Is this just a question of VW Ag not wanting to cause brand name confusion with the 911 twin turbo? Or is there an actual difference, like awd vs. 4wd? <p>Thanks!<p>-Stu Koch<br>99 2.8 qts

Hey Stu,<p>Often, when a manufacturer uses the term Twin Turbo, they mean that there are two turbos placed one after another (sequential). A smaller turbo spools up faster to help low end response, while the other turbo kicks in at higher rpm. This setup is used in the Toyota Supra Twin Turbo for instance, as well as the Mazda RX-7. Is this also the case for the Porsche?<p>In the S4's case, the two turbos are being driven by the two sides of the "V" configuration 6 cylinder independently. Basically there's a K-03 at the end of the exhaust manifold on both the left and the right side of the engine. This is what Audi means by Bi-Turbo.<p>This is as much as I know. Anyone else wishing to join in?<p><br>James Wu<br>A4 Club of Vancouver<br>1998 Black/Onyx 1.8tqa<ul><li><a href="http://vancouver.a4.org">A4 Club of Vancouver</a></li></ul>

The 993 turbo has the same layout as the S4. One turbocharger for each cylinder row. The 959 on the other hand has a sequential system if memory serves me.<p>AT

There is no difference, bi-turbo is just another term for twin-turbo. For instance, the late model RX-7's and Supras do use a sequential setup, but the late model 300ZX, also called 'twin turbo' had a single turbo per cylinder bank. The 993 Turbo has a sequential setup I believe... <p>Sequential turbo systems almost always have the advantage on the street. The smaller primary turbo spools almost instantaneously. Think of a sequential twin turbo as a 1.8T with ANOTHER big turbo that comes online at around 4000 rpm. These engines tend to have a pretty torquey and linear curve up which tends to start flattening off around the mark where the second turbo comes online. This is because the tiny primary turbo is running out of breath. Then there usually is a short, small drop in power when the second turbo comes online, then a big surge as the second turbo comes online. In many cases, the second turbo is no larger than the first, so it comes online pretty quickly too.<p>With a standard twin turbo system, as on the 300ZX, you get a laggy bottom end at low speeds, but a big surge in torque when they both get going, usually around 3000 rpm. This setup actually produces more torque at lower rpms than a sequential setup because you're not waiting until 4000+ rpm to use both turbos - but you have turbo lag. When you are at highway speeds, however, lag is diminished and the advantage goes to the standard twin setup.<p>Finally, an advantage goes to the standard setup in engineering. This setup is just like a regular single turbo setup, except the exhaust from each head powers it's own turbo. The sequential setups require a much more complicated wastegate and charge gate valve control system, which incidentally, is one of the achilles heels for the RX-7 twin turbo (around 72 vacuum lines, alot of which control the sequential turbo system - the hoses tend to get brittle and pop off under boost and oops! you lose your second turbo altogether).<p>Brad Franklin

Those of you worried about lag in the S4, the S4 will probably use a similar setup to the 1.8T. It won't be sequential, but the S4 will probably use two TINY turbo's that give you minimal lag at the sacrifice of ultimate top end.<p>Brad

(joke)