Make it go faster!
...your get up and go got up and went?
One piece of the performance puzzle that has not been neglected is the lust for more power. Sure, you may lie, and tell yourself it's for better hauling capacity, or better freeway merging, but down deep, you know you just wanna make it faster.
LZ22 swap
Did you know you can effectively bolt in a 2.2 liter engine? Does a near-forty percent boost in displacement and torque interest you?

That engine is called an LZ22, and here is the recipe:

    * Z22 short block - rods, crank, pistons, block, flywheel
    * L20B front end - timing cover, timing chain, timing gears, pulley, water pump
    * Open chambered head - U67 from an L20B is most likely source
    * Original L16 oil pan and pickup - note that it MUST be clearanced for the #1 connecting rod, test rotate without a gasket to see where to adjust it - a few whacks with a hammer does it nicely 

This combo is covered exhaustively at the following sources:

    * Bluebirds mailing list
    * Brendan Parrot's cylinder head site
    * Jason Grey's engine spec site 

Other L series options
I have an LZ22 in my '71, and couldn't be happier with it. Of course, building a Frankenmotor is not for everyone; some other ideas for your consideration:

    * Dual SU carburetors - a must for any serious performance application, a set of 38mm SUs will give you a unbelievable boost in low to mid RPM performance, a gain in economy, and great drivability. The SU carb seems to have gotten a bad rap... this is entirely undeserved. The 38mm SUs (SSS 510's and non-US applications) and round top 46mm models (70-72 240Z and 2000 Roadster) are very simple to work on and maintain. These carbs can be found for $150 to $250 for a complete set, air cleaners to intake manifold. Use of the larger 46mm Z carburetors can be done with some tweaking, and is a good combo for engines larger than two liters, especially with hotter-than-stock cams. FWIW, I use a set of 38s on my stock-cammed LZ22 and it's fine-n-dandy. The only installation issues are the intake ports and the linkages for throttle and choke. I have not yet installed a choke (runs okay without one). I used the throttle bellcrank from the original carburetor, and installed it in the place of the mechanical SU link, see below images.

      38mm SU-equipped LZ22 250K Cable to mechanical linkage 250K Cable to mechanical linkage 230K

      Porting the head is easy - the U67 head has 1.375" diameter ports, and the 510SSS intake manifold has 1.5" ports. Stuff the intake ports with towels, and grind away! Note that the proper bit will make a world of difference - a bit for steel clogged immediately, and went very slowly; the correct aluminum grinding bit did not fill up, and ground the ports like they were warm butter. (well, not quite, but you get the idea)
    * Bigger engine - we all know the old song, cubic inches or cubic dollars. If an LZ22 combo scares you, consider a complete L20B, found in lots of mid-to-late seventies Datsun trucks and cars. They also made an L18, but frankly, if you're going to do a swap, do it for more than .2L.
    * Bigger exhaust - 2" tubing with a low-restriction Turbo muffler will let a L16 breathe easily. Slightly bigger (2.25") might be good for L20B's and LZ22's. I opted for 2" pipe on my LZ22, as I was trying to retain as much low-end power as I could. It sure worked!
    * Electronic ignition - See Jason's distributor site be warned, he likes to abbreviate "Distributor" as "Dizzy". Sheesh.
    * Ditch the smog junk - this is so obvious it goes without saying.... if it's not a carburetor, distributor, or other easily identifyable component, remove it and plug the hole it came out of! Parts include: Smog pump and plumbing, second set of points, etc. 

J series options
What?! The J13 has several strikes against it - it's tiny, heavy, and low tech (OHV vs. OHC). As such, you're unlikely to encouner much to make it go faster except an L series swap. If you've come up with a performance modification for one of these, pass it on to us - link at left.
Other performance mods
"For more speed, add lightness..."-Colin Chapman

Drop some pounds with a fiberglass front end from http://www.up22.com/ranger.htm
69-72 pickup
Hood - $189  18#
Hood w/Ram Air (?) Scoop - $199 18#
Fender - $129 - 8#
Front Gravel Pan - $100 5#
72-79 pickup
Hood - $189 18#
Hood w/Cowl Induction - $199 20#