Now that some basic information has been learned the practical aspect can begin.
Often tuning companies refer to things like `Stage 3` engines, there is no standard definition for such things and you should check carefully exactly what is included. Roughly speaking it goes as follows for cylinder heads:

Stage 1:

Stage 2: (keeping 4-2-1 system and K&Ns)

Stage 3: (still with 4-2-1 system)

Stage 4: (4-2-1 system) or 4-1 system for max. top end power (about 3 extra Bhp)

        This is only a guide and many components can be safely mixed between stages if desired, you CAN put roller rockers and chrome moly push rods on an otherwise standard engine. I would think it to be both expensive and a bit pointless to do but...

Parts Price Guide:

General Cylinder Head points to save money & sanity:

- Bigger exhaust valves are pointless for all but a race engine
- Roller rockers are very expensive and are not ever a must have item unless trying to win races.
- Chrome moly push rods are only there to save a little valve train weight to allow very high rpm and quicker spin up; the same goes for alloy valve caps.
- Only the exhaust valve seat needs to be replaced to use unleaded fuel.
- Grinding the ports out as big as you can get them will NOT give you lots of power.
- 9.75/1 is a good practical compression ratio limit for a street engine running 92 octane fuel.
- Skimming to increase compression will alter rocker geometry. This will need shorter push rods or very thin spacers under the rocker pedestals. Don't forget that there is the rocker oil feed through the end pedestal though!!
- Stiffer `uprated` valve springs are ONLY needed for higher Rpm. If you stick to less than 5500 Rpm you don't need them
- Slightly Shorter valve guides are fine to an extent to get some of them out the port but go too far and the limited valve>valve stem contact area gives rapid wear and possibly over heated exhaust valves as the heat has a harder time dispersing.
- Use valve stem oil seals, just about every engine on the planet except Spitfire engines use them. They slip over the top of the valve guides. These are Ford Pinto items (UK Ford), available from Canley Classics in the UK. They cost about £12 a set.
You may need to loctite them in place to the top of the valve guide.

Advanced Head Modifications:

- Waisted stem valves:

        These are just valves with the stem narrowed slightly near the valve tip where it passes through the port. This just limits it infringement into the port and makes the valve stem less of an obstacle for the gas. These are available from Triumph Tune (of Moss Europe).

- AR exhaust manifolds. Stands for ANTI REVERSION. I know of no company offering such a manifold for the Spitfire so if you want one its going to be a one off custom job.
Firstly this is of very little practical use in a standard engine, it only becomes useful when more and more radical camshafts are used with more progressively highly tuned engines. Maximum benefit will only result in a full race engine.

        When a really radical camshaft is used often the `valve overlap` is considerable. This means that there is quite a bit of time when both inlet & exhaust valves are open in the SAME head chamber. This is basically to keep the valves open as long as possible to allow the maximum time for fuel to go in and waste gas out.

        The problem with this is that its very wasteful of fuel as some goes straight out the exhaust (ever wondered why many race car exhausts flame?), also some used gas gets sucked back into the chamber. This obviously reduces power, race cars are generally not interested in good fuel economy so the AR exhaust manifold just tries to limit waste gas suck back into the chamber. Only a problem with big overlap race cams.

        To do this it has a 1>2" long bit of tube INSIDE the beginning of the exhaust manifold tube. The outer tube is flared to allow no reduction in bore diameter. This is a sort of `Gas Flow Barb` like on a fishing hook, gas flows out easily but if it tries to go back then the sharp tube end inside the outer tube creates turbulence. This makes it more difficult to reverse gas flow back into the chamber and makes a small but useful increase in race car power.

- Triple or even 5 angle valve seats:

This just makes a less difficult path for the gas to flow.
That equals less turbulence which = more flow which = which = more power.
These days its not really a big deal and most head shops will do it at very modest extra charge. Some even do it as a standard procedure. Five angles is only slightly better than three and should ONLY be used on race cars, this is because it limits too much the valve seat area. This means that it wont give sufficient longevity for a road car.

- Twin Spark plugs per cylinder:

        More common in the motorcycle & aero piston engine world. The idea is to burn the fuel/air mixture more fully.
This as far as I know is of most benefit in Hemispherical shape combustion chambers, not the sort the Spit has.

        Having said that the Alfa Romeo company made a quite superb 4 cylinder car engine with twin plugs called (rather unimaginatively) an Alfa Romeo T-Spark.

