Twins for dumbies

As the title says im more than willing to learn how to do "twins" on my truck. Everything involved i.e. difference between primary and atmosphere, what to gate and what not to, where to intercool, what exhaust housing may work with what combo, oil to the cartridge's and so on.

I dont know a whole lot on this and maybe everyone can learn from the experts. Try to keep this clean. Thanks and hope for alot of information on this.

So here is a QUICK rundown.
Primary vs atmosphere charger- the primary charger is the smaller of the two chargers. It does the majority of the work. When choosing a primary charger, you want a turbo that spools quickly and doesnt have a super tight exhaust housing so that drive pressures get high. The atmosphere charger feeds air to the primary charger. This charger needs to be sized appropriately to achieve a couple things: First, you dont want it to never spool, but you also dont want it running 40psi either (in a dd application). Most atmosphere chargers run along between 20 and 25 psi- just feeding a large volume of air through the primary.

Wastegating- You want to use a wastegate to keep the turbos (usually the primary charger) from overspeeding. Nitrous speeds chargers up which makes a bigger need for wastegates. You want to keep back pressure as close to boost pressure (1:1) as you can.

As for intercooling, depends how big you go. If you are running a big volume of air, you may want an aftermarket intercooler. The reason being is that if you are compressing air inside the intercooler, it is getting heated up. If your egts are high after that, you can add a stage of water after the primary before the intake manifold. Or, you can add a stage of nitrous at the track.

As for exhaust housing, that is all dependant on your charger setup/injectors/needs. The bigger the housing, the lower the egts, and the laggier it will be. Tighter housing= higher egts and lower spool up times.

Oil- Need oil going into the top of any turbo and draining out the bottom... as long as oil is flowing through you should be ok.

Got most of that.

Why do you only want around 20ish psi from the atmosphere. I thought having the bigger one was for higher boost. Or do you add the two boosts together. I see brain on the other thread is running 100psi. What does that mean for each turbo.

So... Not to derail, but in my instance on a cr 5.9, im probably goin gt42/gt55, but i want it to be daily driven as well as heavy tow... So should i stick with my 1.44 housing on my 42? Im thinking ddp 150 sticks with a modded single cp3 looking for 900+ at the wheels. What would you suggest as a wastegate setup, and would suggest "s" turbos (ie: s380, s?) As my turbos. This question may also help the op in some way too

juniort444e said:

Got most of that.

Why do you only want around 20ish psi from the atmosphere. I thought having the bigger one was for higher boost. Or do you add the two boosts together. I see brain on the other thread is running 100psi. What does that mean for each turbo.

Click to expand...

No, the larger turbo is for a larger VOLUME of air. Not pressure. A bigger volume of air= more power. Just adding more boost only works to a point. There is a point where a turbo leaves its optimal range in the compressor map. This means that at XXpsi, the turbo is running out of air to move. This is heating up the air that is going into the motor which isnt good either. So, a bigger turbo will move more air at the same psi.



Think about this: Say you take air coming out of a stock charger- just for fun we will call it 500 cfm. All that means is the amount of air moved, in cubic feet, per minute. Say that this 500 cfm is entering the motor at 30 psi. Now, you change turbos and step up to a bigger one. Now, this turbo moves 900 cfm at 30 psi. See where the power gain is? Its in the volume of air (measured in cfm) not boost pressure. The larger atmosphere charger supplies extra air to the primary. So, the primary is making its normal amount of boost with more cfm.

As for brian running 100psi, he has 2 large turbos, and they are set up for track purposes... He is moving an ungodly cfm at 100psi. Boost pressure should be a byproduct really, not a goal. In other words, he moves the air to make HUGE power, and his truck makes 100psi.

There is a killer thread where Charles explains it prefectly. He is talking about a triple turbo setup, but the same principles apply here.

Charles said:

Since I obviously cannot know exactly where you might be getting hung up, I'll try to explain everything in detail, simply ignore the parts you have a firm grasp on.


12psig (psi gauge) is 26.7psia (psi absolute), 12psi + 14.7psi of atmosphere. Since the gauge is set to zero at 14.7psi, a reading of 12 tells us we actually have 26.7psi.

