jonnylaw

iTrader: (3)

Ok, so I'm confused on what formula to use to calculate impeller speed on a SC at a given rpm. I am sure, however, of the following figures for my HKS GT SC current setup:

Max engine rpm: 7000
ATI Crank Pulley diameter: 6.610"
GTM SC pulley: 3.346" (85mm)
SC drive (step-up) ratio: 9.444

The stated max rpm limit for the HKS GTS7040 SC is 110,000rpm.

One formula found on the net:
http://www.928motorsports.com/servic...ingpulley.html
I=(Crank pulley size / Blower pulley size x step up ratio) x max rpm
I get 130,596. That's ~20,000 rpm over HKS stated max limit.

The other, posted by Mr.Rizk:
I = E x ((Dc+2.8)/(Dp+2.8)) x drive ratio
I get 101,216. That's ~10,000 rpm below HKS stated max limit.

So there's a difference of ~30,000 rpm between the 2 formulas.

I'm not sure which formula is correct or if there is another one that should be used. But I would like to find out my true impeller speed, so I know whether to swap out the larger ATI crank pulley for a stock sized one (if I am overspinnig the blower with the ATI pulley that is ~1" larger in diameter than stock).

Can anyone help calculate my current impeller speed at 7000 rpm with the values stated above?

Thx!

cdoxp800

iTrader: (12)

I found this one:

http://www.forcefedperformance.com/T...lowerSpeed.asp

and I get the same value as the 928motorsports.com one.

Also here is the Formula these Calculators are using.

jonnylaw

iTrader: (3)

^Thanks for confiming. I need to take off the ATI and retune with a stock size crank pulley (5.745"). Now I am wondering if I should spend the extra money for a dampened/balanced pulley (if it helps protect the crank/bearings/extend engine life), or a OEM one if it is still effective under the higher stress/load of the SC.

With the 6.610" ATI crank pulley that is currently on, the impeller speed is calculated to be near its max limit by 5800. So I have been, and will continue to keep the revs below that until I can get a stock size pulley and retune by the end of the month (hopefully). The only time(s) it's seen above this rpm has been on the dyno. Didn't drive that much at all during the winter. And most my street driving isn't aggresive. So hopefully the blower will be ok...

Wondering if I'll get a more linear power band to redline, albeit even if I lose a bit of boost and peak power.

cdoxp800

iTrader: (12)

Jonnylaw,

Are you having some kind of issue? I have seen your dyno posted and it's fantastic. So why the pulley change now?
Your supercharger is one of the ones that convinced me to get my HKS.

jonnylaw

iTrader: (3)

^^The power's great. It's just that its of concern that its spinning ~20,000 rpms over stated max limits at 7000 redline. I'm not sure what factors go into a max stated impeller rpm limit; and how much potential wear/damage that can occur, but due to the GT SC drive ratio of 9.444, the impeller speeds really jump up after 6000rpms on a 1" larger than stock crank pulley.

I'd rather not put potential stress/damage to the blower by overspinning it by that much on a dyno or when driving spiritedly. HKS does not rebuild its superchargers; only option is to buy a brand new one, unless GTM or another shop can rebuilt it.

HKS recommends putting back stock size crank pulley. I haven't heard back from Sam@GTM, so I'm not sure what his take is.

A few thousand rpm over stated max limit for brief period of time near redline is not a big deal imo, but 20,000 rpm over it seems to be. Unless of course the GTS7040 can really take higher impeller speeds than its stated max limits (not sure if anyone knows but I don't want to be the guinea pig). If I had to guess, I'd say the calculations you posted (with the stock size crank pulley) is the optimum setup for reliability and power...

cdoxp800

iTrader: (12)

I can totally understand that. Better safe then sorry.

d_tizzle05

What about the intermediate pulley that is between the drive shaft and SC input shaft?



I'm trying to do something a little different but the math is the same. The equation should be;

Blow RPM = Engine RPM x (Crank Pulley dia/Intermediate pulley 2 dai) x (Intermediate pulley 1 dia/SC Pulley dia) x SC Drive ratio

The problem is I can't find the Intermediate Pulley 1and 2 dia or the Stock supercharge pulley dia. I'm assuming that the 85mm pulley is an upgrade pulley.

Does any one know these diameters?