07-09-2013, 11:25 AM
People are getting horribly confused with "big" turbos...
There's more than one thing that affects the "bigness" of a turbo - I'll list the ones that really affect us anyway.
1. Compressor wheel size - on GT series this is in -indicated by the last two numbers i.e. GT1752 - That means the compressor (i.e. sucky cold side that everyone sees) has an EXDUCER size of 52mm (that's the widest part - google Inducer and Exducer if you want to know more, some people refer to the inducer size, but seeing as we're mostly Garrett, we'll stick to exducer size). The Compressor wheel size almost directly translates to how much air the turbo will move at a given RPM - but obviously the bigger the wheel the heavier, therefore less air. Also the "generation" of wheel makes a massive difference, an old T25 compressor ends up turning into a hair dryer most of the time, it can't move that much air and makes it hot - less efficient - whereas a modern "GTB" i.e. 3rd generation GT series can move a serious amount of air, whilst being very small and not making things very hot.
2. Turbine wheel size - this isn't as easy to see - it's GENERALLY implied by the "frame" size - i.e. GT17 or GT20 - but isn't directly comparable - obviously the bigger the turbine wheel, the more air you can pump through it - great! But the bigger the turbine wheel the more air you need to have to be able to pump through it, when you've only got 1.9 litres of displacement and an 8v head, that becomes a bit of a challenge. That's not the end of the story.
3. A/R Ratio - You'll have heard me bang on about this, essentially, get one of those pocket fans - grab a straw and blow as hard as you can into the fan - it'll probably begin to spin up! Grab a piece of drain pipe and blow as hard as you can into the fan - notice it doesn't move for shit... It's all to do with the velocity of the gas that enters onto the tips of the turbine blades. But notice also, when you blew down the straw, you really struggled to get much air through it, you can blow as hard as you like, but all that happens is you're putting loads of pressure in (and going red in the face..) and not really getting anywhere, but with the drain pipe you can put shitloads of air through, but it's not really moving very quick at all...
That's exactly what affects turbos - that's why my GT17 performs worse on a 1.9 8v than a GT22 with a small A/R. This is why it's always best to get a turbo off a Diesel engine - Petrol turbos tend to have big A/Rs - they usually have 16v heads, spin to 8000+ rpm - that's a LOT of airflow - whereas Diesel has an 8v head and spin to 5000rpm max. But of course, the other way round, if you get a baby HDi GT15, you end up strangling the engine because it simply CANNOT get the gas out fast enough.
This A/R is exactly what VNT turbos adjust - therefore you have all the advantages of a big hotsided GT28 being able to throw shitloads of air through it at higher RPMs, but you're able to get it spinning low down by forcing the gas through it at high velocity.
Essay over - essentially it's more than "get a gt20" or "get a td04" - remember a TD04 would actually be a pretty good turbo, if you had a small A/R on it - it's purely because it's off a 2.0 16v 7000rpm petrol that it lags a bit! However, if you make it much smaller, you'll run into Surge... And that's next weeks lesson.
There's more than one thing that affects the "bigness" of a turbo - I'll list the ones that really affect us anyway.
1. Compressor wheel size - on GT series this is in -indicated by the last two numbers i.e. GT1752 - That means the compressor (i.e. sucky cold side that everyone sees) has an EXDUCER size of 52mm (that's the widest part - google Inducer and Exducer if you want to know more, some people refer to the inducer size, but seeing as we're mostly Garrett, we'll stick to exducer size). The Compressor wheel size almost directly translates to how much air the turbo will move at a given RPM - but obviously the bigger the wheel the heavier, therefore less air. Also the "generation" of wheel makes a massive difference, an old T25 compressor ends up turning into a hair dryer most of the time, it can't move that much air and makes it hot - less efficient - whereas a modern "GTB" i.e. 3rd generation GT series can move a serious amount of air, whilst being very small and not making things very hot.
2. Turbine wheel size - this isn't as easy to see - it's GENERALLY implied by the "frame" size - i.e. GT17 or GT20 - but isn't directly comparable - obviously the bigger the turbine wheel, the more air you can pump through it - great! But the bigger the turbine wheel the more air you need to have to be able to pump through it, when you've only got 1.9 litres of displacement and an 8v head, that becomes a bit of a challenge. That's not the end of the story.
3. A/R Ratio - You'll have heard me bang on about this, essentially, get one of those pocket fans - grab a straw and blow as hard as you can into the fan - it'll probably begin to spin up! Grab a piece of drain pipe and blow as hard as you can into the fan - notice it doesn't move for shit... It's all to do with the velocity of the gas that enters onto the tips of the turbine blades. But notice also, when you blew down the straw, you really struggled to get much air through it, you can blow as hard as you like, but all that happens is you're putting loads of pressure in (and going red in the face..) and not really getting anywhere, but with the drain pipe you can put shitloads of air through, but it's not really moving very quick at all...
That's exactly what affects turbos - that's why my GT17 performs worse on a 1.9 8v than a GT22 with a small A/R. This is why it's always best to get a turbo off a Diesel engine - Petrol turbos tend to have big A/Rs - they usually have 16v heads, spin to 8000+ rpm - that's a LOT of airflow - whereas Diesel has an 8v head and spin to 5000rpm max. But of course, the other way round, if you get a baby HDi GT15, you end up strangling the engine because it simply CANNOT get the gas out fast enough.
This A/R is exactly what VNT turbos adjust - therefore you have all the advantages of a big hotsided GT28 being able to throw shitloads of air through it at higher RPMs, but you're able to get it spinning low down by forcing the gas through it at high velocity.
Essay over - essentially it's more than "get a gt20" or "get a td04" - remember a TD04 would actually be a pretty good turbo, if you had a small A/R on it - it's purely because it's off a 2.0 16v 7000rpm petrol that it lags a bit! However, if you make it much smaller, you'll run into Surge... And that's next weeks lesson.
