Yes I know that this question has been asked before but I'm asking again as it never truly gets answered, I dont believe this belongs in advanced tech but correct me if im wrong.

The question- Is it possible to run 2 different size turbos in a parallel turbo setup, I know there are no benefits to joining the exhaust before the turbos but if I did would it be possible to run a very small turbo and a large tubro. As long as both turbos are rated for the boost pressure they will see the small turbo wont over spool/spin correct?
If this is correct if the smaller tubro was a VGT turbo would the small turbo be able to act as a waste gate or would this over spin that turbo. I'm only speaking in theory as a setup like this would be wildly not efficient but purely for the wow factor but still function 100%

 

Need more clarification. Are you taking like a v8 engine with a small turbo on one bank, and a large one on the other bank? Or how exactly would you set it up?

 

Sounds like sequential maybe? Requires a valve to keep the larger turbo closed off until boost builds, but yes it can be done. I think this is how the MKIV Supra was setup.

 

Yes V8 and no not separate on banks, I specifically asked if I had joined the exhaust together

Yes going for something like a sequential setup for my wife's sport trac. Ive always wanted to turbo it and was thinking maybe if I can run a small enough primary turbo maybe get some mpg benefits on the highway.

Getting to specifics though, yes its a sequential for argument purposes. If the small turbo was a variable vane turbo, can the small turbo be utilized as the "waste-gate" for peak boost or would a normal waste-gate be required to bleed off manifold pressure. I would imagine if the turbo is too small the vanes might not flow enough to allow pressure in the exhaust manifold to escape or is this not the case for variable turbos

Sorry I wouldve replied sooner but left my laptop at work, I might as well describe my end goal if anybody is interested. Like I said above Im shooting for some form of sequential setup but without the end goal of power alone. I intend on bringing the same setup over to a coyote if the 4.6 blows. If the small turbo is sized correctly specifically for low end under 4k engine rpm range/fuel economy I should see some benefits to mpg. Im aware that the increase could be barely anything but I still want to see if it does change at all.

 

I see. You can look at the ford 6.4l Powerstroke (Navistar) for an example. It was a sequential VGT turbo, specialized use case but it would give you something practical to research. What works or is necessary on a diesel is very different from a gasoline engine.

Truth be told I don't think its even desirable on a gas engine with todays turbos. A good modern single will spool very quickly and give you all the boost you need to blow up either engine. The sequential turbo systems were fallout from the high lag, inefficient turbos of the first turbo era. It's a lot of complexity: sizing the turbos is fiddly, you need valves that aren't all that easy to find, and a way to control them. Even just controlling a VGT turbo properly is a challenge, aftermarket VGT control isn't really popular (probably because its unnecessary).

Not to mention the expense, you can get into some fairly exotic turbos when you are only buying one, instead of two, plus valves, VGT control, etc. And you can run a large frame turbo in the same or less space as you would run the two smaller ones.

Since it is your first build try setting it up simple, there is a lot to deal with already between actually fitting the turbo, all the necessary plumbing, fuel and spark management, etc. You will learn a ton from building it, tuning it and driving it. If you aren't satisfied you can always change it later.

As far as the coyote goes, they drive around really well even without boost, especially if you are running a 6r80 or mt82, you have a very deep first gear that covers up any lack of torque. You don't really need it to spool instantly, you are making enough power NA to keep the turbo focused on the top end.

You also don't want to run boost real low in the RPM range on either engine, it will break things. Keep the boost up in the RPM range and enjoy.

 

Im very aware that a diesel engine is different and I know this setup is expensive and not "cost effective" and I know that running boost under low loads and low in the rpm is bad, very bad. This isnt my first build, I boosted a few cars before and one includes a saturn twin cam. I simply asked if this would work, Im not looking for reasons to not do this.

Yes modern turbos compared to that time are much more efficient but a physically smaller turbo still will always spool quicker under smaller loads. In some sequential setups I've seen the wastegate to the larger turbo can be opened to "prespool" the larger turbo negating the need for an extra throttle plate to isolate the larger turbo but I imagine this still has an effect on lag but my end goal is not avoiding lag.

Again my end goal is not power or avoiding turbo lag. Im trying to achieve being as efficient as possible with this engine(s) and still be able to produce a good torque curve and peak power numbers. This is purely a science experiment to me as I can find any good info purely regarding turbos being effective for mpg and a turbo that small will choke the motor or over spin.
Just trying to be clear

edit: I forgot to add that only the smaller turbo would be a vgt and the larger a standard frame with no integrated wastegate

Edit 2: Looked up the 6.4 as I thought i covered all my bases on sequential setups but the 6.4 is a compound not a sequential

 

The simple, short answer is no. To get the small turbo to spool with vgt means closing vanes which increases drive pressure. Problem is you can't increase drive pressure, it'll just bias the flow towards the big turbo.

Also, vgts don't cooperate with the higher egts gas engines produce. The newer ones from ~2014+ might tolerate it the longest, but they're electronically controlled.

For sequential you have to have a valve to separate both exhaust and intake. If both turbo outlets are connected together, the boost from one turbo will blow out the other one.

