Why is a PD blower harder on a bottom end??

[scv_guy]

Like the title said, why is a PD blower harder on a bottom end then say a turbo?
Say both are making 800whp the blower will have to make the same power at a higher PSI. That being said the boost number should not matter it all comes down to cylinder pressure.
Any input would be great!

[Punisher_ATX]

Parasitic loss. A PD blower will take X amount of power just to turn the blower. So where a Turbo car making 700 crank HP has only the regular accessories to drive say a whipple or something to make the same power the motor itself must make say 775 or whatever it may be just to overcome the drag of the accessories now including a supercharger pulley. Same goes for Centri Blowers. I believe someone tested an F1-X and it takes around 300 or so HP just to turn the blower itself (dependent on boost level). In that example youd need a motor to handle 1000hp just to make 700 to match a turbo setup.

[scv_guy]

That makes sense

[scv_guy]

My first thought was that due to the harsh characteristics of the PD blower hitting the bottom end with all that low end torque. That might be a limiting factor?
any thoughts on that

[dreksnot]

The PD blower is always compressing air and always making boost. It's belt driven and is a constant "air pump." The PD blower is always working and requires a prime mover (the engine) to operate (mechanical direct belt connected). If you know anything about your garage air compressor, it take a lot of current to pack air, and likewise, a lot of "engine power" to work the blower, and requires more and more "power" to make more and more boost. Using "engine" power to make "power," of sorts. Parasitic loss.

Turbos use exhaust and are more or less freewheeling with normal (cruise or no-load) driving. There is no mechanical load on the motor during this time but back pressure does increase some. When throttle down (WOT), the motor is making a lot of hot exhaust gases thus more "airflow" through the inlet turbines, connected to the compressor side of the turbo and makes boost. Still, no extra load placed on the engine but back pressure does increase. Larger exhausts allow for more efficient operation.

So, a engine making 700rwhp output with turbos would match a engine making 700rwhp with a PD blower. HOWEVER, the PD blower engine needs to make an additional, say 150rwhp to drive the PD blower, so, in total, the PD blower engines needs to make 850rwhp (850 - 150) to output the same 700rwhp that the turbo engine is makeing. Those are somewhat made up numbers, but are relative. By the engine having to make more power, that's harder on the motor.

After I typed this, I looked up this. It probably has a better explanation: https://en.wikipedia.org/wiki/Turbocharger

[SSE 4 2SS]

Originally Posted by scv_guy

Like the title said, why is a PD blower harder on a bottom end then say a turbo?
Say both are making 800whp the blower will have to make the same power at a higher PSI. That being said the boost number should not matter it all comes down to cylinder pressure.
Any input would be great!

One of the biggest hard on the bottom end is the forces placed on the crank snout.... We have a car in our shop that runs the Mammoth Kenne Bell.


We have built billet brackets for some of the pulley items. The forces on the belt are tremendous.... This car runs an eight or ten rib belt.... The alternator bracket kept breaking due to the forces.... We built a billet bracket to withstand this.... Then just when the motor is making max power, you decelerate and the belt snaps from one direction of pull to the other.... The motor is then slowing and the blower is still trying to spin due to inertia....

The higher the boost, the higher the forces....

Blowers are considered to be 45-55 % efficient. Turbos are considered 10-20 % efficient. All of this is dependent on the design and build quality....

[scv_guy]

Originally Posted by SSE 4 2SS

One of the biggest hard on the bottom end is the forces placed on the crank snout.... We have a car in our shop that runs the Mammoth Kenne Bell.


We have built billet brackets for some of the pulley items. The forces on the belt are tremendous.... This car runs an eight or ten rib belt.... The alternator bracket kept breaking due to the forces.... We built a billet bracket to withstand this.... Then just when the motor is making max power, you decelerate and the belt snaps from one direction of pull to the other.... The motor is then slowing and the blower is still trying to spin due to inertia....

The higher the boost, the higher the forces....

Blowers are considered to be 45-55 % efficient. Turbos are considered 10-20 % efficient. All of this is dependent on the design and build quality....

I'm running a Whipple 2.9 with a 10 rib setup.
The crank is double pinned with an ATI 6% OD and 3.250 upper

[SSE 4 2SS]

Have you, or do you ever throw the belt or cut the sides of one for no apparent reason??? or break any brackets?

[scv_guy]

Originally Posted by SSE 4 2SS

Have you, or do you ever throw the belt or cut the sides of one for no apparent reason??? or break any brackets?

So far knock on wood no issues. The 10 rid is the ATI kit. I'll be on the dyno next week fingers crossed!

[10aSSee D]

I use the ADM LSA belt drive that puts my Whipple on a belt by its self that way if it breaks I can still drive the car home at least.