Twin scroll vs twin turbo

SS_Firehawk

I've been looking around the world wide Google for the past week looking for information on this topic. All I get is N54 vs N55 BMW BS. I've only gotten bits and pieces on information, and not the whole picture. I'm hoping someone in here can help shed light on this topic. Option for forced induction is expanding on our vehicles, and this is definitely something I think can be expounded upon for the rest of the community who are like me and asking this question.

So as I understand it (very basic), twin scrolls divide the cylinders to different ports on the turbo to prevent backflow. With a 4 or 6 cylinder motor, I can see this working great. With a V8, I think one twin scroll per bank will be the most beneficial. It's most likely the reason why every single turbo V8 that has come to market has come with two twin scroll turbo's, and not one.

Taking this out of context, this may be part of the reason BMW moved from TT's to single scroll. They made similar power and more torque at lower rpm's. Could it be because the exhaust is only powering one turbine as opposed to two? It seems like the right size turbo for the job is crucial regardless. So what are the benefits of a TT system (on our vehicle) over a twin scroll if the primary reason people went TT was to reduce the lag from one large single turbo, and the twin scroll seems to have taken care of this? The one big advantage I can see with TT's is getting it closer to motor.

Thanks in advance for the help in understanding.

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Nixlimited

Twin scroll means a divided turbine inlet, but only one turbine. Twin scroll takes advantage of cylinder firing order and helps spool a turbo faster. Given the same turbo specs, a twin scroll will typically spool faster than a single inlet. I am not sure, but my guess is that the single inlet would be a bit more efficient up top.

BTW, I believe BMW moved from TT to a single turbo using a twin scroll exhaust housing.

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Boosted Performance

Here is a great article:

Twin Scroll Turbo System Design - Modified Magazine

It compares the open housing to a twin scroll housing...but you can see the advantages of the twin scroll here.

With a twin turbo system, you do not get the crucial scavengeing effect that makes the twin scroll housing that much more efficient. This same priciples apply when you are comparing the twin turbo log style turbo manifolds to the twin scroll system with equal length primaries. With a log style manifold there is a lot of interfeerence between the exhaust pulses in each bank because the runners are all different lengths. There is no scavenging. Now, if you had a twin turbo system mounted to equal length primary headers, you would have the same scavenging effect as a twin scroll kit with equal lenth runners. This would be ultimate setup for high, efficienty power levels.

Like this one:



The setup above is a twin turbo kit, but with equal length headers. This is very important when it comes to the scavenging effect. OEM headers on the 370z are equal length, and this is why the twin scroll kit works great.

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SS_Firehawk

So I read the article, I understand how the twin scroll works and it's benefits. I guess I'm trying to see where TT's would be more advantageous than the twin scroll. You mentioned that TT's with equal length headers would be the best combo, but all I see are short manifolds coming with the kits. Is it a packaging issue? If it were the case, why not install the turbo's farther down or create an equal length header package that fits? Is it because we can just kick up the boost to make up for the inefficiencies?

I'm definitely more of a function over form kind of guy. I prefer, and would pay the extra coin to run at a higher efficiency than take the easy road. Turbo guys always say just turn up the boost to make lost power, and I think to myself, that methodology only goes so far. It's the same methodology of the 60's, stuff a bigger motor in it to improve performance. If we stayed that route, we'd all have big blocks and getting 5mpg. Don't get me wrong though, there is a reason why those cars are still coveted. When we have companies like Mazda pushing 14.0:1 compression on a 2 liter motor and can run 87 octane, what are they doing they everyone isn't?

Sorry, went off on a tangent and got lost in my thoughts. Am I making sense here?

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roplusbee

Now that my build is complete, I may look into twin scroll manifolds when I get my motor build going. Maybe even go with 25Gs to replace the 20Gs. I don't want to loose response though............

