| Actually Getting Started: The induction system The standard SU carbs are fine providing they are in good condition, and are suitable for all but a highly tuned or race engine. A range of needles are available and correct choice is essential if the potential of your engine is to be reached, the choice is different for each engine depending on its state of tune and capacity. A knowledgeable supplier or rolling road will know which to try. They are not expensive at about £10 a pair. Performance Mods for SU Carbs: Tools, Parts & Operations:
-A 1500 as standard will need 1.5 SU`s and
the 1300 engine when tuned will benefit from these too. Modified as above (minus any grinding etc.), twin 1.5 SU`s will provide a 1500 with up to about 110 Bhp. Although rolling road tuning is only a must have for Weber carbs SU`s will benefit too and to achieve maximum potential you should consider it. Here is a very rough guide to choosing needles for your SU carbs: 1.25s on a 1300 and 1.5s on a 1500: The three letter code is stamped on the big end of the needle:
Weber DCOE carbs have an almost legendary reputation in European racing circles, in the days before injection it was a common sight to see an impressive bank of Weber carbs strapped to the latest racing machine. Weber have put in a large amount of research into the design of their carburettors and make probably the best carbs you will find anywhere in the world. This might sound a bit stupid but they are called `sidedraughts` for a good reason, they must be mounted horizontal. This is because of the float chambers. If you want a different arrangement get different carbs. Weber also do downdraughts comparable to DCOEs. Most if not all
horror stories about these carbs I am convinced come from people either putting them on
set up wrongly or putting them on an engine not set up to benefit from them, I read a
magazine story about a chap with a standard 1800cc MGB. He thought that twin Webers would
make the car fly, he fitted them and low and behold it just used lots of petrol and wasn't
really any more powerful at all. If your engine is not modified to take advantage of
superior carburation it is a pointless and expensive exercise so think carefully. Probably the best analogy I've heard for Weber carburettors is to imagine them as very expensive running shoes. If your engine is old or just bog standard its just like giving 90 year old Granny some Nike Air trainers and expecting her to leap around all over the place. If the basic building block is not up to it no amount of fancy extras will make a bit of difference. With engine building you must start from the ground up and have a solid foundation upon which to build, to do otherwise is not going to get you anywhere fast except perhaps the breakers yard looking for a new engine! If you have any
reservations about Weber DCOEs remember that such classic marques as: They are
considerably better engineered than SU`s and are far more flexible and versatile, as well
as being able to flow more than enough air for any Spitfire engine. If you want to fit a single DCOE then the size you will need is a 45, for twin carbs you need 40s. This is very important and you wont be able to get satisfactory operation if you choose the wrong size. Also if buying second hand look at the number on the top plate like for example "40DCOE-31" . The 31 (or other number) denotes which car it was originally made for, mechanically all `40`s will be the same but will have different venturis, chokes and jets for its particular application. These can all be simply replaced with the correct sizes but that costs money. A Weber repair manual should have a table of the jets/chokes and venturis originally fitted to various carbs originally fitted to factory engines. My Webers say 40DCOE-31, the 31 denotes that they were originally belting around on a Lotus twin cam engine as well as what parts it was (should) be fitted with. Venturis and chokes slip straight out by hand so if inspecting some second hand ones it is a simple matter to check them out, the sizes are stamped upon them. Also take the top cover off to check for corrosion. If any is present in the alloy body leave it well alone. Corrosion only sometimes occurs when they have been left empty of petrol for a considerable time. If buying new supply the dealer with the list of required jets, venturis and chokes. The carb will be supplied ready to go. Although it still needs a rolling road session. Each DCOE has two barrels so you have one choke per cylinder, this gives fantastic throttle response and the reaction of a tuned Spitfire to the main jets opening up is enough to make even the most chronically depressed driver suddenly grin from ear to ear. As far as I know really the only point in fitting a single 45 is to save a bit of money and make it slightly easier to set up. Either way you still need one to two hours on a rolling road to get them really on song. Here is a list of suggested settings for DCOE carbs on a Spitfire, these are initial settings and may be fine tuned by the rolling road. They are much the same for 1300 & 1500s.
