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- A good way to fit the sump so it won't leak is to coat the block flange with a reasonable coating of silicone sealant, then fit it to the block and then do up the retaining bolts with only your fingers. Leave it for a couple of days, then finish the job with a tension wrench. (Also make sure the sump flange is straight; It makes the job so much easier) If the sump plug is looking a bit daggy because it's only ever been molested by a cheap shifting spanner, then while it's out weld a large nut to the head of it so you've got a nice, large surface to grip next time. The same applies for the gearbox and diff sump plugs, of course.

-The oil pump can be made to work far better by taking to it with a very fine grinder or a small file. Take off all the sharp edges that the oil has to slip around, thus making it easier to suck & push around and giving the engine more power. (You'd be amazed how much power the oil & water pumps take to drive) Make sure all the sealing surfaces are straight and true - You may need to spend some time on a fine grindstone to tighten things up again. The oil pump will have a pressure relief valve with a spring in it. You can boost your oil pressure by putting a 1/16" washer behind the spring. Some pumps may need more, some less; They are all different, depending on what the engine needs.

- Good oil filters don't always have to be big. Since the mid 80's or so, Toyota have been using a type of filter where the paper is packed in, in a different manner to the usual filter.They are able to pack in at least as much paper, and hence filtering area, as a normal filter, but in a package far smaller. The stock Toyota oil filters are used on many of their racing engines. So, even though they may cost a little more, they are the way to go if you have a Toyota or are able to use them.

- Quite often the head gaskets supplied aren't so good when it comes to lining up with all the water passages - Take the time to carefully check them all out and if needed trim the holes with a sharp knife so they match the outer edges of the largest hole in question. Sometimes oil return holes as well. It does pay to be careful though, as some head gaskets are deliberately designed to have smaller water passage holes in them that what is in the head and/or block. This is done to try to keep the water from leaving that part of the head too quickly, and thus picking up the heat from the head effectively. It's actually not uncommon to find this sort of thing on an engine that's grown in capacity from a smaller original one.
Copper head gaskets are also often used on high-power turbo engines. They also normally use O-rings to seal the combustion chamber pressures. If you use a copper head gasket, then both the block and head mating surface must be extremely smooth & flat or the gasket will not seal. Here's a 4AGE copper head gasket.

- Trimming weight off the flywheel is a great benefit. The only problem is that with some factory flywheels it isn't safe to cut a lot out of them, and so you'd be better off doing it properly by getting an alloy one fabricated from scratch. In both cases, you want the minimum of metal around the edges of the flywheel so as to reduce rotational momentum. If you can, have just enough metal to support the ring gear for the starter to run on and nothing else. Alloy flywheels will need a steel insert for the clutch plate to run on; The flywheel won't last very long otherwise. If you do reduce the weight of the original flywheel, then after balancing get it shot peened. Change the flywheel bolts every rebuild on a performance engine, or on a standard type engine every second time. Here's one of my 4AGE alloy flywheels.

- Sparkplugs & leads & coils can make a small difference, but only a few hp or so. The easiest way to pick what sparkplug you need is to simply go one heat range cooler than standard. The best plugs seem to be the ones with the copper cores and 'U' shaped electrodes. As for how long they last - When the car is getting even slightly hesitant to start in the mornings, change them. Don't redo the gap or clean them, just throw them away a put a new set in. Sparkplug lead efficiency depends on how much electricity the lead lets get to the sparkplug, and how much gets radiated away into the other leads (Causing mis-fires sometimes) and people's radios. The coil can make a difference, so it's worth getting a good one. How to tell if you've got a good one? How far the spark jumps is a good sign, I reckon. There are many brand name coils that are very expensive, but I am quite happy to run one of the newer type of coils that's shaped like a electrical transformer. They have a pretty healthy spark and are cheap enough to have a spare lying around.
As has been said more than once, "bang for buck, ignition systems add-ons are the worst."  Just make sure that what you have is in top condition and you can't go wrong.

