Twin Turbo vs. Single Turbo V6: A Dissertation

Mike@GTM

Lately I've noticed quite a few threads about single turbo setups on the 370Z. I've been wanting to share some insight with the community regarding the mechanics of single turbos as compared to twin turbos as it is related to the 370Z specifically.

This is some pretty heavy reading and goes into quite a bit of detail, so be warned.

There are several aspects of turbochargers that define how it operates. Before we get into that discussion, we need to define a few terms. Those of you with a physics or engineering background can skim/skip this part.

• Adiabatic Efficiency: air temperature increases when compressed. Adiabatic Efficiency tells you how much more the air temperature increases when compared to the ideal gas law. Nothing is 100% efficient. The more efficient a compressor is, the less extra heat it generates when compressing the air.
• Exhaust Enthalpy: Enthalpy is the total energy of a substance. Enthalpy includes heat, pressure, and velocity. Enthalpy is what drives the turbine of the turbocharger. The more enthalpy the turbine gets and the larger the difference between the enthalpy on the inlet side and the enthalpy on the outlet side, the more mechanical energy the turbine extracts from the exhaust gasses. This is extremely important as this affects boost threshold, response and top-end power.
• Polar Moment of Inertia: this describes the effect of mass in a spinning object. Simply weighing an object that will be spinning doesn’t tell you the whole story. If you have a 10lb mass that is 1ft in diameter (evenly distributed mass) and compare its polar moment of inertia to a 10lb mass that is 2ft in diameter (also evenly distributed), you will find that the smaller object is easier to spin as it has a lower polar moment of inertia. Also, if you have the mass concentrated near the axis of rotation, it will have a lower polar moment of inertia than an object with the same mass and diameter with its mass evenly distributed.
• Heat: not to be confused with temperature. Heat is energy and the ability to do work. Temperature only tells you how fast the atoms/molecules of matter are moving (root mean square of the velocity). Heat (expressed in BTU's or Joules) can be calculated from the temperature, mass, and specific heat of a substance. Since exhaust gas is mostly hot air with a little bit of water vapor, it has a pretty low specific heat and therefore, a small change in heat makes a pretty significant change in temperature.
• Heat Transfer: basically, nature abhors a vacuum and something that is hot wants to be the same temperature as its surroundings. Therefore, it will transfer heat to the surroundings. The bigger the difference in temperature, the faster heat will transfer. The transfer medium also affects the transfer rate. Some materials transfer heat more effortlessly than others. Aluminum transfers better than steel and steel transfers better than ceramic. Also, the thicker the transfer medium is, the higher the resistance value is (commonly called “R-value”…home insulation is rated R-19, R-8 etc.). The higher the R value is, the more resistance there is to heat transfer. A thick iron casting is going to have a higher R value than thin stainless steel piping.
• Thermal Mass: pretty self explanatory. The more mass you have, the more heat will be stored.
• Boost Pressure: often (mistakenly) thought to be the cause of horsepower in forced induction. This is simply a measure of resistance to airflow. The higher the boost pressure is, the harder the turbo is working to get more air into the cylinders. Also, as boost pressure increases, air temperature also increases dramatically.
• Air Flow: This is what makes horsepower. In naturally aspirated engines, it is usually measured in cubic feet per minute (CFM) because air density doesn’t change much in atmospheric engines. In forced induction, however, it is better to measure it in terms of mass flow or pounds per minute (lb/min). A rough guide to converting lb/min to horsepower is 9.5 – 10.5 horsepower for every lb/min. On the VQ37, it’s closer to 10.5hp for every lb/min thanks to VVEL and high stock compression.
• Turbo Lag: Often misused terminology. I will define two other terms to differentiate two different aspects of turbo performance.
• Boost Threshold: (what people incorrectly tend to use the term “Turbo Lag” to describe) this is the engine rpm that the turbo makes positive boost pressure at in WOT operation. This is a number like 3800rpm.
• Boost Response: (what Turbo Lag is supposed to be). This refers to the time it takes for boost to build once the engine is above the boost threshold and WOT is applied. To illustrate, imagine that you are going through a corner and are at part throttle through the corner. As you approach the exit, you floor it and there’s a brief moment where nothing seems to happen, then, very quickly, boost comes on like a freight train. That brief moment of nothing is the true definition of turbo lag. For this article, we’ll refer to it by its inverse: boost response. The more responsive a turbo system is, the less “laggy” it is.

