NZ Performance Car - - Tech Guide -

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.

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