Another installment of the 1.8T big turbo project. A full 76mm exhaust has just be installed but is 76mm enough for 300bhp+?
Background – Project 1.8T
A bit of background on the project first.
The project had a 76mm downpipe, 70mm 200CPSI cat and a 70mm cat back with a small rear silencer, absorption type.
This was changed out for a stock cat-back exhaust of 55mm for noise reasons.
I was interested to see if 55mm was actually okay so I checked the back pressure. (Find out how you can check exhaust back pressure here).
The result of that testing showed back pressure at the turbo exit, with the 55mm cat-exhaust, was around 1 BAR/15PSI at 4000RPM and around 15 PSI of boost. At that point I lifted off, I learned what I needed to, 55mm was chronically undersized. Hence the change to a 76mm system…
76mm Exhaust System
Downpipe stayed as it was (76mm), cat was opened up to 76mm inlet and outlet and a 76mm cat-back exhaust system was fitted with a centre silencer and a huge rear silencer (55cm x 22cm by around 20cm). Both silencers absorption type.
I’ve just got back from testing the back pressure of this system and the results are not exactly what I was hoping for.
Getting up to 5000-6000RPM with 15-21 PSI boost, back pressure was around 0.4-0.5 BAR (6PSI – 7.35PSI)
At around 4000RPM 0.3 BAR.
Huge improvement but still far from perfect.
Why do we care?
A quick recap why back pressure is a problem in turbo engines.
The power of the turbo, the amount of work it can do, is dictated by the pressure differential between the turbine inlet and turbine outlet. The bigger the difference the more power the turbo has to create boost.
So 7 PSI in the exhaust is a 7 PSI reduction in the pressure differential, so 7 PSI in power that is being wasted due to exhaust back pressure.
If we can get the back pressure from the exhaust down to 1-2PSI the turbo won’t have to work so hard, so it will open the wastegate sooner which will reduce the back pressure in the exhaust manifold which will reduce EGTs and make the engine more efficient and happier. A win-win situation all-round.
Next Steps
Given all this, what are the next steps available to get the back pressure down?
Well the cat could be a problem. Could. It has a wide body and the cells are quite big. While it could be a problem I’m not going to start there.
No, the next step will be the downpipe. It’s 76mm at the moment but the turbine outlet flange is 89mm. We’ll put a 89mm downpipe in the car and reduce it down to the 76mm cat.
The exhaust gas coming out of the turbine is the most turbulent in the exhaust system and the hottest so logically, the downpipe could probably benefit the most from a bigger diameter.
If you want to see how that downpipe performs when it’s fitted, subscribe to the DriveTune newsletter and YouTube channel. I’m in no rush at the moment but I expect in the next 6 months it will go on.