Mike@GTM

Now that we have all the definitions and important terms squared away, let’s continue to talk about single vs. twins on a V6. Fortunately, the firing order of the VQ37 alternates bank to bank and front to rear. So that means that passenger side front cylinder fires, followed by the driver side front, followed passenger middle, driver middle, passenger rear, driver rear. This type of firing order is fantastic for twin turbo setups (one turbo on each bank) as each turbo gets a pulse of exhaust gas in a nice even rhythm. In addition, there’s a nice amount of time between each exhaust pulse to allow maximum scavenging of the cylinders.

With a single turbo, on the other hand, the dynamics get funky. Depending on the arrangement, the exhaust pulses from one bank can collide with the exhaust pulses from the other bank. If you think of the exhaust pulses like waves in a tub or pool, when they collide, they make bigger waves and smaller waves (positive interference and negative interference). Now, you might think that bigger waves are better. Yes, and no. If the wave is too big, not all of it can fit through the turbo at the same time…resulting in back pressure and wasted energy. At low rpm, even the big waves resulting from the interference aren’t overly large and so you get a slight benefit from helping the turbo spin up easier. So you can get improved spool and mid-range torque with the single, but it’ll choke at high rpm. To remedy that, you can use a larger turbine housing, but then you lose the faster spool in exchange for a small benefit at higher rpm. This is the big problem with the Subaru boxer engine using a single turbo on the stock un-equal length headers and part of the reason why Subarus have such a hard time making the horsepower that the Evos do.

The other part of the problem is enthalpy and heat transfer. Remember how I said that enthalpy is the driving force of a turbocharger? Well, the longer the distance between the cylinder head and the turbocharger, the more surface area there is for heat to transfer out of the exhaust gases…reducing enthalpy. The less enthalpy you have going into the turbocharger, the slower it is going to spool, the poorer your boost response is going to be and your boost threshold is going to be at a higher rpm as well.

It gets worse. So, lets’ say you’re wanting to run the same big turbo as your Supra buddies. Have you ever looked at the size of the downpipe on a big power Supra? 4” is the minimum entry fee for big power. Where are you going to put a 4”+ downpipe in your 370Z’s engine bay? Why 4” you ask? Well, remember how turbos operate on the difference in enthalpy? The bigger the pipe is after the turbo, the lower the enthalpy is. Velocity drops off in a big pipe, more surface area to transfer heat out of the exhaust in a big pipe, less backpressure, so less enthalpy after the turbo. Because you cannot realistically put a big enough downpipe in the 370Z engine bay, you are further decreasing the turbocharger’s ability to spool and make power. Again, poorer boost response, and a higher yet boost threshold.

Let’s also take a look at turbo manifold construction. On a twin turbo kit, it makes sense to cast an iron manifold to tuck the turbos as close to the cylinder head as possible. The cast iron has a higher R value, more thermal mass (stays nice and hot between shifts and through corners), and less surface area for heat to escape.

The single turbo manifold construction usually retains the stock headers (to reduce cost) and a relatively thin crossover pipe. So, you have more surface area to transfer heat out of and significantly less thermal mass to store heat. This means that a lot of enthalpy is lost before the turbo even has a chance to capture it.

Mike@GTM

Now, on a street car, the things that matter most, is how much fun the car is to drive. Usually, good boost response, low boost threshold (boost comes on at lower engine rpms) and decent top-end power are what make a car fun to drive on the street or occasional track days. A twin turbo system does these things extremely well and cannot be beat…even by a well engineered single turbo system. The single turbo system has too many compromises to make as effective of a street setup.

On a full on drag race car (tube frame, gutted, no A/C, no power steering, no ABS, modified firewall, etc.) it makes sense to use a big single turbo to make 1300+ horsepower (even though that much power can still be made on twins). The reason it makes more sense is efficiency. A bigger turbocharger is simply more efficient at compressing air than a small turbocharger (or two). Besides, at the point of building a full on drag car, it’s OK if you have a 5 – 6” diameter downpipe running where your passenger’s feet would be in a street car.

So, wait, a bigger turbo is more efficient? Why yes, but it’s also a bigger diameter compressor and turbine wheel. That means that it has a higher polar moment of inertia and therefore has very poor boost response.

