I simply suggest that the evil gremlin of backpressure in the exhaust system (the exhaust system = anything added after the optimum header or optimum charger outlet plus a possible optimum downpipe) will, and does, in general, limit the overall possible performance potential, unregarding if it might be a NA, or a boost application.
And I also suggest that whatever features/specs that might be added behind the/a optimum format header on a correctly combined, built & set engine, you seldom find any gains...
The key issue is simply to have a design that allow for the least overall losses.
[Even the sort of conservative Volvo oem engineers suggested that a 60mm exhaust on a NA-mode 112bhp B23A was worth +10bhp some 10-15 years ago...]
In normal NA-race applications we're in general quite happy when our customers accepts to build a as straight as possible 100mm x100mm (4"x4") "tunnel" after the header (adapted in the apropriate manner) which, after the rear end, exits into a carefully designed 2 x 3" pipes with equally carefully selected 3"muffler (100dB noise race regulations are common here in Sweden...) and then finally exiting the exhaust system.
This format has proven to yield small-to-none torque/bhp losses, and when trying with, and without, the exhaust system are the needed/optimum a/f & ignition settings typically unchanged.
In a boost application I simply suggest that as soon as the last "wheel" has been passed in the charger has the "exhaust system" started.
Hence does the format of the exit of the charger matter as well as both the radii as well as the dia of the "down-pipe" matter, as well as the rest of the system.
So, in general, and obviously unregarding such practical issues like noiselevels, heat and exhasut fumes, etc; after the best format of the charger exit and the needed (?) optimum format of dia & radii of the added downpipe there are rarely no gains to be found, mainly losses.
Obviously with the proper settings of fuel & ignition vs load & boost in EACH comparisson case.
And the noble art of avoiding this correct m.o. comparison procedure is often the case when oem or smaller (read "too small") exhaust systems
seemingly produce better performance than systems with less pressuredrops...
Best regards
Mike
M.Aaro@mail.bip.net
Lule? (northern part of Sweden)
NEW sept '03 files: Gr10/Engine-kit, Gr11/Head & the EMS/T-file [Zip + PDF]