I would run to a Nissan dealer & have that^^^^^checked out.

I noticed my car is fastest when going down hill.

 

Even if invided, I would NEVER join that exclusive club that's being considered...

 

You are just so funny aren't you?

I'm talking about at the same rpm the pressure is higher . When the car would feel sluggish the pressure would be around 35psi at 2000rpm... When it feels powerful it would be around 50 psi at 2000rpm too... Both after driving the car for around 10 min

 

Barnabas thank you for your answer . Unlike Andre your message is useful I actually planned on changing the spark plugs this weekend. As far as the oil I did use full synthetic oil. Keep you updated when the new spark plugs are in

 

You want me to be serious?
2000rpm is just above idle. How the **** can you tell if the car is POWERFUL or not. It's postings like these, not me being a meany that brings out the answers you're getting.

 

The 2000rpm explanation was an example to make your smart *** understand that I was not saying the pressure is higher at higher rpms. And By the way I also feel quite a big difference in responsiveness at 2000 rpm.

 

Like most useful suggestions here;

I'd change the spark plugs, use fully synthetic oil (amsoil is great for the z)

After this if you still experience the dramatic power difference, get your compression checked (could be the previous owner boosted the car in some way)

Also If you haven't changed your transmission and differential fluids I would suggest doing that as well since you are at 100k miles.

Also see if your clutch and brake fluid is clean or dirty, change if needed.

Last but not least check for any exhaust leaks, make sure there are no vacum leaks etc etc.

Best of luck on your Z.