Alright, so here’s my thinking.
Let’s say that in my perfect little theoretical world there’s so much fuel and intercooling for the engine to never hit a boost/fuel cut or detonate (man my theoretical world is awesome). Let’s also assume that my bypass valve on my Super Turbo fails and is stuck permanently closed. What we end up then is a compound charged system. Now the turbo comes first in our system and the wastegate is operated off of vacuum directly after the compressor, so the wastegate will always open at 15 psi (my perfect world allows me to change the wastegate opening pressure for the sake of my math, haha). This means that the turbo, at full song, has a 2:1 compression ratio. The absolute pressure is 30 psi in the pipe. Boost is defined as the pressure above ambient so that math is:
(ambient pressure * compression ratio) – ambient pressure = boost pressure.
(15 * 2) – 15 = 15
Let’s say that we toss the turbo for a few minutes and run the supercharger, unregulated, by itself. Now I’m taking a wild stab here, but I’m guessing that the supercharger, unregulated pumps out say 15 psi at 2000 rpm and up to (to pull numbers from nowhere) 20 psi at 6000 rpm (oh, right, in my perfect little theoretical world the supercharger clutch never kicks off). That means that the supercharger starts at a 2:1 compression ratio and rises to a 2.3:1 compression ratio. So, let’s now put that turbo back on. Here’s where things get interesting. If my thinking is correct neither the turbo or the supercharger care about how much pressure they can create, they only care about how much more pressurized the air leaving is than the air coming in. So, let’s say at 2,000 rpm our little turbo is starting to spool and is creating 5 psi of boost. The supercharger is creating 15 psi of boost. The natural thought would be that the combined pressure would be 20 psi, but if my math is right, the actual combined pressure is 35 psi! Holy crap! What happens at 6000 rpm? The turbo goes into full song and starts pumping 30 psi of absolute pressure into the supercharger. The supercharger sees the turbo’s 30 psi of absolute pressure and raises it to the tune of 2.3. Now we have an absolute pressure of 69. Subtract ambient pressure and you get a piston shattering 54 psi!
So once again the math is:
((ambient pressure * first compressor compression ratio) * second compressor compression ratio) – ambient pressure = boost pressure
((15 * 2) * 2.3) – 15 = 54
So if I got this right, you’d hammer the throttle watch the gauge hit 15 psi of supercharger boost, then the insane exhaust flow would spin the turbo up and you’d watch that 15 psi very quickly rise past 20 psi, then past 30 psi, then past 40 psi and then you’d have to pull over and pick up all the pieces of your engine you just scattered all over the road.
Okay, there’s a good chance I got all this wrong and I’m mixing supercharger characteristics with turbocharger characteristics, but if this were instead a twin turbo compound setup, that logic should be sound, allowing you to create insane amounts of boost with two relatively small turbos.
The reason I went through all that (yes, there is an ulterior motive here) is I was trying to figure out why you have to change pulleys on the AW11 supercharger to see 20 psi, but I saw 20 psi peg on my gauge with just a few mistaken vacuum hoses. As I was playing around with boost (while my vacuum hoses were still wrong) I found that the supercharger could easily slap out 15 psi, but the turbo would soon start making the coolest noise ever (think Group B Rally Car with a modern day muffler on it) and 15 psi would become very difficult to hold, requiring me to come off the throttle bit by bit. My only explanation for this is that my mistaken hose placement caused the bypass valve to never see the proper boost from the supercharger, making it think that the supercharger wasn’t producing any boost, so it would stay closed, turning the system into a proper compound charge setup like I described above. This caused the supercharger to boost as hard as it could and the increased exhaust pressure coupled with the massive draw of air from the supercharger caused the turbo to spool up much faster than normal creating that interesting compound effect where the supercharger is further pressurizing the pressurized air it’s receiving.
So, let’s assume I take a VG30 (cause I love that engine, ya, I’m a Z31 guy too) and run an AW11 Supercharger on it. Piping would be a nightmare (T25 Turbo – Intercooler – Supercharger – Aftercooler – Intake manifold) and the AW11 Supercharger is a pretty small unit meant for boosting a 4AG, so that probably invalidates everything I’m about to say, but let’s pop back into my theoretical world for some fun. The T25 cracks open at 7 psi, that’s a 1.466:1 compression ratio for our first compressor. The AW11 peaks at peaks at 8 psi at 6,000 rpm, that’s a 1.533:1 compression ratio for our second compressor. A little bit of math:
((15 * 1.466) * 1.533) – 15 = 18.7 psi of boost at peak revs.
The VG30 shrugs off 18 psi like Chuck Norris shrugs of… everything. There’s plenty of people in the Z31 world running 18 psi of boost with a simple intercooler and the T3 turbo. Let’s say we’re at 1500 rpm though. The T25 turbo is just dreaming of boost but the supercharger is already putting out 4.5 psi. This 4.5 psi raises the revs quicker and increases exhaust pressure making the turbo spool faster. You could theoretically have 15 psi of boost before 2500 rpm. The T3 turbo is still fighting the good fight struggling to see even 1 psi at that rpm. On a big burley single cam V6 that much boost that soon equals tire-shredding fun.
This all sounds too good to be true and when something sounds too good to be true, I generally end up looking like an ass (although sometimes I think the two are only loosely related cause I end up looking like an ass far more often than things are too good to be true…). So, my question to those of you who held out throughout this damn near dissertation of mine is, what did I miss? Aside from piping, fuel and space issues, this compound twin charged idea seems like the way to go, so why isn’t it more popular?
