More on supercharger HP ratings

["John Fielding" <johnf@domain.elided>]

Having had a chance to study some manufacturers data on Rootes type blowers, the best
information is presented by Eaton on their website.  They are the only manufacturer who
gives curves of throughput cfm and HP to drive the blowers!

Taking our theoretical 1 litre motor which requires approx. 200 cfm of input with a boost
of 10 psi (0.69 BAR) the Eaton M45 blower delivers this at a blower shaft speed of 9000
rpm, which would require a pulley ratio of approx. 1.5:1 for a peak rpm of 6000 rpm at the
crank.  This delivery needs 14 HP for the M45 blower.

The next larger Eaton blower - M62 - delivers the same cfm at 7000 rpm blower speed and
requires 14 HP also.  The larger M90 blower delivers 200 cfm at a blower speed of 5500 rpm
and surprise requires 14 HP.

Of these three blowers the best option is either the M45 or M62 as at the required output
volume the air temperature rise is only 140F whereas the larger M90 temperature rise is
160F.

Extrapolating the HP required from the graphs for 10 psi to deliver a boost of 1 Bar (14.5
psi) the HP figures go up to approx. 20.5 HP

As our example was for a 1 litre motor one can extrapolate the HP required for a 2 litre
motor as being at least twice that for the 1 litre motor.  Therefore a conservative
estimate of blower HP would be twice this original figure, and probably a bit more
allowing for additional losses.  Therefore a blower for a 2L motor would need at least 40
HP at 6000 rpm crankshaft speed.

A 3 litre motor would require a blower HP of approx. 60 HP for the 1 Bar boost level.

IMHO A turbo has got to be a better proposition!

makes you think??

John
Durban
South Africa
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