I have a 6# Powerdyne supercharger on my 95 Z28. However with the cam, heads and free flowing exhaust I'm only seeing about 4-5lbs of boost. I was cosidering upgrading to the 9# pulley and hope to see about 7lbs of boost. Anyone have the 9lb pulley and are there any issues with using it?

 

Much better idea is to switch to a larger crankshaft pulley for more surface area for the belt (contact www.ASPracing.com) ...otherwise you'll encounter a lot of slip if you use a really small blower pulley instead. Only way around it is to overtighten the belt, then your bearings fail. Use this impellor rpm formula to figure out how much to increase your crank pulley by: (Crank pulley diameter / Blower pulley diameter) x 3.05 x max engine rpm

3.05 is the internal step up ratio of the PD....that's where that came from. I don't know enough about your combo but it looks like you'd do well with a ~6.5 inch crank pulley. It will put your max impellor rpm up to 42,627 at 6,300 engine rpm (I just made an assumption about that). The max safe impellor rpm for the PD is suppossedly 43,000.

If you want to go all out then have ASP make you a custom 8 rib setup for both pullies and go with more like a ~3.13" blower pulley and ~7.0" crank pulley. If there's room you might even want to go bigger on both for max contact area. This way you don't have to tighten the belt so much to avoid slippage and your bearings will last a lot longer. You may not be sticking with a PD forever anyway though, so think about how much you want to invest in it. FWIW I switched to a 6.45" crank to use w/my 2.93" blower and got slightly over 6 PSI on my heads/cam/LT combination. Something weird has happened with my stock blower pulley (beaten clearance into it by impellor shaft) ....so the blower's being rebuilt/upgraded by a really nice member in here and while he's at it he's putting a 2.84" blower on it just to bump me up a tiny bit more.

 

I only saw 4.5 lb with the 6 lb pulley.
I put the 9 lb pulley on and the car is had a lot more torque in the lower rpms. The guage went up to 6 but my tranny was programed to shift at 5400 rpm.
I recently put trickflow heads and a blower cam in but we're still sorting out some stuff and it's winter so I haven't had a chance to WOT the car which has the shift points moved up to 5800 rpm.
I'd say go with the 9 lb pulley if you don't want to spend the money for better ones.
For under 100.00 it's definately worth the money.

 

ok ...but you just defeated the purpose of my post ....ok, he can try that then .........

...remember, you have an A4 ....a little different. I'm guessing his is an M6

Belt slip will be a much greater problem for an M6 reving high and shifting hard (if it's an M6)

 

Mine is an automatic. Thanks for the help.

 

Well you could go for better pulleys which would cost money.Then you could get that extra1lb or 1 1/2.
If you had enough you could visit these people.

http://www.928motorsports.com/services/ventshim.html

 

Here's their racer rebuild.

http://www.928motorsports.com/services/racerebuild.html


however the 9 lb pulley will give good hp for the money.
Or you can spend 800.00 for a race rebuild and or the 200.00 or so the larger pulleys may cost.

 

Also, have you looked into other ways to increase boost? Like larger filter, solid intake conversion, descreening MAF, etc. These mods will help reduce belt slip too, as less load will be put on the belt to spin the impellor....your IAT will drop too.

The mods in that link have not been tested by anyone here FWIW.

Since you have an A4, if you don't rev high and want to keep it simple I guess do what 2cub did w/the 9# blower pulley. It is difficult to get the old pulley off the blower though. I would still rather see you go w/larger crank pulley but to each their own. So what about those other mods I mentioned in the beginning here? If you've done none of them you could possibly pick up another ~2.0 with those alone.

 

Cooling the bearings with airflow seems like a good idea
and tightening up the tolerances can't hurt.
Raising the operating range and reliability of the Powerdyne would allow it to put out 9 or 10 psi IMO
by pulleying it to spin faster.
The venting 139.00 price is good but it's probably really just a teaser to get you to buy the racer rebuild.
Maybe we should see what the 928 powerdyne guys have to say about this.
Maybe next year I'll have it redone and get bigger better pulleys.
The cam and Trickflow heads and the CPT trans have taken some time and now I just want to drive the car.