How does a turbo effect intake airflow?

[Anlushac11]

For instance the sticky thread for Log head airflow shows 149cfm intake airflow at X inches of vacuum.

All else being equal if we then subject the same intake port to say 8lbs of boost does that increase airflow or does it just increase the density of the air?

For instance if one cfm of intake air has say one thousand air molecules per cubic feet then 8lbs of boost will still mean one cfm but there will now be ten thousand air molecules in that same cubic foot.

Is that correct?

 

[Lazy JW]

I think you are on the right track but you are assigning way too many molecules for the 8 lbs of boost; Naturally Aspirated (NA) we have about 14.7 lbs (psi) of pressure available to "push" the air into the engine (sea level, WOT and near zero vacuum), forced induction would need about 14.7 lbs of boost to double the mass (ignoring friction losses, etc.); many WW2 fighter planes routinely ran about 45 lbs of boost but they termed it "three atmospheres of manifold pressure".

At any rate, stuffing twice as much air MASS in will result in roughly twice as much torque at any given rpm, hence twice the horsepower. Provided it doesn't detonate
Joe

 

[StrangeRanger]

Instead of "CFM" think in terms of "SCFM" Standard Cubic Ft per Minute which are cubic feet of air as measured at 14.7 PSIA (absolute) which is also 0 PSIG (gauge) and a specified temperature, usually 70°F for industrial purposes. Work all the pressure vs. volume vs. mass calcs in absolute pressures and it becomes much, much simpler

http://en.wikipedia.org/wiki/Pounds_per_square_inch
http://en.wikipedia.org/wiki/SCFM

 

[80broncoman]

I had always heard that a 600cfm carb when the air is blown though at 14.7 psi is now a 1200 cfm carb.

 

[Lazy JW]

...... or does it just increase the density of the air? .......

Yes, it just increases the density of the air (ignoring any temperature rise).

The word "volume" has been much over-used and abused when discussing engines; air MASS is the main thing that your engine cares about, and it it fairly easy to have One Cubic Foot of air that contains twice as much MASS as another cubic foot of air sitting right beside it.

The Germans used the term "Kompresser" when referring to a supercharger, and to my mind it gives a more proper description because it really is just compressing the air so that a given volume will contain more mass.
Joe

 

[Anlushac11]

OK thanks for the info.

Reason I was asking is shop who built my shortblock recommended porting exhaust side only and to not touch the intake side since its a turbocharged motor. Said the intake didn't need it.

Seems to me anything that improved airflow into the motor would be beneficial to increasing power.

 

[Thad]

Anything that helps airflow in a NA system will also help airflow in a blown system, but more so.
A restriction to airflow is a restiction in both cases as would be any improvement..

 

[Lazy JW]

..... recommended porting exhaust side only and to not touch the intake side since its a turbocharged motor. Said the intake didn't need it......

That's just silly. Any intake restriction will cause the turbo to work harder which then increases back pressure in the exhaust.

Do both sides.
Joe

 

[Emerald 74 4X4]

Now polishing the intakes is no-no...

 

[Anlushac11]

Yeah you want to keep texture satiny to slightly rough to get boundary layer effect.