TT vs Single Turbo Clarification

[rudypoochris]

SleeperZ wrote this "With twins you have only half the exhaust to drive the turbo. With half the exhaust flow, your twin turbo turbine must have 1/4 the inertia of the single turbo turbine to spool just as fast, and for twins, though they are smaller, they are not 1/4 the inertia. More like 1/3 to 1/2 for equal flowing twins. Result: Twins spool slower than an equivalently-flowing single turbo." somewhere.

 

Just wondering where the 1/4 inertia number came from? Also does this apply to TT setups set to make equal psi as the single. Or does this apply to TT setups trying for 2X the psi?

 

Also if anyone has links to good tech articles on why a single turbo set to make = psi as a dual turbo setup would spool faster, please post here. much appreciated.

[ngrimm]

I had never considered the mass of the compressor and turbine wheels as a factor before but what Sleeperz said makes sense to me after looking at our twin turbo sbc 350. The turbos we used are from a 2.3 liter Ford so you would think that the twins' spoolup on a 5.8 liter engine would be nearly instantaneous when in reality it is similar to the single 2.3. Don't know if you have checked out Turbomustangs.com but there is lots of info on turbo choices there. Norm

[260DET]

Look I'm no expert on turbos, far from it, but I can't see the real world relevance of any such 'formula'. Maybe I'm missing something?

[ngrimm]

I'm not so sure after talking to some people at Turboford.net, Here is a quote from there: " Rotational mass has very little to do with the lag/rpm threshold, but has quite a bit to do with the lag/time threshold. For instance, with a massive wheel, you might get full boost by 3500 rpm in a tall gear (that would be rpm lag). But the time lag may be so bad that you would get no boost in 1rst." Norm

[Pop N Wood]

Is it the rotational mass, or the design of the impeller blades that makes the turbo take so long to spool?

 

A smaller turbo is designed to flow less air. Thus only makes sense it will come on earlier (less exhaust flow), but then run out of breath quicker.

[thehelix112]

http://www.performanceforums.com/forums/showthread.php?t=67187674&highlight=twin+single

 

Good thread on the topic if you can be bothered registering.

 

Dave

[desert dog]

Correctly sized twin turbos on a V-type motor, will spool faster then one correctly sized single turbo.

[rudypoochris]

Correctly sized twin turbos on a V-type motor, will spool faster then one correctly sized single turbo.

 

Now explain why.

[Brad-ManQ45]

Assuming the same amount of overall flow for TT vs single T, the biggerr turbo's diameter has to increase a pretty good amount - ~1/2 to 2/3's bigger for the same amount of flow.

 

The increased diameter causes rotational intertia to go up not by a straight factor, but exponentially.

 

Hugh MacInnes covers this in his book on turbocharging.

 

Road racing a V engine, he suggests using two turbos for quicker spool.

 

Drag engines want to keep the turbo going as fast as possible between shifts, so the use of one BIG mamma-jamma is the norm (think like the latest TT Supras going to one big one)).

[Ed260Z]

So far I see 2 deciding factors as too which way to go. Apllication & Prefferance.

Aplications like Drag Racing would benefit from a Big Single. Auto X would benefit from a TT set up. There is also how much power you want to put out a Big Single would be good for huge boost 500HP +, while a quicker spooling TT making 20PSI or less would would be good for around 500Hp. I also like the more linear feel of a TT set up.

 

I've seen argument from all sides, and I'm more confused about it now, then when I first started looking for answers.

[Pop N Wood]

You guys are a little hung up on rotational inertia. Surely it plays a part, but making the rotational parts lighter (say out of ceramic materials) will reduce the rotational inertia but won't necessarily increase the flow through the unit.

 

Think of a variable pitch boat or airplane propeller. The pitch of the blades needs to match the speed of the boat/plane. Too much pitch at too slow of a speed and the motor just bogs. Too little pitch at too high of a speed and the motor over revs without making the boat/plane go faster.

 

Turbine impellors are designed to work best when a certain velocity and volume of air flows over them. The turbo must first convert exhaust pressure into the required airspeed through some type of nozzle. Smaller nozzles produce higher air speeds, but are also more restrictive so they flow a smaller volume of air.

 

An engine produces a certain volume of exhaust at any particular RPM. If the turbo is designed to work at that volume of exhaust flow, then maximum boost is created. If a bigger turbo is used, then the engine will have to rev more before enough exhaust flow is developed to drive that larger turbo.

[Michael]

I'm not a turbo guy, but I've done some work here and there on propellers, so Pop n' Wood's analogy is worthy of some further comment...

 

The issue with propeller pitch is with the effective angle of attack [at each spanwise station] of the propeller blade. The angle of attack comes from the relation between the blade geometry (pitch) and the vector sum of blade rotational velocity and the oncoming flow speed - or the speed of the boat or airplane - or the speed of the flow in the nozzle leading to the turbine disk. For a given rpm, low oncoming speed and high blade pitch means that the blade angle of attack (again, angle of attack varies along the blade radial location, but assume that it's a constant for now) is high, so the blade is at or near stall - and maybe even beyond stall. Efficiency will be low. At too high an oncoming speed for a given blade pitch and rpm, the so-called advance ratio is above its critical limit, meaning that the blade angle of attack is effectively below the zero-lift angle - meaning that the propeller isn't making any positive net thrust. Or in the case of the turbine, the turbine isn't extracting net positive work from the oncoming flow.

 

However, these are all issues of steady-state efficiency; what the turbine does once it spools up to a steady-state rpm for a given throttle setting. The original issue, I think, was less about turbine efficiency than about turbo lag, which is mostly concerned with how fast a turbine spools up. And this, I think, has as much to do with turbine wheel moment of inertia as it does with blade design and optimal blade-nozzle matching. Keep in mind that moment of inertia goes as the 5th power of a linear dimension (mass * radius ^2, or density [of the turbine material - steel, ceramic, whatever] * linear dimension ^5); double the size of the turbine wheel, and moment of inertia goes up by a factor of 32!

[260DET]

At a practical DIY level it seems that most choose a single turbo rather than twins. Heat loss, space, weight, clutter, complication, cost are all relevant factors, as is the performance of current larger BB turbos.