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Old 01-13-2012, 06:13 PM   #2 (permalink)
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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.
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