: >9" Crank pulley: point of diminishing returns?



GMX322V S/C
12-31-09, 04:45 PM
According to the supercharger efficiency map for our superchargers: http://www.eaton.com/ecm/groups/public/@pub/@eaton/@per/documents/content/ct_127899.gif, efficiency seems to drop precipitously above approximately 17,500 SC RPM.

If the snout pulley is 3.09" in diameter, a 9" crank pulley will spin the SC at (9/3.09)*6200=18,058 RPM at redline. Using the same math, every 0.5" increase in crank pulley diameter increases max SC RPM by approximately 1,000 RPM at redline--further into the less efficient area.

By the same token, with a 10" crank pulley, 18,000 SC RPM would be reached at 5,556 crank RPM; all other factors being equal, could the guys running >9" crank pulleys be reaching a point of diminishing returns somewhere below redline? Perhaps this is where the LPE SC porting job could help...

Vrocks
12-31-09, 08:38 PM
Unless you want massive hp and torque you don't need to push the sc very far. Doing a head and cam package with a 9" pulley will give you around 575 at the wheels (auto, cai, very minor exhaust change).

If money wasn't an issue there's alot I'd do but with a manual trans for more control. If I wanted to put 10K into the car I'd get the zr1 sc or the other one (can't remember the name), put a 10" ring on the crank (with a 2.5" upper pulley), mild head and cam and long tubes from American Racing or Kooks headers.

Nine Ball
01-03-10, 09:13 AM
Do you have test variables used for that pump chart? Keep in mind that blowers also produce different psi based on what is after the blower, the engine. Less efficient heads, cam, exhaust etc.. can also increase backpressure on that blower, resulting in a higher boost psi reading. But, that also could mean less hp produced. Too many variables, it seems.

Also, you can't just consider redline rpm. Increased boost during low/mid range rpm will also make the torque and hp curves jump sooner in the powerband, resulting in quicker times at the track. Getting these huge cars moving requires all the torque we can get to the ground.

Gotham CTS-V
01-04-10, 07:52 AM
In colder weather, traction is impossible/dangerous in any gear under 4th. So I would say returns can be diminishing as far as a fast daily driver goes. I drive the escalade faster than the V around town, actually. Highway is a different story. Same with summer time.

Also, a heat exchanger is necessary with over 9".

Vrocks
01-04-10, 09:15 AM
In colder weather, traction is impossible/dangerous in any gear under 4th. So I would say returns can be diminishing as far as a fast daily driver goes. I drive the escalade faster than the V around town, actually. Highway is a different story. Same with summer time.

Also, a heat exchanger is necessary with over 9".

Yep, this is the first car I've owned that broke the back end loose at 55mph on a dry highway (around 36 or 40F). I went to pass someone and gave it enough gas for it to kick down two gears... tires broke free and it went a little sideways - I'd be lying if I said I wasn't smiling after I passed the other car.