So I have an 04 coupe 6MT with a bunch of power modifications listed below

Z1 Plenum Spacer
Lightweight Flywheel
Z1 Lightweight Crankshaft Pulley
eBay Headers
HKS Replica Exhaust

I know the differential is supposed to slip when turning because one wheel is spinning slower than the other but recently I noticed the diff losing traction or spinning quite a lot compared to how fast the car was moving. If I kept the car in a straight line the diff wouldn't slip or at least not nearly as much. After installing my aFe Takeda Stage 2 Cold Air Intake System my butt dyno instantly noticed an uptick in power. At first I thought I was breaking traction in first and second and even third gear but after driving a while longer what I noticed is my RPMs jumping up really quick and the differential will take a moment to "catch up". I want to take a video today so hopefully someone can explain better what is happening. I haven't gotten the car tuned after any of my modifications and I think a tune will greatly increase my power even more. I have another modification coming in today, and that is Motordyne ART pipes. I'm afraid that after installing the Motordyne pipes this problem with the differential not being able to handle the power will only increase.

Anyone with this same experience please chime in with your input. And thank you to the G35 Driver community for all the help you've given me in the past!

 

Sounds to me like your clutch is slipping (6MT) or your transmission (AT)

If 6MT, Z1's "mild performance" OEM replacement clutch/PP has done well on my motor which is similarly modified, along with a ebay 19 lb steel flywheel that I had machine balanced to the pressure plate before installing. You'll also want a new clutch pivot ball, there are two kinds you can get from Z1, you want the shorter one for your '04

 

Differential slip is when one tire is doing a burnout but the other one is not.

Your clutch is shot.

New clutch, resurface the flywheel, new CMC/CSC and fluid, throw out bearing, pilot bushing. If you have the shitty stamped steel clutch fork you should probably upgrade to the cast steel one.

https://www.z1motorsports.com/trans-...it-p-5400.html

 

Ohh and you should probably replace the RMS while you have the flywheel off.

 

Okay, so small update for you guys. I took a look at my clutch reservoir and it was filthy. The fluid was completely black and the plastic tank had a bunch of sediment on the bottom. So I used a turkey baster to suck out as much of the old fluid as I could, washed the plastic reservoir and filled up with DOT4 instead of DOT3. Attached a clear vinyl hose to the slave cylinder and pumped the clutch by hand a few dozen times while topping off the reservoir when needed. After about 30 pumps the fluid in the vinyl tube was clear as well so I shut the drain off and took her for a test ride. The clutch definitely feels much better, the pedal and the actuation. I'm still getting wheel spin in 2nd but not so much in 3rd now. It feels like the car is gripping the road much better. You guys are probably right and I could use a new clutch, there is a bunch of play especially when starting from a stop in 1st gear. Got my Motordyne ART pipes installed over the weekend and I love them already. The car had aftermarket catalytic converters then ebay "resonated test pipes" which made the car scream I wasn't abnle to keep those on for daily driving. These pipes fall somewhere inbetween the two. Louder than cats but much quieter than regular old test pipes.

There isn't too much left for performance modifications, I want to put the 75mm throttle body on, get a 1 piece driveshaft, and then go to the engine internals possibly. Does anyone have any input on sprung vs unsprung friction discs. I was thinking about going with the unsprung full face from Z1 but want to do my due diligence and read up on all the different combos of pressure plates/friction discs/flywheels.

 

here is a short clip of a few pulls some in 3rd some in 4th you can hear what I'm talking about with the rpms

 

Yes that is definitely a slipping clutch, at the rate you're beating on it you probably only have about 1 week left before the vehicle will be 100% not driveable.