single or twin turbo for RB26

[R31Nismoid]

wouldnt you get the same area with a big single and a big exh wheel?

[DoughBoy]

Oh and for inertia:

Inertia = radius of gyration^2 x mass

As you can see, a large wheel will dramatically increase inertia.

[dojobi]

Ahh if you want to quote friction loss from twin turbos, how about the inherant flow advantage from twice the turbine exit area on twins, and the cooler running from having twice the coolant capabilities?

You certainly know your physics DoughBoy

[DAN00H]

Rexbo

Come on mate - when was th last time F1 was turbo!! they are not running 'a big single' they are NA!... Audi ran twins in the LM cars, as did toyota & nissan THIS YEARS WINNER Bently speed 8 - OH LOOK!! TWIN TURBOS - SHOCK!!!. Ever look at the serria cosworth 500? comments from the drivers of the time speak of the difficulties when the thing came onto boost - it ran a single and raced against the TT GTR... JGTC - the R34 ran the V6TT that is now in the Zed.... I think you will find twins are used for more than just packaging or sales hype.

One way to put this to bed, is get two IDENTICAL engines and slap on a set of twins on one and a single on the other. Careful attention would need to be paid to the manifold design of both to ensure they both got the best gas flow. Otherwise, same came, ecu injectors etc.....

[DoughBoy]

I guess looking at a performance application on a Skyline, you could run twin 3 inch pipes off twins, or a single 4 inch off a single? Maybe even a 4.5inch, although this would impinge on your arguements that a single has more space - ever tried to fit a pipe that size anywhere on a car?

Twin 3 inch area = 14 sq inch

Single 4 inch area = 12sq inch

Also the flow on the turbine inlet/ turbo flange could be another comparison. Funny side story, they have actually started to put flange size restrictions in on turbo drag classes in the states, guess there must be some hidden horsepower in having the largest flange possible?

[DoughBoy]

You certainly know your physics DoughBoy

I failed physics, so I know this is a sarcastic stab at me

[R31Nismoid]

Oh and for inertia:

Inertia = radius of gyration^2 x mass

As you can see, a large wheel will dramatically increase inertia.

would be interesting to see just how much

[DoughBoy]

The problem is actually getting the wheels seperate to weigh them - you can extrapolate as much as you want but in the end it's only going to be an approximation.

I think in the end - large single for drags, small twins for circuit/street. It's been proven time and time again, but like I said you can make any combination work with sufficient resources and planning.

[Roy]

I failed physics, so I know this is a sarcastic stab at me

LOL...you learn what you want to learn, and forget the rest, because of that the bastards fail you:)

[DoughBoy]

AFAIK cones weren't part of the curriculum

[Rexbo]

Rexbo

Come on mate - when was th last time F1 was turbo!! they are not running 'a big single' they are NA!... Audi ran twins in the LM cars, as did toyota & nissan THIS YEARS WINNER Bently speed 8 - OH LOOK!! TWIN TURBOS - SHOCK!!!. Ever look at the serria cosworth 500? comments from the drivers of the time speak of the difficulties when the thing came onto boost - it ran a single and raced against the TT GTR...  JGTC - the R34 ran the V6TT that is now in the Zed.... I think you will find twins are used for more than just packaging or sales hype.

One way to put this to bed, is get two IDENTICAL engines and slap on a set of twins on one and a single on the other.  Careful attention would need to be paid to the manifold design of both to ensure they both got the best gas flow. Otherwise, same came, ecu injectors etc.....

The last time F1 was turbo was 1988... running single turbos. The Le Mans cars now run twin turbos due to packaging constraints and rule mandates.

As to the rotating inertia, that is a very good valid point, which is why i say twins have better response at already high rpm, because they do have less rotating inertia. However there is a critical key missing from this, and its not pulsematching or inertia, its the pressure differential and efficiency on the turbine side of the turbo.

A single turbo will have a larger pressure differential across the turbine wheel than twins, causing faster spoolup than equivalent twins. The spoolup capability due to the pressure differential is also largely due to turbine efficiency. Larger wheels will always have higher efficiency than smaller wheels for the same reason that large combustion chambers have higher burning efficiency in and engine. There's less internal loss due to heat transfer and friction. Twin small turbos create lots of drag on the exhaust gas from the turbine housing walls and the turbine wheel blades. Also, more of the heat from the engine is transferred to the metal in the turbo, taking away from the energy of the exhaust gas. Now when i talk about this, im talking about terms of 3-5% less efficient than a single turbo. Its not much but everything helps, thats why extrude honing your exhaust housing is beneficial.

Something else that someone brought up at work is that the advantage of a single turbo dissipates as the number of cylinders of the engine goes up. If you look at the combustion cycles of engines, a 4 cylinder engine takes 2 rotations of the crank to fire all 4 cylinders, and the subsequent exhaust gas pulses are easily tuneable to hit the turbo efficiently, more so than tuning for 3 cylinders that don't fire evenly. Also when dealing with V-engines, a single turbo setup requires long exhaust manifold runners and a lot of space, which would be beneficial on a V6, but on a V8, V10 or V12, the story would be different.

