VG30DETT- 3.0 liter, 24 valve DOHC twin turbocharged/intercooled V-6 from 300ZX-TT
Pros: easy to find, inexpensive given it's performance, easy to modify
Cons: Requires custom fabrication, losses some "Z-ness", requires lots of extra parts
RB30DET- 3.0 liter, 24 valve DOHC turbocharged/intercooled in-line 6 from JDM Nissan's
Pros: Large power pand, easy to upgrade, great power potential
Cons: hard to find in USA, requires a little fabrication, worth the hassel?
Okay, lets say you want to keep you stock engine so an engine swap isn't for you. You can either rebuilt it, swap heads (i.e. E31, E88, N42, P79, P90, etc), convert to fuel injection or carbs, or build up the motor. But, if you have an early 70-78 Z car, you won't be able turbocharge it without the '81-'83 L28ET motor. So, unless you swap, you going NA (Naturally Aspirated).
A popular thing to do is to take a P30 block and fill it will flat top high-compression forged pistons and an E31 cylinder head to put compression into the high 9's. You can get compression into the mid 10's with an F54 block and E31 head, but it would require a swap as the F54 block only came on '81-'83 280ZX turbo models.
Transmissions
If you want more useable power from your naturally aspirated L-series engine, swap in a 5-speed manual transmission. I know what you're saying if you have a later 280Z, "I already have a 5-speed!". Well, I hate to break it to you, but the 5-speeds in 280Z's and early ZX's (years 77-80 ) are bad. These are called "Type-A" transmissions and it was one of the first 5-speeds Datsun ever made. It is basically a 4-speed with a 5th gear sharing the reverse fork, which made the fork weaker. When these transmissions see abuse, such as in racing, the fork as been known to blow-out of 5th gear and in many cases it requires an entire transmission rebuild. Truth is, Datsun/Nissan never made very good transmissions. The Type-B 5-speed as made in '81-'83 and is a genuine 5-speed, and is a wiser choice for your Z-rod.
The best 5-speed Datsun ever used wasn't made by them- it was a Borg Warner T5 5-speed manual trany. This thing is bullet proof. Along with Datsun Type-B 5-speed, these are both true 5-speed transmissions, not just converted 4-speeds. The Borg Warner T5 trany was only made for the '83 280ZX Turbo and wasn't fitted in every one produced, so they can be tricky to come by.
All Datsun-made transmissions from 1970-'83 can be bolted up to the L-series block. 5-speeds in Series 1 240Z's, however, require modifcation to the center console so you can access 5th gear. Otherwise, it is a pretty simple bolt on.
When swapping transmissions, you need to keep in the mind the extra parts you will need. There are two types of flywheels on all 5-speed transmissions, a coupe and a 2+2/turbo flywheel. You can use a 5-speed on any year L-Series block, but the throwing bearing collar must match the type of flywheel used. Also, coupe and 2+2/turbo flywheels aren't interchangeable because the pressure plate dowels won't line up to the flywheel dowels.
Also, a lot of people get confused when swapping the transmission how to keep the speedometer correct. Well, the speedo is metered by a plastic-toothed cog in the differential, not the transmission, so it does not matter what year or type of transmission you have; only differential.