This represents a pressure ratio across that compressor of 1.8163:1, 26.7psia / 14.7psia. The meaning of this is that the compressor is taking in air at 14.7psia and compressing it 1.8163 times such that it is discharging air at a pressure of 26.7psia. That is the pressure ratio. It is technically described as P2/P1 where P2 is the outgoing and P1 the incoming pressure.


From here the easiest way to see this might be to simply take a ride through the system.

The first stage takes in air at 14.7psia at sea level. If it's producing a pressure ratio of 1.8163, then it is compressing that air from 14.7psia to 26.7psia on it's outlet.

The second stage compressor then takes in this air at 26.7psia and compresses it another 1.8163 times such that it takes the air from 26.7psia and compresses it to 48.5psia.

The third stage compressor then takes in this air at 48.5psia and compresses it yet another 1.8163 times, such that it takes in the air at 48.5psia and compresses it to 88.08psia.


And with 3 total stages, each running a pressure ratio of 1.8163:1 and starting at 14.7psi, 88.08psi is exactly what you will end up with.

Converting back to gauge pressure, our boost gauge would read out 73.38psi (88.08psia - 14.7psia worth of atmosphere).



I hope that does it.

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swamp_ratt said:

So... Not to derail, but in my instance on a cr 5.9, im probably goin gt42/gt55, but i want it to be daily driven as well as heavy tow... So should i stick with my 1.44 housing on my 42? Im thinking ddp 150 sticks with a modded single cp3 looking for 900+ at the wheels. What would you suggest as a wastegate setup, and would suggest "s" turbos (ie: s380, s?) As my turbos. This question may also help the op in some way too

Click to expand...

Im not familiar enough with cummins setups but here is one way you could go about it. By looking at other successful setups, you can decide approximately what size charger thos injectors will spool. Say a 66mm for the primary- should spool ok, especially with some help from an atmosphere charger. So now, you have to find an atmosphere charger that will also spool easily so that you keep your throttle response for towing. You dont want the truck to fall under the chargers. Then you get smoke and heat. Maybe an 82 mm? Just throwing numbers out there I really have no idea. Now, depending on how that drives (hot, laggy etc) you can change wastegating and exhaust housings to combat that.

I would think that a 42/55 setup would be too big for any real towing, and towing at 900+ horsepower fuel only will be an art.

That helped out a ton. I always wanted to ask but felt i would sound stupid, but my question would have been, if my stock turbo flows 25psi and my 4202 flows 25psi is it still the same or do i get more. And you and charles answered that pretty well.

Now like swamp ratt was asking, how do you figure out what housings work and what to do with out buying a ton of different parts that you cant or wont use.

Jd that was a awesome explanation of it, cleared up a lot of stuff for me! :rockon:

Also about waste gating these turbos. Since say my 4202 can hit 50+ psi and i only want roughly 20-30psi would i have to gate it that low, and i dont know where to begin with the primary since its getting fed. And how come some go with gating it back into the exhaust and some just dump it. I know every setup is different in everyway but what would work best for say a DD/weekend warrior.

juniort444e said:

That helped out a ton. I always wanted to ask but felt i would sound stupid, but my question would have been, if my stock turbo flows 25psi and my 4202 flows 25psi is it still the same or do i get more. And you and charles answered that pretty well.

Now like swamp ratt was asking, how do you figure out what housings work and what to do with out buying a ton of different parts that you cant or wont use.

Click to expand...

Better to ask and learn than assume and be wrong. As for sizing turbos and stuff, you can compare compressor maps (which shows how much air a turbo can move at what psi etc) from your stock turbo for a reference, up to a turbo you think may work. Basically, you want your primary charger to be toward the top of its map and your atmosphere to be near the bottom of it's.

More pirated from the other thread:

Charles said:

Pretty much.

Since the stock chargers are only going to be able to be held at a given PR with any practical amount of wastegating, the resultant room between that PR and the target manifold pressure is all that's left for the first stage to make up. If that value is low, then the first stage must be bigger to move the same volume of air with equal efficiency than if it were able to spin up higher and run more pressure.

For instance, on a 7.3 a GT47-88 first stage charger will make similar power to a 70 some odd mm single charger because the single is much higher in the map, where it can move to the right without exceeding the choke point of the wheel, whereas the 47 is just loafing along near the bottom of the map in comparison. Seeing as the maps lean to the right, you can move the same air at a higher pressure with a smaller charger as you can with a bigger charger running less pressure.

