Discussion
V8RX7 said:
Mr E said:
V8RX7 said:
Yes but one turbo - twice the size but being fed twice the air (from all 6 cylinders) would surely do exactly the same ?
But would be larger for the same peak flow, and therefore laggier and have higher boost threshold. Sorry you've lost me.
3 cylinders feed a turbo size 1
6 cylinders feed a turbo size 2
What exactly is the advantage ?
I presume there must be one but I don't see how doubling the turbo size and doubling the flow affects anything.
V8RX7 said:
Mr E said:
V8RX7 said:
Yes but one turbo - twice the size but being fed twice the air (from all 6 cylinders) would surely do exactly the same ?
But would be larger for the same peak flow, and therefore laggier and have higher boost threshold. Sorry you've lost me.
3 cylinders feed a turbo size 1
6 cylinders feed a turbo size 2
What exactly is the advantage ?
I presume there must be one but I don't see how doubling the turbo size and doubling the flow affects anything.
Elysium said:
Screechmr2 said:
Elysium said:
And they are wrong. Twin means 'matching' my last car a first generation 335i was a twin turbo because it had two matching turbos.
Bi just means two. My current car is an Audi BiTDI. It has one small and one large turbo arranged sequentially. It is a biturbo, but not a twin turbo.
The article is also wrong regarding BMW twin power turbos. They say twin power because they use a single twin scroll turbocharger.
Twin doesn't mean matching. Therefore twin turbo can be two different sized turbos. Bi just means two. My current car is an Audi BiTDI. It has one small and one large turbo arranged sequentially. It is a biturbo, but not a twin turbo.
The article is also wrong regarding BMW twin power turbos. They say twin power because they use a single twin scroll turbocharger.
Dictionary said:
something containing or consisting of two matching or corresponding parts
Warby80 said:
V8RX7 said:
Mr E said:
V8RX7 said:
Yes but one turbo - twice the size but being fed twice the air (from all 6 cylinders) would surely do exactly the same ?
But would be larger for the same peak flow, and therefore laggier and have higher boost threshold. Sorry you've lost me.
3 cylinders feed a turbo size 1
6 cylinders feed a turbo size 2
What exactly is the advantage ?
I presume there must be one but I don't see how doubling the turbo size and doubling the flow affects anything.
The downside is that as they are physically smaller they are only able to compress a smaller volume of air and tend to peak early in the rev range and limit HP.
Usually a larger turbo is used in tandem and is set up so when the smaller turbo is maxed out then the larger turbo has spooled up and can take over with higher boost pressure.
A single turbo with variable vanes has a similar effect - the vanes extend higher up the rev range allowing more boost to be made.
On the N54 (pre-2010 335i and 535i, 1M and Z4 35i) the two small turbos are single scroll and are fed from cyls 1-3 and 4-6 respectively as to feed from all would mean that there would be reversion with exhaust from one cylinder flowing back into others and preventing scavenging. The N55 uses a twin scroll turbo which separates the exhaust flow from the two halves of the engine so you avoid the problems of a single scroll turbo connected to all cylinders and can set the timing so you get exhaust scavenging.
The E63 for example uses two twin scroll turbos rather than two single scroll turbos in the C63, which is presumably partly why it can be tuned to produce 100bhp more.
The E63 for example uses two twin scroll turbos rather than two single scroll turbos in the C63, which is presumably partly why it can be tuned to produce 100bhp more.
Mines a straight 6 with 2 equal sized turbos each acting on 3 of the 6 cylinders, they are quite small, overall the engine doesn't have too much lag as it keeps the turbos spinning when you lift off, max torque arrives at 1850rpm and stays there until 5000rpm, max power is from 5000-7300rpm which perhaps shows that the 2 smaller turbos whilst great at reducing lag and shortening the boost threshold can't keep up at higher revs.
Edited by Wills2 on Saturday 3rd December 19:30
I'm no particular turbo expert, but the trade off between quick spool up speed and limited flow at high revs versus high flow at high revs but with slower spool up is also affected by blade pitch as well as physical size.
Hence why development in variable vane turbos have enables a broader power band without having to use different sized turbos.
Ian
Hence why development in variable vane turbos have enables a broader power band without having to use different sized turbos.
Ian
Screechmr2 said:
Elysium said:
Screechmr2 said:
Elysium said:
And they are wrong. Twin means 'matching' my last car a first generation 335i was a twin turbo because it had two matching turbos.
Bi just means two. My current car is an Audi BiTDI. It has one small and one large turbo arranged sequentially. It is a biturbo, but not a twin turbo.
The article is also wrong regarding BMW twin power turbos. They say twin power because they use a single twin scroll turbocharger.
Twin doesn't mean matching. Therefore twin turbo can be two different sized turbos. Bi just means two. My current car is an Audi BiTDI. It has one small and one large turbo arranged sequentially. It is a biturbo, but not a twin turbo.