Cheers
David
Let’s say that in my perfect little theoretical world there’s so much fuel and intercooling for the engine to never hit a boost/fuel cut or detonate (man my theoretical world is awesome). Let’s also assume that my bypass valve on my Super Turbo fails and is stuck permanently closed. What we end up then is a compound charged system. Now the turbo comes first in our system and the wastegate is operated off of vacuum directly after the compressor, so the wastegate will always open at 15 psi (my perfect world allows me to change the wastegate opening pressure for the sake of my math, haha). This means that the turbo, at full song, has a 2:1 compression ratio. The absolute pressure is 30 psi in the pipe. Boost is defined as the pressure above ambient so that math is:
(ambient pressure * compression ratio) – ambient pressure = boost pressure.
(15 * 2) – 15 = 15
Let’s say that we toss the turbo for a few minutes and run the supercharger, unregulated, by itself. Now I’m taking a wild stab here, but I’m guessing that the supercharger, unregulated pumps out say 15 psi at 2000 rpm and up to (to pull numbers from nowhere) 20 psi at 6000 rpm (oh, right, in my perfect little theoretical world the supercharger clutch never kicks off). That means that the supercharger starts at a 2:1 compression ratio and rises to a 2.3:1 compression ratio. So, let’s now put that turbo back on. Here’s where things get interesting. If my thinking is correct neither the turbo or the supercharger care about how much pressure they can create, they only care about how much more pressurized the air leaving is than the air coming in. So, let’s say at 2,000 rpm our little turbo is starting to spool and is creating 5 psi of boost. The supercharger is creating 15 psi of boost. The natural thought would be that the combined pressure would be 20 psi, but if my math is right, the actual combined pressure is 35 psi! Holy crap! What happens at 6000 rpm? The turbo goes into full song and starts pumping 30 psi of absolute pressure into the supercharger. The supercharger sees the turbo’s 30 psi of absolute pressure and raises it to the tune of 2.3. Now we have an absolute pressure of 69. Subtract ambient pressure and you get a piston shattering 54 psi!
So once again the math is:
((ambient pressure * first compressor compression ratio) * second compressor compression ratio) – ambient pressure = boost pressure
((15 * 2) * 2.3) – 15 = 54
So if I got this right, you’d hammer the throttle watch the gauge hit 15 psi of supercharger boost, then the insane exhaust flow would spin the turbo up and you’d watch that 15 psi very quickly rise past 20 psi, then past 30 psi, then past 40 psi and then you’d have to pull over and pick up all the pieces of your engine you just scattered all over the road.
Okay, there’s a good chance I got all this wrong and I’m mixing supercharger characteristics with turbocharger characteristics, but if this were instead a twin turbo compound setup, that logic should be sound, allowing you to create insane amounts of boost with two relatively small turbos.
The reason I went through all that (yes, there is an ulterior motive here) is I was trying to figure out why you have to change pulleys on the AW11 supercharger to see 20 psi, but I saw 20 psi peg on my gauge with just a few mistaken vacuum hoses. As I was playing around with boost (while my vacuum hoses were still wrong) I found that the supercharger could easily slap out 15 psi, but the turbo would soon start making the coolest noise ever (think Group B Rally Car with a modern day muffler on it) and 15 psi would become very difficult to hold, requiring me to come off the throttle bit by bit. My only explanation for this is that my mistaken hose placement caused the bypass valve to never see the proper boost from the supercharger, making it think that the supercharger wasn’t producing any boost, so it would stay closed, turning the system into a proper compound charge setup like I described above. This caused the supercharger to boost as hard as it could and the increased exhaust pressure coupled with the massive draw of air from the supercharger caused the turbo to spool up much faster than normal creating that interesting compound effect where the supercharger is further pressurizing the pressurized air it’s receiving.
So, let’s assume I take a VG30 (cause I love that engine, ya, I’m a Z31 guy too) and run an AW11 Supercharger on it. Piping would be a nightmare (T25 Turbo – Intercooler – Supercharger – Aftercooler – Intake manifold) and the AW11 Supercharger is a pretty small unit meant for boosting a 4AG, so that probably invalidates everything I’m about to say, but let’s pop back into my theoretical world for some fun. The T25 cracks open at 7 psi, that’s a 1.466:1 compression ratio for our first compressor. The AW11 peaks at peaks at 8 psi at 6,000 rpm, that’s a 1.533:1 compression ratio for our second compressor. A little bit of math:
((15 * 1.466) * 1.533) – 15 = 18.7 psi of boost at peak revs.
The VG30 shrugs off 18 psi like Chuck Norris shrugs of… everything. There’s plenty of people in the Z31 world running 18 psi of boost with a simple intercooler and the T3 turbo. Let’s say we’re at 1500 rpm though. The T25 turbo is just dreaming of boost but the supercharger is already putting out 4.5 psi. This 4.5 psi raises the revs quicker and increases exhaust pressure making the turbo spool faster. You could theoretically have 15 psi of boost before 2500 rpm. The T3 turbo is still fighting the good fight struggling to see even 1 psi at that rpm. On a big burley single cam V6 that much boost that soon equals tire-shredding fun.
This all sounds too good to be true and when something sounds too good to be true, I generally end up looking like an ass (although sometimes I think the two are only loosely related cause I end up looking like an ass far more often than things are too good to be true…). So, my question to those of you who held out throughout this damn near dissertation of mine is, what did I miss? Aside from piping, fuel and space issues, this compound twin charged idea seems like the way to go, so why isn’t it more popular?
Cheers
David
