In 2013 a new NA owner asked which part of the standard exhaust is most restrictive, and when the standard backbox becomes a bottleneck. The answers are owners' estimates and recollections, not controlled tests. See also NA exhaust upgrade and Exhaust manifold options.
Where the restriction is
| Part | What owners said |
|---|---|
| Manifold | One owner calls the standard one rubbish and says you could gain 5 to 10 bhp changing to a 4-1. Another says the manifold and the U-bend are the key parts and everything else is minor increments. An NA owner who fitted a 4-1 and had the pre-cat removed at the same time reported a very noticeable increase in mid-range torque, and the engine revving out better. |
| Downpipe | Said to be too small. |
| Cat | One owner says the standard cat is restrictive and you could gain 2 to 5 bhp with a sports cat and 2.5 inch pipework. Another suggests swapping the 400-cell cat for a 200 or 100-cell one. A third cut out the main cat and welded in a 200-cell sports cat at an early stage and says it made a noticeable difference; he puts the cat at about £80 plus welding. |
| U-bend (between cat section and backbox) | Very restrictive, according to one owner who was surprised how much difference a 2.5 inch U-bend made. |
| Backbox | One owner who fitted a 2.5 inch Tullet backbox after a manifold swap says it didn't do much for performance, but the engine revved better and felt de-restricted, "almost like having a couple of kg off the flywheel". |
One owner who tried it in stages said a 4-1 works well with the standard inlet: the mid-range stays solid and the manifold helps the top end. He says he likes the standard car's mid-range torque and that a 4-1 keeps it while helping the top end. He followed a Cup 240 round Snetterton and says there was little between the cars, with his mid-range so much better that he had to lift in places.
Turbo backbox on an NA
A few owners swapped the NA backbox for the Turbo one. One calls it a straight swap that should flow better. Another says you also need the Turbo U-bend, and then it bolts straight on. A third says use the Turbo one because it is bigger in diameter and has the flange in the same place.
Power figures people quoted
- One owner's 2.4 inlet and remapped ECU, with a decent exhaust, dynoed at about 170 bhp.
- Supercharged cars: 249 bhp with a standard manifold, a VSE backbox and the pre-cat removed (loud), and 249.8 bhp with a Piper manifold, 2.5 inch downpipe and a Turbo backbox ("nice and quiet"). Another owner says 240 to 250 bhp is not uncommon on a supercharged car with the standard manifold and a Milltek exhaust.
- One owner says the Piper manifolds were prone to cracking, at least the one he had lying around.
Practical points
- One owner says you can get better results from the intake than the exhaust, and advises reading up on it. Another keeps the standard airbox but modifies it rather than fitting a K&N, though the cone filter sound is addictive to its owner.
- Most NA tuners in the thread went on to supercharge. One owner calls the supercharger conversion the most cost-effective upgrade and says to make sure any exhaust changes suit a later supercharger. He thinks exhaust changes should be fine. See Supercharged exhaust advice.
- Tullet parts: one owner asked whether they bolt straight onto the standard sections or need adapters. The thread doesn't record an answer.