turbo - why improving LOW-end torque
 

OK, so it may be a stupid question, but I just don't understand it.


So people say that turbo helps in particular with low-end torque. And if you look at car spec sheets, it seems to be the case, NA cars tend to have max torque between 4.5 and 6 k rpm, and turbocharged have max torque from 1.7 - 2 k.


But turbo is driven by exhaust, so it should be particularly effective at high rpm, because high rpm equals a lot of exhaust to spin the turbo to high speed.


What am I missing here?

What is your question?
Small turbo responds faster but peaks faster also.
Larger turbo opposite.
Look at the shape of the curve.
For this car, most of the turbos are small because most people use them on stock motors. No point in putting a huge turbo when it is just going to blowup.

Sorry if I wasn't clear.


My question is: why turbos seem to improve torque in particular at low rpm, while they seem to be driven most effectively (so they should also have the largest effect) at high rpm

Efficiency vs max power. Curves are different, not everything is linear.
You need to word your question better. It is hard to understand.

It's an interesting question, and one that I understood from reading your first post.

Unfortunately, I have no idea.

The size and the efficiency of the turbo determines how well it does at higher RPMs. The long and short of it is that turbos get less efficient the faster they're spinning. You're compressing air violently with exhaust gasses, so things get hot. Hot air = bad.

You can choose larger turbos which tend to be more efficient at a higher RPM range than smaller turbos (I'm oversimplifying, but the general principle applies) and make more peak power. The trade off is that these turbos will take longer to spool.

I kept thinking about it and a theory came to my mind.

Here it is:

While at high rpm the turbo receives more power (because high rpm means more exhaust, which drives the turbo), at the same time the demand for the turbo output increases as well (because high rpm means more intake strokes per unit of time so the turbo must deliver more air). So the two effects would basically cancel out, and the benefit of an ideal turbo would be independent of rpm. But in a real turbo energy losses from friction, turbulent flows and such would be higher at high rpm, so IRL the net benefit would be higher at low rpm.

I am not sure though if my thinking is correct.

Thanks, I typed my previous response without seeing yours.

MDM here you go.


A turbocharger is an air pump. So down lower in the RPM when the turbo gets spooled up you see a spike in the torque because the compressor is within its most efficient CFM zone. If you look at a compressor map it will show you the efficiency curves across the range of the compressor wheel on the turbo your using.



Here is a TD04-15G compressor map for example.
http://www.stealth316.com/images/td04-15g-cfm.gif

As the turbine spins faster from increase exhaust flow you'll get pushed up higher into less efficient curves which is why you see boost taper off and power start to trail off because the air mass required to generate the power begins to diminish for all sorts of volumetric efficiency and thermodynamic properties.


To your other question about why is it that on a NA car that the torque maxes out in the 4.5 to 6k range. It is because of the formula for calculating horsepower partly. There is a standard constant in the denominator of 5,252 and the numerator is (LbFt * RPM). So you will generally always see the inflection point where HP and Torque curves cross within an acceptable deviation of the 5,252 RPMs. Torque begins to fall off after this because as the piston speed increases there is less time per combustion event for the force to act on the crankshaft.


So to recap. A turbo/supercharger increases the torque curve and broadens it increasing the amount of work done over the revband (area under the torque curve) and helps push air mass needed for generating horsepower beyond the inflection point.

Another thing to take into consideration is that turbos are designed to take advantage of the lower speed exhaust gases. Once they start really spinning and creating boost some of that pressure will be drained off through the blowoff valve. So technically the high rev pressure creates more boost then you really want or need.

Thank you Tcoat, can't believe I forgot to talk about that. So everything Tcoat said about the wastegate control is spot on.


So going back to the compressor map we'll use the 150,000 rpm curve for the turbo. If you let it run at its max speed at a pressure ratio of 2.4 (20.5 PSI gauge) you'll be getting 200 CFM where if you keep the pressure ratio down to 2.0 (14.7 PSI gauge) you'll get 428 CFM which will make you more horsepower the higher you push the motor. The wastegate helps you maintain that by ensuring the turbine isn't over speeding. The BPV, if electronically controlled, can then also be used by the ECU to help maintain the pressure ratio making the control even more efficient.

Because compromise.

small turbo = boost comes in fast & early, big turbo = boost comes in slow & late. O.O

It's a manufacturer choice. When you're driving on the street, you're usually not revving the engine that high so low end torque is more useful. You use a small turbo to do this, because at low rpm you have a relatively smaller flow rate of exhaust, so you want to use a small turbine to restrict the flow a bit to produce more pressure to drive the turbine.

What happens with the small turbos used to achieve this is that at high rpm, the turbine becomes a major restriction, and the compressor is maxed out in terms of airflow, so the torque drops early. Adding a bigger compressor doesn't really help, because the turbine isn't big enough to produce the extra power to drive it, no matter how much power the engine is attempting to transfer to the turbine.

The small turbo at high rpm is like trying to power a pinwheel sized turbine with a leaf blower. Most of the leaf blower's power is going to be wasted.

Another way to think about it is that power is proportional to airflow. A bigger turbo allows more airflow, but it needs more power to run. Thus it allows you to produce more torque at high rpm. A smaller turbo allows less airflow, but it also requires less power to run. It will hit its airflow (and thus power) limit at a lower rpm.

The high power Supras and GTRs and whatever all use big fat turbines and compressors to optimize for high rpm torque, or maximum power.