Both of these replace the same restrictive factory downpipe on a New Age Impreza WRX STi with a straight 3" stainless item, but they take a different approach to the catalytic converter: one fits a higher-flow sports cat in its place, the other removes it entirely.
| Spec | Sports Cat Downpipe | 3" Decat Downpipe |
|---|---|---|
| Catalytic converter | Replaced with a built-in 200 CPI sports cat, a higher-flow cat, not a full delete | Removed entirely, replaced with a straight-through pipe |
| Construction | T304 stainless steel, 3" bore, TIG welded, CNC-machined flanges | T304 stainless steel, 3" bore (2.5" outlet also available), TIG welded, CNC-machined flanges |
| Lambda/O2 sensor | Not specified as a separate boss, cat housing has its own sensor provision | Fitted with a boss to accept the O2 (lambda) sensor |
| Road use | Retains a cat, generally the more emissions-conscious of the two options | No catalytic converter, check current UK MOT emissions rules before fitting for road use |
| Catalogue price | From £329.99 (Japspeed UK) | From £189.99 (Japspeed UK) |
The sports cat downpipe is the middle ground. It still opens up the restrictive factory downpipe to a straight 3" bore, but the built-in 200 CPI cat means you're not running with zero emissions control. It costs more than a straight decat, but it's the option most people reach for when the car still needs to be reasonably road-focused.
The decat is cheaper and removes the catalytic converter entirely for the least restriction. As with any decat, check current UK MOT emissions rules for your car before fitting one if it needs to stay road-legal. This one is commonly run as a track-day part rather than a daily-road setup for that reason.
A sports cat downpipe still has a catalytic converter fitted, just a higher-flow one (200 CPI here, versus the denser factory cat), while a decat downpipe removes the catalytic converter completely and replaces it with a straight pipe.
A sports cat downpipe is generally the better fit for a car that still needs to be practical on the road, since it retains some emissions control while still improving flow over the factory downpipe. A full decat gives more flow again but removes the cat entirely, so it's worth checking current UK MOT emissions rules before running one on a road car.
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