Turbo Lag

[Guest nevarmore]

This thread and I can't quite wrap my head around the idea of turbo lag. The street knowledge I'm reading in that thread and the linked COFBA thread in it seem to conflict with the hard physics that pubik skewl has learned me. Pressures in a closed system (like an exhaust) should be constant, there is a high pressure at one end (engine) and a low pressure at the other (exhaust tips) which should lead to a constant velocity from the continual difference in pressure. The HowStuffWorks explanation doesn't help much either.

 

Can someone explain this?

 

Fuck, if I had gone to work today I wouldn't have had all this time to sit and ponder this technical crap.

[iwishiwascool]

You’re not taking into account the lack of initial pressure. Lets say there is vacuum between the Turbo and the TB, if you increase the gross volume of the piping it is going to take more pressure/air and therefore time to fill up the increased volume to fill it to X psi.

 

Now there isnt a vacuum there but thats the gist of it.

[20G TSi]

if you remember well enough from publik skooliing, you'll recall a word called VELOCITY, or the speed at which air (or a liquid, they behave the same) moves.

 

In a turbo application, cutting out all other variables, the exhaust housing and wheel size (and its aggressiveness) determine a turbos ability to spool. The smaller the housing, the faster the air is travelling, therefore the faster the turbine is spinning the compressor wheel and creating boost. This will almost always mean quick boost/less spool, but also limits the amount of air that can pass through at high RPMs.

 

Think of it as blowing as hard as you can through a straw or through a toilet paper roll. The air travelling through the straw is going to be moving much quicker than through the toilet paper roll, yet the paper roll flows more and leaves no backpressure.

 

Take those examples and relate them to a turbine housing on a turbo.

 

You wouldnt want to use the straw to flow a big V8's exhaust due to the straws inability to flow enough air. The air will be moving extremely fast, yet the limit the straw can flow is quickly overcome by the huge exhaust flow output of the V8. In this situation, the turbo would spool very quickly, but wouldnt make much high RPM power due to lack of exhaust flow at high RPMs.

 

The opposite, you wouldnt want to use the paper roll on a small 4cyl, since the marginal exhaust flow would be moving through the tube at a rather slow rate, and therefore wouldnt have enough velocity to spin the turbine/compressor wheel quick enough to create boost. Sure, at high RPMs, the 4cyl might create enough flow and velocity to create boost, but spool time would be slow and high in the RPM band.

 

Now, imagining the straw situation is on the 4cyl. It will have enough velocity at low RPM's to spin the turine/compressor wheel and create boost, and (if matched right) the 4cyl would not flow more exhaust than the straw can.

 

 

Thats about as simple as I can put it... That ofcouse doesnt address the thermodynamics involved, so there's more to it than that, but hopefully that helps somewhat.

[20G TSi]

Originally posted by Nevarmore:

Pressures in a closed system (like an exhaust) should be constant, there is a high pressure at one end (engine) and a low pressure at the other (exhaust tips) which should lead to a constant velocity from the continual difference in pressure. The

With a turbo, there are TWO systems, not a single 'closed system'.

 

As for that kit, I feel its a pisspoor idea b/c (as unaddressed above) it makes absolutely NO thermodynamic sense (huh?). By the time the exhaust gasses have travelled through that length of pipe, they have slowed and cooled significantly. Sure they can compensate the slower velocity by using a small exhaust housing, but refer to my example above of flowing a v8's exhaust through a straw... High RPM output is going to be limited by the smaller turbine housing.

 

They could help situations a bit by using a ball-bearing centersection (instead of sleeve type bearings) and a lightweight ceramic wheel, but from reading the thread on the Z28 board it didnt sound like they were doing that.

[SupraGlue]

Here's a quickie answer. The exhaust side between the manifold and the turbo is NOT a closed system.

 

There are pressure losses due to drag against the pipe walls, and there is thermal energy loss through the pipe walls. Longer pipes = more pressure and heat loss.

[OGRE]

to me his question reads :

why is there turbo lag, according to physics shouldn't it be under boost all the time?

 

just a thought but everyone addressed why there is lag between when you stomp the throttle and when the turbo(s) spool up very well. But the explination of why the motor isn't constantly under boost was not, (which is what i think he is asking) of course

and the answer to that would be just to say that the turbo doesn't make boost while it's not under load. again, i have been known to be wrong before.

[Mensan]

Stick your hand behind your tailpipe with the car idling. Then have someone floor the car. The difference in pressure you feel is the same difference in pressure the turbo sees. The lag is the time it takes to make the heavy metal blades of the turbo spin faster.