What are the benefits of twin turbo?

[327ire]

Would 2 turbos producing a collective 8psi make more HP than one turbo producing 8psi? Or are the benefits purely to do with lag?

[327ire]

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In a twin setup , one small one big sized,... could they work properly if they got 3 cylinders each from a m20b25 or would the small one begin to make problems after 4-5-6000rpm?

[tomtomiS]

the long and short of it is using a twin setup with small and larger turbo is that the smaller one spools up quicker to give more low down grunt while the larger one spools up shortly after and takes over, giving more boost than the smaller one. they're all a bit fancy these days with ecu's controlling boost pressures and wastegates etc.
vw do a TSI engine in the mk5 golf which is a 1400cc with a supercharger to aid the low end and a turbo that comes in later in the revs.

[mhuk]

the long and short of it is using a twin setup with small and larger turbo is that the smaller one spools up quicker to give more low down grunt while the larger one spools up shortly after and takes over, giving more boost than the smaller one. they're all a bit fancy these days with ecu's controlling boost pressures and wastegates etc.
vw do a TSI engine in the mk5 golf which is a 1400cc with a supercharger to aid the low end and a turbo that comes in later in the revs.

OK a little off topic but reminds me of a old mate of mine who built this engine (b18ft) for his volvo 480.

[Turbo-Brown]

There are two types of twin turbo setup.

The parallel type (as used on the current petrol 335i) where you have two equal sized turbos, each feeding from three cylinders. This system doesn't have any clear advantages over a larger single turbo in terms of power/psi and spool up won't be all that different to a large single as each of our two little turbos only gets half of the engine's exhaust gas.

The other main type is where you have a small turbo and a large one (as used on the current 535d) where all of the engine's exhaust is sent through the small turbo at low engine speeds and gradually, as engine speed increases, the smaller turbo is blended out and the larger one takes over.

The advantage of this type of system is that you can largely eliminate lag but not suffer a power cap due to small turbo sizes.

There are variations on a theme like the Supra twin turbo system which used equal sized turbos but could sent all the exhaust through one at low engine speeds, again to reduce lag.

[327ire]

Thanks guys. Turbo-Brown what turbos were you running with your twin setup? IV seen guys selling twin manifolds and claiming 270+bhp with 8psi. This has got me thinking about it, but after the replies above there isnt a great advantage of having 2. (considering you have 2 turbos that can give trouble rather than 1)!

[327ire]

So how about this,..

2 turbos each getting 3 cylinders, both ct26's from supras. One has one of the inputs covered so the exhaust housing is getting about .20a/r. The other has both inputs open (twin scroll). Would the closed one suffer after 3-4k revs?

[Turbo-Brown]

I was running a pair of Skyline Garrett T28 turbos, and my system did make 270bhp on 7psi.

There was nothing my system could do that a single turbo couldn't have done I don't think, I just used two turbos as I had them kicking about.

Think your engine would suffer more from the increased exhaust restriction.

The supra system is able to divert all gas to one turbo for low speeds, and then divide it equally between turbos for higher speeds.

[jaistanley]

[insert a page of complicated thermofluids here]

The above complicated maths that I CBA to find in my uni motes and type up tells you that a single turbo is more efficient and properly tuned will out-perform twin (parallel) turbos. At the extremes smaller turbos will add more heat and can cavitate at the compressor tips and cause damage.

In all honesty unless you want to go down the route turbo brown has (and put in all the fantastic engineering he has) it's pointless on our M20's. Just go for a decent, well specced single.

Jai

[E30BeemerLad]

I thought the turbos on the twin supra were different sizes on the sequential setup with the 2JZ engine found in the MK4 supra?

The 2.5 1JZ engine found in the Soarer Twin Turbo uses parallel setup of CT12a's I believe

The MK3 Uk spec car had a rather large CT26 and was one of the biggest turbos fitted to a standard production car. I believe the MR2 turbo also has a CT26 but it is a totally different animal to the unit fitted to the MK3 sup

[ShakeyC]

A complicated exercise in achieving nothing better than a simple one could, the difference is only really in how they drive on the road. Most twin setups drive like donkeys VW TSI is shockingly bad to drive the Legacy twin seq turbo's are thirsty beasts compared to older single snails and all seq turbos have a torque trough in the midrange no matter how smooth the dyno graph looks.

[Turbo-Brown]

A complicated exercise in achieving nothing better than a simple one could, the difference is only really in how they drive on the road. Most twin setups drive like donkeys VW TSI is shockingly bad to drive the Legacy twin seq turbo's are thirsty beasts compared to older single snails and all seq turbos have a torque trough in the midrange no matter how smooth the dyno graph looks.

Well that's a sweeping generalisation!

My old twin turbo would return 29mpg and certainly didn't have a hole in the midrange torque, nor is there any reason why a well thought out twin turbo system should have!

[ShakeyC]

I had legacy's personally the older single snail is way better drive and more frugal than the newer GTB 36mpg opposed 22mpg, VW TSI i had for few weeks in 07 was rubbish, old audi+volvo TT conversions all sequential setups are the same. Sequential only really works on engines bigger than 3.0l as turbo can be better spec'd so when transitions occur its smoother but on the whole torque trough always present.

The serial turbos like old and new Toyota's, Lexus, Nissan and diesels are much better to drive and run because in essence these TT setups are treated as single ones on half the engine volume.

[Turbo-Brown]

Aah sorry, thought you were talking about twin turbo setups including parallel systems.

Certainly there's potential for there to be a drop of (or massive spike!) in torque when the transition between small and large turbos is happening.

[Rodderz]

I can confirm that the Legacy twin turbo does have a flatspot between the 2 turbos that you can get caught in,and they have a horrendous thirst! I now have the latest Legacy GT with the twinscroll turbo which is a fantastic setup and car!

[Omi]

They call it the Valley of Death on the Leggy boards.

Actuator moves exhaust feed from smaller turbo to the larger one, resulting in instant drop-off from the smaller turbo and a moment of lag while the bigger one spools up.

Can be helped a lot by raising the RPM of the swap, so the smaller turbo has already peaked and there's faster flow to spool up the 2nd turbo...

I get my GT-B in a week or so, 1996, white, estate, manual, decent miles and fresh import... Anyone want to tell me about engine swaps?

[ShakeyC]

To be honest Leggy was best all round car i ever owned, did you know in 90's it was worlds fastest estate car money could buy at the time! GTB was developed around the fatherland's ring piece.

Only 2 things i recommend converting it to single turbo which doubled my fuel mpg and waxyoil the hell out of it, leave suspension and all that alone its bang on and heavy clutch and control you get used to quickly its heavy yet slick.