Hey guys,
This has been bugging me the past several days and I did a search but got no final answer. People have claimed that turbo Y makes more power than turbo X on the same engine at the same boost because it flows more at that pressure and go on to the big straw/ little straw analogy. Now, wouldn't the straw be the entire engine rather than just the turbo? -seems like the turbo would be the 'mouth' blowing on the straw rather than the straw itself.

Lets setup a hypothetical example. Say we have an engine that we're going to test 2 different turbos on. Everything on the engine remains unchanged except for the turbo compressor itself, no changes to the exhaust manifolds, turbine housing, or turbine wheel. Lets go on to say the the discharge temps from the 2 turbos are identical. Now, boost is regulated to say 15 psi by a wastegate. Am I the only one seeing thats its impossible to flow more air into the engine without exceeding the wastegate regulated 15psi? Woudn't the only difference be the bigger compressor just spinning slower than the smaller one? I inderstand that spool and powercurve will be slightly different. Thoughts? Or did I overlook something huge?

thanks!

 

Think of it this way,
If you have two differently sized fans, both running the same rpm and pushing the same air pressure, the bigger fan will push more air.

 

But the wastegate wouldn't allow them to turn the same rpm, you're working with a closed loop system. I buy that one compressor can push more air at the same pressure, but when you have it hooked to an engine, the engine can only hold so much air before the pressure builds and the wastegate then regulates it -so wouldn't it fall back to lowering compressor rpm?

 

I think the main factors are:
- back pressure (larger turbo offers less backpressure at a given boost pressure)
- compressor outlet temps (a big turbo in its sweet spot heats the air less than a small turbo that it past its sweet spot)

For a given engine and port configuration, backpressure is the only thing you can change to change the volume of air that flows through at a given pressure. Backpressure opposes the incoming charge flow, so it matters a lot in determining how much air flows through the whole engine system. Temperature affects air density, so you will get more oxygen for a given volume if the temp is low. I don't see how anything else could matter. Bigger turbos are generally better in both of these areas at the HP peak, so that is why they make more power.

-Max

 

That sounds right to me.

-Max

 

I don't think you understand all the variables that have to do with this. Thermodynamics dictates how flow flows.
Everything mentioned so far plays a part, but there is alot more to it.

I guess my addition to the discussion would be that air velocity goes up with a turbo that pushes more lbs/min at the same pressure temp and volume, and the engines' volumetric efficiency goes up with a higher intake velocity.

 

That applies to an open system though. If you sped up the fluid(air in this case) you'd just have a higher stagnation pressure and again this couldn't happen because of wastegate regulation.

 

The typical example is not comparing two identical turbines, but two completely different turbos.

In your example, you created a case with the same turbine (same everything, but compressor) and yet you are trying to yield the same pressure and outlet temps. This is impossible. The systems will have to do different amounts of work to achieve the the same pressure. There will be different outlet temps.

You can't have your cake and eat it too, there are trade offs with every turbo.

 

Exactly, I was just setting an example to debunk the whole flow vs pressure thing. IE The straw analogy does not hold up here.

So, the only way that one turbo will make more power over another turbo at the same pressure on the same engine is because of better *exhaust flow* (turbine section and exhaust manifold) and lower compressor outlet temps. Correct?

 

I have been wondering the same thing for a while.
It seems like (speculation) the major factor not considered intuitivly is that the rate of engine power increase is (directly??) related to available airflow. So
if you increase the airflow at given pressure at a given
rpm the rate of power increase will increase. I don't
know if that makes a lick of sense but that is what I
have been thinking.
jp

 

Sure that would make more power, but my argument is that this is not possible unless you get into engine porting. I don't think you can increase airflow to the engine without a rise in pressure if the engine itself is left alone, and since the pressure is regulated by the wastegate it will simply bleed more air around the turbine and slow your compressor back down to whatever flow gives the preset pressure.

Its like porting heads on a supercharged car, due to the open loop nature of a supercharger, the boost pressure will drop because the flow went up. If it were turbocharged, thus closing the loop with wastegate feedback, the pressure would remain the same, but the compressor would be spinning faster.

 

You can increase flow at a given boost pressure and port configuration by lowering the backpressure. That is one reason why big turbos make more power -- they tend to be less restrictive on the exhaust side, which increases flow, which makes more power.

