Does Boost Blow the motor?
Feb 12, 2010, 11:25 AM
#16
Yes I can make my stock turbo hold 23psi to peak power (had it out to 7600) between playing with how the WG arm is, the BCS, and MIVEC. AS airflow drops so does torque, as torque drops so does power. A small turbo doesnt supply enough air. You are using the term VE to mean the basic principle of CFM I think. So even though it is holding the boost out to redline (we have one car that we tweaked and held 25psi to 8k on a stock IX turbo) it doesnt make power because there isnt enough cubic feet per minute flow.
CFM is far more than VE though. If it were that simple that would mean I need really efficient aero and keep the VE and charge cool to make the same power on any setup. VE doesnt apply to a turbo the same as it does the engine. We have adabiatic efficiency (following the Ideal Gas law), inducer/exducer size, and the air mass a turbo is capable of moving. The air mass (in lbs/min) is the function of VE vs CFM. How much air gets sucked in versus how much mass after compression is available for the engine to use and at what temperature.
To add one more turbo to the mix, the 4202 on our drag cars makes about 600whp (632 is the highest I seem to remember) at 23psi on a 2.0l. It is making that boost level before a stock turbo car would reach peak power. The compressor is physically larger, that is why CFM has gone up not the turbine size/backpressure ratio. Stock IX turbos are 48mm I think? The 3586 is low 60s, the 4202 is 74mm so at a given pressure ratio there is more mass being compressed.
CFM is far more than VE though. If it were that simple that would mean I need really efficient aero and keep the VE and charge cool to make the same power on any setup. VE doesnt apply to a turbo the same as it does the engine. We have adabiatic efficiency (following the Ideal Gas law), inducer/exducer size, and the air mass a turbo is capable of moving. The air mass (in lbs/min) is the function of VE vs CFM. How much air gets sucked in versus how much mass after compression is available for the engine to use and at what temperature.
To add one more turbo to the mix, the 4202 on our drag cars makes about 600whp (632 is the highest I seem to remember) at 23psi on a 2.0l. It is making that boost level before a stock turbo car would reach peak power. The compressor is physically larger, that is why CFM has gone up not the turbine size/backpressure ratio. Stock IX turbos are 48mm I think? The 3586 is low 60s, the 4202 is 74mm so at a given pressure ratio there is more mass being compressed.
Last edited by JohnBradley; Feb 12, 2010 at 11:28 AM.
Feb 12, 2010, 11:29 AM
#17
It's true that larger turbos are more efficient at higher boost levels and produce lower charge temps...
But you're still missing the key reason why a larger turbo at the same boost makes more power, it's purely because there's a much bigger wheel pushing more air threw a bigger STRAW! More Air Flow!
Feb 12, 2010, 11:32 AM
#18
Yes I can make my stock turbo hold 23psi to peak power (had it out to 7600) between playing with how the WG arm is, the BCS, and MIVEC. AS airflow drops so does torque, as torque drops so does power. A small turbo doesnt supply enough air. You are using the term VE to mean the basic principle of CFM I think. So even though it is holding the boost out to redline (we have one car that we tweaked and held 25psi to 8k on a stock IX turbo) it doesnt make power because there isnt enough cubic feet per minute flow.
CFM is far more than VE though. If it were that simple that would mean I need really efficient aero and keep the VE and charge cool to make the same power on any setup. VE doesnt apply to a turbo the same as it does the engine. We have adabiatic efficiency (following the Ideal Gas law), inducer/exducer size, and the air mass a turbo is capable of moving. The air mass (in lbs/min) is the function of VE vs CFM. How much air gets sucked in versus how much mass after compression is available for the engine to use and at what temperature.
To add one more turbo to the mix, the 4202 on our drag cars makes about 600whp (632 is the highest I seem to remember) at 23psi on a 2.0l. It is making that boost level before a stock turbo car would reach peak power. The compressor is physically larger, that is why CFM has gone up not the turbine size/backpressure ratio. Stock IX turbos are 48mm I think? The 3586 is low 60s, the 4202 is 74mm so at a given pressure ratio there is more mass being compressed.
CFM is far more than VE though. If it were that simple that would mean I need really efficient aero and keep the VE and charge cool to make the same power on any setup. VE doesnt apply to a turbo the same as it does the engine. We have adabiatic efficiency (following the Ideal Gas law), inducer/exducer size, and the air mass a turbo is capable of moving. The air mass (in lbs/min) is the function of VE vs CFM. How much air gets sucked in versus how much mass after compression is available for the engine to use and at what temperature.
To add one more turbo to the mix, the 4202 on our drag cars makes about 600whp (632 is the highest I seem to remember) at 23psi on a 2.0l. It is making that boost level before a stock turbo car would reach peak power. The compressor is physically larger, that is why CFM has gone up not the turbine size/backpressure ratio. Stock IX turbos are 48mm I think? The 3586 is low 60s, the 4202 is 74mm so at a given pressure ratio there is more mass being compressed.
Feb 12, 2010, 12:22 PM
#20
Yes I can make my stock turbo hold 23psi to peak power (had it out to 7600) between playing with how the WG arm is, the BCS, and MIVEC. AS airflow drops so does torque, as torque drops so does power. A small turbo doesnt supply enough air. You are using the term VE to mean the basic principle of CFM I think. So even though it is holding the boost out to redline (we have one car that we tweaked and held 25psi to 8k on a stock IX turbo) it doesnt make power because there isnt enough cubic feet per minute flow.
