Discussion: Turbo size in relation to size, flow, and power

[TSG59_Steve]

yes you're right TSG59_Steve about a bigger turbo does flow more CFM. however an engine will only require a larger volume flow rate IF you increase volumetric efficiency.

In an engine the only air factors that will variate performance are temperature, pressure, and volumetric efficiency. At low boost the charge temperatures between the two turbos are about the same, the boost levels are also constant...therefore density of the air is equal between both setups (simple ideal gas law). the last factor is the volume. This is multiplied by the density to determine the mass of the air in the cylinders. From there the ECU can then calculate how much fuel to inject to obtain the desired A/F. Usually a bigger turbo will increase volumetric efficiency slightly, which would increase perfectormance however the increase between those two particular turbos in question would be minimal...Air flow is not a factor at all in computing the net power output of the system. And just like a said, all the larger airflow will allow you to do is keep supplying the motor with the amount of air needed to maintain a certain intake pressure for a farther range in the rpms. For example, as the engine climbs to say 7000rpm, the motor is intaking a much greater volume of air then at 4000rpm. This is why many smaller turbos loose boost in the high end, it just simply cannot continue to supply the engine with the amount of air needed at that rpm, and therefore the pressure begins to drop off. at say 11psi both turbos will supply the same air flow (cfm) throughout most of the powerband, however the large turbo will just be spooling slower as it does not have to work as hard to maintain that boost level due to its larger compressor wheel design.

haha I could continue to elaborate and will be happy to if you want...I used to think the same thing as well until I took fluid mechanics in my mechanical engineering program this year. If you question anything I said, please ask. I'd just hate to change the origional purpose of this thread though

Please continue.

I may be misreading it, but could you please explain what you mean by volumetric efficiency? What factors determine the v.e.? I would expect volume (duh) and pressure but you mentioned pressure seperately.

 

[GVR4592]

Volumetric efficiency is the actual amount of air being pumped into the cylinder compared to the theoretical maximum amount of air that could be pumped into the cylinder. A 2.0 engine with 100%ve would ingest 2.0 liters of air.

Everything that flows air into and out of the cylinder, affects VE, cylinder head ports, valves, cams, intake manifold, exhaust manifold turbine housing, etc. Pressure doesn't have any effect on VE.

 

[TSG59_Steve]

Volumetric efficiency is the actual amount of air being pumped into the cylinder compared to the theoretical maximum amount of air that could be pumped into the cylinder. A 2.0 engine with 100%ve would ingest 2.0 liters of air.

Everything that flows air into and out of the cylinder, affects VE, cylinder head ports, valves, cams, intake manifold, exhaust manifold turbine housing, etc. Pressure doesn't have any effect on VE.

Awesome, thanks for the response.