I think your best bet would be "wonky" set of compounds, they'll do exactly what your wanting to do, and are simple.

 

Okay I will dig more into compound setups, just from what I understood off hand was that compound setups were more for peak boost numbers that are difficult to achieve with spooling a single large turbo.

Expanding on the sequential setups, I was under the assumption that as long as the larger turbo doesn't stall/backspin that the turbo itself would prevent air from escaping. If that is the case then the next issue would be building too much pressure causing the compressor to surge because of the low turbine speed, I would imagine balancing the 2 is difficult but can be done.

As for the VGT I have a 2018 vgt2056 from a eco diesel at my disposal so I think it would work, plus I got it free so I'm not afraid to burn up the seals. The setup I had sketched up had a vacuum/pressure actuated exhaust valve to direct the flow from just the small turbo to both turbos, using that valve to build drive pressure for the small turbo and once that turbo is "spooled" to its operating range the valve will start to open starting to drive the larger turbo. Unless the exhaust gas volume isnt enough to spin both turbos fast enough for the large turbo to not stall a vacuum actuated or electric throttle body could be used as the block off for the larger turbo

 

It's do able, especially if your able to write some code. It's not going to work well just mechanically controlled.

Your only thinking about under acceleration, what about cruising? Or coasting? Boost would be low, so boost priority would switch to the small turbo, the vanes would close (assuming you use a wastegate actuator to control it) making it try to make your desired boost. Which is hard on the engine, and efficiency.

 

Isnt that true for a standard turbo too? While cruising waste-gates are always closed unless the waste-gate sees boost then it opens the vanes to slow the turbo down. The vanes should never be fully closed on a gasoline engine because its designed to be an exhaust brake for diesels.

The whole point of turbos is that they are efficient, they take the wasted exhaust gas to supply extra oxygen to the motor in simple terms. Thats why under cruising conditions the fact that the turbo is pushing air into the motor instead of the engine doing the work to pull the air in. If the small turbo housing is too restrictive with the vanes in its "smallest" acceptable setting then the bypass can be opened to help spool the larger turbo which is the root of my original question of running the 2 different sized turbos

As for coasting, I dont see how it would be any different than any street single turbo with a traditional setup

 

Regular turbos use the housings tongue for the nozzle, vgts close down and produce a far smaller nozzle than regular turbos could ever think of. It's not even a comparison.

Turbos are not always the most efficient. Below 10psi modern superchargers with intercoolers are usually best. This is because under 10psi, there really isn't alot of exhaust energy to power a turbine. To power you offset that lack of energy for pressure, and you end up with higher than needed drive pressure.

It will not be a simple system. A simple system might work, but you'll find it'll take a ton of tinkering for a system that won't ever end up being right.
It might be simple in your head, but that's because you seem to lack the experience/ knowledge to see the total picture of it. You have to isolate the flow paths and pressures, as well as figure out how the system will make boost when it needs to, and keep from making boost when it doesn't. I love the out of the box thinking on this, and it could be a good learning experience for you. Something you should look up is the v8 cummins that was on the '16-'20 nissan diesel titans. That was a sequential/ compound mixed setup, that should hopefully show you the complexities your could be dealing with.

 

I think you are confused about what it means to be efficient, you are only looking at the efficiency of the turbo, not the engine as a system. Even though popular internet myth is that turbos spool for free, that isn't the case. They are using waste energy but they are still a flow restriction. The worse you make the flow restriction (tiny vgt turbo on a large engine) the harder the efficiency hit.

A modern turbo engine is efficient because it is small, and consumes less air and fuel off boost. They consume more fuel on boost that a naturally aspirated engine of the same horsepower. They work "more efficiently" because more often than not the operator does not have the vehicle in boost, and the power is available only as required.

There is nothing efficient about sticking a too small turbo on a 5 liter 400+hp engine. It's even less efficient to use multiple turbos to do the same thing. More turbos does not equal more efficiency, once you have scavenged enough energy from the exhaust system to make the required power that's all you need. The system will be more efficient if the turbo doesn't spool until demand is high.

You are contradicting yourself. And no, you won't see an increase in MPG bringing boost in early like that. You would be better off letting the engine downshift.

Nothing about this is practical. A too small turbo is not efficient, sequential or otherwise. Use the right size turbo for the application. Engines need to breath when not in boost.



Analyze your actual goals for the build and post them, concisely. It seems like you are all over the place.

 

I dont think youre getting the picture, Ive stated multiple times what im trying to achieve with this and i really dont think its all that complicated to grasp. If u want to attack me just say so.

Increasing the effective compression ratio of engine increases efficiency and horsepower this is exactly why headers are known to increase mpg. Im trying to reduce the pumping losses experienced under cruising this will increase mpg, if the air from the small turbo is being pushed into the engine by the exhaust gases will allow the engine to do less work pulling the air in. Superchargers do the exact same thing but instead use up some of the efficiency by driving the blower, instead if you use a turbo it works of the energy of your exhaust.

