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If you want to improve the performance of a 3.9 +8 without major engine surgery, then the mods worth doing are:

  • Replace the exhaust manifolds, and silencers, and possibly tailpipes
  • Fit a free flow air filter, (or fit a K & N filter cartridge inside the standard cannister) there really is not much difference between the options
  • Modify the ecu with a new chip from Mark Adams/John Macdonald or John Eales, Optimax for economy, Tornado or John Eales for performance.

Anything else is really highly debatable, fitting new trumpets, whether carbon fibre or ali, whether shorter or fatter, is not worthwhile unless you also enlarge the manifold and head ports to match, opening up at one end only is not going to improve the flow.

Any other performance is going to need the engine out, a new cam and followers, a lightened flywheel, stronger clutch, balancing all moving parts, porting the head and matching all the ports between the inlet/exhaust manifolds and the head, some stronger bottom end mods, and that is going to cost money.

Submitted by: Andy Garlick

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Exhausts are almost an area of personal preference rather than Hobson’s choice.

Just one or two pointers

1 I think I know the cross over system you are referring to , and I think it has 2 catalysts with a Y piece joining the pipes just behind the cats , can you confirm this?

2 Always use stainless , it will save ever having to do the job again if you fit a reasonable quality.

3 You don’t have to fit any one particular silencer box. I have even heard a +8 running at MogSport with no silencers at all , just the cats, although rather noisier than usual it was not unbearably loud.

4 £1000 does seem a lot for the section downstream of the cats, when a complete Librands system is only twice that.

5 I would steer clear of Kwik-Fit etc unless you really want a bodged system.

6 If you want to spend some money I would look at using the new manifolds being produced by P Mulberry, then use a separate system down each side of the car rather than the cross-over.

7 The top choice for power noise and long life would be either a Librands, or the system sold by Mike Duncan (which I suspect may be the one developed by RPI?) these will cost £1500 to £2000 complete (and fitting them is NOT a DIY job) but will outlast the car and make it sound and go so much better.

8 Do not use the “resonator” tailpipes, they increase the cost and do not work on road cars.

9 Keep your current cats if they are good condition. Some people fits “sports cats” which are just a lower number cells, 100 cell cats are purely for racing , you won’t pass an MoT emissions test with these. 200 cells are probably as low as you dare go for emissions testing , but 300 cell would be better , I think the standard are 400 to 500 (but others may know better?

Submitted by Andy Garlick

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