L24
The L24 was used on the '69 to '73 240Z sports coupes. The early 240's used the E31 block with flattop pistons and might have used an E31 head. The E31 is the uncommon but very desirable head used on early 240Z's. They had high-quench 42-cc chambers and produced a little more power despite having smaller valves than the E88 that replaced it. Only roughly 10,000 were made, and most are gone having been used in high performance racing applications. Good luck finding an E31 head, and if you do, it can command a very nice price. The rest of the 240Z's use a P30 block and E88 head- not a bad combo. The L24 motor on 240's from '69 to '72 have the desriable 3 or 4 screw SU carbs- the high performance models.
In 1973 the 240Z got hit with the new emissions in the United States and was forced to use more restrictive carbs, known as "boat anchors" in Datsun circles, and are worthless. These carbs carried over the 260Z was well and were dropped in 1975 for modern fuel injection.
However, something interesting of note- the intake manifold on these early 240Z's with the better carbs isn't has high-flowing as the intake manifold on the L26 motor- the engine in the 260Z. The stock 240 intake manifold is the E88 (see a patern?) and the stock 260Z intake manifold in the N36. The N36 intake manifold is rumored to produce 10 more horsepower than the E88 intake manifold. But, the 260Z, and thus the L26, was only produced for 1 year (roughly), so they are somewhat hard to come by, but not nearly as hard as the E31 head. The 240Z also benifits from a very simple balance tube setup (less complicated than the monstrous 260Z setup).
L26
The L26 was used in the 1974 260Z and is the last carbureted Z's. The 260Z is the "layover" model for emissions while Datsun kicked out a higher performance model (280Z). The 260Z falls inbetween both in terms of performance.
To Gain more horsepower, Datsun included a stroker crankshaft on the L26, which stroked it's displacement to 2562-cc or 2.6 liters. The L26 uses the same flattop pistons. The block is the P30 paired to an E88 head with an N36 intake manifold. The only thing inhibiting the L26's performance is tho crappy SU carbs. For a lot more performance, slap on a set of rebuild or remanufactured 70-72 3 or 4 screw SU's and you'll be pleasantly surprised.
L28
The pinicale of true Z engines- the last in-line 6. The L28 is a bored and stroked version of the L24, increasing overall displacement to 2.8 liters. All L28's also utilize modern Bosch fuel injection, so no more SU's. The '75-'80 280Z's used the N42 block with dished pistons to reduce emissions. In '81-'83, with the addition of the turbo model, Datsun cranked out a bitching block- the F54. It used either flattop or dished pistons and had siamesed cylinders- meaning that instead of the coolant flowing to each cylinder, it flowed connect 1-2, 3-4, 5-6, with cast webbing for more ridgity. But, don't think that the F54 is a "turbo" block- it is really up to the head to decide that.
Speaking of heads, there are a lot of choices on the L28. For starters, there was the short-lived N42 on 280Z's from '75 to '77. Then there was the crappy N47 until 1981. Then from 1981 till 1983, you could either get the P79 or P90 head. In 1983 you could get the interesting P90a turbo head as well. The P90a used hydralic valve lifters instead of standard rocker arms, making the engine ghostly quiet. With a low-lift, higher-duration cam the P90a is a very good setup- only problem with P90a- you can't get replacement lifters anymore.
The P79 head is a 53-cc head for use with only flattop pistons on an F54 block. It uses an internally oiled cam and has the exhaust ports/liners the same as the N42 head. The P79 head also used an exhaust shim that heated up red hot to reduce emissions. Racers have long argured that it inhibits air flow and hurts performance so they remove it. It is difficult to remove, however, and doesn't always leave a smooth surface, so it could end up damaging your air flow as a result. My advice- leave it be.
The P90 is the turbo head used on ZX Turbo models '81-'83. It has the same 53-cc combustion chambers as the P79 but sqaure exhaust ports like the N42. The P90 is said to have the highest flow of any cylinder head. Makes since, because it was a turbo head!
A naturally aspriated L28 with flattop pistons and an E31 head will give you a killer compression ratio of 10.8:1, just a bolt on. If you're going turbo with L28, you must have the P79, P90, or P90a as all other head will increase your compression ratio too much and blow your engine! All E31 head are known to raise compression on all L-Series engines, so I don't advise using one on a turbocharged motor.
Combos
The great thing about Z engines is their interchangeabiltly. You can use an '83 F54 block with a '71 E31 head and convert it back to carbureation by using an N36 intake manifold from a '74 260Z. The possibilities are endless. Here are some of my combos:
The Retro combo: P30 block, E31 head, N36 intake manifold, dual 3 screw SU, mild lift/duration cam. Good midrange punch and revs freely. Not too finiky, easy on gas, and keeps the "old school" theme.
New Age combo: F54 block, shaved .080" N42 or P79 or P90 head, triple Weber carb setup with Canon intake manifold, hot cam with .460 lift on intake/exhaust. This engine will pull like a bat out of hell between 3,000 rpm and 6,500 rpm and smoke the doors off most new "tuner imports". Very good setup.
Turbo combo: F54 block, P90 or P90a head, Bosch fuel injection, hot cam with .460 intake/exhaust lift. Paired to a modern T3/T4 ball bearing turbocharger, a large front mounted intercooler, and of course use a compressor bypass valve, you can achieve mid to low 11 second quarter mile times with a built rear end. Use an R200 diff.
Needless to say, I favor the F54 block by far as it incorperates some of the best design features, such as the siamesed cylinder walls, additional cast webbing, and they are rather easy to come by. The N42 head is also easy to come by, and recommended for high performance applications and even mild street application. If you want a beast, the P79, P90, or P90a heads are the way to go. Stay clear of the N47 head.