Its all a tradeoff between cost and benefit, as is all racing. Cost not being money, but performance, packaging, weight, and in the end, its all speed in a race car.

This is a really good thread, I like it!

[DoughBoy]

This is about street cars, that might see a mild amount of circuit work. Drawing comparisons to F1 and "ideal" systems is bollocks, cause it ain't going to happen this weekend. Oh how large is an F1 motor? 1500cc? Have you seen a F1 turbo? UAS John has had a couple come through his hands from Keith Carling's car, and they aren't that big at all - hell two were used on a 3,000cc VG30 engine. So what's the deal there? They also spun at unrealistic levels to maintain sufficient boost pressures - I think the comp map went up to 4 bar?

Use a real world example - say 1000hp on an RB26. From the Garrett catalogue, this would see a combination of either

1/ Twin GT30R turbos or

2/ One GT42RS turbo

Looking at the max turbine efficiency graph the GT42RS comes in at 69%, the GT30R at 72%. I'm not sure I can trust Garrett's published figures, but take it with a grain of salt they are correct. Where is the uber efficient large wheel here?

If the pressure differential created say "x" amount of force, bearing in mind that both the inertia and turbine efficiency of the smaller turbo is superior, how in hell would that spool a large turbo quicker?

[Butters]

both the inertia and turbine efficiency of the smaller turbo is superior[/qoute]

Am i missing somthing ..... yes they intertia is less but dont you have 1/2 the amount of exhaust gas to move it ???

Hence it would have to be twice as effiecent "inertia" wise then a single turbo to be the same .......

[dangerous_daveo]

So is a split pulse turbo better or worse???

Im guessing in the case of twins normal would be best.

Also is a even or uneven split pulse turbo better than the other??? Assuming split pulse is better.

[DoughBoy]

Yes but since inertia is a square of the radii it is not proportional. If you half the radii you quarter the inertia.

Split pulse housings have a higher efficiency than normal open housings.

[Sydneykid]

Oh and for inertia:

Inertia = radius of gyration^2 x mass

As you can see, a large wheel will dramatically increase inertia.

Yes but since inertia is a square of the radii it is not proportional. If you half the radii you quarter the inertia.

Some more to think about............

The 2 X turbines/shaft/compressor of my 2530's weigh more than 3 times as much as the 1 X titanium turbine/shaft/ compressor of my T66. Plus the ceramic ball bearings used in the T66 have ~30% less running friction than the plain ball bearings used in the 2530's.

I'm too tired ot work it out..................

[DoughBoy]

So in effect your trick, no doubt cheap and mass produced turbo combination, weighs approximately 1.5 times a single 2530 rotating combination. Apples for zuchinni flowers maybe?

Doing a gross injustice and highly wrong calculation on it,

2530= exducer dia = 60.1mm, mass = 1, Inertia 903

T66 = exducer dia = 91.5mm, mass 1.5, Inertia 2093

So 2x 903 =.... 1806?

Also, I'm assuming that the measured reduction in frictional losses are from equivalent sized bearings? Take into account that most Txx series use the large 3/8th (9.5mm) shaft, where a 2530 has a 5mm shaft size? So the actual bearing surface is twice as large assuming same width, which is probably wrong as well since the larger shaft would need a wider bearing surface too? So if you assumed the bearing was only 1.5 times the width, that would still give you a surface area 1.5 times the amount found on 2x 2530's, less 30% for innovative design of the ceramics. So 20% more friction end result on the T66?

Yeah it's late and I am playing devil's advocate, a well as making absurd guesstimates. The T66 probably does everything the 2530's do, and is easier to work on - and gets alot of ooohs when you pop the bonnet

[DoughBoy]

T66? Why would someone use an old truck turbo design on such magnificent late model technology cars? Surely one would see better use on a Canter than a Skyrine? >

[slow13dude]

OK, I see Mazda is also populated by morons:

FD RX7

Years Available: 1992 to 2003

Engine: 13B (1308cc) twin rotor (2 x 654cc) EFI Twin Sequential Turbos with Intercooler

Hardly 30 years ago.

[DAN00H]

Now i like the twin turbo idea where they're sequential sequence turbos. You have one large turbo spool into another smaller turbo. Say your "large" turbo is a GT28 on an RB25. That would spool up plenty fast for everyone, agreed? taken from post 73.

Now having had a very detailed look at the bandag bullet and discussing it with the owner, it doesn't work this way... If anyone is not familiar with it, it runs 2 V8's in tandem, 2 x superchargers and 4 turbos. it is the 2 smaller turbos that are feeding the 2 larger units, not the other way around.