CFM is far more than VE though. If it were that simple that would mean I need really efficient aero and keep the VE and charge cool to make the same power on any setup. VE doesnt apply to a turbo the same as it does the engine. We have adabiatic efficiency (following the Ideal Gas law), inducer/exducer size, and the air mass a turbo is capable of moving. The air mass (in lbs/min) is the function of VE vs CFM. How much air gets sucked in versus how much mass after compression is available for the engine to use and at what temperature.
To add one more turbo to the mix, the 4202 on our drag cars makes about 600whp (632 is the highest I seem to remember) at 23psi on a 2.0l. It is making that boost level before a stock turbo car would reach peak power. The compressor is physically larger, that is why CFM has gone up not the turbine size/backpressure ratio. Stock IX turbos are 48mm I think? The 3586 is low 60s, the 4202 is 74mm so at a given pressure ratio there is more mass being compressed.
CFM is far more than VE though. If it were that simple that would mean I need really efficient aero and keep the VE and charge cool to make the same power on any setup. VE doesnt apply to a turbo the same as it does the engine. We have adabiatic efficiency (following the Ideal Gas law), inducer/exducer size, and the air mass a turbo is capable of moving. The air mass (in lbs/min) is the function of VE vs CFM. How much air gets sucked in versus how much mass after compression is available for the engine to use and at what temperature.
To add one more turbo to the mix, the 4202 on our drag cars makes about 600whp (632 is the highest I seem to remember) at 23psi on a 2.0l. It is making that boost level before a stock turbo car would reach peak power. The compressor is physically larger, that is why CFM has gone up not the turbine size/backpressure ratio. Stock IX turbos are 48mm I think? The 3586 is low 60s, the 4202 is 74mm so at a given pressure ratio there is more mass being compressed.
Feb 12, 2010, 12:26 PM
#21
That said, my points about the WHY in this debate are true I believe. The only reason the larger turbo makes more power is because VE has been improved becuase of a less restrictive hot side and also because of lower charge temps because of a more efficient comrpessor wheel.... NOT because the stock turbo is pushing boost but not CFM.
Two motors with the same VE and boost pressure = same airflow.
Feb 12, 2010, 12:29 PM
#22
Increasing the size of the compressor only reduces charge temps and the maximum possible boost while still being efficient.
Feb 12, 2010, 12:29 PM
#23
First off, if you have two identical cars, but one is running a stock turbo, and the other is running a 35r. And both run the same boost, there will be VERY LITTLE difference between the two cars speed wise. The 35r car will make a tiny bit more power boost for boost because of improvements in VE due to less exhaust back pressure, and also a slightly cooler inlet charge due to improved compressor efficiency. But these gains are minor.
The biggest gain you will see from going to a larger turbo is that you can run higher boost levels more efficiently. The stock turbo simple cannot run 30 psi very efficiently, and it bleeds down if you do hit that boost. A 35r can run 30 psi efficiently and hold it. Not to mention you will really see gains for boost levels over 30 psi.
The biggest stress on an engine, if you forget about a bad tune and detonation, is the inertial forces from RPM. The mechanical forces from combustion are not as strong as the inertial forces created when a piston has to stop, accelerate, stop, etc.
Larger turbos generally will give you more top end, and therefore, your likely to want more rpm from a larger turbo, especially if you are on a 2l and large turbo, where you might not be making full boost until 6k rpm.
Anyway, all food for thought. Nothing to be afraid of in terms of boost. But the tune needs to be safe, and don't push a turbo beyond it's efficiency. That means make sure as you add boost while tuning, you are making sure the car is going faster. If you add boost and don't go faster, then you're running too much boost.
The biggest gain you will see from going to a larger turbo is that you can run higher boost levels more efficiently. The stock turbo simple cannot run 30 psi very efficiently, and it bleeds down if you do hit that boost. A 35r can run 30 psi efficiently and hold it. Not to mention you will really see gains for boost levels over 30 psi.
The biggest stress on an engine, if you forget about a bad tune and detonation, is the inertial forces from RPM. The mechanical forces from combustion are not as strong as the inertial forces created when a piston has to stop, accelerate, stop, etc.
Larger turbos generally will give you more top end, and therefore, your likely to want more rpm from a larger turbo, especially if you are on a 2l and large turbo, where you might not be making full boost until 6k rpm.
Anyway, all food for thought. Nothing to be afraid of in terms of boost. But the tune needs to be safe, and don't push a turbo beyond it's efficiency. That means make sure as you add boost while tuning, you are making sure the car is going faster. If you add boost and don't go faster, then you're running too much boost.
Id consider 7-10 mph in the 1/4 mile much more than minimal..
Mike
Feb 12, 2010, 12:33 PM
#24
It's not because the stock turbo isn't supplying enough air. It's because the VE of the motor is BAD because of a very restrictive turbine housing choking things up. The compressor has nothing to do with it provided it's operating in it's efficiency range.
Increasing the size of the compressor only reduces charge temps and the maximum possible boost while still being efficient.
Increasing the size of the compressor only reduces charge temps and the maximum possible boost while still being efficient.
Mike
Feb 12, 2010, 12:39 PM
#26
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it's pretty common for my customers to max out a evo9 turbo 400awhp at 30psi on e85, they can swap in a 35R and make the same 400awhp at 20psi and do it on 93 octane.
Feb 12, 2010, 12:49 PM
#29
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Feb 12, 2010, 01:01 PM
#30
That surprises me... because if you look at track times, your average 35r runs 10.0, and your average IX turbo runs 11.0. And the 35R is normally running about 35 psi, and the IX turbo around 28 bleeding to 22 psi.