Boost Killing Engines? Pressure Or Volume Of Air?

[MarkE]

Hi guys

Read alot of stuff about X PSI killing engines, and turbos running more efficiently at Y PSI.

Firstly - engines. I wouldve thought the volume of flow of the air throughout the engine, and not the PSI reading (merely a measure of restriction?) wouldve been the critical factor here? ie. removing the exhaust from the engine, whilst reducing the PSI reading, isnt suddenly going to reduce the stresses placed on the engine, right?

Secondly, turbo efficiency. Surely its the speed at which the turbo is spinning, and thus the volume of air it is pushing out which is indicative of its efficency, not the level of restriction?

Also, why does everyone measure their PSI reading, instead of say CFM? Since PSI is entirely relative to the engine, and the restriction in the engine, why isnt the yardstick CFM?

Thanks

Mark

[chops]

PSI is easy to measure, CFM is not so easy.

You're right though, CFM (or flow) makes power, not boost.

However for a fixed volume higher boost = higher CFM, since flow is related to pressure.

[Sydneykid]

Hi guys

Read alot of stuff about X PSI killing engines, and turbos running more efficiently at Y PSI.

Firstly - engines.  I wouldve thought the volume of flow of the air throughout the engine, and not the PSI reading (merely a measure of restriction?) wouldve been the critical factor here?  ie. removing the exhaust from the engine, whilst reducing the PSI reading, isnt suddenly going to reduce the stresses placed on the engine, right?

Secondly, turbo efficiency.  Surely its the speed at which the turbo is spinning, and thus the volume of air it is pushing out which is indicative of its efficency, not the level of restriction?

Also, why does everyone measure their PSI reading, instead of say CFM?  Since PSI is entirely relative to the engine, and the restriction in the engine, why isnt the yardstick CFM?

Thanks

Mark

<{POST_SNAPBACK}>

My 20 cents on your accurate observations;

Boost is simply a measure of restriction, remove restrictions and power goes up and boost goes down.

The AFM measures airflow.

Without removing restrictions, increasing boost generally means more power and lower boost generally means less power. So boost is a relevant measure, comparatively, but not as an absolute measure. I can have one Skyline engine that makes 500 bhp at 1 bar and another one that needs 2 bar for it make 500 bhp.

The exhaust question is more complex, removing the restriction has a number of effects and they are all somewhat interrelated. Firstly the exhaust back pressure reduction is reflected in more airflow at the same boost and hence more power. Plus the turbine can spin at a higher rpm because of the lack of restrictions downstream from it. The compressor (running at that higher rpm) can generate even more airflow. It may even generate more airflow at lower boost pressure, hence higher efficiency. The combustion temperature, as a result of the above, may well decrease allowing more ignition advance to be used and more power to be generated. That's why exhaust is the best/first upgrade on a turbo engine.

cheers