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ZMan8

sub'd

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Boosted Performance

It is a price and fitment issue. To do a proper manifold idealy you want short equal lenght runners. This is not possible with the VQ because of the engine layout. The next best thing is mounting the turbo at the end of a properly designed equal length header, like the picture I posted above. Power per PSI with that setup will be higher than with the log style manifolds every time, mainly due to the scavenging effect.


If you want the best twin turbo kit, go and buy a good set of equal length headers. Have a good fabricator cut the three bolt flange off, and have him extend the collector, so that you can fit a twin turbo kit like the one I posted above. With a proper engine build, and turbos, you can get in to the tripple digits.

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G37sHKS

Well speech!

I wonder if Tony@Fastintention will take this advice and make equal length headers for his TT kit??

Damn, I thought GTM did their homework and deliver the best kits for the community!! Me want equal length headers right now!!

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Nixlimited

Speaking of which, what ever happened to the FI TT kit?

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SS_Firehawk

Knowing this makes me wish I could run my long tubes even more. When I get real internet that allows me to see pictures hosted on photobucket, I want to get a good look how The twin scroll kit is being routed. I swear that the stock header and cat combo takes so much more space than my PPE''s. I guess there is one way to find out...

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Boosted Performance

PM me your email and I can send you pictures..if you wish.

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DIGItonium

Give it a few more weeks...

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roplusbee

It is actually in the works, as in, being contructed. Not sure how far he has gotten with testing though. I didn't want to pry.........especially since I just got my tune fixed @ UPREV.

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Mike@GTM

I'd like to add my two cents here if I may.

First off, I'd like to say that a twin scroll turbo is an excellent choice for applications that can make the most use of them. The article mentioned in this thread is a good example of that, but fails to address the drawbacks nor is it specific to the 370Z which has its own unique set of constraints and effective solutions.

The critical factor they left out is heat transfer. Since an exhaust gas turbocharger is a thermodynamic device that operates on the Brayton cycle, it relies on heat to do work. The more heat that is in the exhaust gas, the more work the turbocharger can do. The job of the turbocharger on an engine is to force more air into the engine. That "forcing" of air requires work...work derived from the exhaust gas.

In the past, I've basically stated that a cast iron exhaust manifold is better than a stainless steel pipe. This time, however, I'm going to let the math do the talking for me. So let’s get going.

Heat transfer through a cylindrical pipe is expressed as follows



Q is the rate of heat transfer in BTU’s per hour from the inside of the pipe to the outside. T2 is temperature in Fahrenheit of the fluid inside the pipe (exhaust) and T1 is the temperature outside the pipe (engine bay). The ln stands for the natural log function. R2 is the radius of the outside diameter of the pipe and R1 is the radius of the inside diameter of the pipe. Obviously, pi is 3.1415... The k is the thermal conductance of the material and L is the length of the pipe.

T304 Stainless Steel has a k value of 103 BTU's / (hr * sq ft * Degree Fahrenheit)

Cast Iron has a k value of 381 BTU's / (hr * sq ft * Degree Fahrenheit)

You will notice that T304 Stainless Steel has a lower k value than cast iron which means that it actually conducts less heat than cast iron. So what’s the deal? Why do I state that the cast iron manifold is better? Well, let’s answer that.

The more material that heat has to transfer through, the slower it will move. This is thermal resistance. In the case of a cylindrical object such as a pipe, heat transfers radially from the inside out and thus its intensity decreases significantly the thicker the wall thickness is (sidebar: if you remember from physics, radial intensity decreases in proportion to 1 / square of the radius)

So let's make a few assumptions concerning the boundary conditions to analyze the differences on a level playing field.

Let’s assume that the exhaust gas inside the pipe is 1200 degrees Fahrenheit and the engine bay is a toasty 150 degrees Fahrenheit and steady state conditions*. That gives us a Delta T of 1050 degrees (T2 - T1).

Since most mandrel bending machines are limited in what wall thickness tubes they can bend effectively, and that number tends to be 0.065", we'll be using that in our analysis. Also, limitations in effective casting techniques put the minimum wall thickness of cast iron in the 0.375" range, so we'll use that number in our analysis as well.

