Parallel vs. Sequential vs. Compound - Twin Turbo systems explained - Boost School #11

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In today's episode we're talking about twin turbo setups and we will be doing a detailed explanation and comparison of three different setups. Parallel, sequential and compound twin turbo setups. No, sequential and compound turbo are not the same thing and I will explain how the two differ.

But first, the basics. Why would you ever use two turbos over one? After all a single turbo does the job just fine. It hooks up to the exhaust manifold where it relies on the heat energy of the engine's exhaust to spin up the turbine wheel which is directly connect to the compressor wheel which suck in air compresses it and sends it into the engine to be burned together with the fuel.
But as with most things in life, there's a compromise and that's the turbo size choice. Naturally you would assume that a larger turbo makes more power than a smaller turbo, all other things being equal, and you would be correct. But a larger turbo also needs more heat energy to be spooled up to generate it's maximum boost.

Although it may sound weird a parallel twin turbo setup is in reality very similar to a single turbo setup, just with twice the turbos. You will find parallel turbo setups most often on v6 and v8 engines. A classic and well known example is the twin turbo V6 engine on the Nissan GTR R35.
A parallel turbo features two turbocharger of equal size operating completely independently. This means that each is hooked up to a separate exhaust manifold and each has it's own waste-gate. Exhaust gasses are not diverted from one turbo to the other in any scenario. Each turbo also has it's own intake piping. The intake piping from the two turbos may join before or after the intercooler or it may not join at all, instead each turbo may be feeding a separate intake manifold.
Now when it comes to V6 and V8 engines a parallel setup has many benefits. The first benefit is that dramatically simplified and reduced exhaust piping.

So then why are all the 2jz, RB, LS and other engines which are often used for making big power all running big single turbos and not twin turbos? Well the main reason behind this is that OEMs have incredible R&D capabilities at their disposal and prioritize driveability whereas car enthusiasts do not. What I'm trying to say is that the big single is the easiest setup to get right when chasing big power.

However, twin turbo setups still face the same compromise of the single turbo. While they ease packaging costs and help preserve responsiveness and efficiency by allowing shorter and simpler piping they ultimately can't escape the turbo size choice compromise, they're just splitting it in half.

But a sequential twin turbo setup promises to achieve both, a very low boost threshold and great low rpm performance together with power at the top. Originally invented for the Porsche 959 to get rid of the sudden onslaught of boost present in previous Porsche Turbo models the sequential system gets rid of the compromise by spooling up the turbos in sequence, so one after the other instead of both at the same time.
A common misconception is that a sequential turbo system consists of one small and one large turbo. This doesn't have to be the case and a sequential turbo system can feature two turbos of the same size. What makes a system sequential is not the size of the turbos, but rather the fact that they are spooled and after the other and not at the same time as in the parallel system.

Now this is where all the confusion begins because the terms compound and sequential are often used interchangeably and they're really not interchangeable because not every sequential system is compound but every compound system is sequential.
The way to differentiate between the two is this. In a system that is only sequential we have one turbo spooled before the other but both turbos feed into the intake manifold of the engine. In a sequential compound system one turbo is spooled before the other but one turbo also feeds into the other turbo, and one turbo is always larger than the other. Proper compounding cannot be achieved with two turbos of the same size.

So how does the system work? Well we have two turbos, one small and one large different people will call different turbos primary and secondary so you can largely ignore that distinction. What you have to remember is that the smaller turbo is always the high pressure turbo and the large turbo is always the low pressure turbo and the large turbo always feeds into the smaller turbo.


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#d4a #boostschool #twinturbo

00:00 The problem with singles
03:06 Parallel
09:39 Sequential
15:00 Compound