single or twin turbo for RB26

[Rexbo]

Twin turbos are put on cars for many reasons:

- Packaging. Notice that all the large single turbo setups require expensive manifolds and piping to get it to fit in the engine bay? Why not just cast simple small collectors and put two small turbos without packaging problems.

- Marketing. Two turbos MUST be faster than one... because there are TWO of them, right?

- Cost. Cast manifolds are cheap and easy to make. Tubular ones with reinforced flanges and pulse-matched length runners arent.

- PROFIT! You can sell two turbos instead of just one, so you make more money!

- Driveability. Single turbos have a habit of "hitting" harder than twins even if they make full boost at the same RPM. Not good in the eyes of tuning companies or OEM companies.

- Other... sometimes there are reasons you can't even think of off hand for why there are twin turbos on a car. Take the 2jz supra for example, toyota wanted two turbos so they could cast their own turbine and compressor housings especially for the car, and call them "toyota turbos". Dont ask me why!

180b is right, a single well matched ball bearing turbo will always outperform ball bearing twins for equivalent HP levels. There's a good reason for this, and its called "twin scroll" or "divided turbine housing". A well designed exhaust manifold will coordinate the exhaust pulses from the cylinders to hit the turbine wheel all sequentially, creating a steady and constant stream of exhaust to spool the turbo. Now think about that... which will spool more efficiently, a turbo that will get 6 tightly packed pulses of energy in addition to the normal flow volume, or two rotating masses with proportionally similar volume but 3 longer spaced pulses. Years of research have gone into this.

Thing is... thats usually fairly hard to do unless you do a lot of design work, which most companies don't want to do, especially aftermarket ones, so they slap two on that work well and call it a day.

Believe me, twin turbos work, just singles work better when done correctly. This is the key word: correctly. More often than not, its not done right, so it doesnt work right.

If you don't believe me, feel free to contact me with questions, I can find any answer you want here at the office where we make turbos for a living...

EDIT: Think of this argument as similar to the difference between a 300hp SR20DET or a 300hp RB20DET... that should raise someone's blood pressure!

[slow13dude]

So basically Rexbo, a single turbo is preferable if done properly, but a lot harder to get right (tuning, development, engineering) than just using a twin-turbo system?

[Rexbo]

slow13... almost always.

A co-worker brought up a good point though. There are really two types of spool times (transient response) that we're talking about here. The first is low RPM response where the turbo is spooling up as the engine is going from low to high rpm, and the second is instant response while the engine is at high rpm but the throttle goes from full closed (no boost) to full open, as happens on road courses frequently.

A single turbo setup will always be more efficient, you can't argue that, so you'll get higher HP at a certain boost level with a single turbo than with twins. Singles will always also have better response from low rpm to high rpm revving conditions, which usually happens on the street and on the drag strip.

Twins however, do have an advantage in one situation, and this is really only an advantage depending on the driver, car, and conditions. This is when you're already at high rpm, and are going from no throttle and no boost to full throttle and full boost, an example being coming out of a low-speed corner that the driver has had to brake hard and downshift a couple times then accelerates out of. The turbo has spooled down, but the engine is still at high rpm because of the downshifts. In this situation, the lower rotating inertia of the twin turbos will cause them to spool faster than a similar single.

This can be circumvented though. Anti-lag systems that may not work on street cars or many racing series can keep the turbo spooled up, allowing you to keep the efficient single turbo. Driver technique can be altered to account for the spool time of the turbo, such as getting on the gas earlier in the turn or even left foot braking in the case of rally cars. Sometimes it can even be ignored, because the benefit of higher horsepower at the expensive of driveability is acceptable. I put this under the "tuning" catergory of a single turbo system. Whether it be the fuel map, the driver, or the vehicle, its harder to "get perfect" a single turbo system.

So in the case of the Nismo Z-tune, the JUN cars, the Mines cars, the HKS cars, those are primarily built for time attack and tuner battles, which would favor twin turbos over a single slightly laggier turbo at high rpm, because traction is a limiting factor so increased power wouldnt necessarily cause faster lap times.

Really... its up to you.

[pnblight]

Rexbo how can you say a single is almost always better than twin when in your own words the twin perform better (recovery spool up)(less lag between gears) and are perferred all the major time attack teams. I don't know a race team in the world who would want to install a turbo system with more lag in it. (ie thats why there are basically none).

Twins give best on road drivability and spool up lag and that is why they are the system of choice of every major R&D team in the world. Nissan ,Porsche ,Toyota ,HKS ,Tomei ,Apexi how can they all be wrong????

[Beer Baron]

the only advantages I can see for singles is: cost, simplicity, looks and sound. that's it. in all other areas twins eat them.

