Boost - Flowrate - Power Relationship

[Exploded_Muffin]

I need some education on the relationship between boost pressure and corresponding power generated

Let's assume we have two exact same cars (A and B).
Car A has a Procharger P-1X making 10 psi at 7500 rpm and car B has a Procharger P-1SC making exactly the same 10 psi at 7500 rpm.
The max flowrate of the P-1X is 1275 cfm while that of the P-1SC is 1200 cfm (Procharger specs sheet).
The P-1X will make more power at 7500 rpm at 10 psi than the P-1SC at the exact same rpm and boost pressure.

Why is this?
Is it because of the difference in efficiency between the two head-units?
Or that the P-1X is able to flow more air at same psi? (Not sure if this is even a correct assumption)
Or something else?

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[engineermike]

Interesting question. If both of the superchargers are making the same boost and have equivalent intercoolers, then the difference lies in compressor efficiency. Presumably the P1SC is lower efficiency, which will require more crank power to spin and heat the air more.

 

[illtal]

If ATI would provide a compressor map we could see what exactly are the differences, especially in the efficiency and flow rates at certain impeller RPM. The higher CFM comes from a bigger inducer/exducer setup and likely a revised volute.

 

[Cory S]

If both engines are at 7500, and manifold pressure on both is at 10psi, same spark advance, IAT etc, then they will make the basically the same power. The larger compressors shine when you need to move more air (ie: more manifold pressure levels) for more cylinder fill at given RPM. In MOST cases, a larger compressor designed to flow far more air, can make slightly less power and often have less response because its operating below its sweet spot. Like someone running a D1X at only 13psi VS. a P1X at 13psi. Now a D1X at 20-22psi would be far more efficient than a P1X at those levels.

 

[SolarFlare]

Mak already did this a few years ago, they swapped from P1sc to P1x and i think they gained 30 something whp.

 

[Exploded_Muffin]

Interesting question. If both of the superchargers are making the same boost and have equivalent intercoolers, then the difference lies in compressor efficiency. Presumably the P1SC is lower efficiency, which will require more crank power to spin and heat the air more.

I see. So a two-fold effect of greater parastic power loss to make the same boost AND higher IATs (hence lower air density?) by the less efficient compressor so less power overall.
Could there be a difference in flow rate as well across the head-units?

If both engines are at 7500, and manifold pressure on both is at 10psi, same spark advance, IAT etc, then they will make the basically the same power. The larger compressors shine when you need to move more air (ie: more manifold pressure levels) for more cylinder fill at given RPM. In MOST cases, a larger compressor designed to flow far more air, can make slightly less power and often have less response because its operating below its sweet spot. Like someone running a D1X at only 13psi VS. a P1X at 13psi. Now a D1X at 20-22psi would be far more efficient than a P1X at those levels.

My question to you - Is (Or can) the P-1X flowing more air through the engine at 10 psi/7500 rpm than the P-1SC at the same 10 psi/7500 rpm?
If yes then the part I highlighted might not hold true across the different head-units.
If no then this ties in with engineermike's response

Mak already did this a few years ago, they swapped from P1sc to P1x and i think they gained 30 something whp.

Have you got a link to this please? Or what's the forum member's screen name so I can do a search. Thanks

 

[Cory S]

I see. So a two-fold effect of greater parastic power loss to make the same boost AND higher IATs (hence lower air density?) by the less efficient compressor so less power overall.
Could there be a difference in flow rate as well across the head-units?



My question to you - Is (Or can) the P-1X flowing more air through the engine at 10 psi/7500 rpm than the P-1SC at the same 10 psi/7500 rpm?
If yes then the part I highlighted might not hold true across the different head-units.
If no then this ties in with engineermike's response




Have you got a link to this please? Or what's the forum member's screen name so I can do a search. Thanks

The engine can only draw in X amount of air at any given RPM, unless pressure behind the valve is higher. So if a compressor is trying to compress more air (obviously more volume before the intake valve is needed) than no matter what, manifold pressure will have to read higher in order to force more intake charge into the cylinder.

 

[SolarFlare]

Have you got a link to this please? Or what's the forum member's screen name so I can do a search. Thanks

https://www.makperformance.com/The-all-new-Procharger-P1X-on-the-MAK-S550-Mustang_b_8.html

 

[Exploded_Muffin]

The engine can only draw in X amount of air at any given RPM, unless pressure behind the valve is higher. So if a compressor is trying to compress more air (obviously more volume before the intake valve is needed) than no matter what, manifold pressure will have to read higher in order to force more intake charge into the cylinder.

Ok, makes perfect sense to me and simplifies things a bit.
So it really just comes down to compressor efficiency and IAT differences (For the same car, same tune, different head-units)

 

[Cory S]

Ok, makes perfect sense to me and simplifies things a bit.
So it really just comes down to compressor efficiency and IAT differences (For the same car, same tune, different head-units)

Correct.

 

[engineermike]

I see. So a two-fold effect of greater parastic power loss to make the same boost AND higher IATs (hence lower air density?) by the less efficient compressor so less power overall.
Could there be a difference in flow rate as well across the head-units?

When comparing the two, if they both produce 10 psi in the manifold and the P1SC has an effective intercooler to offset the higher discharge temp, then the flow rate is the same and the only difference is parasitic drive power.

In practice, the manifold temp will likely be higher with the less efficient unit. If they are both at 10 psi then density, being PV/RT, will be lower and flow ingested by the engine will be lower as a result.

If we were talking PD blowers, where the flow rate is determined by the blower size, speed, and inlet pressure, all this works very differently….

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