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Discussion Starter · #1 ·
I Know most of you Supra guys wouldnt care to read it but IDC!!!!....So anyways I've been seeing this Around SupraFourms and Rx7Club Whats The Differences Between the Two Sequential Twin Turbo Set-up....So to save time i got this off An Rx7 Fourm that i thought might be helpful.....

"For a long time I’ve been wondering about the differences between the Rx-7 sequential twins and the Supra sequential twins. Both systems were originally designed for luxury cars (Mazda Cosmo and Toyota Aristo/Lexus GS) and then adapted for sports car use. So are the 2JZ sequential turbos simpler than the 13B/20B sequential turbos? Are they inherently more reliable? Having developed the 2JZ turbo system after Mazda, what did Toyota do differently and did Mazda “miss” something in their system design that Toyota figured out later?

To answer these questions, I searched through Mazda and Toyota technical papers on the respective engines. I searched though technician training/service highlights documents. I looked through workshop manuals and websites. I also physically examined the actual components in-person.


The Short Answer

Both systems have the exact same number of actuators and each actuator has basically the same overall purpose. The 2JZ has fewer solenoids and less plumbing compared to the 20B/13B design, while the actuators themselves are simpler. The 2JZ sequential twins are hardly bulletproof but they do have less failure points. However, the design of the 2JZ sequential turbo system inherently requires more space, and thus its architecture may not have even been possible with the packaging constraints of the Rx-7 and Cosmo.

Overview of how each engine's turbo system works

I know this is going to be hard to follow, but here is the basic idea of how the 13B-REW twin turbos work. As the driver starts from low rpm all exhaust gas flows into the primary turbocharger. As rpm increases and boost begins to reach a predetermined level, exhaust is diverted away through a small valve upstream of the secondary turbo. This valve regulates boost while allowing gases from the exhaust manifold to feed the secondary turbo.

As the secondary turbo spins up, on the compressor side a valve prevents its pressurized air from entering the engine. Instead another valve in the Y-pipe diverts that compressed air back to the intake until the secondary turbo is fully ready. When it is time for both turbos to run together, yet another valve opens upstream of the secondary turbo exhaust housing in order to feed it as much exhaust gas as possible. The compressed air from the secondary turbo compressor outlet is then allowed to flow into the engine. Finally a wastegate becomes the sole means of controlling boost by diverting exhaust gas upstream of the primary turbo and out towards the downpipe. The basic operation of the secondary turbo is controlled by two valves upstream of the exhaust side, before the turbine inlet."









"Here is the basic idea of how the 2JZ twin turbos work as the driver starts from low rpm. At first only the No. 1 turbo is active. The No. 2 turbo remains inactive by completely choking off the exhaust stream with two valves downstream of the No. 2 turbo. Unlike the 13B, on the 2JZ-GTE there are no valves upsteam of the No. 2 turbo to block exhaust from entering the turbine housing. As rpm increases and boost begins to reach a predetermined level, a small wastegate-sized passageway opens downstream of the No. 2 turbine housing. This valve controls boost because the wastegate is still closed. Some exhaust can now flow out of the No. 2 turbo and the turbo will begin to spin up. As the No. 2 turbo spins up, on the compressor side a valve prevents its pressurized air from entering the engine. At the predetermined time, a second and larger valve downstream of the No. 2 turbine housing opens up to allow full exhaust flow from the turbine housing. The secondary turbo comes online completely, and pressurized air from the No. 2 turbo is then allowed to enter the engine.

Note that unlike Mazda’s system, the 2JZ engine’s No. 2 turbo compressor side is never vented back to the airbox during prespool. This implies that the No. 2 turbo intentionally compressor surges during prespool—it builds boost bu the air has nowhere to go. This compressor surge continues until its boost pressure exceeds that of the No. 1 turbo and a valve opens in the intercooler piping. I know this whole compressor surge part is hard to understand and on its face completely counter-intuitive. However according to the literature Mazda engineers carefully engineered the REW sequential turbo system to also intentionally run in surge during prespool. As far as I know this has not been confirmed in real-world testing on production rotary cars.

The basic operation of the No. 2 turbo is controlled by two valves downstream of the turbo, after the turbine inlet in what we would normally consider the path of the exhaust."








Heres a Pic showing the 2JZ turbo system and most of its actuators



Heres the Chart comparing the two



"That is a very detailed and comprehensive chart with more information than most people are interested in. So here are the most important differences in actuator design. Note that all of the 2JZ’s main actuators for sequential turbo control run only on pressure; there are no vacuum-controlled actuators.