The article is also wrong regarding BMW twin power turbos. They say twin power because they use a single twin scroll turbocharger.
Dictionary said:
something containing or consisting of two matching or corresponding parts
Dictionary said:
analogous or equivalent in character, form, or function; comparable.
Since analogous means comparable, similar, or like and equivalent means equal in value, I think we have to conclude that 'twin' turbos must be the same size.Sequential turbos are not the same size - so they should not be described as twin turbos.
Mine has three 
Small one at low revs to get things going which is then helped by a big one until at high revs it should run out of puff......
But id doesn't and at that point third then takes over at high revs and the engine goes all the way to a dizzy 5.5k
It's a diesel and the power just keeps on coming (and in fact up to 390bhp and 740nm torque)
You'll probably know that most diesels struggle to get past 5k revs (there is just no power gain to having a high revving diesel) and the Tri turbo BMW Diesel engine is a work of art
Small one at low revs to get things going which is then helped by a big one until at high revs it should run out of puff......
But id doesn't and at that point third then takes over at high revs and the engine goes all the way to a dizzy 5.5k
It's a diesel and the power just keeps on coming (and in fact up to 390bhp and 740nm torque)
You'll probably know that most diesels struggle to get past 5k revs (there is just no power gain to having a high revving diesel) and the Tri turbo BMW Diesel engine is a work of art
If we are talking about purely parallel turbocharging and we take a single turbocharger of "size = 1", then take two turbos of "size = 0.5". The total turbo size is the same in both cases, but the "twin" set up has more lag! Because despite what you might think, turbo inertia does not scale linearly with size ( because there are bits of the shaft / wheels that are spinning that don't contribute to energy capture or release (the shaft, wheel centres etc)) And as each turbo has half the flow to drive it, if you do nothing else but fit "twin" turbo's you gain nothing.
So why bother you may ask, well there are 2 reasons:
1) You can connect those turbo's to optimum cylinders in terms of firing order and hence capture more of the EVO blow down pulse, which does reduce boost threshold
and
2) You can generally get a lower volume of exhaust manifold between turbine and exhaust valve, so there is less heat loss and the pressure pulses are less dissipated by the volume of the exhaust manifold - again, lower boost threshold / lag
Twin scroll turbines effectively take a "twin turbine" and fit it to a single turbine, by dividing the exhaust manifold all the way to the turbine periphery. You get all the benefits of twin turbo's with none of the downsides (cost, complexity, greater lag etc) This is why most OEMs (like BMW with the later straight 6 engines) moved to single, twin scroll designs.
Of course, the better option is to move to some sort of sequential turbocharging arrangement, where the whole engine exhaust goes through the small one up to a certain mass flow, and then is routed through the big one. But this is complex, expensive, and for gasolines that run rich after start, has an impact on exhaust emissions (because it's tricky to get the catalyst up close to the head, and all those turbines and valves etc absorb heat during a cold start slowing down cat light off. For low volume, expensive niche models (911 turbo etc), gasoline variable geometery turbines bring a lot of benefits of sequential charging with a smaller and more emissons friendly package.
So why bother you may ask, well there are 2 reasons:
1) You can connect those turbo's to optimum cylinders in terms of firing order and hence capture more of the EVO blow down pulse, which does reduce boost threshold
and
2) You can generally get a lower volume of exhaust manifold between turbine and exhaust valve, so there is less heat loss and the pressure pulses are less dissipated by the volume of the exhaust manifold - again, lower boost threshold / lag
Twin scroll turbines effectively take a "twin turbine" and fit it to a single turbine, by dividing the exhaust manifold all the way to the turbine periphery. You get all the benefits of twin turbo's with none of the downsides (cost, complexity, greater lag etc) This is why most OEMs (like BMW with the later straight 6 engines) moved to single, twin scroll designs.
Of course, the better option is to move to some sort of sequential turbocharging arrangement, where the whole engine exhaust goes through the small one up to a certain mass flow, and then is routed through the big one. But this is complex, expensive, and for gasolines that run rich after start, has an impact on exhaust emissions (because it's tricky to get the catalyst up close to the head, and all those turbines and valves etc absorb heat during a cold start slowing down cat light off. For low volume, expensive niche models (911 turbo etc), gasoline variable geometery turbines bring a lot of benefits of sequential charging with a smaller and more emissons friendly package.
Max_Torque said:
If we are talking about purely parallel turbocharging and we take a single turbocharger of "size = 1", then take two turbos of "size = 0.5". The total turbo size is the same in both cases, but the "twin" set up has more lag! Because despite what you might think, turbo inertia does not scale linearly with size ( because there are bits of the shaft / wheels that are spinning that don't contribute to energy capture or release (the shaft, wheel centres etc)) And as each turbo has half the flow to drive it, if you do nothing else but fit "twin" turbo's you gain nothing.