        The modification might be interesting to mention though, it can be performed by THE CYLINDER HEAD SHOP in the UK. They have some very advanced machinery there and practically nothing is `no can do` with them.

        This chamber shape is however that used by both the Dolomite Sprint and Rover/Buick V8. Perhaps owners of Spits with these conversions might be interested. The machining work will cost about £150 per head and a special ignition system will cost anywhere between £350 and £1000. Such as custom twin spark system is available so if you're the try anything for power mentality no matter how lunatic then perhaps its worth some investigation. The maximum power gain you are likely to get is about 5%.

Cylinder Head FAQs:

Why doesn't grinding out the ports to make them really big give more power?
        Answer: The amount of mixture reaching the chamber is dependant ultimately upon the valve size, calculations have shown the optimum port diameter to be exactly 0.8 that of the valve. If the port is smaller than this then enlarge it to be the correct diameter. Any more and no more power will be produced.

        Also if you make it too large the velocity of the gas slows too much and fuel drops out of suspension in the air, at this point low end power will be reduced considerably. Only at very high Rpm could sufficient flow velocity be maintained. Optimum gas velocity for a race engine is about 230 Feet per second, for a road engine it is about 170 and upwards.

What's the difference between porting & gas flowing etc.?
Answer: Nothing, its all just different terms for the operation of altering the ports & combustion chamber shape to flow more air into the engine.

What is manifold matching and do I need it?
Answer: Quite simply the inlet and exhaust manifolds are crucial for the induction and removal of fuel and waste gasses. Now as standard the openings in them to match the head ports to manifold ports are pretty vague and a measurable improvement can be had by a good manifold matching job. Both exhaust and inlet manifold can be beneficially modified in this manner.
To do it the shop may insert dowel pins into the head and drill location holes for them in the manifold flanges. This ensures correct line up which can be a hit and miss affair with the standard studs & nuts approach.

       Then the manifold gasket is placed over the front of the manifold flanges, a fine scriber is used to mark its outline on any intruding material. The gasket is removed and a die grinder used to carefully open the manifolds ports to the gaskets dimensions. When the cylinder head is gas flowed they should as a matter of course perform this operation on the head itself.

Why are exhaust valves smaller than inlet valves?
Answer: The bigger one valve is the smaller the other has to be, the pressure expelling the exhaust gas is considerably higher than the force sucking in air through the inlet tract.

Why are overhead camshafts better than OHV heads?
Answer: They throw away several moving parts reducing frictional losses and allow much more accurate valve timing as the camshaft acts almost directly upon the valve.

Why are 4-2-1 headers better for a road engine than 4-1s?
Answer 4-1s supply more peak Bhp but at the cost of mid range power, which is what you need for a practical road engine. Also 4-1 headers are also almost impossible to fit to a right hand drive Spitfire because of the steering column.

What is optimum exhaust back pressure?
Answer = None! The reason most engines (except top fuel dragsters and the like) produce more power WITH an exhaust system is than believe it or not air has momentum.

        The air rushing down the exhaust manifold actually sort of `sucks` the last of the spent fuel out of the cylinder, big supercharged or turbo engines running high boost don't apply as the pressures are so high that the suction factor doesn't really have any more effect. You can make an exhaust manifold with too big pipes for much the same reason why you can have too big inlet ports.

The gas speed drops and any momentum is reduced, (minus the point about fuel suspension).

What about the external rocker oil feed kit?
        The only really concrete thing I can say is that every Spit engine I have stripped down had totally knackered rocker shafts. This makes valve clearances impossible to adjust accurately and is not good for oil pressure. Possibly connected is bad wear to the end camshaft journal as it supplies the rocker shaft, as the oil gaps increase more oil would be taken from the cam.

        Some say rocker feed should be sacrificed to maintain the more important main bearings, some say synthetic oils can make the standard feed fine. I don't know for sure but I will be using the external feed with grub screws blocking off the original feed. It was a weak point in the head gasket too and often leaks oil out from the rocker feed.

        I have also heard from owners of 6 cylinder engines that the oil pressure drop when fitting the feed was too much. My personal opinion on this is that this may well be so on the 6 cylinder engines. Why? you ask me.
Well the bigger 6 cylinder engine has the SAME oil pump as the smaller 1496cc Spitfire engine. That means its pretty much stretched to capacity, adding the oil feed may be just a little too much for it. The smaller 4 cylinder 1496cc engine has extra capacity to spare over the bigger 2 litre engine to I personally wouldn't worry about that aspect.