Note that the Aux. Venturis and Chokes are
by far the most expensive items. The principle behind these stems from the fact that the colder air is the denser it becomes. More density = more air/fuel mixture which = more power. The hot air under the bonnet created by the hot engine and more specifically by the hot exhaust manifold can reduce potential power outputs by perhaps as much as 5%. There are several things you can do to reduce both the temperature of inducted air and that of the fuel being delivered to the carburettors / injectors even before you think about the cold air system.
If you intend to fabricate an airbox you should use a feed tube of no less than 2 1/2 inches internal diameter, preferably go for 3". Mount the intake as close to the front of the car as possible. You should also ensure that the airbox leaves at least 3 inches between the front of the air inlets and the wall of the box, you can do without any further restrictions. This box isn't exactly going to be structurally vital so make it out of very thin aluminium sheet. Simply cut out with tin snips, drill, line the edges with silicone water sealant (just to make it air tight) and pop rivet together. You have two choices about air filtration: Use the existing filters and build the air box around them, scrap the original filters and mount one big one at the front of the inlet piping. I doubt there is any performance differential in these two options. If you have spent money on good ones already just use them. If you don't have any then it may be cheaper to get one big one for front mounting. I recommend K&N air filters, they have been Dyno tested and officially make power with good particle filtration. Fuel injection works as it sounds, fuel is injected rather than sucked out the jets. This provides superior fuel atomisation and the ability to cunningly attach it to all sorts of expensive computer gizmos to give more power and economy. They tie the fuel directly to the air flow, ignition and engine temperature too. Injection also has fewer moving parts which means that despite its complexity its very reliable. This is the ultimate in efficiency but is probably just a big waste of money and time for most Spit owners who just want a simple and fun car to fling around. Nevertheless I think its sufficiently interesting to write about. There are no excuses to be made here, fuel injection is the best (theoretically). It can provide the most power, lowest emissions and best fuel efficiency. It can also break the bank and be only very slightly better than a well set up twin Weber installation. I have certainly heard of and read about injected Spitfires (with Triumph engines), usual procedure is to modify the 6 cylinder Triumph TR mechanical fuel injection set up. This is perfectly possible and when correctly set up should be very slightly more powerful than twin Weber set up and have lower emissions. Bear in mind that modifying injection systems even the more simple mechanical type is likely to be very tricky work. There are allegedly problems with the seals on the Lucas injection pump with unleaded fuel too, unleaded versions are available new for about £200. It can be an attractive proposition as you may be able to pick up second hand injection kit cheaply at a show, also the manifolds bolt right in. A luxury you won't have if you try bolt on an injection system from a local Toyota breakers yard. Modern fuel injection is electronic and has as mini computer built around the engine it is fitted to with a three dimensional fuel/air/rpm chart tailored exactly to the engine it was fitted to at the factory. For this reason it will be very hard to use a system from a different car, changing the fuel map is possible occasionally but ask me if you can change it to suit a Spit engine and I honestly don't know. The electronic systems do have about a +/- 10% margin of error correction built in so a common mod is to replace the air filter with a K&N air filter. This lets more air past the air meter and the electronics supply more fuel to cope. I think its a bit of a risk to assume your Spit engine will be within that 10% band of error, even the fuel map itself will still be incorrect and cancel out the `edge` that injection gives you in the first place. Even that assumes you can fabricate your own bespoke inlet manifold. For the rich there is a solution (like £1500>£2500), injection systems are available that bolt straight on to any engine with the immortal Weber DCOE manifolds. They have fuel maps that can be changed to suit a 1 litre shopping car to a 500 Bhp 5.7 litre small block Chevy (though the Chevy will obviously require twice as many injection runners). Some can even be plugged into a laptop PC so a knowledgeable passenger can try the effects of different fuel air maps while you are driving it! Wow, sadly I have not the cash for such a system, I certainly wouldn't bother with my 1500 engine either. It is an interesting thought though! The Japs have
again beaten us by making marvellous Carbs for those engineering masterpieces that are
modern Jap bike engines. I know of one US company that does these conversions, as for
