* Pulse Interference: 4-2-1 manifolds are designed to merge exhaust pulses in a way that creates scavenging effects in naturally aspirated engines. This improves low-end torque. However, turbocharged engines already rely on the turbocharger to scavenge exhaust gases. The carefully timed pulses of a 4-2-1 might interfere with the turbo's ability to efficiently spool and build boost. This is especially true at lower RPMs.
* Reduced Exhaust Velocity: The merging of exhaust pulses in a 4-2-1 can reduce the overall exhaust gas velocity compared to other designs, like a 4-1. A lower exhaust velocity can reduce turbocharger efficiency.
* Engine Specifics: The ideal exhaust manifold design is highly dependent on factors like engine displacement, camshaft profile, turbocharger size, and intended powerband. A 4-2-1 *might* work well on a very specific setup, but it's less likely to be optimal compared to other designs.
* Better Alternatives for Turbos: For turbocharged engines, a 4-1 manifold or even a dedicated equal-length header design are often preferred. These designs typically provide better pulse separation, maintaining higher exhaust gas velocity and improving turbo spool. Equal-length headers are particularly effective at minimizing pulsation interference and maximizing consistent flow into the turbo.
In short: While not automatically a performance killer, a 4-2-1 manifold is usually not the best choice for a turbocharged engine. It's more likely to hinder performance compared to other manifold designs specifically optimized for turbo applications. The potential negative effects on turbo spool and overall efficiency are significant enough to generally make it a less desirable option.
