Single vs. twin turbos ??

[Turbo231]

What are the advantages of twin turbos over a larger single turbo? If I have no space limitations and no rulebook to follow, at what engine horsepower should I move from a large single to a twin turbo setup on a typical V8?

[SupStk]

Good question. Figured it was easier to package two small turbos than a large one on V engines. Seems that the smaller turbines would spool faster and be more efficient closer to the exhaust source.
But then again what I know about turbos would fit in a thimble with lots of room left over....

[Frankshaft]

The advantage to 2 turbos, is they can be smaller, which makes them spool quicker. They also are easier to fit, I think. 2 small turbos that provides the same volume of air and boost as 1 big turbo, is easier to tune, drive and manage the power better. Plus like I said above, they spool quicker. Or, you can use 2 big turbos and make even more power. But personally, I don't like them. I like my engines to make power because they want to and can, not be forced to.

[Newold1]

From what I understand from the turbo experts, on a V engine (two heads) the twin turbos are also closer to the exhaust valves and the extra hot exhaust also produces more power in a turbo.

[gruntguru]

Advantages of single turbo.
Lower cost
No boost balancing issues
Higher efficiency/horsepower
Simpler plumbing (except possibly headers)

Advantages of twins.
Shorter exhaust runners (especially on Vee engines)
Fewer exhaust ports/turbo. Blowdown energy capture not possible for single turbo beyond six cylinders.
Less inertia, less lag.

[mekilljoydammit]

First off, a lot of the time when people talk about lag they really mean boost threshold. Boost threshold is illustrated by what RPM the turbo can start to spool / etc, lag is illustrated by how fast it comes up to speed if the engine is over boost threshold RPM and you nail the throttle. Sure two smaller turbos will have less inertia but they also each get half the exhaust energy.

In the real world it gets down to particulars. Plumbing may or may not be easier with twins, etc but for performance either can work. A thing that gets overlooked sometimes is that a lot of bigger single turbos are designed to work more efficiently at higher boost so for some engine configurations running two smaller turbos of the same flow potential might be more efficient where the engine is designed to work... or vice-versa. Pulling stuff out of thin air with the first turbo maps I opened for an imaginary V6 vs I4 at 600hp, 600hp on the V6 might be 15psi of boost where a pair of GT2860RS would be more efficient while on the I4 600hp might be 25psi where a GT4088R would be more efficient.

Nowadays there's lots of turbo options - there's off the shelf options that will do up to about 3000hp worth of air flow per turbo so "needing to move to twins above a certain horsepower" isn't necessarily a thing if packaging isn't at play.

[Turbo231]

Great comments here. Let me throw out a more focused scenario. I am building a G-body that weighs 3500 lbs and I want to drag race it. Building a decent 350 SB that would be about 300 hp NA with bottom end built for boost (8.5/1 CR). Automatic trans and convertor still to be bought. I have space to do either a single or twin turbo system and a great fabricator to route tubing. I want the best ET I can get. And I want to limit boost to 25 psig. So, big single or twins?

[mekilljoydammit]

Doing lazymode numbers, 25psig is a pressure ratio of 2.7, so around 800hp or 80lb/min. Call it a PR of 3.0 at the turbo. 80 lb/min and 3.0 PR matches nicely to a S364SX-E BW turbo, maybe a S467SX if you want a bit more headroom. For twin turbos it would be basically any of the S200 turbos and you'd have headroom. Or for Garrett a pair of GT30s or so. I'd do a single BW journal bearing one in this case (actually helping a coworker spec stuff out for similar) as it's not into the huge expensive ones and the BW journal bearings turbos seem like pretty good bang/buck compared to the ball bearing Garrett stuff lately... I'm just doing the numbers though.

[rayhall]

What are the advantages of twin turbos over a larger single turbo? If I have no space limitations and no rulebook to follow, at what engine horsepower should I move from a large single to a twin turbo setup on a typical V8?

Forget any comments about lag. The real problem is this,

With one turbocharger on a V type engine the exhaust back pressure will be a lot higher then two turbocharger's when mounted close to the cylinder heads. High exhaust pressure means problems with cylinder filling, less power and a lot more stress on the engine. The reason for the high pressure is due to exhaust manifold design. It is almost impossible to make a single turbo manifold that is efficient. Your test is always exhaust backpressure.

If your exhaust back pressure is two or more times the boost pressure, your turbo system is badly designed. You want exhaust pressure less then boost at WOT. A good design system on a high boost setup will have exhaust backpressure 15 psi or more lower then boost pressure.

Ray.

[ptuomov]

What’s the rpm range, what’s the bore spacing, and are you willing to plumb two twin scrolls (one twin scroll per side)? The two twin scrolls is the very best one can do with a cross plane V8 if the bore spacing is wide and peak power rpm is high. If the bore spacing is close and if the peak power rpm isn’t too high, then one can do well with two single scroll turbos, too.

