I'm not familiar with the growl you are experiencing. However, upon recommendations from others at the time I installed the Fidanza, I also installed a sprung hub clutch disc as well.
As I recall, the purpose of the sprung hub was to quiet the transmission rattle at idle. I don't recall any discussion about quieting the engine impulses in the transmission when lugging at lower rpm too,
but it makes sense, I suppose.
As it happens, the disc I'm using is the disc used on 93-97 LT1 Camaros.
Sprung hub disc part # 381039 (Centerforce) from Summit Racing @ $199.xx
With regard to the pressure plate bolts, the people on this forum that successfully installed the Fidanza also recommended using longer bolts from
SRP. The reasoning for that is because the flywheel is aluminum and the longer threads spreads the holding force over more thread area - a good thing when bolting to aluminum.
In addition, the
shank on the factory pressure plate bolts fit into a recess at the surface of the flywheel. This is to add a measure of lateral support
to resist shearing between the flywheel and the pressure plate. Some have used longer grade 8 bolts from the hardware store, but the shank of hardware store bolts may or may not fit well enough to provide any additional support against shearing.
It is recommended (by the same people that recommended the sprung hub to go along with a successful Fidanza installation) to buy the longer bolts.
SRP has the bolts with the proper thread and shank length, and mine came as a kit containing the proper drill bit and thread tap along with the bolts.
FWIW, I did NOT rebalance the flywheel after drilling and tapping it for the longer bolts. However, I did take care to drill the depth exactly the same on each hole. I don't believe I have any vibration issues (e.g., "buzzing") - at least none that I can feel. Maybe I'm just lucky on that one. But, if I had it to do over, I'd probably have it balanced...for peace of mind (at least).
So, to summarize the Net Registry's contributions to the Fidanza installation recommendations for quiet operation with the ZF and the LT5...
- Use a sprung hub clutch disc (mine is the Centerforce #381039 at Summit Racing)
- Use bolts with proper shank and thread length and hardness (SRP has them)
- Balance the flywheel after drilling and tapping the flywheel for the longer bolts (I didn't, but maybe got lucky)
- raise the idle by 100 rpm.
- Replace injectors if the least bit out of spec or marginal function
The result (for mine) is a car that idles and pulls as quiet as it did with the stock flywheel. Even Marc H commented on how nice my Z is to drive. If it is, it is because of the experiences and recommendations I followed from those on the Registry. Nothing against learning by the school of "Hard Knocks", but when possible, I'll try the proven path first.
So, if all this doesn't help, then at least you can probably rule out the Fidanza installation as the cause. That would bring us full circle -
back to the transmission.
Good luck. Keep us posted.
P.