By decreasing the size of the turbo to increase the amount of air trying to be forced under lighter loads in theory should maximize this affect and thats exactly what BBI turbos is talking about in their lastest response and they seem to grasp the concept just fine

 

this is exactly what I was looking for, theres no real info I could find about turbos when it came to offboost performance like I'm seeking.

I would be doing more builds so I can further my real world experience but I lack funds to do it for fun on my off time and I don't live in an area where anybody is looking to hire anybody into their shop that hasnt already worked in a place like it so here I am. I appreciate you taking the time to educate me, thank you

 

Engines are more efficient with more air when they are making power, but when they're being forced to compress the extra air when they don't need to, efficiency drops.

This is really evident in long haul diesel engines. (I know diesel vs gas, heavy duty vs car, apples to oranges etc, it gets the picture across)
Decreasing boost while cruising lowered temps, and increased mpg.
So it's not always about just shoving more air in. That's also what makes s/c more efficient below 10psi, they have bypasses so that the engine isn't wasting energy making boost when it doesn't need to. And then you have wide open exhaust system also.

 

Even though thats diesel I'm sure the affects would be worse on gas because of ignition timing that needs to be pulled to avoid detonation. A twin charger setup (turbo into supercharger) is definitely more ideal

 

Stop being so arrogant. I'm trying to help you and point you in the right direction. Did you just come here to have your ego stroked, or do you want to get this right?

Headers have little or nothing to do with effective compression ratio. Maybe your thinking of scavenging, which turbochargers completely eliminate.

I think you are confused by how forced induction works and what is happening inside an engine when a turbo is spooled. You are talking about a very specific circumstance that almost doesn't exist. You might be able to make it happen at a very specific load in a very small RPM window, but that's not how a gas engine works, you are rarely in that exact sweet spot that's why the OEMs don't do it. The trade off for that little sweet spot is going to be a loss of efficiency everywhere else.

Reread BBI's post about why a supercharger is more efficient at low boost levels, I think you missed a couple of key points.

 

If I was being arrogant I wouldn't be asking questions hotshot. I had a theroy about a setup I planned on doing and I asked if it would work. My responses even consist of me saying "I thought this is how it worked" and thats why I assumed it would work coming here is just my fact checking when I couldnt find info anywhere else

If you take a non s/c motor and put a s/c on it gas consumption decreases when cruising, its a fact theres evidence of this everywhere of this.

I dont give a shit how accurate you are, Im not going to prolong a conversation with somebody who thinks so lowly of somebody questioning how something is going operate when theres little info about the subject. I'm not sure where u got the air headed idea but I have my answer from somebody else so I dont need your help being pointed in the right direction

 

Calm down guy. Take a step back, reread what I posted earlier in this thread, none of it was intended to be personal, most of what I wrote was in a kind tone and meant to be helpful. I don't know where you got the impression that I was attacking you. My responses may have been cold but none of them were harsh until post 17. If I feel like you are wasting my time I am going to let you know.

 

I am calm? Im not sure how its possible to be saying things in a "kind tone" but be cold at the same time. Nobody is forcing you to respond so if you feel like my questions are trivial to you and waste your time then dont respond, somebody who has more patience will reply.

Differences aside im going to try and sum up everything to see if theres something I missed. I will try to reword it so that it makes sense because I do truly believe you are misunderstanding what I am trying to say just as much as I am misunderstanding u.

Ill start by correcting a few things I said- yes I know headers have no affect on effective comp, not sure why I used that to support the claim headers improve mpg but I did. Headers do increase mpg but not in that way, they use the vacuum scavenging provides to assist in oxygen being pulled into the cylinder (this helps mpg and power)
I am not trying to run boost while cruising, the pressure (if thats what u want to call it) I was planning to run would be under 1 psi.

My idea behind this is that the turbo is using the exhaust gas to push the air in that otherwise would have been pulled in by using energy (pumping losses) from the engine. My thought was by using a smaller turbo wouldnt the amount of air the turbo was trying to push in would be greater under lighter loads further reducing the energy lost pulling the air in. Superchargers pull air in just the same but are belt driven, yes they have a bypass valve but there is still an increases to total efficiency otherwise they wouldnt get better gas mileage.

Exhaust manifold pressure hurts efficiency I know this but I thought the drag of the supercharger might be greater than whats used up by having higher exhaust pressures with turbos providing better benefits.
The only "good" to come of exhaust back pressure is the natural egr affect in can bring but thats emmision related

Now this is the setup I had in mind to test this theory
Using a Y pipe to join either side of the banks with a short straight section. A larger standard frame turbo intended for higher rpm use and peak numbers. A much smaller VGT turbo mounted as if it were a wastegate off the side of the short straight coming off the Y. Between the small and larger turbo would be a "cut-out" flap to block off the larger turbo. Cold side wise both turbos would be hooked to the intake together (NOT larger into the smaller like a compound)

Having started this thread Im now understanding that the exhaust pressures needed for the small turbo to flow the amount im asking would be too high and counteract any efficiencies the added air brings

I hope I split this up so that its easy to read so that Im understood best. I apologize for any hostility, I just dont understand how I could come off as ego stroking. I thought I had an understanding and I tried to explain my reasoning behind it.