Twin Turbo vs. Single, difference?

[Guest InterSpool]

Is there any difference between the output potential of a TT or single turbo setup?? The way I see it, exhaust is the limiting factor when it comes to spooling turbos, so what is the point of having two? Shouldn't one big turbo with the same CFM as two smaller turbos do the SAME EXACT amount of work? Enlighten me.

-Andrew

[Brad-ManQ45]

Total output would not be the deciding factor. Spoolup behavior and shifting under acceleration are the factors you would make the decision on.

 

What size engine and amount of boost? If you can get the same efficiency w/one turbo as you can with 2, then whaht is the car used for?

 

If drag racing, the single would probably be better, because during shifting the bigger will not lose as much speed (even in the short time of a shift).

 

For autocross/roadrace, two smaller turbos would spin up faster for better response when starting to accelerate.

 

Please note the condition that I assumed - the same effeciency and airflow from each setup.

 

You would probably have better midrange on the twin and better top end on the single.

[SleeperZ]

If drag racing, the single would probably be better, because during shifting the bigger will not lose as much speed (even in the short time of a shift).

 

For autocross/roadrace, two smaller turbos would spin up faster for better response when starting to accelerate.

 

Please note the condition that I assumed - the same effeciency and airflow from each setup.

 

Given the conditions you assume, same airflow and efficiency, I believe the single will spool faster. I am assuming you are running parallel twins, each driven by three cylinders. In order to spool a turbo with half the exhaust flow, you need a much lighter turbine to equal the spool characteristics of a single with full exhaust flow. And I'm not talking a turbine with half the inertia, I'm talking twins with 1/4 the inertia of the single.

[Guest InterSpool]

So parralel turbos are in a sense, inferior to a good single setup?

[Clifton]

So parralel turbos are in a sense, inferior to a good single setup?

 

I wouldn't say inferior just more costly and complicated. The 2JZguys all seem to go single and they make silly HP. You can have a big single spool fairly fast, TimZ has a BIG turbo and has a nice torque curve. http://hybridz.org/nuke/index.php?name=PNphpBB2&file=viewtopic&t=32009.

[Brad-ManQ45]

Nathan - I'm just repeating info from Hugh MacInnes "Turbochargers" on spool up.

 

What you are not taking into consideration is the very large diameter of the single turbo's turbine and associated weight further from the shaft.

 

His example stated that in a typical scenario that the moment of inertia for the large turbo was 2.5 times greater than for the small turbo.

 

By my math, that means that there is a 20% hit on how quickly the single turbo will spool given double the exhaust volume...but I'm no math expert.

 

I will quote him now:

 

"Where turbocharger acceleration is important, I recommend you use two small units instead of one large one."

 

This was my reason for making the statement about roadracing and autocross.

 

Andrew did not mention what his application was.

 

In my case, it will be a TT SBC - no I won't be racing it, but on a V engine there are less hassles with exhaust leaks with twin systems - and I'm not sure I'd have enough room for a single big one even if I move the battery.

 

I have no desire to TT my 280ZXT, just get a bigger compressor section for the T3/T4 I've already got...even with only S3 trim and clipped turbine IT IS FAST.

 

You should note that the Skylines are inline sixes and have RULED with twin turbos - banned from rally competitions even. Racers that go for top speed and drags generally go one big one on small engines because the spool characteristics are not an issue.

[SleeperZ]

Brad, I am not contesting twin turbos cannot be made to have good spool characteristics. In order for equal flowing twins to spool the same as a single, the inertia of one twin must be 1/4 the inertia of the single. If the single has 2.5x the inertia of one of the twins, then the twins will be much slower spooling. I get my numbers from "Maximum Boost" by Corky Bell.

[Dnaught14]

I hav seen far more TT setups on a v8 applications than single turbo setups. Maybe its becuase people think 2 are always better than 1, or that the setup is the only way to turbocharge a v8 etc. Regardless, for money and power a single turbo seems far more logical. Dont get me wrong Twins look fun on a v8 , but as the others have said, it would be more expensive and more custom to do 2 than one. In my eyes if you wanted a cheaper and easier way to generate similar power numbers, a single turbo setup would be the way to go.

[majik16106]

depends on the engine, its easier on a v8,. plus twin turbo's on a v8 are usually t/3 t.4 hybrids or full to4s, which arent small. Like a Vg30dett, it makes more sense because that engine is set up as twin bank, two manifolds, two throttle bodies.

 

Heres the supra argument, big twin kits on 2jz ( hks 2530) start to spool a bit later in the power band, but they come on REAL hard. a big single has a much smoother spool up, goes over the entire band, thats why most road racing turbo cars in japan, gtrs and supras switch to big singles, in fact the most popular is probably the t78. But on the street the supras get real crazy with true twins (sequential from the factory) and the back end breaks loose real hard, so most go single, but its the opposite for the skyline gtr, they keep the twins more often because the awd keeps the car straight when the smaller twins come on and they launch like a $&$*#@(@) on the highway, ever seen mario's car?

[majik16106]

I hav seen far more TT setups on a v8 applications than single turbo setups. Maybe its becuase people think 2 are always better than 1, or that the setup is the only way to turbocharge a v8 etc. Regardless, for money and power a single turbo seems far more logical. Dont get me wrong Twins look fun on a v8 , but as the others have said, it would be more expensive and more custom to do 2 than one. In my eyes if you wanted a cheaper and easier way to generate similar power numbers, a single turbo setup would be the way to go.

