bigbird01

iTrader: (3)

If single turbo are good for top end spool and twin setup is good for bottom end spool, what would be the downfall in a twin setup with a large cam so the top end power would be there also?

Jake@EPP

The smaller turbo would run out of boost up top. A larger turbo with a big cam is a win win setup, just not street friendly as the smaller cam would be. If you roll-race then the big/big setup is perfect.

engineermike

A large single will not make any more top-end hp than equivalently sized twins. The twins offer the advantage of earlier spool for any given hp potential.

Phil99vette

iTrader: (11)

I just don't see how a smaller turbo spools faster. You've got 2 turbos so the mass of the wheels has got to be at least 25% greater than a single large turbo. Your exhaust energy is being split in half but the A/R of the turbo is not going to be half that of a single big one. Please let me in on what I am missing.
Phil

THE_SUPRA

thats just how it works...


...also phill99vette, that kind of out of the box thinking is not welcome here at ls1tech.com

engineermike

Dammit, I really didn't want to think this hard about it. . .

Okay, let's compare a single 76 to it's equivalent in twins. That would be approximately twin 54 mm turbo's (76 squared, halved, the square root, gives the same eye flow area). So, you have half the exhaust gas to spool a turbo that's 54 mm.

Now, we'll look at the energy required to spool the turbo's. The energy required is proportional to the moment of inertia. For simplicity's sake, we'll call the wheel a disk. Moment of intertia of a disk is 1/2 x mass x radius^2. The mass of a 54 mm impeller is roughly half of the mass of the 76 mm impeller. However, the radius is smaller too.

Running some assumed numbers, I get that the moment of inertia of the 76 is about 4 times that of the 54. So, with a twin setup, you have half of the exhaust gas to spool up 1/4 the rotational inertia.

Make sense?

Mike

THE_SUPRA

im gonna go ahead and stick with my theory...stupid junior college physics class.

daniel6718

iTrader: (36)

how does the posi-trak on a plymouth work? dont know it just does!

haha always wanted to say that

Phil99vette

iTrader: (11)

Your theory makes some sense but I disagree. Does anyone have a connection to the impeller and turbine weights? I *think* that the 54 and 76 are on the same frame from precision and the turbine wheel/housings are similar. The turbine wheel should be the same while the housing should have a smaller A/R. If you told me that the 54 exhaust side wheel is 1/2 the weight
of the 76, I'd call BS. Lets face it, its a turbine that uses exhaust energy to spin it. You've got 1/2 of the exhaust energy trying to spin something that is not 1/2 the weight or A/R of the larger one. I can tell you this, my S95 takes what feels like the same energy to spin it compared to a 76mm.
Phil

Pro Stock John

iTrader: (34)

Phil, are you saying that you think twins don't spool up faster than a big single? Why wouldn't you believe that two smaller turbines wouldn't start spinning faster than one big one? Have you ever seen T and TT cars at the track?

99Z28LS1

iTrader: (4)

this is all too much for me, you guys are thinking to much. but i will say i would rather have a BIG turbo and decent cam, because i roll race

Pro Stock John

iTrader: (34)

Why wouldn't TT's be better from a roll too?

Phil99vette

iTrader: (11)

Yes I have seen both at the track. One thing that makes me wonder is some of the turbo guys I know switched from a 106 to twin 76s and they reported slower spool by a couple hundred RPMs. I'm not here change anyones mind but I can tell you this... I'd rather have a 106mm turbo on a 400 cube motor than a 76mm on a 200cube motor. You guys can duke this one out.

engineermike

They would be!

engineermike

First of all, even though the exhaust gas is just working on the turbine wheel to spool it, it still has to speed up the compressor too. So, the mass of both wheels combines is what needs to be accelerated.

Secondly, it's not simply the mass of the wheel that affects spool time. It's the moment of inertia. That is, the mass and it's relationship to the centerline. For instance, a 1 lb flat plate will take more energy to get it spinning than a 1 lb shaft. So, changes in wheel diameter can have a drastic affect on spool time since mass is added to the outer edge, which is the worse possible place.

Pro Stock John

iTrader: (34)

This is kind of a non-sequitur, we would also want the first combo and not the second...

bboyferal

iTrader: (9)

^ There is no non-sequitur since there was no statement that was conditional.

Superman09

iTrader: (65)

Some large *** vocabulary going on here, impressive, haha. All i gotta say

SSilverSSurfer

iTrader: (8)

"your using these big words around me, i dont know know what they mean so ima take them as disrespect....now watch your mouth and help me with the sale" -40 year old virgin

VCV13

iTrader: (9)

Hi, I'm new here. I've got an 02 WS6, and I've been building a twin turbo setup myself. I just wanted to say that I agree with EngineerMike. He knows what he is talking about. I'm a mechanical engineer myself, and stand behind his explanation. It's all about the moment of inertia. The A/R has nothing to do with why a twin setup will spool faster then a single turbo setup. Choosing the A/R will change spool time, but it is a design criteria that should be correctly chosen for each application.

If you have a properly sized twin turbo system designed to run an LS1 engine at 500hp, and a properly sized single turbo system designed to run the same engine at 500hp, the twin turbo will always spool up faster. That's the only way to compare them. Also the thing to remember is "properly sized"