However, once you start exceeding the pressure ratios where the single stage can maintain equivalent efficiency, the multistage setup starts to pull away. And even when they are equal in full power efficiency, the multistage system will have a much broader powerband and much better driveability.

Click to expand...

Here is the actual thread. The good reading for you starts at the bottom of page 8. Pay close attention to Charles' posts. He is VERY good with this stuff. Again- this is in response to a triple setup but there is great info about turbos/sizing and wastegating in there.

http://powerstrokearmy.com/forums/showthread.php?t=3545&page=8

swamp_ratt said:

So... Not to derail, but in my instance on a cr 5.9, im probably goin gt42/gt55, but i want it to be daily driven as well as heavy tow... So should i stick with my 1.44 housing on my 42? Im thinking ddp 150 sticks with a modded single cp3 looking for 900+ at the wheels. What would you suggest as a wastegate setup, and would suggest "s" turbos (ie: s380, s?) As my turbos. This question may also help the op in some way too

Click to expand...

You would want a lot tighter housing on your gt42. Probably a 1.12 on the gt55, and even with that, I think you would have a time towing. An s400 or s300 would work as your primary, and then an s510 as the atmospheric, but an s510 is noticably laggier than a gt55. Honestly if you want a hot towing set up, I would look at something more like an s362 and s478. I know of at least one guy who has made 800 or so horse with that and still towed. Wastegate set up is simple, gate before the primary and have it dump into the hot pipe before the atmospheric turbo. Then I would run a second gate before the atmospheric that dumps into the downpipe.

Thanks matt. Who makes the s turbos?

juniort444e said:

Also about waste gating these turbos. Since say my 4202 can hit 50+ psi and i only want roughly 20-30psi would i have to gate it that low, and i dont know where to begin with the primary since its getting fed. And how come some go with gating it back into the exhaust and some just dump it. I know every setup is different in everyway but what would work best for say a DD/weekend warrior.

Click to expand...

Well that depends. Are you using the 4202 for a primary or an atmosphere? For an atmosphere, a 4202 may be on the small side. You can have it make as many psi as you want, you just have to take into account that the air that the 4202 is feeding the primary will be compressed again. So say that 30 psi is compressed agin going through the primary, you could have 100 psi going to the manifold pretty quickly waste gates will control the pressures there. Another thing to keep in mind is waste gates can only flow so much. You may need a couple gates to be able to bleed off that much pressure. If thats the case, your back pressure is getting kinda high- gotta strive for that 1:1 ratio.

As for dumping the gates- the gates will be dumping exhaust. Most DD trucks will want to dump back into the exhaust so that they dont cover their fender wells, engine bay and undercarriage with soot. Personal preference i guess.

Ok stupid question but how do the gates know when and how much pressure to dump?

When you dump back into exhaust, do you route it in the downpipe or can you reroute it back into the exhaust to help keep turbos spooling. Or does that defeat the purpose.

I was just giving an example with the 4202, i figured it would either be too small for atmosphere or to big for primary. What i was thinking tho, was using it for the primary with a tight housing to spool faster and something around a 45 or bigger for atmosphere. Just a thought. Ultimately i wanted the gt40 series and a gt45 something for the atmosphere.

Gt40/47 would be a decent setup. is it a 7.3? Ive heard of a few running 38r/47

swamp_ratt said:

Thanks matt. Who makes the s turbos?

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Borg warner. I would call Engineered Diesel and tell them your goals and let them set you up.

Yeah i found the turbos. Looking at em on bullseye right now

Yes 7.3. And thats what i ment to type, mind is crunching alot right now. I thought about a 38r/47 but wasnt sure on the 38r living long. But thats why im here asking.

So a 40/47 would be ideal. Now about housings. You want somewhat loose on the primary, and somewhat tight on the atmosphere. Maybe a 1.19 for the 4088r and a 1.08 in the 4708r

weekendwarriorfsw32 said:

Ok stupid question but how do the gates know when and how much pressure to dump?

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Boost controolers/regulators. Plumb from the wastegate to a boost source, then you adjust the regulator to get your desired pressures.

I don't know how to multiqoute, but yes you can dump the gates back in to help spool. Most gate the primary and have it dump before the atmospheric to help spool it. Then if you run a gate on the atmospheric just dump it into the exhaust.