 

Not really true, because a big single on a v8 would mean making like a gn style set up,but to me all that pipe re routing, and back through an intercooler is pretty intense, id rather mod two manifolds and do some basic piping and keep the turbos to the side, plus they spool plenty fast bieng that close to the block. BUT its a lot cheaper to pick up two t4's on ebay versus spending 2k for something big enough to make what twin t4s would.

[Ben]

One thing not mentioned here is the fact that;

"Turbos respond better to pulsing exhaust flow. However, whan many cylinders dump into a common manifold the individual exhaust pulses are smoothed out. When we run two turbos we effectively spread the pulses further apart & increase their amplitude. Therefore, when the turbo is spooling up to speed rather than receiving a constant push it will get a series of vigorous shoves that help it accelerate faster. It is for this reason that twin-turbos frequently manifest reduced lag than that they have smaler low-inertia turbine wheels. "

....

"In V-type & flat cylinder layouts it is easy to visualise how superior exhaust flow will be into twin turbos. However the same is equally true with straight-sixes. The other aspect, of course, is exhaust flow out of the turbine. Generally we will be hard pressed to find space down the side of the engine for anything larger than a 3" dump pipe. with two turbos, even small 2.5" dump pipes provide 37% more flow area to help turbulent turbine flows to more quickly settle, while 2.75" pipes provide 67% more area. "

....

"Also with exhaust flow divided, each turbo will run a touch cooler, which ultimately improves their service life."

 

(quotes taken from "Forced Induction Performance Tuning, pp 94-96)

 

I have manifolds & a pair of T28's for my 'L28TT project'. But with my current engine setup, my T3/T4 hybrid is not yet being used to it's full potential.

 

One day....

[Turbo Meister]

Andrew,

According to Corky Bell, "An engine of 180 c.i. can benefit from two turbos. Two turbos can slightly cut turbo lag, as opposed to one large turbo, and allow a better balance of low-speed and top end performance. Over 350 c.i., twin turbos become a virtual necessity."

My 383 Chevy uses two turbos but thats because a friend is letting me use them. They are slow to spool up but I've ordered smaller turbine housings to overcome that situation.

Hanns

[CableSrv]

Ok not to add more info to arguement but I believe it's first needed to know what he wants this for.

 

First single turbos are notorious for making BIG boost over longer periods of time or rpm.

 

Second there are many ways to setup twin turbos. both equal size or one small and one medium size. Equal size twins doesn't do much and I would agree a single big turbo would out perform it. Now a small and medium twin setup would out perform where small spool up time is required.

 

Here is the reasoning of small and medium twin setup. A small turbo is used to get quick instant boost for low psi, while the second medium turbo is used for picking up where the smaller one was at a higher rpm to finish the boost cycle off. Take for example a small turbo setup only to get 9-10psi relativly quick while the second is setup to do around 8-20psi.

 

The small turbo would spool up around 1500-2000 rpms and the second turbo would kick in at around 5-8 around that same time. then the small turbo would basically be stuck at the max psi using a wastegate dumping the rest unused boost while the second uses it's max potential.

 

and both working together to get a 20+ psi! this is normally used in road races or autocross because it typically has quick spool time which means exiting corners tend to keep you in boost if not quickly build it back up.

 

drag you need max boost so rpm doesn't matter nore does spool up time. in a sense it does but that's a bit more technical detail to go into that well I don't really care cause I don't drag race for the most part.

[silicone boy]

Well, when I started the TT V8 buildup, I didn't consider a single turbo setup enough. I'm going batty routing all that plumbing. theory aside, it's become a real world practical problem running all that intake and exhaust plumbing through what I thought was a spacious Z engine bay-mine now looks cluttered and spark plug changes won't be fun. It would have been a lot easier with a single turbo setup. Spoolup would not have been that much an issue since I have a healthy stroker motor with plenty of low end grunt anyway. I will make the twin turbo setup work, it's just not as elegant as I thought it would be.

[SleeperZ]

And I agree a twin setup on a V8, especially a Z makes good sense. The point was well made that the turbos are normal sized for a V8, and cheap, where a big single would be big bucks. The plumbing seems easier to contain, but it sounds like maybe not. I am soooooo looking forward to someone completing a successful V8 turbo Z setup!!

[Scottie-GNZ]

There seems to be a general concensus that a "big" single turbo means you will have lag. That is absolutely untrue if care is taken to not just buy a "BIG" turbo but select the right components for that big turbo. One of the things that has a big influence on spoolup is the exhaust housing. For some reason it seems like only the GN crowd takes advantage of that fact.

 

A L28et with a "BIG" 66mm compressor and a big and heavy P-trim exhaust wheel can eliminate lag by choosing a small a/r on the exhaust housing. Yes, you sacrifice a little top-end but with a 66mm wheel for 2.8L, how much does that amount to? For me, I run a 63mm compressor and P-trim exhaust but use a .63 exhaust housing and I can spin my tires at 60mph with a downshift. Another big factor that causes lag with big singles is being too rich in the lower RPMs. I try to run as lean as possible in the 1st 2 gears then add fuel on the top-end when the load increases. That is the beauty of a programmable system.

 

I think Z-Gad can testify to this with his "Big" turbo. Nothing wrong with a big single on a V-8 if the turbo is configured correctly.