Carburation - Initial considerations and manifolds 

When trying to improve your Min’s breathing efficiency, carburation and inlet manifolds are open to as much speculation as other ‘performance enhancing’ components. A great deal of which falls into the b*llsh*t category. As is my wont, I’ll try and outline some basic guidelines. 
Many facets of our lives are governed by one particular idiom - big is beautiful, and it seems from what many folk are told carburettors are no exception. This is all very well, but it can have drastic effects on how well the engine performs over the broad spectrum of its use. For maximum power, i.e. foot firmly buried in the carpet pile, the method of introducing air/fuel into the engine could be carburettors, injection, man and garden hose, milk pail, or whatever, as long as the air/fuel ratio is correct and the fuel is properly atomised for a complete and efficient ‘burn’. I’m not going to get into a discussion on air/fuel ratios and what’s best for power/economy here - a full subject in itself - but the metering device. And that, mes amies, is the carburettor. Then, having selected a suitable one, a decent inlet manifold is a definite requirement to maximise its performance. To cover all eventualities would take a tome, so we’re going to concentrate on road use in this article. 
Before shelling out hard earned beer vouchers on another carb, a nanosecond's contemplation as to why wouldn’t go amiss. Just why do you need to change it? If it’s solely to come tops in the bar room b*llsh*t competition, or aesthetics, then this article is not for you. If maximum performance - be that economy or power - is the goal, read on. 
Your Mini engine is no less for it’s ancient design than any modern unit. They all accomplish the same thing - turning heat into mechanical energy to propel the four-wheeled wonder down the road. To do this the heat has to be developed from somewhere, in our case by burning (that’s burning, not exploding) a fuel with an oxidant - petrol from a pump and oxygen from the air. To achieve our goal of either maximum economy or power, or combination of both, these two parts need adding in a certain ratio. Years of development and tests have come up with certain well-defined limits as to which is relevant where. The carb is designed to complete this duty with efficiency given its mechanical/cost limitations. To whit, the vehicle manufacturers have carefully considered the carb chosen for each engine application, of which size plays a big part. 
We fit modified cylinder heads to improve volumetric efficiency. Greater power outputs are achieved through maximising air consumption. The carb therefore needs to be sized so as not to be restrictive. However, the most common mistake made is fitting a carb that is too big. What you need is a carb sized to provide adequate airflow for the expected power output, NOT the size of the engine. This is based upon the fact that a certain volume of air is needed for a certain amount of power. To illustrate, air consumption of a 65 HP 998cc engine is going to be relatively similar to a 65 HP 1275 - so the carb size needed is going to be more or less the same within certain limits. To stress the point - if the correctly sized carb is used, there is absolutely no more power to be had from going bigger. 
Bearing this in mind, on a bigger capacity engine, fitting a carb a shade on the small side will make the engine work a tad harder to draw air through it to achieve that power output. It’ll mean a slight reduction in outright power, but the benefits are worthwhile - greater fuel economy and greater torque. Torque is what accelerates the car in 99% of road use, so optimising this is far more beneficial than tuning for maximum power. Parting with a sizeable chunk of money for a big carb is a waste, as it’s full potential isn’t being used and bottom end flexibility is severely compromised. Check out the relevant chart below for sizing. 
The solid coloured area shows the carbs strongest operating envelope. When getting into the shaded area, consider going up on carb size - BUT carefully consider the text when doing so. This is not hard and fast, but a good guide, and presuming a decent flowing manifold is used. It is possible to get really good performance out of a bigger than seemingly necessary carb on a smaller engine (say HIF6 on a 998) - but the knowledge of how to do so is limited to the most learned ‘wizards’! 
How many? 
Throwing more carbs at an engine isn’t going to make one iota of difference to maximum power output unless the engine was drastically under-carbed to start with. In which case a bigger single carb would have the same effect. 
In the days of yore, multiple carbs were seen as automatic power improvers - a legend that still lingers in the minds of the unknowing/misinformed. The main reason for their apparent huge power output improvements was more to do with manifolding than carb quantities. Modern technology has proven and developed the art of maximising airflow/velocity. As can be witnessed by some of the horrible/frightening aftermarket manifolds produced some years ago for the single SU in comparison to those on the market today, but we’ll consider this more later. 
