Posted 25 September 2009 - 06:53 AM
Sorry folks - just double checked my site and that piece is missing. Have asked site man to sort it but here it is for those that are interested -
Camshafts – considerations for selection
When selecting a camshaft or contemplating such a purchase, many folk start asking questions about which is the best one to fit. That is almost an impossible question to answer since it entirely depends on what the individual is looking for in terms of performance. Asking others will invariably end up with them recommending a specific cam because that is what they have used and liked. Asking vendors will generally end up with them recommending certain cams, largely because it is what they keep in stock. I am as guilty as the next in this – but then the cams I keep in stock are the result of much testing and use over some 30+ years, having tried all the generally available proprietary cams then moved on to develop more to get what I was looking for. There are, however, a number of things that really need to be considered very deeply before continuing the search.
The very first and most important thing is that the A-series engine is only a small capacity 4 cylinder engine – even when stretched to 1400cc without going to the extra expense of long stroke cranks. As such, aiming for a smooth idle, with low emissions that will accelerate like a scalded cat when you nail it in top gear at 1500rpm is pure fantasy. For that you will need a big V8. So first shot of reality is that you can not have everything. Generally for the road you can have either a smooth idling, low emissions performance that will pull from low rpm with decent low to mid range torque and a performance envelope that will be all over by 5,000 to 5,500rpm, or something with a slightly lumpy idle that doesn't get on the boil until the rev counter sees 2,500rpm+ and will wail to around 6,500 to 7,000rpm in quick fashion. There are cams that will straddle aside both of these aims – but fall short in over-all performance for trying to do so. There are one or two that manage simply startling performance between the higher take-off rpm and lower peak performance rpm.
The second thing to consider is just what is meant by the question 'where does it pull from'? Cam manufacturers often quote power bands for each cam saying '1,500rpm to 6,000rpm' and the like. What it does not state is in what gear this performance is likely to actually work in. When somebody states that such and such a cam 'pulls hard from 1,500rpm' they often fail to state what gear they are using when doing this. It is extremely optimistic to say the least to think that the A-series will achieve this in top gear. 1,500rpm in top gear is likely to be around 25 to 30mph depending on what final drive is fitted to the car. And the higher the gearing, the worse what I am about to say will be. Just how many folks drive their Mini around at that speed and expect the car to launch towards the horizon when they nail the go pedal? Or, more exactly, how many folk look for blistering acceleration at that speed without first changing down at least one gear? Not very many at all I suspect. Trying to bimble along at 30mph in top gear with a final drive any higher than a 3.44 on 10” wheels is madness. The engine tugs and rocks on its mountings/engine steadies and is not at all lively or 'happy', let alone fuel efficient. Using third gear makes a huge difference all round. Try it. So, trundling along at 30 mph in third gear now has the rpm level closer to 2,000 to 2,200rpm, and the engine can apply more effective torque to the wheels, which will aid more lively acceleration when the loud pedal is jumped on. Gone is the mad requirement for the car to take off from 1,500rpm in top gear. So now the application of a more sporty cam becomes more feasible.
And lastly – the real, on the road performance envelope. You need to be very honest here otherwise you can end up hating your Mini because it becomes a pain to drive. Warm and fuzzy thoughts of blasting down country B-roads, or beating up the opposition at the local hill climb or sprint are pure pipe dreams when your Mini is mostly used in urban driving. You will end up hating the car. And oddly enough, seemingly 'under-camming' the engine when the car is likely to see a fair bit of competition or track day use is not as bad as it may seem. The Minis speed comes more form i's cornering ability than its engine power. Fitting a very sporty cam can make the car very difficult to drive quickly, whereas fitting a slightly 'tamer' cam may give you the confidence to really wring the last drop of cornering speed out of the car. When driving around, make a concious effort to analyse the rpm range you generally drive in – there are not many that will constantly see more than 5,500rpm on their tacho. May be 6,000rpm at the most So why fit a cam who's performance envelope fades out at 7,000rpm? Pointless. You are better off trading that higher rpm level for a cam that will give a bigger mid range torque bulge. And that is CRUCIAL.
TORQUE is what acceleration is all about. TORQUE accelerates the car, not BHP. BHP is a tool for higher speed. Not something the Mini is designed for, being as aerodynamic as a barn door.
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