[rayhall]

What’s the rpm range, what’s the bore spacing, and are you willing to plumb two twin scrolls (one twin scroll per side)? The two twin scrolls is the very best one can do with a cross plane V8 if the bore spacing is wide and peak power rpm is high. If the bore spacing is close and if the peak power rpm isn’t too high, then one can do well with two single scroll turbos, too.

If you treat the engine as two four cylinders, with dual entry turbine housing and dual wastegates, then exhaust backpressure will be better. Doing this with a engine that fires twice on one bank, is even more important. Still no where as good as two turbos on well designed manifolds. Even a road car can get exhaust back pressure 2psi or less then boost.

I have had a engine that made a disappointing 400 HP. Moving the wastegate from the original position by 2-1/2", dropped exhaust back pressure by 9 psi and the engine made 100 more HP at the same boost.

I was told by a Garrett engineer in 1980 about the importance of exhaust backpressue. Through all my years of turbocharging and racing the exhaust backpressure gauge was my most important tool.

[user-23911]

It's a common argument with Mitsubishi V6s.


A well built TT (stock engine) setup will have a good power band from 2000 RPM to 6000 RPM.....or in my case 3000 RPM to 8500 RPM with good heads and cams and bigger turbines.
A big single setup will have a good power band from 4000RPM to 6000RPM on the same stock engine or a higher redline if the heads will breath enough.

There's no comparison.
2 is always better than 1.


The only downside is finding the extra money for the other turbo.
A V engine and single turbo.... with all the extra piping, what usually happens is that the pipes end up cracking somewhere, heat shielding is often a problem followed by fire.

I've built both sorts.........more than once.

My next one (Cleveland V8) is going to have 4 small turbos.....only because I've got them spare.
Small ones will give instant boost at 1000 RPM upwards.

[ptuomov]

What’s the rpm range, what’s the bore spacing, and are you willing to plumb two twin scrolls (one twin scroll per side)? The two twin scrolls is the very best one can do with a cross plane V8 if the bore spacing is wide and peak power rpm is high. If the bore spacing is close and if the peak power rpm isn’t too high, then one can do well with two single scroll turbos, too.

If you treat the engine as two four cylinders, with dual entry turbine housing and dual wastegates, then exhaust backpressure will be better. Doing this with a engine that fires twice on one bank, is even more important. Still no where as good as two turbos on well designed manifolds. Even a road car can get exhaust back pressure 2psi or less then boost.

I have had a engine that made a disappointing 400 HP. Moving the wastegate from the original position by 2-1/2", dropped exhaust back pressure by 9 psi and the engine made 100 more HP at the same boost.

I was told by a Garrett engineer in 1980 about the importance of exhaust backpressue. Through all my years of turbocharging and racing the exhaust backpressure gauge was my most important tool.

In my experience, the bore spacing matters with single scroll twin turbos (one per cross plane V8 bank). I have 122 mm bore spacing, which means that with compact manifolds the engine runs great in the 3500-6000 rpm range. At low rpms, the 180 degree exhaust blowdown interference screws up the valve overlap of four cylinders. At high rpms (above 6000 rpm) the 90 degree exhaust blowdown interference screws up the overlap of two cylinders. But in that 3500-6000 rpm band it runs like a dream, and that’s a wide enough power band for a five speed street car and my driving.

[In-Tech]

What are the advantages of twin turbos over a larger single turbo? If I have no space limitations and no rulebook to follow, at what engine horsepower should I move from a large single to a twin turbo setup on a typical V8?

Forget any comments about lag. The real problem is this,

With one turbocharger on a V type engine the exhaust back pressure will be a lot higher then two turbocharger's when mounted close to the cylinder heads. High exhaust pressure means problems with cylinder filling, less power and a lot more stress on the engine. The reason for the high pressure is due to exhaust manifold design. It is almost impossible to make a single turbo manifold that is efficient. Your test is always exhaust backpressure.

If your exhaust back pressure is two or more times the boost pressure, your turbo system is badly designed. You want exhaust pressure less then boost at WOT. A good design system on a high boost setup will have exhaust backpressure 15 psi or more lower then boost pressure.

Ray.

If this is Ray from Australia, hello from Carl in California that worked with GT. I think you mis-typed. There is very little way possible for the boost to be higher than the back pressure or the turbo will spin backwards. Yes I am quite aware that the pulses are critical and although hard to achieve boost lower than back pressure, it can be done slightly, most STRIVE to achieve 1:1.

[Atom]

I have had a engine that made a disappointing 400 HP. Moving the wastegate from the original position by 2-1/2", dropped exhaust back pressure by 9 psi and the engine made 100 more HP at the same boost.

Would you care to elaborate on this?

You always come across these pics when talking about wastegate placement.
But I can see the location for best control being worse for back pressure.