The main benefit of running more carbs, twins in our case, has already been intimated earlier - smaller carbs produce better drivability. So using a smaller choke sized twin set-up instead of a large single with similar airflow capabilities for the same engine power output will generally produce better drivability. The downside is the extra cost (twice the price), setting up, and manifold efficiency. However, contrary to popular opinion, twin carb set-ups do not go out of tune/sync quicker than a single. Once correctly set up, there’s no difference whatsoever! 
That’s for SUs. The much-hallowed Weber is effectively a twin carb in itself - having two ‘chokes’ in a common body. Deemed to be the BIG power producer - yet another fallacy. Again we’re talking mainly about installation/manifold efficiencies and more appropriate sizing than any super-natural capability. Tuning a Weber is easier for most as calibration can be easily altered through the use of a myriad of components even when the ‘crème de la crème’ is used - ‘split Webers’. Tuning an SU can be awkward because of the use of the tapered needle, made more laborious when there are two of them. This split Weber business is an abhorrent thing to do to such a well designed piece of equipment and completely unnecessary on anything other than the absolute out-and-out circuit racer. Not to mention the rude amount of money needed to do the job properly. Getting the benefits out of a Weber on a Mini means bodywork surgery, which is hassle and far from simple. It therefore isn’t really worth considering for a road car, and would need article on it’s own - so will be left for another time! 
Manifolds - singles 
Having selected a carb suitable for the job, choosing a compatible manifold is of great importance. After all, if the carb is right and a crap manifold used, much of the sought-after power potential will be lost. It needs to meet a number of criteria - carry the carb at the correct angle, allow fitment of a suitable air filter, and provide uninhibited passage of the fuel/air mix to the engine without compromising velocity. This last feature being the most important for usable power, the others a definite bonus to avoid fitting hassles and frustration. 
There are a variety of manifolds on the market, generally cast in aluminium-alloy. Few understand why this is, generally believing it’s mainly down to ease of production and is certainly one of the major considerations. Aluminium conducts heat faster than iron, the benefits of this being two-fold. As far as maximum power is concerned, the cooler the inlet charge, the better. Aluminium’s ability to dissipate heat faster keeps the intake charge down in comparison to an iron one. For economy, quicker warm up is essential, again aluminium's rapid conduction of heat gets it up to running temperature quicker - hence Rover’s introduction of an aluminium-alloy manifold on the MG Metro engine to utilise these principles. The mistake they made was making it slightly too big in the ports. 
Port size needs to be designed to achieve maximum airflow without compromising velocity for the given space the manifold has to fit in. In a Mini this equates to precious little! Big ports mean lower air speeds, and as a consequence bottom end/mid range drivability suffers. In days gone by, there were only a few alternative manifolds on the market for single SUs. The best ones being mainly produced to suit one particular engine build type - namely the 1380cc big-bores. The small-bore engines were considered the poor cousins. A couple of the better manufacturers ended up making two manifolds to alleviate the situation, but were based on the big bore item. The big problem here was the fit - the big-bore block is 3/8” taller than the small-bore. 
To achieve maximum performance, manifolds were conceived to be as flat in side plan as possible. Consequently on the big-bore engines, the jet tube was perilously close to the bulkhead. I remember from the pained experiences of others that if the engine wasn’t kept rock still, the jet tube invariably ended up being whacked against the bulkhead, bending it and rendering it useless. A situation made worse by using the choke!! When this design was used for the small-bore engines, mayhem ensued as the fragile jet tube was practically resting on the bulkhead before the engine was even started. One manufacturer tried countering this problem by using a much steeper angle on the carb-mounting flange. This worked after a fashion but necessitated the carb float bowl angle to be re-set to avoid fueling problems. Not many folk realised or understood this, so ended up frustrated by apparently strange and inconsistent engine performance! Nearly all manifolds needed finishing off by hand to achieve acceptable airflow performance, so were not really consistent ‘off the shelf’. 
