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Supercharger Speed Question

BigTom73

BigTom73

New member
I was running some numbers and I'm not sure if my math is wrong or one of my assumptions.

The Eaton M90 "redline" speed is about 14,000 rpm. Using a 3.0 pulley, the supercharger will be above redline before 6000 rpm.

Is that right?

That means the supercharger is redlining before the engine is?

If that is the case, then why run a pulley that small or smaller?
 


the m90 really has no max rpm that its supposed to run and it isnt limted. there is just a point where youre making more heat than you need to, which is in the 3.2-3.0 pulley area. eaton pretty much tells us we shouldnt be going below a 3.4 or so.
 
BigTom73 said:
I was running some numbers and I'm not sure if my math is wrong or one of my assumptions.

The Eaton M90 "redline" speed is about 14,000 rpm. Using a 3.0 pulley, the supercharger will be above redline before 6000 rpm.

Is that right?

That means the supercharger is redlining before the engine is?

If that is the case, then why run a pulley that small or smaller?
Click to expand...

My brain's being lazy this morning. Could I trouble you to show your work, please? (Not doubting, just intrigued)
 
Not a problem

If you want the speed of the supercharger all you have to do is multiply the engine speed by the drive ratio. This ratio is equal to the diameter of the harmonic balancer divided by the diameter of the supercharger pulley.

You get that by the following:
In one revolution of the crank the belt moves a distance equal to the circumference of the harmonic balancer. One revolution of the supercharger results from the belt moving a distance equal to the circumference of supercharger pulley. Since circumference is equal to pi times the diameter, the drive ratio can be expressed in terms of the diameter of the harmonic balancer to the diameter of the supercharger drive pulley.

For an engine speed of 6000 rpm with a 3.0" supercharger drive pulley and a harmonic balancer with a diameter of about 7.16" the supercharger speed would be 14,320 rpm.
 
The limit of realisticness on the GenIII stops at a 2.6"...honestly...if you go lower...its just overkill IMO. Anything lower than that is just one of those..."just because I can" things. Also, like the GenV...it stops at the 2.8"...proven time and time again the GenV takes too much power off of the crank to MAKE power when going lower than the 2.8".
 
BigTom73 said:
The Eaton M90 "redline" speed is about 14,000 rpm.
...
For an engine speed of 6000 rpm with a 3.0" supercharger drive pulley and a harmonic balancer with a diameter of about 7.16" the supercharger speed would be 14,320 rpm.
Click to expand...

"...the redline of the supercharger is 14000, but that doesnt mean you cant spin it to 16000 at your car's redline. Obviously you want to not overwork the supercharger, but the redline is more a consideration for marine applications where the motor is constantly moving at high rpm. In a car, you spend very little time at the redline even when racing. " Found here
 


SlowNA06 said:
Okay, so what are we looking at here?
Click to expand...

Lets use the OP 3.0 pulley. Your flowing roughly 650CFM with a 3.0 pulley, 55hp or so is ate by the blower and a temp of 250 degrees. This is where intercooler start to really help


Now if we can convert CFM to Lbmin we could get a HP number But my math sucks and the equation is huge.where P is absolute pressure and T is temperature in degrees Rankine (Fahrenheit + 460):


P(psia) x V(cu.ft./min) x 29
(10.73 x T(deg R))
 
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