17-01-2012, 05:35 PM
It's all to do with efficiency in relation to the shaft speed.
That's where the compressor map comes in.
A GT1546 compressor will put out enough air to maintain 26psi at 5000rpm on an XUD9, but the problem lies with the shaft speed, to produce that pressure with the airflow required at 5000rpm, you'd spin the compressor WAY past it's design max, theoretical max and you'd be down to something like 30% efficiency. Turning it into a hairdryer.
A T2 is a "bigger" turbocharger, but is of an older design, so it'll put out a bit more air when comparing compressor maps to a GT15 at it's design max, BUT it'll be even further down on efficiency, putting out even hotter air. When you actually work out how much power you'll get, that's why the T2 and GT15 are surprisingly similar - BUT - BIG MASSIVE BUTT (Just like your mum...) this DOESN'T take into account the hotside.
The hotside of the turbo also makes a big difference to whether you can get the air out of the engine - this is down to the "size" in cm^2 and it's A/R. I could go heavily into what A/R is and it's relations to other dimensions, but basically it's how "tight" the housing is, how small the gap is for the gasses to get into the turbine wheel, which affects the velocity of the expanding gasses. The smaller the A/R the tighter the housing is, i.e. the gas will be moving faster, which makes the turbine wheel spin up quickly... The bigger the A/R the slower the gasses will be going into the turbine, therefore spinning up slowly, BUT it's to do with backpressure - a small A/R will give high backpressure and not let much gas out, strangling the engine, giving high exhaust mani pressures and affecting inlet pressures, a larger A/R will spool slower, but allow more gas through.
That's where a VNT turbo comes in, you've got a VARIABLE A/R which means that at low engine speeds and lower volumes of exhaust gas, the vanes are up close, you have a small A/R the turbine will still speed up quick, but when the engine gets quicker, more hot gasses, the vanes open up, giving a really big A/R - loads of gas out, improves flow out of the engine MUCH better, but the turbo is spooled up, so it doesn't matter about spooling it up...
That's where the compressor map comes in.
A GT1546 compressor will put out enough air to maintain 26psi at 5000rpm on an XUD9, but the problem lies with the shaft speed, to produce that pressure with the airflow required at 5000rpm, you'd spin the compressor WAY past it's design max, theoretical max and you'd be down to something like 30% efficiency. Turning it into a hairdryer.
A T2 is a "bigger" turbocharger, but is of an older design, so it'll put out a bit more air when comparing compressor maps to a GT15 at it's design max, BUT it'll be even further down on efficiency, putting out even hotter air. When you actually work out how much power you'll get, that's why the T2 and GT15 are surprisingly similar - BUT - BIG MASSIVE BUTT (Just like your mum...) this DOESN'T take into account the hotside.
The hotside of the turbo also makes a big difference to whether you can get the air out of the engine - this is down to the "size" in cm^2 and it's A/R. I could go heavily into what A/R is and it's relations to other dimensions, but basically it's how "tight" the housing is, how small the gap is for the gasses to get into the turbine wheel, which affects the velocity of the expanding gasses. The smaller the A/R the tighter the housing is, i.e. the gas will be moving faster, which makes the turbine wheel spin up quickly... The bigger the A/R the slower the gasses will be going into the turbine, therefore spinning up slowly, BUT it's to do with backpressure - a small A/R will give high backpressure and not let much gas out, strangling the engine, giving high exhaust mani pressures and affecting inlet pressures, a larger A/R will spool slower, but allow more gas through.
That's where a VNT turbo comes in, you've got a VARIABLE A/R which means that at low engine speeds and lower volumes of exhaust gas, the vanes are up close, you have a small A/R the turbine will still speed up quick, but when the engine gets quicker, more hot gasses, the vanes open up, giving a really big A/R - loads of gas out, improves flow out of the engine MUCH better, but the turbo is spooled up, so it doesn't matter about spooling it up...
