Single or Twin Turbo

[Ashe]

I know that the Straight Sixes see power gains and faster spooling when going from twin to a big single turbo.
However if I were to turbo a 3VZ would I see better gains from running a simple twin turbo into a single intercooler, or would a larger single with the more complicated exhaust piping be a better choice? Not talking about extreme numbers like 3 bar boost or 1000HP, just a decent 1.5 bar boost with a fairly quick spool up.

This is more of a "how would that work?" than "I'm getting ready to do it right now" thing.

I'm thinking that 2 decent sized turbos (GT35R size) with the short simple piping would be better. but I know there are people much much more experienced than me here.

[jonra23]

If both systems are of the same quality and sized for similar top end flows twins will generaly spool quicker than singles for a couple reasons.

1. Turbos get their energy to accelerate (spool up) from the pulses in an exhaust system rather than just mass flow, by having two seprate systems you get greater pulsing, this is why on single turbo setups a "spilt pulse" turbo will spool faster, especialy if plumbed so that each consecutive firing cylnder goes to diferent side of split housing.

2. Two small turbos with the same flow rating as a larger single have less mass to accelerate (not as influencing to spool up as above)

Spool up speeds in any system can be generaly increased by running a decent electronic boost control rather than a bleed system.

Being a Vee engine will be much easier to plumb up a decent twin turbo system than trying to get reasonably equal runner lengths to a single turbo system.

regards
jon

[JustCallMeOrlando]

Straight sixes will only see power gains if they go to a bigger turbo. If the twins are of comparable size then they'll make as much power.

Same goes for spool up, the twins have a theoretical advantage in spooling.

As Jon said, due to it being a V, it's always going to be easier to do a twins setup if you have the room.

As for your turbo setup, that's just nuts. One GT35R would be capable of 300-400rwkw on a 2JZ or the like, and you want to use two! I reckon you'd make full boost by about 7000rpm.

[mic*]

The pulsing theory is highly dependant on exhaust manifold design. If the manifold does not provode equal length runners to the flange, the pulse frequency will be irregular and dis is bad. Generally the less pulses hitting the wheel, the more imporatant it is to get the manifold spot on. Eg 4cyl vs 6cyl.

Jon, i disagree with point 2. Each turbo has half the exhaust gas flowing. Inertia is proportional to the square, and flow is proportional to the cube of exhuast energy.
(EDIT: Meaning that halving the flow, more than halves the inertia of the gas, which is what mostly determines spool up speed or lag)

Probably the biggest advantage of twin is that the exhuast (turbine) discharge area will always be bigger for two turbos of equal flow to one large one. Ie lessback pressure. Means more outright power potential. This is often traded off for smaller turbo's reducing boost threshold significantly, and lag somewhat (in the area that was below, or near the big single's boost threshold ).

Further advantages are that boost is more stable, and that the heat is absorbed by two units so in some situations they run cooler = longer life & maybe a couple more kW's.

As a general rule a bigger turbo is more efficient than a smaller one. But total power is the sum of all your efficiencies.

Plumbing an exhaust on a V style engine to one turbo is very difficult to do without losing more efficiency than you stand to gain. So to speak...

[chris davey]

And the biggest disadvantage of twin turbos is that there is twice as many things to break!

However, in a v engine twins do make sense. Mate had a single turbo v6 with a crossover pipe and it always had exhaust leaks no matter how well it was braced.

[jonra23]

The GTR skylines come with a RB26DETT, 2.6 litre twin turbo, they would be pretty good for a 3.0 litre V6.

Come in ceramic and steel rear wheel setups, the steels are generaly on N1s or rebuilt ceramics.

The ceramic rear wheels have abit of habit of falling off if getting adventurous with boost.

[lumpy]

When thinking of turbo sizing with twins, you need to split the engine capacity in half. ie for twins on a 3vz, you need a turbo that will be in it's efficiency island when flowing exhaust from a 1.5lt engine generating 1.5bar of boost. I don't think a GT35R, even with a small exhuast housing will be suited to a 1.5L 3 cyl engine so two on a 6cyl 3lt won't do too good either.

I think power gains and faster spooling when going twin to single have more to do with the design of the single turbo (and new manifolding) than anything else.

[jonra23]

Sorry, my mistake.
didnt read properly, thought you were talking about a pair of turbos off of a GTR skyline.

[Ashe]

Heh, for some reason when I did my calculations on turbo size I forgot to divide engine capacity in half.
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Well running twin turbos would definatly make the turbo piping simple, I was racking my brain for a way to have the flows arrive at aropund the same time as opposed to one side taking 3-4 times as long to get to the turbo.

[mic*]

Gt28R would be my choice from the Garrett range. For a twin setup @ 2.5 pressure ratio on the 3VZ that is.