Increasing performance and noise with Prodrive backbox?

[Matt106]

I've recently delved into the world of turbo power in the form of an Impreza wagon. It's already had a couple of subtle modifications to increase the power but I'm hoping to unleash some more horses.

The main issue is what to do with the exhaust system which is probably restrictive. It's currently running a Prodrive backbox, Prodrive decat, decat up pipe and sports cat down pipe. The centre section is standard and I'm hoping changing this for a straight through 2.5" will increase the flow and noise?

I like the looks of the Prodrive box so fairly reluctant to splash out on 4" Jap can! Will the 2.5" system be free flowing enough to reach 330-350bhp? I already have supporting modifications to reach this power (in theory) apart from injectors.

[TEKNOPUG]

Matt106 said:

I've recently delved into the world of turbo power in the form of an Impreza wagon. It's already had a couple of subtle modifications to increase the power but I'm hoping to unleash some more horses.

The main issue is what to do with the exhaust system which is probably restrictive. It's currently running a Prodrive backbox, Prodrive decat, decat up pipe and sports cat down pipe. The centre section is standard and I'm hoping changing this for a straight through 2.5" will increase the flow and noise?

I like the looks of the Prodrive box so fairly reluctant to splash out on 4" Jap can! Will the 2.5" system be free flowing enough to reach 330-350bhp? I already have supporting modifications to reach this power (in theory) apart from injectors.

I don't think that the exhaust will be a restriction at the power levels you are looking at. Pushing past 350bhp will see gains from upgrading to a 3" exhaust. Below that and the difference between a 2.5 & 3" will be neglible. If you HAD to buy a whole new exhaust system, I'd say go for 3" but there are better things (returns) to spend your money on that upgrading the 2.5" ProDrive system, IMHO.

Trying to remember the full spec. of your car. VF34 turbo but conservatively mapped to 280bhp - is that due to still running stock injectors? It may be best to pick a mapper first and then ask them what they advise. Fuel pump, bigger injectors, probably a 3-port boost solenoid should be sufficent to run 330+. Cooler plugs also. If you want to spend some money on the exhaust, then look to get an improved "Harveyy trick" up-pipe. Reduced spool time on the turbo by 300-500 rpm.

However, first thing I'd do is uprate the brakes - they are marginal at PPP power levels!

[paulmoonraker]

I wouldn't go for a 3in exhaust at that power level. My STi is running 350bhp and had a 3in exhaust, I changed to a 2.5in and had a noticeable improvement in low end torque after a remap.

The centre section change will make a negligible difference to power. It's done mainly for the sound.

[paulmoonraker]

Oh, and if you want more noise, by an Afterburner vortex

[Matt106]

It's already got a harvey up pipe and 225 fuel pump however still need injectors, solenoid and possibly plugs.

I may swap out the centre section for an aftermarket one and see if it increases the noise. Most aftermarket system look a bit out of place on the wagon in my opinion.

I've got Thwaites, Roger Clark and Andy Carr all within easy reach so it'll no doubt be one of those guys.

[paulmoonraker]

Matt106 said:

I may swap out the centre section for an aftermarket one and see if it increases the noise. Most aftermarket system look a bit out of place on the wagon in my opinion.

It will be a bit louder, but also raspier and have more drone on motorway runs... Stick an Afterburner Vortex on and be done with

[Ennoch]

Doubt you'd notice any difference in power unless mapping although if it's been decatted then it's likely to already have been done, in which case you'll be unlikely to see further gains. I got 297bhp from the standard TD04 (about it's limit) with just a full decat; the rear silencer and STI backbox were still on there, and the STI backbox is marginally less flowing than the Prodrive item. If you're looking for power then you'll be disappointed but if it's for noise then go for it, just expect more noise off WOT as well as on.