HKS SC + 85mm Pulley = Out of Breath?

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10-09-2009, 09:12 AM

#21

z_333

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Quote:

Originally Posted by MR RIZK

On the original HKS SC kit the smallest pulley size you could use in theory was about 89mm from the OEM 95mm based on a 142.6 crank pulley size and 6600 rpm cut. This kept the pulley rpm at 10453 rpm and the impeller at 99131 rpm which were just under the warranty limits.

Formula as supplied by rotrex:

I = E x ((Dc+2.8)/(Dp+2.8)) x 9.483

I = Impeller speed max
E = Engine Speed max
Dc = Crank Pulley Diameter
Dp = S/C Pulley Diameter

Pulley(mm), Impeller RPM, Pulley RPM

95 93049.76 9812.27
94 94011.01 9913.64
93 94992.34 10017.12
92 95994.37 10122.78
91 97017.76 10230.70
90 99166.89 10457.33
89 99131.44 10453.59
88 100223.20 10568.72

Michael

Thanks Michael for the input. Do you know if the SC needs to be modified to run smaller pulley? I've read in bimmer forums that they actually add restrictors on the intake when downsizing pulley. Theorically it would reduce power. But would it help in reducing impeller load which may cause the SC to slip?

Here are my findings. Rotrex SC has 60mm intake. HKS, however added a restrictor on the intake therefore its only 50mm. I've seen dyno runs with both SC using 95mm pulley. HKS offers a nice gain all the way till the readline without boost dip compared to rotrex as it would suffer before redline.

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10-11-2009, 04:41 AM

#22

MR RIZK

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to be honest don't know but I cannot see how the would reduce impeller load as it is belt driven. the rotrex doco just talks about rotational speed hence the formula above.

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10-11-2009, 08:56 AM

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z_333

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Yes its belt driven. But, if you look at the internal design of the SC it is not gear or belt direct transfer. It secures the friction between rollers using the traction fluid. When the load on impeller is high which makes it heavier to turn, the internal of the SC will slip. I could be wrong. Anyone who tried this please let me know. Thanks.

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