[williamsf1]

Rexbo is right, and a lot of you guys are missing the point.... in a perfect world the single is always going to be better.... BUT few of us ever live in a perfect world, so I will take my above average twins thank you very much.....

1 = less drag/ resistance/ and a more powerful exhaust pulse

2 win because we never are operating in an engine lab at a certain RPM

[Mik]

Rexbo is right, and a lot of you guys are missing the point.... in a perfect world the single is always going to be better.... BUT few of us ever live in a perfect world, so I will take my above average twins thank you very much.....

1 = less drag/ resistance/ and a more powerful exhaust pulse

2 win because we never are operating in an engine lab at a certain RPM

Actually...no, it has been proven that in a perfect world, one turbine per cylinder would be best.

[Rexbo]

Rexbo how can you say a single is almost always better than twin when in your own words the twin perform better (recovery spool up)(less lag between gears) and are perferred all the major time attack teams. I don't know a race team in the world who would want to install a turbo system with more lag in it. (ie thats why there are basically none).

Twins give best on road drivability and spool up lag and that is why they are the system of choice of every major R&D team in the world. Nissan ,Porsche ,Toyota ,HKS ,Tomei ,Apexi how can they all be wrong????

If your read closely, a single has two VERY important things over twins: overall power and faster low-end spoolup time.

1) Overall power. What can I say, every race team needs more power unless you're traction limited. Look a champ cars, look at Le Mans cars, look at JGTC cars, Formula Atlantic, and many many others. All single turbo for that single reason. Time attack cars are race cars, but very very specialized for a certain type of racing, i.e. they are the exception to the rule.

2) Low-end spoolup time. If twin turbos were the most efficient way of making boost, WRC cars would have them. But they dont. Why is that? Its because single turbos provide the fastest spoolup from low rpm that rally cars sometimes see. To compensate for the "recovery spool-up" as you call it, they have an anti-lag system installed.

I'm not saying that all those race teams are wrong, I'm saying that they have different motivations for going twin turbo. Porsche: have you ever looked at the engine bay of a mid-engine flat-6 engine? Draw me a diagram of where a large single turbo will go. Nissan and Toyota: I dont know where they use twin turbos on any race cars, only in factory cars and again, its for driveability or packaging. The supra came with twin parallel sequential turbos so that it had a big broad driveable powerband, as did the RX-7. The 300zx came with twins for packaging, and the skyline for marketing reasons really... you can't expect your top of the line car to have less turbos than the Z do you?

HKS, Tomei and Apex'i are driven by something else: profit. They build time attack cars, which are a relatively low-budget race car (in comparison to the big boys). But more importantly they build those cars to sell their product. Now everyone knows that the higher volume of a product you sell, the more money you make. I can't THINK of an easier way to sell your product than to make your car twin turbo, so everyone else out there wants to now buy two of your turbos instead of one. You've doubled your volume! Its almost genius.

I'm not bashing on Time Attack cars, but they aren't in the top level of motorsports, where you do whatever you can, regardless of money, to go as fast as you possibly can, and that is the job of the team. Time Attack cars are not those cars. F1, WRC, Le Mans, Touring Cars, JGTC, those are top level teams. Those teams are built to beat the other team, not sell products. If you look in the top level of motorsports, you'll have a hard time finding twin turbos that are there for a performance advantage over packaging cionstraints or rule mandates.

We can argue around forever about how I said twins are almost as good as a single, but the original statement stands:

A correctly set up single turbo will always outperform twins.

Call or email any Turbo engineer you know. Call Holset, call Garrett, call Borg-Warner, call Mitsubishi, call IHI. I'd be interested in hearing a response contrary to what has been said already.

http://www.holset.co.uk/files/

http://www.turbobygarrett.com/

http://www.bwauto.com/menu01.html

http://www.ihi-turbo.com/

[slow13dude]

Nissan and Toyota: I dont know where they use twin turbos on any race cars.

The GTR was designed to win the Group A racing class.

[Sydneykid]

Wooohoo, what a fun thread...............some random thoughts......

Shoot Out, versus Non Shoot Out.....SO mode attempts to equalise the power readings by correcting for temperature and barometric changes and adjusts back to a common. For example, forget baromnetric pressure for a moment and let's say 20 degrees. If you have a hotter day than 20 degrees then the SO power will be higher than NSO power. If you have a colder day than 20 degrees, then SO power will be lower than NSO power.

The idea of SO is so someone in Darwin in summer could compare their cars power with someone in Hobart in winter. It also removes MOST of the tricks that dyno operators get up to to fudge power figures.

Speaking of tricks, I notice that it is becoming a common trick to select a high starting speed or RPM for the power graph, makes the engine look good from an average power point of view. But it doesn't fool me, if it hasn't got 50% of its maximum power within 3,000 rpm of its rev limit, then it's a DOG with a standard ratio gearbox. Stick a close ratio dog box in it and it's a different matter.