- Synthetic oil is the only way to go. No excuses! The only thing to check is to see that it is in fact a full synthetic, not a blend like the early Mobil One was. (Now a full synthetic though) They last longer, protect better, and make more power than mineral oils. 

- Oil filters vary in quality quite a bit, but if you have a Toyota you're laughing - They are expensive but the best for your engine and are quite good enough to go racing with. Oil filters either come with a bypass valve or they don't - The bypass valve is there in case the pressure differential (Between the incoming and the outgoing oil) in the filter gets so high, possibly because of debris blocking the filter, that some oil, dirty as it may be, will still get to the engine. This is not a good thing for performance engines. At high revs, it's possible to get so much oil flow that the bypass valve will open anyway and so the oil is not being filtered! If you take the time, it is possible to get a filter for your engine that doesn't have a bypass valve. The only thing you must be careful of is to take great care in warming up the engine before running it at high revs. (It takes me at least 15 minutes on an average day to warm the engine before I'll rev it hard) One small trick some people use it to fit an adjustable clamp (The sort that's made from metal and adjusts in & out with a screwdriver) around the filter base to stop it from blowing a side out.

- Superchargers are great for making lots more power quickly. They give the engine immediate response, unlike a turboed engine, and also a lot more power at low revs. They are, however, a bit of a compromise to set up. If you want lots of power at low revs, you need to spin the blower quickly, vice-versa for higher revs. But, the faster you spin the thing, the more power it sucks from the engine. Ok, it compensates by making lots of boost, but the power required to drive the thing goes up something like the square of the rpm of the engine, so even if you have set the thing up to make boost at high revs, all things being equal a turbo engine that is making the same amount of boost will make more power. The other problem they sometimes have is that if you spin the rotors too fast then it's possible that on certain types (eg, Toyota) the tips of the rotors (which are made from a form of teflon plastic to help seal the air behind the tips) will start to melt.

- Turbo's are a real blast, and a turbo engine is ever so easy to get more power from, simply by upping the boost. A couple of basic rules for turbo's - The bigger the dump pipe from the turbo's turbine housing the less backpressure you'll have and so the earlier the turbo will spin up in the engine's rev range. A more efficient intercooler is always good. I say 'more efficient' instead of 'bigger', because in the quest to fit a bigger intercooler the air plumbing can often get all too long and so cause excess lag when wanting boost. To make an intercooler more efficient, you have to get more airflow through it and so that often means bigger scoops, etc, to get the air moving through it. What a lot of people forget is that the exit from the intercooler is just as important as the entry, so the airflow is still not as good as what it should be. One possible solution is to use a water-to-air intercooler, as they are often more flexible with the installation and still provide good cooling. They're also a lot better around town where you often don't get a lot of good, clean, cool, airflow.
If you're going to add a blow-off valve to the turbo engine, then don't expect any noticeable improvement in performance. The vast majority of installations don't make the car faster at all; they just sound faster. ;) If your engine is a MAP sensed one then you can vent the blow-off valve to atmosphere no problems, but if you have an AFM'd engine them you must vent the outflowing air back into the inlet system between the AFM & throttle butterfly. The reason for this is when the blow-off valve opens it'll let a lot of air still pass through the AFM, which then signals the computer that the engine is still making a lot more power than what it actually is, so, the computer makes the engine run waaaay too rich and it'll blow clouds of black smoke out the exhaust.

