Wastegate plumbing to relieve drive pressure?
- Thread starter mech2161
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BallSoHard U
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They are not independent. 1 has a direct effect on the other. You can control the boost by diverting exhaust pressure around the turbine wheel. But there is no magic wastegate that will allow you to run 10psi. more boost but keep the drive pressure equal if you are already on the edge of the turbo's MAP. Gate is only going to allow the engine to expel more gas than the turbo is capable of using efficiently. Keeping the boost flat and leveling off the drive pressure is the way they all work. Unless you hit the limit of the gate then right back where you started.
Big Blue24
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Usually internal wastegates are used to prevent overspeeding the turbo. In lay-man's terms, the wastegate is used to limit maximum shaft speed which is very closely related to intake boost and exhaust gas drive pressure.
The only way to maintain boost with lower drive pressure is to increase heat in the exhaust system= higher EGT's.
Think of it this way, to make 60 lbs of boost, your turbo needs to develop 30 shaft horsepower. The shaft horsepower is supplied by the turbine wheel. The turbine wheel converts exhaust heat, exhaust velocity, and exhaust pressure into shaft horsepower. If you remove pressure by opening a wastegate, the only way to maintain the needed 30 shaft horsepower is to provide more exhaust heat and exhaust velocity. Technically, exhaust heat and velocity are very closely related since temperature is really just average velocity of the molecules of exhaust gas.
If you wanted to take wastegate theory a step further, you need to start looking at compressor maps. A compressor map will show the approximate efficiency range of the turbo. Consider a stock HX35 being driven to 60psi boost. I would require a lot of shaft horsepower, say 35hp to drive the compressor way off it's map to 170,000 rpm and create 60psi. Making 35 shaft horsepower with a small stock exhaust turbine wheel would require extremely hot exhaust gas at probably 100psi of drive pressure. That 100psi of drive pressure really chokes the motor and robs horsepower. If a wastegate were introduced to this example setup, it could be set to lower boost to say 40psi where the required shaft horsepower might only be 15-20hp. Drive pressure would drop dramatically to say 45 or 50psi and shaft rpm would probably be somewhere around 130,000 rpm. The engine would breath much freer and you'd probably end up with a small increase in HP from the freer flowing exhaust system. An HX35 will flow a tiny bit more air at 60psi vs 40psi but at the expense of super high drive pressure that would rob more horsepower than it can provide with the small increase in airflow. A turbo map shows the efficiency of the compressor at different pressure ratios which can easily be translated into boost. Generally speaking, the higher the boost, the less efficient the turbo's compressor becomes.
Do I think a wastegate will add horsepower to a well balanced turbo system that is operating efficiently in the middle of the compressor map, No!
Why do stock trucks have wastegates? To allow them to run undersized exhaust housings for faster spoolup and less turbo lag.
Can you get a large enough exhaust housing to not need a wastegate? Yes, however, spoolup will be slow and laggy because the housing will be sized large enough to handle exhaust heat and pressure at full load at rated horsepower. A perfect example of this is an 89' Dodge Cummins with a 21cm exhaust housing. Very slow to spool up but no need for a wastegate.
Does a stock 94-2002 truck need a wastegate? No. Why is boost limited to around 21psi? Probably to limit shaft speed to ensure turbo longevity for 300K miles. In fact, a stock 94-98 truck will actually run a tad cooler and make a little more horsepower with the wastegate disabled when the motor is running at full load such as towing up a grade.
In short, wastegating is very dependent on overall setup.
The only way to maintain boost with lower drive pressure is to increase heat in the exhaust system= higher EGT's.
Think of it this way, to make 60 lbs of boost, your turbo needs to develop 30 shaft horsepower. The shaft horsepower is supplied by the turbine wheel. The turbine wheel converts exhaust heat, exhaust velocity, and exhaust pressure into shaft horsepower. If you remove pressure by opening a wastegate, the only way to maintain the needed 30 shaft horsepower is to provide more exhaust heat and exhaust velocity. Technically, exhaust heat and velocity are very closely related since temperature is really just average velocity of the molecules of exhaust gas.
