Backpressure - Can big wastegates help?
04-17-2015, 06:00 AM
#141
I was interested purely about power, not backpressures. Let's say we have 500cfm of exhaust going through turbo and 200cfm through wastegate, and that's choking the engine. If we put larger wastegate capable of 400cfm, won't power go up, if turbo still have enough energy to push those extra 200cfm?
engine cfm at x rpm and x psi
turbo required cfm to make x psi at x rpm
wastegate cfm at max valve lift
What are your 3 numbers?
04-17-2015, 07:55 AM
#142
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Way i view this is turbines are driven by energy in the system not just pressure ratio. There are systems where boost pressure is higher than exhaust preturbine backpressure so a pressure ratio less than 1:1. By some accounts that turbo shouldnt be spinning and making boost.
Energy that drives that wheel is exhaust mass velocity. My opinion is a bigger wastegate can help an undersized turbine wheel by bypassing the extra mass flow the turbine wheel doesnt need.
One turbo can make same hp on two different engines. It just operates at different speeds and the exhaust mass gets used differently. Say a 2.0 liter can make 700 hp on a given turbo. Turbo pressure ratio is higher with the small cubes so it operates at a higher turbo speed line. Higher speed commands more turbine drive energy so more mass flow is required thru the turbine. A 6.0 making 700 hp with same turbo does it at a lower pressure ratio so the turbo compressor wheel operates at a lower rpm. Requires less drive energy as less boost is required. Turbine wheel requires less exhaust flow. So more exhaust mass has to bypass the turbine hence more wastegate duty. Here a bigger gate helps. If you choke the gate, exhaust mass backs up in the turbine and you have backpressure
Energy that drives that wheel is exhaust mass velocity. My opinion is a bigger wastegate can help an undersized turbine wheel by bypassing the extra mass flow the turbine wheel doesnt need.
One turbo can make same hp on two different engines. It just operates at different speeds and the exhaust mass gets used differently. Say a 2.0 liter can make 700 hp on a given turbo. Turbo pressure ratio is higher with the small cubes so it operates at a higher turbo speed line. Higher speed commands more turbine drive energy so more mass flow is required thru the turbine. A 6.0 making 700 hp with same turbo does it at a lower pressure ratio so the turbo compressor wheel operates at a lower rpm. Requires less drive energy as less boost is required. Turbine wheel requires less exhaust flow. So more exhaust mass has to bypass the turbine hence more wastegate duty. Here a bigger gate helps. If you choke the gate, exhaust mass backs up in the turbine and you have backpressure
04-18-2015, 02:07 PM
#143
TECH Resident
04-18-2015, 02:43 PM
#144
Open the gate shaft speed and boost drops, not that hard to understand
Take an ls1 with a stupid single turbo like a tc76. It has a single 44mm gate
To deliver 15psi of boost it shows 45psi of drivepressure and no creep, so the gate capacity is adequate
Stick a 60mm gate on it won't change a damn thing, that gate will just be open less, but still dumping the same mass flow
Open the gate more....boost drops
Take an ls1 with a stupid single turbo like a tc76. It has a single 44mm gate
To deliver 15psi of boost it shows 45psi of drivepressure and no creep, so the gate capacity is adequate
Stick a 60mm gate on it won't change a damn thing, that gate will just be open less, but still dumping the same mass flow
Open the gate more....boost drops
04-18-2015, 02:46 PM
#145
The more efficient the turbine (less drive pressure) the more gate capacity you need. Not the other way around.
The orifice of the gate ismore effective at higher pressures
The orifice of the gate ismore effective at higher pressures