As for big singles and drag cars, last time I looked the Croydons GTR had a big single and it ran 8.5 sec at 165 mph.

Many regard the BMW F1 Turbo engine as the most powerfull of the 1.5 litre turbo era, 1300 bhp in qualifying and yes, it ran a big single.

WRC cars run singles because they are required to have a SINGLE 34 mm RESTRICTOR located not more than 50 mm from the compressor, that's pretty hard to do with twins.

The only Japanese time attack circuit that has any meaning is Tsukuba and that suites twins because of its layout. If it was Philip Island, then maybe a big single would be the go. Sure the 2 fastest GTR's at Tsukuba last year were running twins, but they were both beaten by an Evo running a big single.

Vehicle manufacturers liked sequential twins because they can more easily pass emmissions standards. The very first Porsche 911 Turbo had a single KKK turbo mounted behind the LHS rear wheel.

Champ cars have a single turbo because the regs say they have to.

For every example of a twins being used effectively, there is an example of a big single being used equally effectively. It depends on which side of the fence you sit as to which example you will quote.

[Nugget_GTR]

Low-end spoolup time. If twin turbos were the most efficient way of making boost, WRC cars would have them. But they dont. Why is that? Its because single turbos provide the fastest spoolup from low rpm that rally cars sometimes see

im not saying one is better than the other i would go single if i wasnt so lazy with having to change everything and ive seen awesome results from both, but you cant include WRC in this discussion they are running small single turbos not a big single turbo so of course they will have quick spool up times. hell once there running they dont go off boost due to the anti lag system.

so if they but on two smaller turbos to equal the power of the single they use.. hell they would be small... rally teams are wanting reliability over anything you add another turbo in the mix thats one more thing that can go wrong, in a car that is getting the rapping of its life not to mention extra cost in turbos and plumbing. so its sensible to have one, plus there restricted to rules they must keep it close to factory.

[R32-GTS]

Toyota was also another manufacturer who changed from twin turbo;s on there 1jz to a single turbo version later on. You hear everyone talking about how much better the later model 1jz is with quiker spool , more torque , and more top end power so all this talk of twins coming on quicker , less lag between gears is utter bullshit.

[Rexbo]

yeah rally cars are an interesting example... take the group B era for instance. Thats where I think most of the good research came from. There were cars that were super-turbo'd (supercharged and turbo charged), twin-turbo'd, single turbo'd and just about everything else you can do. What came out of it all at the end? A single turbo and a restrictor. The 30mm restrictor within a certain distance of the turbo is specific for the WRC class of cars which was created in 1998, before that, the Group A cars were allowed any number of turbos that passed through a single restrictor that had no distance limit behind it.

Same goes for champ cars, they used to not have to run a spec turbo, but now the weenies who make the rules say that everyone has to run a single Garrett TR30R turbo. How lame.

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? Now have that compressed air, say 5psi, flow into a little GT22 or GT25 that was also being spun by the exhaust. Say that's also set at 5psi of boost. its not just 5+5 is 10 psi, you would have a total of 5+5+14.7 = 24.7psi of boost goin into the engine! A ridiculous setup and far less efficient than just getting a gt3071r or something and doing the same thing, but still it would be a trick little setup hahah.

[dangerous_daveo]

So would say a split pulse either even or offset be better than say one big mouth? Obviously with a well set up exaust manifold that also pulsates.

[dojobi]

I used to always think that twins were better than singles. 2 small turbos MUST spool faster than 1 big one, because they're just so much smaller (at least that was my thinking). The one thing that I was always forgetting was that you only have 1.3L of engine capacity to spool up each turbo, so it's going to spool up a lot slower than you'd think. I bet a lot of people that blindly assume twins are faster forget this.

Good thread btw.

[DoughBoy]

What ever happened to my good friend Inertia?

[JAGR33]

What ever happened to my good friend Inertia?

You wanna elaborate on that Chris???

[DoughBoy]

Twins have got the advantage over singles due to lower inertia, something that no one has made reference to or even acknowledged in this thread. Lower moment of inertia, quicker spooling time.

And another point in the large single's favor is the maximum pressure ratio is significantly larger than a set of twins. All the more pressure to boost you with, my dear.

Both have their good and bad points, and either will work pretty well as long as it's matched to the application. I think alot comes down to preference (rice factor?) and budget.

[dojobi]

I'm sure Inertia would make quite a difference. Have you got any idea of how much?

One other thing that hasn't been mentioned (as far as I can see) is friction loss from having 2x the moving parts. The twins would have to be less efficient. Even if it is just a matter of a 2 or 3%.

[DoughBoy]

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?