Twin scroll vs twin turbo

Originally Posted by 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.