So why bother you may ask, well there are 2 reasons:
1) You can connect those turbo's to optimum cylinders in terms of firing order and hence capture more of the EVO blow down pulse, which does reduce boost threshold
and
2) You can generally get a lower volume of exhaust manifold between turbine and exhaust valve, so there is less heat loss and the pressure pulses are less dissipated by the volume of the exhaust manifold - again, lower boost threshold / lag
At last an answer that makes senseSo why bother you may ask, well there are 2 reasons:
1) You can connect those turbo's to optimum cylinders in terms of firing order and hence capture more of the EVO blow down pulse, which does reduce boost threshold
and
2) You can generally get a lower volume of exhaust manifold between turbine and exhaust valve, so there is less heat loss and the pressure pulses are less dissipated by the volume of the exhaust manifold - again, lower boost threshold / lag
Thanks
All twin turbo cars are different.
Most twin turbos have 2 turbos the same size, Usually one on each exhaust bank.
However sequential turbos are where there are 2 in line. A small turbo and a big turbo.
The advantage of sequential turbo is theres no lag.
I think the first was the 959? But not sure, then Subaru developed it on the Legacy.
The problem was the "valley of death" changeover between the 2 turbos howver its been overcome with particular success on some Bmw models like the 335d.
The old Toyota MR2 had a twin entry turbo. Only one turbo but it must of entered the head in 2 places.
Most twin turbos have 2 turbos the same size, Usually one on each exhaust bank.
However sequential turbos are where there are 2 in line. A small turbo and a big turbo.
The advantage of sequential turbo is theres no lag.
I think the first was the 959? But not sure, then Subaru developed it on the Legacy.
The problem was the "valley of death" changeover between the 2 turbos howver its been overcome with particular success on some Bmw models like the 335d.
The old Toyota MR2 had a twin entry turbo. Only one turbo but it must of entered the head in 2 places.
HoHoHo said:
Mine has three
Small one at low revs to get things going which is then helped by a big one until at high revs it should run out of puff......
But id doesn't and at that point third then takes over at high revs and the engine goes all the way to a dizzy 5.5k
It's a diesel and the power just keeps on coming (and in fact up to 390bhp and 740nm torque)
You'll probably know that most diesels struggle to get past 5k revs (there is just no power gain to having a high revving diesel) and the Tri turbo BMW Diesel engine is a work of art
One of them is an electrically driven Turbo too. Small one at low revs to get things going which is then helped by a big one until at high revs it should run out of puff......
But id doesn't and at that point third then takes over at high revs and the engine goes all the way to a dizzy 5.5k
It's a diesel and the power just keeps on coming (and in fact up to 390bhp and 740nm torque)
You'll probably know that most diesels struggle to get past 5k revs (there is just no power gain to having a high revving diesel) and the Tri turbo BMW Diesel engine is a work of art
Yep the truth Turbo unit is a work of art probably £15k just for the engine.
nottyash said:
The old Toyota MR2 had a twin entry turbo. Only one turbo but it must of entered the head in 2 places.
Basically it's a 4-2-1 manifold with the turbo being integrated into the 2-1 bit. Air from cylinders 1 and 4; and 2 and 3; are merged by a pair of primary exhaust manifolds then fed into separate turbines (scrolls, hence the setup also being called "twin-scroll") mounted on the same shaft. The exit side of the turbo housing them combines the flows from the two turbines into a single exhaust outlet. The intake side still consists of a single compressor feeding all four cylinders.
Edited by kambites on Saturday 3rd December 20:51
V8RX7 said:
At last an answer that makes sense
Thanks
I'm a bit dim so I find pictures and a video helped me.Thanks
https://www.youtube.com/watch?v=3wFuR1x-Hes&sp...
Welshbeef said:
HoHoHo said:
Mine has three
Small one at low revs to get things going which is then helped by a big one until at high revs it should run out of puff......
But id doesn't and at that point third then takes over at high revs and the engine goes all the way to a dizzy 5.5k
It's a diesel and the power just keeps on coming (and in fact up to 390bhp and 740nm torque)
You'll probably know that most diesels struggle to get past 5k revs (there is just no power gain to having a high revving diesel) and the Tri turbo BMW Diesel engine is a work of art
One of them is an electrically driven Turbo too. Small one at low revs to get things going which is then helped by a big one until at high revs it should run out of puff......
But id doesn't and at that point third then takes over at high revs and the engine goes all the way to a dizzy 5.5k
It's a diesel and the power just keeps on coming (and in fact up to 390bhp and 740nm torque)
You'll probably know that most diesels struggle to get past 5k revs (there is just no power gain to having a high revving diesel) and the Tri turbo BMW Diesel engine is a work of art
Yep the truth Turbo unit is a work of art probably £15k just for the engine.
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