In stock form the twin turbos operate sequentially. One spools first, quickly. Pressure from that one helps spool the second, and at a certain point it comes online and both turbos work together. Proper operation is verified by a “boost pattern test” done under wide open throttle in 2nd and higher gears. This test should show 10psi by 3000rpm when the primary turbo spools, a slight drop to 8psi around 4000-4200rpm at the transition, and a surge back to 10psi by 4500rpm when the secondary turbo comes online. This is called a 10-8-10 boost pattern.

This is a great way to increase response and power, but it has it’s drawbacks. The biggest of which is that it rarely works 100% properly for very long at a time. The system is controlled by a maze of vacuum solenoids, check valves, hoses, plugs, and actuators. IF any one of these parts is not 100%, the system will not boost properly at all times and power will suffer. This makes the car difficult to predict and a lot less fun when you aren’t making 100% of the power you should be.

Other problems associated with stock sequentials are the transition lag followed by a hard surge of power, which can actually cause an inexperienced driver to crash the car. And you usually see some boost “bleed” off above 6000rpm, back down to about 8psi.

As a result, some people choose to go non sequential. The same turbos are retained, but the controls are all removed. This allows them to operate “naturally” in a parellel mode. They both spool at the same time, and they both continue to work 100% until redline. The result is poorer low rpm response (boost comes on slower, usually around 4000rpm) but a much more linear, predictable, and stronger powerband from there all the way to redline with little to no boost “bleed”. In other words, the car is slower from a takeoff, but once the boost comes on, it hits hard and doesn’t let off. There’s no more worries about transition surge in the middle of a turn, and no more worries about the sequential control not working properly.

Once you convert a set of turbos/manifold to nonsequential, you can’t go back to sequential operation later, because some of the welded-in flapper doors have to be cut out to do the conversion, and they cant be put back once removed. To convert a car back to sequential you would need to find another stock set of turbos and manifold with the flappers still in place.

If you intend to keep the stock restrictive catalytic convertor and/or catback, nonsequential might not be best for you. The more exhaust restriction present, the slower the NS turbos will spool.

For the FD engine, half of the junk under the manifold is for emissions, and half is for the sequential twin turbo control system. In my mind, there isn’t much merit in removing only half of the stuff under there, so I usually recommend that people ditch it all, or leave it all stock. That is, go with a no-emissions AND non-sequential setup and eliminate all the hoses/solenoids/etc. possible. IT is possible to JUST remove the emissions while leaving sequential controls intact. It is possible to JUST remove sequential controls while leaving the emissions intact. I suggest that you read up on my writeup about the emissions removal elsewhere on this page for details about that, so you can make a more informed decision.