l just bought one and my friend who have a 1985 nissan 300zx tell me that my car is a piece of junk and l just wanna know can my car beat it on a track.
its a 1985.5 with headers and after market muffler and his nissan has a intake and racing springs

depends on the condition of the car really.
well porche does not have a limiter aboard any of ther cars so u can probably acheive a higer top speeed than your friend.
By the way all porches handle well. that is why porches are soo good.. their handling is just soo smooth ( beware of bold tires there is no point having a porche if the tires are bold cos if it is ud just crash if u go through a corner at 70 mph.)

If your porsche is 1985 or newer and in around the same condiotion as his then it will shit all over it on a curvey track, in a strait out drag it could be close. And u bought a porsche of course it is going to handle well.

There r really too many unknown variables to answer this question properly.

What year is ur porsche?

Which version of the Porsche do u have? 944, 944S, 944 Turbo S, 944S2? (There is over a 100 HP difference between some of these models)

Is ur Porsche stock? Are there any aftermarket mods?

Your friends 300ZX: Is it stock?

Is it the VG30E engine or is it the turbocharged VG30ET?

All of these variables would have a huge impact on track performance, & there r combinations of these variables that would result in either ur Porsche winning or the 300ZX. Therefore the question is impossible to answer with any certainty.

If yours is an '85.5, then it has about 140hp & weighs about 2800 pounds. Maybe an extra 10hp with the exhaust, if you're lucky. The Nissan should have 30+ more hp & torque, but weighs a good bit more (like 3550 for 2+2). The 944 with stock suspension will handle OK on the track, but not stellar. Upgraded springs rates (torsion bars) & shocks will make a big difference.
l have about $8000 into my suspension on the 944 race car with about the same power as yours & l can beat some very fast cars on the track - mostly due to differences in driving ability.
You only need to spend about $2000 to get 80-90% of the benefits of a suspension like mine.
The biggest differentiator on a road track in a situation like u describe is the driver is ability. If u have 10 track days in a yugo, u can whoop someone on their first or 2nd track day with a new Z06.
If drivers r equal, look at suspension (probably similar on those older, tired cars) & hp/weight ratio (also seems to be similar). Then tires ( l user Hoosier race tires on the track, similar to slicks), brakes, etc.
There is no magic in a Porsche suspension, particularly a 22 year old stock 944.
My wife & l both love the way our car handles on the track, even compared to our 911. We've driven the 944 and/or the 911 at Watkins Glen, Lime Rock, New Hampshire Speedway, Mont Tremblant, Road Atlanta, Barber Motorsports Park, & Roebling Road.

Finally, to answer ur question definitively: Yes - ur can can beat his on the track. Its simple: just make sure u have the edge with driving ability!