as above cg13's are about 75 bhp (manufactours always give bhp at the flywheel).....i decat would probs give you 1bhp (proven on the primera owners club but thats a bigger engine) i would say, 4 branch manifold into a 2" de-cat then into a 2" centre section then a backbox...idealy you would need the keep the secondary cat, or fit a silancer, its entierly upto you. just to give you abit of back persurre. but then agian the back box might work for back persurre.
then a tunned length airfilter (near battery is a tunned length, the pipe as to be a certian lentgh to give you the best performance) you would/should see near 100bhpn at the flywheel and around 80bhp at the wheels. witch isnt bad! the best performnace for your money is a 4branch manifold.
after i have all that setup and working/running with no peoblems, i would move on to uprated cams, then get a ga16 throttle body and injectors/fuel rail....also probs by an apexi just for the fact its there if needs be, then have it mapped, then you should be looking at around 95-100 at the wheels, and near engough 120bhp at the flywheel.
also i would look into stripping out of the interior, harness, light weight seat, plastic windows, lightweight wheels, plastic windows etc....uprated brakes (ga16 brakes would do fine if on a budget) obv uprated springs and shocks, antiroll bars and decent tyres, tyres are everything! so is handling, berofre modifying the engine i would start with handling.
then should have a good setup for track/fast road use on your hands.
