twins light hard at 6 psi. whats the deal?

Diesel freak, id like to hear more about twins tuning, and wastegating...
 
Diesel Freak said:
Wastgating is a HUGE issue. Most production twins are somewhat lacking in this regard. Both turbos need gates....

Using a OEM style gate in addition to hot water heater reliefs to control boost, while effective, actually wastes power.
Click to expand...

We can't have trucks wasting power for no good reason can we? Edjumacate us sir =P What is the right way to set up the wastegates on twins? I'm guessing there's a divorced wastegate on the hotpipe?
 
At 6000' you are going to have issues with spool...regardless of what you do.

A HT3B is used in many street trucks, but it is an old slow turbo. I went with the big GT42 (74mm inducer). A S400 (75mm inducer) with a 1.00 A/R housing from Forced Inductions would also work well as a primary.

On my gates, I am running a 38mm external (20 psi spring combo shimmed 3/16") on the top turbo and a 50mm external (15 psi spring with a manual ball and spring boost controller) on the bottom turbo. The top turbo runs on a pressure differential (primary boost to the top of the dome with secondary boost to the bottom). This is tuneable from ~38 psi all the way to 60+ with just one boost controller. Works very well.
 

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