And I think Marshall is on the right track here -- it is the stuff after the compressor that determines the air flow, not the compressor itself. At a given RPM, boost level, port configuration, and back pressure, the engine will flow some fixed amount of air. If the turbo supplies more air, the boost will rise, which the wastegate will regulate back down to the previously given fixed level. If you magically snapped your fingers and swapped a different compressor in that flowed more, the compressor would flow more air and cause a boost pressure rise, the wastegate would slow the turbo down to get back to the "right" boost level, which would reduce the backpressure, which would increase the flow, which would make more power.

So, yes, in a sense a bigger compressor will increase the air flow. But it isn't like the compressor can just shove more air in the engine at the same pressure. The change takes place on the exhaust side, allowing more air to go through at a given pressure.

At least I think that is right. There was some article I read about losing energy when the wastegate opens. Gotta look at that again to see what it's point was. My memory is that you want a turbo that doesn't have to vent a lot of exhaust out the wastgate. So, I think you want to be sure to choose a turbine and a compressor that is suitable for your goals.

-Max

 

I belive your definition of "airflow" needs to be spelled out here. If you mean airflow in a volumetric sense, then yes the volumetric airflow into and out of the engine is dictated by the engine itself and its relative RPM for a given instant in time.

However, the important measure is *mass* air flow. Engine power is made with the amount (mass) of air and fuel, not just volume. The way to increase the mass flow of air (in this case) for a given volume flow, you need to increase the density. This is where turbo efficiency comes into play. A larger turbo will do less work on the air to compress it to a given pressure. Less work=less heat input. Therefore a larger (hopefully more efficient at a desired boost level) turbo will heat the air less than a small turbo trying to match the same volumetric flow and pressure.

So larger turbo=less heat input=greater air density= greater mass of air=more power for a given volume flow rate and pressure.

make sense?

~Chris

 

Yes, that makes perfect sense Chris. I've just heard sooooooooo many people say one turbo will flow so much more cfm at a certain psi (on the intake side of things -and they were NOT referring to the density increase from the temp drop) and thats why its making more power -when it seems exhaust restrictiveness and comp. outlet temp are making the difference. I was curious as to why people were upgrading the compressor wheels on the stock twins when intake temps were still resonably cool at 17-18 psi (see Anthony's datalogs) -people claimed it was the "flow" and to think of a "big straw and a little straw and which is easier to blow through" The turbo seals I can understand, but I'm really not convinced now that upgraded compressors are worth it for a 400 rwhp goal. A better exhaust manifold? -absolutely.

 

Right. I believe we agree; there are turbo's that flow more cfm at a given pressure, and they may be more efficient with lower outlet temps, but you can only be as good as the weak link which would be the back pressure and is realted to the exhaust manifold and housing.

While I am an FC owner, and thus don't have twins, I am sure that when given the option for a high HP goal a larger turbo is called for... no just a modified small turbo. The exhaust is just too restrictive.

 

Look at maps from different compressors and you'll see that at constant pressure ratio flow varies with turbine rpm and compressor efficiency. Assuming two turbos have same efficiency and therefore same out temp but one needs higher rpm to make same boost, higher turbo rpm means more work to spin the turbo and higher backpressure ... engine flows less air.

 

Right on, now since running the turbo faster is causing more backpressure, would it be better to route exhaust through the wastegate? Max said that was worse than going through the turbine. I'm guessing there's a happy medium somewhere between it all going through the turbine and the wastegate opening.

 

Routing more exhaust through the wastegate changes the boost, so you can't change that without changing the boost for a given turbo.

I think the moral of the article I read with the "exhaust going through wastegate" info was that you should choose a large enough turbo (compressor and turbine) that you will get your intended boost without having to route too much through the wastegate.

Here are my guesses about what happens at high RPM under boost for various turbo setups:

Running a big compressor with a small turbine seems like you would get a ton of backpressure from the small turbine trying to spin the big compressor.

Running a small compressor with a big turbine seems like you would end up with high compressor outlet temps as the little compressor spins madly to meet your required flow levels at a given boost pressure.

Running a small compressor with a small turbine seems like you would both choke the exhaust with the small turbine housing and get high compressor outlet temps as the compressor overworks itself to meet the required flow for a given boost pressure.

Running a big turbine with a properly sized big compressor seems like you get just the boost you want without having to dump too much exhaust out the wastegate.

Of course, unless you have a CVT, you need the engine to operate over a wide range of RPMs, so you have to make some compromises to get good performance over the full operating range of the engine. Obviously, there are some things to balance here, and no turbo will be perfect under all conditions.

We all know that small turbos give a relatively wide power band, but definitely fall off at high RPM and don't make big power. And that big turbos make big peak power, but also tend to be peaky.

-Max