One of the unfortunate aspect of all this is that some ill-informed or less-knowledgeable manifold suppliers/manufacturers today have copied these manifolds exactly, and therefore all the inherent problems. There are only a couple of really good, thoroughly investigated and developed manifolds for the single SU on the market, the best by some margin being the Mini Spares/Mania components. So when deciding on which one to go for, before you look at the cost consider the design. It should fit easily, take a standard air-cleaner box and hold the carb high enough to avoid bulkhead/speedo cable fouling. The ports should taper nicely from manifold to carb mounting face and have a reasonable cast finish. There should be provision for water heating, the tube size compatible with the Minis heating pipes (1/2” bore as opposed to the 5/8” bore of the Metro). As for port sizing, for road use on practically all engines 1.35” at the manifold face is good. For big engines, i.e. 1400cc plus or 1380cc where maximum top end power is wanted, 1.4” at the manifold face is needed. 
Single SU inlet manifold flow test comparison 
96.50 cfm - MG Metro standard alloy manifold as cast. 
99.00 cfm - Howley 1.75" - previously the best available - this one 
was quite heavily modified to achieve this flow figure. 
105.00 cfm - Titan Motorsport 1.75" as cast. 
108.10 cfm - Mini Spares C-AHT770/A small-port 1.5/1.75" as cast. 
112.11 cfm - Mini Spares C-AHT771 large-port 1.75" as cast. 
The higher the cfm (cubic feet per minute) figure, the more power potential. All manifolds tested on a known-performer cylinder head with a manifold-less maximum flow 138cfm. 
The manifold to head mounting flange thickness is also an area where most have fallen down. Some have copied the MG Metro manifold that has a raised lump where the retaining washers sit up against to be level with the cast iron exhaust manifold used only on the MG Metro. Others have gone for a thickness that suits the budget exhaust manifolds they sell where the flange thickness varies depending on what material was cheapest at the time of manufacture. This mish-mash of flange sizing causes manifold gasket sealing problems. Either the exhaust is blowing or the intake is leaking in air, causing erratic running. The better ones therefore have a flange thickness compatible with the more popular, and generally higher quality exhaust manifolds - that's currently 8mm thick as per Maniflow and Janspeed. 
Manifolds - twins 
We already know the benefits of twin carbs, and have already commented on the fact that the manifold type was responsible for the early horsepower gains. You only have to look at it to see why - the carbs are almost running a ‘straight shot’ into the intake ports; far more efficient than the early single carb counterparts despite pretty awful castings - especially internally - that really were not at all as efficient as they should have been. 
Apart from the factory fitted manifolds of the Coopers and S, there are only a couple of other options. Those fitted to the Sprite/Midget, MG1100 and Austin 1300GT have a lower carb-mounting angle; so can cause installation problems in a Mini. The main problem was the manifold design - basic and not especially efficient. They hadn’t had the benefit of the development that the single carb one has in more recent years, simply because the demand is far smaller. Many of those using twins want it to look ‘original’ or where racing’s concerned have to meet homologation rules, so could be construed as looking original. Mini Spares took some time out when looking at reproducing the original 'ST' twin carb manifolds to make sure the internal finish and shape were maximised given the criteria of a seemingly 'standard' reproduction manifold. Consequently the 'off-the-shelf' performance is much better and more consistent. 
For maximum performance out of a twin carb set-up, the best manifold is manufactured in steel by those manifold fabrication wizards at Maniflow. Unfortunately it is made to favour the bigger carbs, so isn't really suitable for twin inch and a quarters - but probably works better than the original cast aluminium ones! BUT - there is a big price penalty. If you decide to run twin carbs, then this steel manifold is a must to maximum performance potential. 
In conclusion 
Apart from the foregoing and stressing yet again that big is not necessarily best, I would like to point out that if you’re considering a carb change, the best SU to go for is the HIF variety. Space precludes me from depicting why. Suffice to say much development was put into it - making it far more effective than the older HS versions. I know I always bang on about this one particular point, but it really makes a big difference in driving pleasure - ALWAYS, ALWAYS, be honest about your car’s main use and how you mostly drive before deciding what components you buy. 
It is entirely feasible to use an alternative servo take-off as per the injection Minis and all Metros. This was simply the oil transfer pipe banjo bolt from the engine block and the transfer pipe cut short. 
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