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Exhaust Back - Pressure Ratio

Updated: Nov 1, 2021

We did some testing of exhaust back pressure pre-turbine in the exhaust manifold on our 07 Subaru Liberty GT.


The standard design turbine housing for Subaru stock location turbos are, in a word "shocking". Not only does the exhaust gas get choked from the 90º turn it has to make but also the overall design of the housing is far less superior compared to other turbo manufactures. Because of this design, the pressure you see in the exhaust manifold can be as high as x2.5 what the boost pressures are. On a stock turbo with slightly more than standard that might be ok, but as you target more boost you are making the turbo work harder, so in effect the manifold pressure goes up.


For example Stock Turbo:

Boost pressure target - 18psi (peak RPM)

Exhaust manifold pressure - 30-40psi (peak RPM)


These are the results:


We custom Fabricated a 44mm External Wastegate to lower the Exhaust Manifold Air Pressure (EMAP) and did it drop.


It went from 1:1.5 (21psi IMAP: 31psi EMAP) to 1:0.66 (27psi IMAP: 18psi EMAP)

Yes you read correctly - 1:0.66


We didn’t push it hard on 98ron - 312awkw @21psi

But on e85 we went as much as the MAF could take - 409awkw @27psi

We are going to start making these external wastegates kits for 6speed - WRX/STI/LIBERTY - Stay Tuned

Big Huge THANKS to Michael South for your skills

Engine Specs: 2.6L Forged Bottom End 9:1 Comp Closed Deck K1 83mm Billet Crank 14mm Studs O-Ring’d CNC’d Heads 274 Cosworth Cams Blouch Dom 3xtr 10cm


We are working on finalising our 44mm external wastegate up-pipe kits for the EJ20 and EJ25 platform.


Watch for updates









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