- You'll often hear of long-stroke engines 'making more torque' than short-stroke. This is wrong, stroke has little to do with the generation of torque, it's almost purely the capacity of the engine. Consider two cylinders, one with a long stroke/small bore and the other with a short stroke/big bore but both having the same capacity. If you apply the same (combustion) pressure to the top of each piston then the instantaneous torque on the crankshaft will be exactly the same. 
Here's the proof, take a ...
4" bore 3" stroke (37.68 cu") and put 1,000psi on the top of the piston with the crankshaft sitting at 90°. The piston area works out to (2^2 x 3.14159 = ) 12.56sq inches so 1,000psi on top of that = 12560lbs acting on a 3" arm =  3140ft-lbs.
2" bore 12" stroke (37.58 cu") still with 1,000psi, etc. Piston area is 3.14159 sq inches so 3,141 lbs on a 12" arm - 3141ft-lbs or practically exactly the same torque. (Note that these are just instantaneous figure, the engine won't really make that much torque when running)
The benefit of the shorter stroke/big bore is that you can fit larger valves for better breathing, lose less friction for any given revs due to the lower piston speed, and rev the engine harder due to the lower piston speed. The con-rods will also likely be longer and that too is a useful benefit for the majority of engines.

- Which is better, a 2.5 litre four-cylinder engine or a 2.5 litre six-cylinder?  Most people automatically poo-poo the four-cylinder engine but as I just demonstated above the bigger the bore the better for breathing.  True enough then the six should have a shorter stroke and that's also a good thing .... so which is best? Even though the four will have bigger bores and so breath quite well, the six will actually have more total valve area, and here's the maths behind it.
A reasonable general assumption is that for a four-valve engine the inlet valves will be about 1/3 the diameter of the bores. If we give the four-cylinder engine a 100mm bore (like a Subaru 2.5 litre EJ25) and the six-cylinder an 86mm bore (like the Toyota 1JZGE) then the valves work out to 33.3mm and 28.6mm respectively. The four will have a total of eight 33.3mm valves or a total of 6967sqmm. The six will have a total of twelve 28.6mm valves or a total of 7709sqmm.
So the six will be better in terms of breathing ability, but it will be a heavier & longer engine.

A Renault RS9 Formula One racing engine. Approximately 750hp @16,000+ rpm
 
 

A BMW Formula One 1.5 litre turbo racing engine. In excess of 1,000hp at the wheels and the BMW engineers reckon they had about 1300hp in qualifying trim which is rather healthy to say the least.

This is a fairly rare photo - It's a 1995 to 1996 (I think) Illmor/Mercedes 2.65 litre V-8 Indycar cylinder head. You can see the inlet ports pretty clearly here, and perhaps surprisingly they are still bifurcated this far out of the inlet valves. If you look carefully you can see the sides of the cam buckets are cut away to allow for very high lift cam lobes to pass. These engine are required to use conventional coil springs instead of the more advanced pneumatic valve springs that the Formula One engines use.

Speaking of pneumatic valve systems, this is a basic diagram of how they work

Also of interest is the 1986 (?) Honda RA168E twin-turbo V-6 1.5 litre Formula One engine. It was rated at 'only' 685hp, but with the rules & restrictions at that time it was heavily derated. (Note the BMW engine of a couple of years before!) You can see the extremely strong con-rods, crank, pistons, etc, that help it stay together at high revs and a fair old amount of boost.

This is supposed to be the actual power figures from a 2000 spec Ferrari Formula One engine. I don't know how accurate it is, but I think it'd be pretty close.

These Toyota engines are interesting. The one on the left is the very rare 1.3 litre 3K-R twin cam 16 valve racing engine. I don't know how many were made but it appears there was only a small handful. Note the mechanical fuel injection & slide throttles. The second engine is the 1.6 litre 16 valve twin cam 2T-G racing engine, also very rare. (I've seen one though, at the 1984 Bathurst 1000 in Aus, at the Toyota tent).  The third engine is a 2.0 litre 16 valve 18R-G racing engine. I took that photo at a performance shop in Los Angeles. At the same shop was the engine at the far right, which is the 2.2 litre '503' engine, and it's not developed from any commercial Toyota engine. It's loosely based on the 3SGTE, but has no common parts. It was mainly intended to run at the famous US Pikes Peak Hillclimb, but Toyota have also used them a fair bit in various sports cars. They make around 900hp odd running on methanol.

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