LG John

Simpleton question:

I have a reasonable understanding of the basic workings of a turbo. However, I've always been puzzled why small turbos run out of puff at high revs Surely at high revs the exhaust gases will be flowing fast therefore turning the turbo fast and in turn sucking the air in fast What have I missed?

NotoriousREV

I beleive it's because the turbo "overspeeds" therefore becomes much less efficient. The vane angles and wheel size etc are chosen to work within a specific speed range. Start upping the boost, therefore increasing turbine speeds and sooner or later your going to get the tips of the vanes supersonic. On top of this, the compressor housing itself can only flow so much air.

MorayMackenzie

The Tiny little exhaust housing and turbine that lets the TD04 spin up so quickly and early simply cannot flow as much gas as a larger one would. This causes a build up of backpressure as the turbo exhaust side effectively begins to strangle the engine when exhaust gas flow reaches a certain level.

MorayMackenzie

In fact, the TD04, as with any traditional turbo, is a compromised choice. Subaru want to make their road cars feel responsive and strong in general everyday driving... the TD04 spools up well and makes reasonably good boost in the engine ranges that will be used in average everyday roadcar driving (or this is what subaru will have chosen it for, i suspect). The negative side of this is that the turbo, as with any other traditional turbo, becomes inefficient when you try and push it beyond its capabilities.

MorayMackenzie

Rambling rubbish part: the third!

A turbo with larger compressor housing+wheel and exhaust housing+turbine should flow more than a turbo with smaller bits. The TD05 has larger bits than the TD04, and will, as expected, flow more too (assuming straight inlet on both turbos ).

Mind you, you are less likely to suffer from "fuel-surge" related "moments" with a smaller turbo like the TD04 as (a) you will not have as much power and (b) consequently you will not be attempting to suck as much fuel up. (that last bit was definitely a totally lame argument that holds no real technical justification )

LG John

Cheers guys Got a better understanding now. Fuel surge isn't a problem now, I nearly always remember not to chuck her around with low fuel

STI MAN

The old "Fuel Surge - where's Saxo Boy?" Chestnut!

Pavlo

In addition to anything that happens on the compressor side:

The exhaust back pressure will also increase when you run the turbine at very high RPM. It can still act as a impellor, so the faster it spins, the more the gas will want to flow to the outside rather than the centre. Should be more or less proportional to shaft RPM squared, so you really hit a brick wall with the flow.

High exhaust back pressure means higher cylinder pressure, so the actual boost relative to the cylinder will be higher, and the there will be residual exhaust gas still in the cylinder, leading to charge pollution, and less power.

Paul

DaveD

iTrader: (2)

A smaller turbo will generally spool up more quickly than a large turbo because of it's smaller inertia.

However, a feature of the turbine (and all turbines in fact) is that at high flows it will become choked. That is to say (very simplistlicly), that no matter how much more air you try to force into it, the air simply will not go through the hole the turbine represents. Unfortunatelly, this choking happens at peek efficiency, so a turbo has to comprimise where it's working most efficiently, along with how quickly it responds.

A small turbo will have a quick response, but will effectively start to strangle the engine at high flows (ie; high rpm), while a bigger turbo will present more lag, but allow more gas to flow (ie; allow the engine to rev higher).

Nick200

Sorry for the hi-jack but what bhp figure is attainable from a standard td04....?

Adam M

about 300 max.

dowser

On a good day.....

Richard

EMS

And on which rolling road.....

Mark.

Pavlo

As john banks pointed out, Stephen done got 300hp, but he had done everything else, ECUTEK (obviously), rolling road setup (obviously), exhaust headers, inlet pipe, full exhaust, huge induction kit.

But he still chose to use a hybrid turbo, based on a TD04, but holding the power over a wider rev range.

Safer with a bigger turbo too.

Paul

john banks

Fair to say that Steve's power was sometimes over 300 BHP on the original TD04 and regularly over it on the hybrid TD04. Lots of little things brought his power and torque up - hundreds of hours of mapping with access to a dyno and a wideband lambda, and every possible breathing mod done.