Exhaust sizes for optimal flow at different power levels, internal and external wastegate
Here are some guideline for you people who have desire for max efforts and want the backpressure staying at minimal or basically having zero backpressure. Based on the testing I have done from real-world scenario and CFD data, this is mainly for 1.8-2.3L engines. This is with taking the exhaust routing of our Saabs into account as piping length and where the bends are placed affects backpressure significantly.
If you manage to keep the EGTs down on petrol/pump gas you get sometimes a tiny bit more head room here on exhaust system sizing as the exhaust mass produced by E85 is 50.6% larger than petrol/pump gas.
External wastegate with screampipe setups.
340-400hp: 3’’ downpipe to 2.5’’
400-475hp: 3’’
475-550hp: 3.5’’ downpipe to 3’’
550-650hp: 3.5’’
650-750hp: 4’’ downpipe to 3.5’’
750-850hp: 4’’
One thing to address from what has been mentioned earlier here, even tho you have an external wastegate with screampipe drops EMAP and some exhaust mass to the exhaust system. You can still in some cases use larger than mentioned here and benefit from it as flow can still stagnate by poor pressure drop behind turbine outlet due to exhaust pipe routing still allowing for an slight pressure build up. Also as we know from before the full backpressure now goes directly through the turbine housing so even the slightest pressure increase will be able to stagnate flow to an certain degree. Once again external wastegate is only there for one thing and it is pressure control, not for great deals of exhaust mass flow.
Internally wastegated setups.
275-310: 3’’ downpipe to 2.5’’
310-360hp: 3’’
360-450hp: 3.5’’
450-550hp: 4’’ downpipe to 3.5''
550-650hp: 4’’(4.5'' downpipe can even be of use here.)
650+hp: 4.5''
Cross-section area/flow area of each pipe dimension.
Showing how every half inch increases the flow area.
2’’ = 3.14159 in2 / 2026.8282 mm2
2.5’’ = 4.90874 in2 / 3166.9227 mm2
3’’ = 7.06858 in2 / 4560.3651 mm2
3.5’’ = 9.62113 in2 / 6207.1682 mm2
4’’ = 12.56637 in2 / 8107.3193 mm2
4.5’’ = 15.9043 in2 / 10260.8182 mm2
5’’ = 19.63495 in2 / 12667.6843 mm2