General question about FI

phunk

stop thinking in terms of PSI... PSI is just a measurement, it doesnt equate to a static amount of horsepower or gains.

factory built turbo cars have engines that are built specifically for turbos. they have a lower compression ratio and actually require their boost to make any decent power. if you ever drove a evo or a sti, they drive like they have 50hp gokart engines until you hit full boost... a stock civic drives peppier in regular driving... this is because the compression ratio is so low in the turbo cars, drastically reducing how much power the car makes when the FI system is not pushing enough airflow to cause positive pressure.

a higher compression creates more efficient power from the same amount of air/fuel mixture. this is why the NA engines feel so much better when driving around town. Because of this, NA engines, when boosted, make far more power per psi (or more importantly, per airflow volume). They actually need less airflow to produce the same power as factory turbo engines.

This comes with a downside though. Higher compression engines, coupled with the increased heat of pressurized airflow, offer a far less stable air/fuel mixture. It is much more likely to pre-ignite in the cylinder, causing engine damage. This is why NA cars turned to BOOST have to be very precisely tuned/calibrated to not cause engine damage.

Its a compounding issue since NA engine internals are not as structurally beefy. because they use lighter parts to support making power at higher rpm (to somewhat make up for no boost), and because the pistons are typically built around tighter clearances for better emissions and longer engine life, rather then containing mass amount of HP.

NA engines and Turbo engines, while mostly the same, have slightly different specs throughout most major components.

To answer your question simply, why turbo cars run so much boost from the factory and dont really make all that much HP compared to boosted NA engines.. its because they have to. Between usually lower engine RPMs and heavier internal components, lower compression ratio, and camshaft specs... THEY HAVE TO or they would be slow as dirt. Take the turbo off a turbo car and you have redefined SLOW.

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