Next, we're going to compare apples to apples and say that both pipes are 1 foot long and have a 1.5” inside diameter (radius of 0.75”).

So, we plug in our numbers and get the following.

Stainless Steel: q = (1050 degrees Fahrenheit) / ((ln ((0.75 + 0.0625) / 0.75) / (2 * pi * 103 BTU/hr*sqft*F * 1ft))

Cast Iron: q = (1050 degrees Fahrenheit) / ((ln ((0.75 + 0.375) / 0.75) / (2 * pi * 381 BTU/hr*sqft*F * 1ft))

Using a scientific calculator, we get the following results:

The Stainless Steel Piping transfers heat at a rate of 8,180,000 BTU's per hour per foot of pipe.

The Cast Iron Pipe transfers heat at a rate of 6,200,000 BTU's per hour per foot of pipe.

Ladies and Gentlemen, I invite you to notice that Stainless Steel Piping loses 31.9% more heat per foot than the cast iron piping.

Consider now that a cast iron manifold is only about a foot long in the first place and that most stainless steel setups are 2 to 6 times longer than that.

Also, consider that as the exhaust gas loses heat, it also loses velocity. Since enthalpy (previously defined in my single turbo vs. twin turbo thread here: Twin Turbo vs. Single Turbo V6: A Dissertation) includes heat, velocity, and pressure, it is easier to talk about enthalpy instead of just heat by itself or velocity by itself. The loss in enthalpy of the exhaust gas is why a single turbo system (twin scroll or single scroll) using stainless piping mounted far away from the engine simply doesn’t make as much torque as a twin turbo setup. It has very little to do with tuning as someone claimed and has more to do with the physics of how these systems work.

Since OEM’s use cast iron turbo manifolds designed by qualified engineers and given the little taste of one of the many heat transfer formulas I presented earlier, I would say that they are far smarter and more educated than the average enthusiast gives them credit for. They did, after all, design and build an entire car…not just a spaghetti factory of stainless steel pipe with a turbo hanging off the end.

One other thing I’d like to mention is the firing order of the VQ37VHR. This engine fires bank to bank and front to rear. As such, each twin turbo manifold receives an exhaust pulse, pause, exhaust pulse, pause, exhaust pulse, pause (wash, rinse and repeat). Those pauses are the other bank firing and do not cause interference in the critical spool up region as suggested in this thread. Therefore, the Twin Scroll on this engine does not have the same advantage as it does on an inline four or a horizontally opposed four cylinder engine. It is nothing more than a way to compensate for being as far from the engine as it is and the attending enthalpy loss.

*Steady State Conditions means that the system has been operating at the same input and output long enough to be consistent. Since an engine in a car is rarely operated in steady state conditions, there is another important consideration to factor in. That is heat capacity. The more heat capacity a material has, the more resistant it is to fluctuations in temperature. The cast iron manifold wins that contest hands down over the thin wall stainless steel piping and therefore will have better boost response in transient load without me even having to go through the calculations. It’s a no brainer.

SS_Firehawk

I definitely like seeing vendors expertise contributing to this thread. I'm happy to see that both options are available. It seems the performance differences are very minor between the two. I would venture to say TT's have a slight advantage in performance. I think that someone bringing forth a twin scroll option was overdue. It seems a bit more cost effective compared to a twin turbo setup. At least we have a reference as to the benefits of each available to us in one spot. Lastly I can't know for sure unless I had 20k+ burning a whole in my pocket with a lot of time to test both. It does sound entertaining though.

Edit: Just read the majority of that thread. I does a good job illustrating the benefits of twin turbo's. You also mentioned the cost difference as well. I still find both competitive with respect to the market they play. And again, like yuu said, it is preferential to the consumer on what they think is better for them; lower cost and slower boost response, and higher cost faster boost response. But even then, with the twin scroll, I don't see difference in response to be that far off the mark of TT's. Thanks again for the contributions.

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