If you wanted to take wastegate theory a step further, you need to start looking at compressor maps. A compressor map will show the approximate efficiency range of the turbo. Consider a stock HX35 being driven to 60psi boost. I would require a lot of shaft horsepower, say 35hp to drive the compressor way off it's map to 170,000 rpm and create 60psi. Making 35 shaft horsepower with a small stock exhaust turbine wheel would require extremely hot exhaust gas at probably 100psi of drive pressure. That 100psi of drive pressure really chokes the motor and robs horsepower. If a wastegate were introduced to this example setup, it could be set to lower boost to say 40psi where the required shaft horsepower might only be 15-20hp. Drive pressure would drop dramatically to say 45 or 50psi and shaft rpm would probably be somewhere around 130,000 rpm. The engine would breath much freer and you'd probably end up with a small increase in HP from the freer flowing exhaust system. An HX35 will flow a tiny bit more air at 60psi vs 40psi but at the expense of super high drive pressure that would rob more horsepower than it can provide with the small increase in airflow. A turbo map shows the efficiency of the compressor at different pressure ratios which can easily be translated into boost. Generally speaking, the higher the boost, the less efficient the turbo's compressor becomes.
Do I think a wastegate will add horsepower to a well balanced turbo system that is operating efficiently in the middle of the compressor map, No!
Why do stock trucks have wastegates? To allow them to run undersized exhaust housings for faster spoolup and less turbo lag.
Can you get a large enough exhaust housing to not need a wastegate? Yes, however, spoolup will be slow and laggy because the housing will be sized large enough to handle exhaust heat and pressure at full load at rated horsepower. A perfect example of this is an 89' Dodge Cummins with a 21cm exhaust housing. Very slow to spool up but no need for a wastegate.
Does a stock 94-2002 truck need a wastegate? No. Why is boost limited to around 21psi? Probably to limit shaft speed to ensure turbo longevity for 300K miles. In fact, a stock 94-98 truck will actually run a tad cooler and make a little more horsepower with the wastegate disabled when the motor is running at full load such as towing up a grade.
In short, wastegating is very dependent on overall setup.
mech2161
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Ok I did some crude calculations and pushing this turbo much past 40lbs psi would push it out of it's effiency range. That means extra heat and drive pressure with no gain in hp. I guess the next step is get a gate fabbed up and do some dyno testing.
Big Blue24
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Pushing a turbo a little out of it's "mapped" efficiency range is not bad as long as the turbo is strong enough to handle the extra shaft rpm. A perfect example of this is the stock Dodge 04.5'-07' HE351cw turbo. It has a 60mm compressor and is wastegated from the factory at 26-28psi. It has an electronic bleeder valve/solenoid that allows momentary spikes of 30-32 psi. I'd guess the "mapped" efficiency range stops around 40 psi. However, this turbo makes peak HP when pushed well into the 50-55psi range. Anything over 45psi boost and the drive pressure increases exponentially but it's not till the boost nears 60psi that rear wheel horsepower starts to drop. Now this turbo is a little bit of a freak because it has a relatively large compressor exducer=major diameter of 88mm that allows it operate well at high boost. The only reason to wastegate an HE351 in my opinion is for turbo longevity. An HE351 will only last a few weeks if pushed past 55psi, in fact, some only make it to 50psi once and then the compressor wheel explodes. I have mine wastegated at 45psi but it definitely makes more power at 50psi.
Every turbo is going to be different in how well it compresses air at high pressure ratios= high boost.
The HX35 off my 99 had a 54mm by 78mm compressor wheel. It seemed to make peak horsepower around 38-40 psi. I pushed it up to 45 and 50psi a couple of times and it definitely started to lose power over 45psi. A drive pressure guage showed 80-100psi of drive pressure depending on RPM when the turbo was pushed to 45-50 psi. In that situation, a wastegate helps both turbo longevity and makes more power/keeps turbo at peak power.
I honestly think every turbo will be a little different and you just have to play around with it to discover what's best. One of the "appeals" of external wastegates is the easy adjustability.