Tubular manifolds are the most common aftermarket manifold type available. Tubular manifolds come in various lengths, sizes and flange types. Equallength manifolds are great for most applications, and runner lengths can be fabricated to suit your needs, whether it be short for quick boost response with the trade-off being less top end, or long runner for high top end, but less response. Tubular manifolds come in two main designs — twin scroll and single. The single-scroll manifold is the most common of the tubular manifolds. The exiting exhaust gases all merge into one collector, and depart through a single exit flange which the turbo is bolted to. A twin-scroll design is a completely different beast. The entire manifold could almost be seen as a component part of the twin-scroll turbo set-up it will end up being bolted to. Twin-scroll turbos have a very unique turbine housing design. The housing is divided, which greatly improves turbo response. The manifold design is also unique, it’s built so that the runners are paired at 180 degrees apart from each other in terms of their firing order. They’re designed this way to ensure there is very little interference from another cylinder’s exhaust gases. Twin scroll set-ups offer the best boost response by far, but sacrifice peak power numbers. For example, Mike did back-to-back testing with Carl Ruiterman’s SR20DET-powered drift car a few years ago, and found that boost came on 500rpm earlier, but 20kW of peak power was lost up high in the rev range.
Two external wastegates are needed for this set-up as each pair of runners has to remain separated, and that drives the price up. So if you have a bigger budget and want the absolute most out of your set-up, twin scroll is the best option in terms of boost response, but for most applications a single scroll set-up will make great numbers.