Twin VS Single Turbo...

[WydRyd]

OK, I've been seeing a lot of posts about people going with Twin Turbo setups for monstrous HP.

Heck, I'm even considering it myself down the track, however, my understanding was that a Twin Turbo is better because it offers better spool-up, with lower top end HP, whereas a big single has more lag, but gives better top end HP.

Now, if a twin setup can flow high 700HP BUT reaches max boost around 4K rpm, then what's the point?

Wouldn't you save costs and just go with a big single ball bearing setup which reaches max boost at 4K anyway and still gives you high 700's HP?

Is going TT merely for "prestige" or bragging rights, but performs the same as a big single?

Discuss and educate!

[twin plugged targa]

As I understand it a twin turbo setup is usually a twin sequential turbo with the 1st smaller turbo kicking in at a low rev. then a second larger turbo kicks in at a mid range rev. This creates a smoother torque/HP curve and the larger power can better be put down to the asphalt.
A single turbo will have a more aggressive acceleration and more on off power (Incl turbo lag).
My small practical experience with driving each is that the 2001 TT had a doscile streetable feel about it and it did'nt really feel the same as the 79 930 which I tried- Chalk and Cheese to coin a phrase.
- But then again I am only a novice in the turbo dept compared to others here!
rgds Ben

[WydRyd]

Hi Ben. All modern TT 911's (or should I say 993tt's and 996tt's) run parallel turbo's, not sequential (like the 959 did). These are generally smaller turbo's matched to the engine for streetability (i.e: minimal lag), but top end HP is typically limited to 450 or so HP.

I'm referring to those TT setups that generate upwards of 650+ HP with larger turbos/compressors. Obviously two larger turbo's will still result in full boost being reached higher up in the RPM band, around ~4K).

The question is, why bother with twins if you still get full boost at that point, when you can go one big single and save on plumbing costs etc...

[beepbeep]

It's two-pronged problem (it actually has more "prongs" buth these two are the biggest ). It's not just boost-treshold, there is shaft-inertia too.

to state an example: A well made BB single turbo might give you same boost treshold (boost treshold: a point where manifold-pressure becomes positive) and even extra HP on the top (due to better efficiency) but it won't spool up as fast diring short spurts trought rev-band beacuse it's heavier.

If you dyno your car on static bench where you load engine on every RPM point and keep it there you will find your boost treshold accurately. But if you do a quick start in 1:st gear your big turbo just won't have time to spin up before your zing trough your power-range. Remember, those things spin at 120.000 RPM + , there is substantial amount of energy stored in them.

If you drive your 930 at 60MPH in 4:th and floor it wou might ask yourself "what's the fuss all about" as you will get boost quite quickly already at 3000RPM compared to time it takes to reach 120 MPH or whatever. But if you start from standstill you'll just reach full boost when it's time to shift up.

All of these things affect amount of turbo lag:

- Boost treshold (primary a function of turbochargers aerodynamic/pump efficiency, engine volume and amount of ignition advance)
- Impeller inertia (function of shaft mass)
- Bearing drag (BB or oil-bearing? Oil-bearing has higher parasitic losses but is sturdier)
- Amount of ignition advance (dumped ignition will send more heat into turbo, more heat -> more gases -> more spoolup

Get the picture?

[RarlyL8]

Well, there just isn't much left to say.

How 'bout "I got two tubos so there must be twice the power as one".

[A Quiet Boom]

The way I see it let's say you have two engines. One is a 3.0L turbo making 700HP and the other is a 7.0L N/A making 700HP. The 7.0L will almost always perform better with respect to throttle response and available torque than a turbo engine. Super or Turbo charging makes the engine "act" larger by forcing more air in but there are problems with that like parasitic drag, turbo lag, complexity etc. The idea of a twin turbo setup is to get the motor to perform more like the larger engine by reducing turbo lag and building up low rpm response. If the engine were being built for a specific type of racing where a single turbo could be kept on boost at all times it should perform equal to a large engine, the problem lies in the rpm range required for street driving. Low end torque is especially handy on the street and in higher HP turbo setups it takes twins to even approach the lowend of a large engine.

I've been fortunate enough to have gone from a small engine to a large engine with many of my cars in the past. My 912 is one such example, it had a carbed 2.0 911 motor when I bought it and the car had ZERO torque, or so it seemed. I then switched to a carbed 3.0SC motor, keeping the same trans and exhaust and the car is a whole different animal. The lowend of the larger motor allows me to start in 2nd if I choose, jump on the throttle anywhere above 1200rpm and pull smoothly etc. That's the idea of twin turbos, make it feel less like a turbo and more like a larger motor. There are other things to consider as well, twins can be very efficient because they can be located nearer the exhaust ports than a single in a banked engine like the 911 and therefore pickup more energy than if they were farther away like a single has to be mounted. Hope this helps a little.

[fizzzzz]

A single turbo setup will also generate more heat making you more prone to detonation. Two turbos would mean the load is split at the same psi.

[WydRyd]

If you have two smaller turbo's set to 18psi each, wouldn't they not create the same heat as one larger turbo running that same boost level?

[beepbeep]

Originally posted by fizzzzz
A single turbo setup will also generate more heat making you more prone to detonation. Two turbos would mean the load is split at the same psi.

You are wrong. It's actually the opposite. Small turbochargers usually have lower adiabatic efficiency than bigger one, so they will heat up inake air sligthly more. Also, you have issue of dual air- and oil-plumbing and added drag of two shafts spinning instead of one. Two turbos "splitting the load" might be more efficient only if you use too small turbo in you single application and replace it with two of same sort.