Eric,



No problem.. sorry for the quote misuse. Thanks for the response. I guess that when you mentioned the displacement issue you were refereing as the same vehicle.
It is all good.



Carlos

 

No hard feelings. Sometimes it is hard to write post after post and include everything. Sometimes I don't explain things well, and by no means, was I yelling at you.

It's just that some people aren't reading and understanding what others are trying to show them.

I think you understand, but maybe when you quoted me, I didn't include a good enough explanation.


Eric

 

Everyone that doesn't understand needs to reread this quote many times.

That is exactly what many people in this thread are doing wrong. They are simply quoting the capability of a turbo and not the capability of a turbo on our engine. You need to understand the equation for airflow through and engine and then apply that to the compressor map for the turbo. That's what links them together.

Thanks for the great quote Speedlimit.


Eric

 

You missed one part of his explanation: VE. VE stands for volumetric efficiency. Volumetric efficiency for a particulare engine changes with boost pressure and RPM, and represents actual airflow / displacement. So if my car has a VE of 90% at 4500 RPM and 18 psi, then it is flowing 1.8L per cycle at that point.

Changing any airflow component can affect the VE of the engine. This is why changing cams, exhaust, intercooler, etc. increases power at the same boost level; it increases VE. This is also why the 4G63 outperforms the Saab 900 engine, better volumetric efficiency.

 

The incomplete comprehension of VE and gas law is the breakdown, and is why this discussion has persisted.

PV = nRT

Everyone needs to remember that discussions of volume, temperature, and pressure tend to confuse unless it is explained on how they relate to air mass. AIR MASS is what creates power. Volume, temperature, and pressure are ONLY factors that affect power to the extent they affect mass air flow.

The mechanical VE of the engine is what the engine flows at various points in the operating range an indicated pressure of '0'. We are assuming this is unchanged in comparing small and large turbos. We must note however that the presence of a turbo indirectly affects the effective VE, and this is where things become confusing.



Let' say we want enough mass airflow to support a goal of 400bhp.

As rpm rises, a small turbo's lack of volumetric capacity is the limiting factor. On the compressor side, we try to make up for that by increasing pressure, but that only goes so far until the temperature rises dramatically and the compressor becomes inefficient. On the turbine side, as mass air flow increases, the limited volumetric capacity causes a bottleneck, which increases the exhaust backpressure and reduces the efficiency of the intake system. This reduces our effective VE and puts a limit on how much air mass we can get through the engine.

As rpm rises, a large turbo is benefitted by its larger volumetric capacity, which means that its compressor can reach our mass airflow goals with less pressure, which results in better efficiency and lower temperature. On the turbine side, the larger volumetric capacity means that at our mass airflow goal, less backpressure is developed, which improves our effective VE and therefore our ability to move air mass through the engine. The downside is the larger turbine side volume means we need to turn more rpm to get enough exhaust gas velocity to get the big wheel moving.

So we see that at an indicated 20psi, the larger turbo is running more efficiently (lower intake temps), and moving the exhaust gases out more quickly (lower backpressure). So while the indicated boost pressure may be the same, it's misleading because the large turbo setup is moving more air mass through the engine. The boost gauges read the same, but the MAF signals will be very different.

It need not be more complicated than that.

 

Very good explanation. Anyone that has any questions regarding earlier posts in this thread should re-read Ted B's post before asking, as it does a very good job of explaining the major differences between the normal operation of a large turbo and a smaller turbo.

-Paul

 

Correct. And the temperature that I have referred to about 10 times is the temperature in the ideal gas law, given by Ted.

The temperature will go up when the compressor efficiency goes down, and the mass flow rate will go down:

n=PV/RT

In our example, P, V, and R are all constants, so the only way mass airflow, or n, goes down is by raising temperature, which of course is what happens when the compressor is in a lower efficiency island than the other compressor that we are comparing.

Ted also explained the VE once again.

Again, as I stated many times in this thread, there are two basic factors in determining if a bigger turbo will produce more power than a smaller turbo at the same psi on the same engine with the same modifications:

1. VE
2. Temperature



Eric

 

Eric above seems to really have a good grasp on the basics of the math involved in how a turbo operates.

I skimmed through this post so I am not sure if this has already been said, but greatest contribution to why a larger turbo produces more power is due to the turbine side of the turbo.

When you place a larger turbine housing on a given turbo it will operate with a lower turbine expansion ratio. Because of this the backpressure on the engine will be less. With a lower engine backpressure for the same boost pressure the pumping losses in the engine will be less. This will cause the overall efficiency of the engine to increase and the BSFC (brake specific fuel consumption) to decrease.

BSFC = (lbs of fuel flow/hour)/horsepower

Assuming that the compressor flow and A/F ratio are constant (constant fuel flow), a lower BSFC will cause your horsepower to increase.

 

for example: a 16g & a 35r turbo @ 19 psi of boost

The power difference of the larger turbo comes from several area's, this primarily comes with reducing the exhaust backpressure between the exhaust valve & turbine wheel.

By reducing this backpressure, each cylinder has a better incoming charge that is less contaminated by the spent exhaust gases trying to get back into the next cylinder that has it's exhaust valve opening, so you get a better bang/firing event=more power, you also get to run more timing as a result of a cleaner incoming charge=more power, as the incoming charge is cooler too=more power.

Then there is a inlet air temp difference, as less temp is transfered from the turbine wheel/housing into the compressor housing & wheel = more power.

I wish that everyone was able to measure their backpressure when doing all these turbo, exhaust manifold, o2 hosuing tests that we see, as that will give you insight to how hard your working the turbo to get a given boost.

The reason why you get a power increase by replacing your stock cat with a test pipe is due to the reduction in backpressure. The same holds true for upgrading your catback, o2 housing & turbine housing.

^That is where the bulk of the power comes from

 

haha, i read the first post and the first thing that came to mind was PV = nRT. i can't believe this got to 5 pages. high five to eric and Ted for the correct explainations.

and if anyone wants a good explaination of the backpressure issue, just think why the 10.5HS housing is better then the 9.8HS. less back pressure means more power up top at an expense of slower spool.