1) On the 2JZ, the big valve controlling the exhaust side of the No. 2 turbo is called the Exhaust Gas Control Valve. This is a butterfly valve (like a throttle plate) downstream of the turbo which chokes off exhaust flow. Toyota’s implementation of this design requires more space than what Mazda used for the same purpose. On the REW engines, the equivalent valve called the Turbo Control Valve is like a big wastegate flapper. It lies upstream of the turbo and chokes off the feed gases. The REW’s actuator requires both pressure and vacuum to operate.

2) On the 2JZ, the small valve controlling prespool is called the Exhaust Bypass Valve. It lies downstream of the turbo and in its normal closed position it chokes off exhaust flow. When this valve opens, a small amount of exhaust flows out from the No. 2 turbo through a small separate pipe so it can join the exhaust stream of the No. 1 turbo. Toyota’s implementation of this design requires more space than the design Mazda used to accomplish control of prespool using the REW’s precontrol valve. Just like the Precontrol Valve on the REW engines, the Exhaust Bypass Valve controls boost until both turbos come online. On both the Toyota and Mazda systems the valve and actuator are controlled like a wastegate.

3) On the 2JZ, during prespool the No. 2 turbo’s pressurized air does not flow back to the airbox like on the REW engines. During prespool the Reed Valve (as in a reed for a musical instrument) allows the No. 2’s pressurized air to enter the engine by going around the closed Intake Air Control valve. The pressure from the No. 2 turbo must exceed the pressure from the No. 1 turbo for the Reed Valve to open. Diagram of the reed valve"

 

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Discussion Starter · #2 ·
"Advantages of the 2JZ Sequential Turbo System

The 2JZ’s sequential turbo system has the following advantages compared to the system used in the REW engines:

1) Fewer and simpler solenoids – the 2JZ uses four 2-port solenoids. When I say 2-port solenoid, think about the stock wastegate and precontrol solenoids on the REW engines. The REW engines uses three 2-port and three 3-port solenoids. By 3 port solenoids I am referring ot the solenoids found in the rat’s nest. In my personal experience I see 2-port solenoids fail less often than 3-port.

2) No vacuum chamber and no separate check valves are used for the 2JZ turbo system. The 2JZ has a pressure chamber with presumably an integrated check valve. While this tank does leak sometimes, overall there are less failure points in the system.

3) Fewer vacuum lines -- fewer solenoids and simpler actuators require fewer hoses to control.

Conclusion - Why Didn’t Mazda Use the 2JZ’s system design?

Mazda designed their sequential system in late 1980’s originally for the Cosmo luxury coupe. This was after the sequentially turbocharged Porsche 959 supercar but before the Toyota Aristo/Lexus GS and Subaru’s twin turbo Liberty/Legacy. When Mazda was developing their system, Sequential twin turbos had never been done in a mass production car so they had very little real-world data on reliability. For cost savings Mazda had to adapt or re-use existing parts and systems. Some of the vacuum systems and actuators had clearly been modified from the older rotary engines. Mazda appears to have used off-the-shelf 3 port vacuum solenoids—they just added to the rat’s nest found in previous generation rotary engines.

So why didn’t Mazda use Toyota’s design and control the secondary turbo after the turbine outlet instead of before the turbine inlet? Part of the exhaust side of the REW turbo system came from modifying the variable turbine A/R system used on 86-88 model turbo 13B engines, which allowed for cost savings. Using a Toyota-style system may have been more reliable because it has simpler plumbing. The chief disadvantage I see of the 2JZ’s system is packaging. The Exhaust Bypass Valve, Exhaust Gas Control valve, and their associated exhaust piping require more space than Mazda’s compact Precontrol Valve and Turbo Control valve. Rotary-powered cars usually have the engine mounted low and behind the front axle so little space is available. Mazda already had to carefully shape the lower intake manifold on the Rx-7 13B-REW to fit the turbos.

Looking back at it, for the series 6 Rx-7 the engineers did the best they could given their many constraints. They had to use the turbo system that had already been developed for the Cosmo. The revised solenoid rack on later engines supposedly helped reliability but the new parts still failed over time due to other underlying issues such as heat. With the cancellation of the Cosmo and the failure of the Rx-7 to achieve commercial success Mazda would not devote much money to making the turbo system more reliable. The 90s are long over now; on modern gasoline engines twin turbos are coming back but sequential turbo systems are conspicuously absent from these new downsized engines. This is due to the cost and complexity inherent in systems like the ones described in this article."
 

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Very, very cool read!!!!! Thank you for posting.
 

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wow, thanks for putting that together so well. Awesomely informative!
 

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Nice write up!! Very interesting to read through. This still amazes me even though the technology is over 20 years old now.. Thanks for sharing!
 
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