Exhaust for a supercharged NA: what is worth fitting

An owner with an Eaton M72 supercharger, a 75 mm throttle body, 470 cc injectors and a side chargecooler asked what exhaust to fit for about 260 hp, mostly for track use (70% track, 30% road). His questions and the replies are below, with figures from other supercharged owners. They are one-off reports, not tests.

The questions and the answers

• How big? The owner was unsure between 63 mm (2.5 inch) and 70 mm (2.75 inch) outlet, with 44 to 45 mm primaries. One owner says 2.5 inch is more than good enough for 260 hp, and runs a Tullet manifold with the original downpipe, a Turbo cat, a Turbo backbox and U-bend. Another is on 2.5 inch with 335 bhp.
• How much power is realistic? One owner says with stock internals 260 bhp is the limit if you don't want to break anything, and that you can get 250 with just a standard M62 and a stock engine. He says the extra 10 bhp costs relatively a lot.
• Long-primary custom manifold versus a ready-made Tullet? The same owner calls the difference "peanuts on performance", and says it is about the longevity of the system. Going from a 2.25 inch line with a decat to a full 2.5 inch Tullet was called a negligible difference, and just more noise.
• 4-1 or 4-2-1? The poster believed a 4-2-1 moves torque down the rev range, which matters less with a supercharger, and he preferred the 4-1 for easier fitting. Nobody in the thread compared the two.

Cheap routes that owners say work

• Remove the standard pre-cat section and replace the backbox with the Turbo one (2.25 inch). The owner who suggests it says it ends up quiet.
• Fit a 2.5 inch Tullet manifold only, as the standard cast one "gets a beating" and cracks.
• One supercharged car with a Tullet manifold, standard downpipe with the cat removed, and a Turbo U-bend and backbox measured 89 dB at a UK track and made about 252 bhp on the Dutch software power test. The owner says it doesn't sound brilliant but is good enough.
• One owner with a standard manifold and cat on a supercharged car at about 250 hp has a Larini backbox and finds it too loud for most UK tracks. See Exhaust backbox choices and noise.
• For a decent road Stage 2 supercharged car, one owner suggests a Turbo muffler and a bigger-bore downpipe without the pre-cat.

What breaks

• The standard cast manifold cracks. The Tullet manifold can also crack after quite a few track days, according to one owner.
• A full Tullet system by default uses a ceramic cat, which one owner says can be damaged after a few years of track use. A steel HJS cat can be welded in on request, but is expensive.
• A Dbilas manifold and 2.5 inch downpipe with a 200 cell cat made one owner's car louder than before (with a VSE backbox). One owner asked whether anyone had reported a cracked Dbilas manifold on a supercharged car; the only reply was that the supplier says not to expose it to temperatures above 900 degrees C. Nobody reported one cracking.
• The manifold of a supercharged car is said by one owner to see close to 1,000 degrees. See Exhaust manifold options and Exhaust manifold heat wrap and shielding.

Earlier supercharged figures (2013)

One owner got 249 bhp with a standard manifold, a VSE backbox and the pre-cat removed (loud). Another got 249.8 bhp with a Piper manifold, a 2.5 inch downpipe and a Turbo backbox ("nice and quiet"). One owner points out that Piper manifolds were prone to cracking, at least his. Another owner says 240 to 250 bhp is not uncommon on supercharged cars with the standard manifold and a Milltek exhaust.

See also NA exhaust tuning: which parts restrict.