price or performance I couldn't guess except that it will be expensive. Up to you again but I reckon a bit of common sense reveals that there isn't really a lot of point unless you happen to have picked up 4 Mikunis for peanuts at a local parts sale. I have no experience of these but they seem to be roughly comparable to SU`s. I do not recommend them for a performance engine as I DO know that a much less wide selection of tuning parts is available for them than either SU or Webers. Other Weber made carbs: Obviously Weber make a great many different carbs and all are when set up right very very good. The DGV is one
conversion that seems common, I'm sure its superb too. DCOEs are
designed as the perfect performance carb and will always remain so. With most if not all
modern vehicles using injection the chances of anyone else bothering with the huge
investment to make a better carb than a Weber DCOE is not very big. Weber also make those big quad barrel downdraught carbs, these are really for use on big `V` type engines. They are called Weber 500s. These are highly recommended by UK Rover V8 specialists RPI ENGINEERING, who maintain that they are superior to the usual US Holley type carburettors. The big Weber is of course more expensive and Holley carbs have been used without problem for decades on American V8 muscule cars, there is always room for improvement however! Supercharging, Turbochargers and Nitrous Oxide: As I said before the mechanical ceiling has long ago been reached for Spit engines and trying any of this trickery on an already highly tuned engine is likely to be quite a dodgy exercise. If you like the thought of your con rods entering low earth orbit or want an alternative way to achieve more power with an otherwise standard engine then read on. Note that a carb`d engine with forced induction requires no great sophistication from the carbs and many have very successfully cars have used SU carbs. Superchargers and Turbos both do exactly the same thing, they force air/fuel mixture into the engine at above atmospheric pressure. Just how much above is up to the type of apparatus and the installation. A supercharger is mechanically driven, usually by a belt from the crankshaft. This gives it superb response times as it reacts instantly to engine speed changes, it also means it uses up a little power driving its impellers round. The instant response times are very popular with drag racers who don't want Turbo lag. Turbos work by using the exhaust gas to turn a mini turbine, the other end of which is a small air pump for shoving fuel into the engine, they are more power efficient than Superchargers as they harness otherwise wasted energy, they also make the exhaust note quieter! Turbo lag as the turbine runs up to speed was a big problem for early turbo cars, it had the effect that drag racers didn't like it as they didn't get the instant response and road car drivers didn't like it as the car could suddenly violently surge forward when the revs rose above 3000 Rpm and the turbo kicked in. These days turbos are much more passive and with lightweight (some times even ceramic) turbine impellers (known as hybrid turbos) have overcome most of the early niggles. Both systems can potentially be designed to increase the power as much as you like before the engine blows up, it can seem very attractive to fit a supercharger to a standard 1300 Spit and achieve the same power output as someone who has a semi race engine with an impractical wild cam and loads of immensely expensive extras. As before with injection the problem is finding some price practical kit that you can adapt to fit. It can and has been done, in the days of yore supercharger kits were available for various cars such as MG Midgets, Frogeye Sprites and the MGA. The turbo avenue is tricky as it requires some fancy piping to route the gas to the turbo, it also really needs an electronic management system to ensure proper operation. The limiting factors are mechanical failure and fuel pre-detonation. There is no point in fitting a Hybrid turbo with 3 bar (30 PSI) of boost to a Spit as it will fall to bits very soon (although it would probably be a very exiting 5 minutes!). The compression of the charge and other factors raise the temperature of the induction charge and pinking will be a problem at higher boost levels, not to mention your piston crowns melting apart. Intercoolers and Charge coolers are for cooling the hot forced induction charge, Intercoolers use air to cool it and Charge coolers use liquid to cool the intake mixture. A healthy but sensible 8 PSI or so should be a good bet, the rest is up to you and your wallet. A custom supercharger installation with all kit included would probably cost at least £2000. There are companies in the UK who do such conversions for a living. Nitrous is just an oxidising agent which when heated splits into nitrogen and oxygen, this additional oxygen can result in 50% more fuel being effectively burnt. Do the maths yourself but truly absurd power hikes can be achieved if your engine is bullet proof. I wouldn't try it with a Spit engine. |
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