Hey guys,

For the hell of it today we decided to do a comparison test on the stock versus Gruppe-S 4-1 Header. There's been a lot of debate regarding the effectiveness of the stock manifold - we've always known 4-1 headers (such as ours) to consistently make power, but wanted to get some solid numbers on the STI 2.5.

The car in question is an 2004 STI 2.5 with a TD06-20G turbo, Perrin FMIC, Helix Turbo back, running on 91 octane pump gas @18psi. The car put down 292whp/300ftlbs which is pretty much inline with what is expected. With the installation of the Gruppe-S 4-1 uncoated header / Helix uppipe (and no tuning) the car put down 303whp and 318ft/lbs of torque. So gains of 11whp and 18ft/lbs were seen. Keep in mind this is untuned, more power definitely could have been made, but at 6:20pm on friday we just didn't have the hour or so needed to tune it.

The dyno graph demonstrates the following:

1. There is no low-end to mid-range power loss with the Gruppe-S 4-1 headers
2. Positive gains are achieved with headers, with NO LOSS on the 2.5 car

I believe this dyno graph demonstrates the fact that the stock headers are far undersized relative to what a modified 2.5 STI pushes out in terms of exhaust gas, especially one with an upgraded turbocharger (larger turbine). The drop off in torque in higher RPM regions indicates that a car running race gas potentially would benefit from running larger runner diameters (although a larger inducer on the turbine side would probably also be a good idea).

Cheers,

Gary
gary@gruppe-s.com