Parallel vs. Sequential vs. Compound - Twin Turbo systems explained - Boost School #11
853,003
Published 2022-09-25
AEM ECU: bit.ly/D4Ainfinity5
AEM digital racing dash display: bit.ly/D4Acddash
Support d4a: driving-4-answers-shop.fourthw...
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.
A special thank you to my patrons:
Daniel
Daniel Morgan
Pepe
Brian Alvarez
Jack H
Dave Westwood
Joe C
Zwoa Meda Beda
Toma Marini
Nelson
#d4a #boostschool #twinturbo
00:00 The problem with singles
03:06 Parallel
09:39 Sequential
15:00 Compound
All Comments (21)
-
AEM ECU: bit.ly/D4Ainfinity5 AEM digital racing dash display: bit.ly/D4Acddash Patreon: www.patreon.com/d4a Support d4a: driving-4-answers-shop.fourthwall.com/ Versatile: amzn.to/3OpMSRU
-
As a civil engineer that started out as a mechanical engineer. This guy is phenomenal. His explanations are so engineer processed. Thorough through, and through, step by step, and still technical but not confusing. To me it’s like intellectual crack for my brain.
-
Babe wake up D4A posted about turbos
-
I LOVE THE DIESEL CONTENT. So hard to find modern, reliable info with this level of technical detail.
-
Video suggestion: guide to tire dimensions and their effect on grip& performance.Many youtubers will simply say wider tire=more rubber contacting the ground which means more friction and grip.But friction doesn't depend on the contact area because a smaller area will have more pressure and a bigger one will have less,so the overall friction will be the same for both considering mass of the vehicle doesn't change.I would like to hear an explanation from you
-
Really like that you mentioned cylinder mixing/interference being better avoided in some configurations for turbo response and engine efficiency. I would argue that is a large factor that many people skip over when looking at mid/low rpm power/spool differences
-
interesting note on compound turbo setups, you can do it using the same size turbos, but you need 3 of them; 2 to act as low pressure turbo, 1 to act as high pressure.
-
I'm a huge car guy and also big into physics and I like this channel a lot because it gives me the fundemental detail I'm looking for. Love the comments the discussions and the overall community here.
-
Loved the video! I’d love to see one now on “twin charging” or using both a supercharger and a turbocharger.
-
I am an educated car guy, but I still learn. And you help me to tell people such things in a quite easy way. Thanks!
-
As someone who works as a diesel engine developer, I love the way you explain this topic. The only thing you're kind of missing (propably due to not confuse the heck out of your viewers LOL) is that modern compound setups use a bypass valve for the high pressure compressor, which will "turn off" the high pressure turbo at high pressure ratios, making the system practically function as a single stage turbo only (the low pressure turbo).
-
Man the fact that I was totally focused throughout the whole vid makes me feel like a liar when I don't focus on studying
-
Great video as always! I feel like I have a good understanding of single and twin turbos but sequential and especially compound turbos, I did not understand as well. I always thought the smaller turbo in a compound set-up fed the larger turbo. I did not realize that a smaller turbo can still increase boost from a larger turbo feeding it. Thanks again for the video!
-
Frikking love it how you explain things!A perfect learning curve from simple into more complex examples,no unnecessary bs or dumb jokes and no ego sauce either! Thank you <3
-
Another awesome video from D4A! I always learn something new from your videos. Congrats on 500k subscribers! You've earned it! You'll hit 1 million in no time if you just keep up the good work. Thank you for keeping your content efficient and to the point, without fluff, filler or excessive ads
-
Absolutely awesome video, as usual. One thing I'd like to see everyone - not just you - embrace though, is clearly stating "peak power" instead of "power"; because - for example at 7:38 - two setups with "the same power" probably only make the same peak power, but one has greater area under the power curve overall (e.g. higher power at some RPM that isn't the peak), meaning it actually makes more power overall- many people don't seem to realize that's a thing.
-
FIAT/Lancia triflux was also an inline 4 with radial valve setup that had parallen twin turbos each located on the opposite sides of the engine. There was both intake and exhaust on both sides of the engine due to the radial valve setup effectively "turning the inline 4 into a v8" as far as intake and exhaust piping goes
-
instead of mufflers just keep adding compound turbos until every last bit of exhaust energy is captured!
-
I'm a Diesel mechanic from Iran. I have never worked on anything remotely "advanced", because they don't exist in our country or at least are not frequent. But I always had a theory: if small turbo --> lower RPM activation + lower boost and large turbo --> higher RPM activation + higher boost, why we don't invent a system that I call "a turbo for a turbo". It works in a way that we put a tiny and very sensitive turbo that will spin immediately with the lowest amount of exhaust gas before a larger main turbo. That small turbo won't feed the compressor side of larger turbo, but the exhaust side of larger turbo. The large turbo will suck the air from air filter in traditional way. The exhaust side of larger turbo will be also fed by exhaust manifold. There will be a shut-off gates before exhaust sides of both turbos. This system will work this way: we step on fuel pedal, engine revs slightly, small turbo goes crazy and will feed the larger turbo with high air pressure (the exhaust manifold that feeds the same spot in larger turbo is closed now to prevent back pressure) so that it starts to spin before the engine would spin it at normal conditions. Now that the large turbo is kicking in and exhaust pressure is going up, the exhaust manifold that leads to small turbo will be shut (to prevent the turbo from exploding) and the valve before the exhaust side of big turbo will open and big turbo which has enough speed now will be fed by the exhaust fumes as it was intended. But there will be no lag, because the big turbo is already spinning. Please let me know what you all think about this
-
Excellent break down of how both turbo systems in general work, and how the difference between the various combinations of multi-turbo systems work!