Ultimately, for a 370Z on a stock engine, a single turbo is a very poor choice for a street driven car. It will have a boost threshold 1500 – 2500rpm later than twins, have poorer boost response and/or top end horsepower. If you are spending $5-7k+ on a turbo system, why throw away so much performance by using a single turbo system?
I mean, I understand the desire to be different from everyone else, but when you consider that most 370Z’s aren’t FI in the first place, just being FI at all is being different from everyone else all by itself.

One other tidbit I’d like to add. There was a discussion about having cats before a turbo. It is a terrible idea for not only the reasons already laid out here, but also from a reliability standpoint. Coming from the Subaru world where the 2002 and 2003 WRX had a cat mounted pre-turbo (along with two more in the downpipe), that pre-cat was notorious for coming apart and taking out the turbo. Subaru had so many warranty issues that they eliminated that pre-cat in all later WRX’s and STi’s. Also, the cat eats up enthalpy like a sponge and leaves less for the turbo to use. One of the first mods a 2002 – 2003 WRX owner does is change the up-pipe.
Also, I know someone will eventually mention heat wrapping the long exhaust runners in a single turbo system. As a Subaru guy, at one time, I thought it would be fun to install a nice set of stainless steel headers on my car and heat wrap it to retain some enthalpy. Unfortunately, thin stainless piping and heat wrap still have a significantly lower R value than a cast iron manifold with heat shields and I eventually went back to the stock exhaust manifold for more bottom end torque, better boost response, lower boost threshold and overall better drivability around town. Not to mention the top end gains with the headers were marginal at best…they were Tomei headers in case you’re wondering.

All in all, this is just information for people to know. This is a culmination of mechanical engineering knowledge coupled with years of experience in working both in the high performance building industry and high performance car industry and being an enthusiast. I just wanted to share this information with people so they can make informed decisions. This is not to knock other companies’ products as I can appreciate how much hard work it is to bring a product to market.

roplusbee

And Sam does it yet again. Compiled multiple threads questions (and anwers) and broke it down so that the less knowledgable and vets alike can understand (and poddibly relate). You impress me everytime you post Sam!

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2007 350Z NISMO #0618: Zerolift | UPREV | F.I. | Greddy | Competition Clutch | Injector Dynamics | Aeromotive | Pioneer | JL

SAM@GTM

The Credit belong to Mike@GTM sense he is the one that put this info together.

We appreciate you Complements.

Sam

DCNISMO

You brought up bad memories why I hated thermodynamic class in engineering school.....I escaped with a B-.....

Mike@GTM

Thermo I, II, and heat transfer were my favorite subjects by far.

Kingbaby

What is your take on the S&R performance setup on the HR motor?

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Red__Zed

Saving this spot for a reply once I'm back on a computer with a keyboard.

On the surface, looks to be well written and informative

roplusbee

No prob Sam. I know you guys are top notch!

<off-topic>

Looks like weare in the same boat dude. I Majored in Computer and Electrical Engineering, but we wer required to take courses from the other engineer's programs. Lets just say I squeaked by. Minimum to retain credit in the Engineering courses was a B- @ the time when I attended Morgan State University. Since then I have Switched to UMUC and I take most of my courses online now. There are a few that have to be taken in a classroom though.

</off-topic>

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2007 350Z NISMO #0618: Zerolift | UPREV | F.I. | Greddy | Competition Clutch | Injector Dynamics | Aeromotive | Pioneer | JL

Seb@SZ

Excellent, thanks for sharing this information for the public. TTs for life.

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tower74

So the heat issues with the two turbos being that close to, well everything doesn't effect anything? No extreme turbo temps? I thought heat was the enemy for FI? I know when it was warm out the car runs normal, but when it's cold I could tell the diffrence in power. Just wondering.

djpathfinder

Thanks for the write-up Mike! A lot of great info in there, including references to the WRX/STi, another vehicle that we own, and now better understand. Repped!

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2012 GT-R Black Edition Jet Black
2010 370Z 40th Anniversary // SOLD

Kingbaby

I posted this on my club's facebook page.

For those that fall asleep in class!

How a turbo works - YouTube

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07 Silver Alloy Nissan 350Z Nismo #0191 "Lin'da"
2amR--------------------> VQ I.N.C. - Like us!

Cell

"Not a problem" lol....

Also, great post mike.