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SSI Insert revised (no index tag!): 22-Feb-2016
Article on The Internet Mini Encyclopædia |
There are quite a number of cams available for the A series engine and different ones are suited to different engine capacities, specs and applications.
Cams vary in the lobe dimensions and relative angular positioning. Cams which open the valves for longer periods are termed "longer", "hairier", "wilder" etc. These are usually used in higher performance engines. Short duration cams are referred to as "Short", "Mild" or "Tame". There are also scatter cams.
Firstly, a bit of (very simplified) theory: If you open the valves for longer you can get more gas into and out of the cylinder, thus giving more power.
Secondly, the biggest trap to fall into, longer duration cams are not always best. Having a cam which gives a power peak at 6krpm and nothing at all below that will not only be impossible to cope with on the road, but will actually give worse 0-60 times than a similar car equipped with a 'milder' cam.
Remember that the acceleration of a car is not related solely to it's peak power, it is related mainly to the area under the "power vs. revs" graph i.e.: the total power output as the engine speed rises.
Thirdly, you may ask "why did the factory not fit 'wild' cams as standard?". A longer cam has more overlap period when both valves are open at the same time. This happens at the top of the exhaust stroke when the exhaust valve is closing and the inlet is opening ready for the induction stroke. Both valves open together means fuel/air going down the exhaust and also exhaust gas getting up the inlets.
This is not good for economy and certainly isn't good for the MoT emissions hydrocarbon (HC) check. As the revs rise the momentum of the gas flowing through the engine keeps things in check and the amount of inlet contamination and unburnt fuel being wasted is reduced.
This means that an engine with a longer duration cam will have a more lumpy idle as the engine is more unhappy at lower revs. Not only is it experiencing inlet gas contamination from the exhaust gas, but, because the time the valves are both closed for is less, the amount of gas which is left in the cylinder for compression/combustion is quite low. This often means that the idle speed of the engine needs to be set slightly higher to get it to run smoothly.
So which cam should I use in my engine?
There are two main properties of a cam to look at:
Duration: The 'time' in crank rotation degrees (i.e.: 2 x cam degrees) which the inlet and exhaust valves are open for. A standard road cam will be typically 230' and a long duration race cam in the 300' bracket. Smaller capacity engines tend to require shorter durations.
Lift: This is how far the valve is opened. High lift is not always good. Some cams work better with 1.25 ratio rockers than with 1.5 ratio rockers. The lift will be anything from 0.2" for a mild road cam to 0.5" for a race cam. Higher lift means that the gas can flow easier into the cylinder, but higher lift also means lower gas velocity, especially with big valve diameters.
There are lots of aftermarket cams available and also some nice ones to emerge from the BL motorsports development arena. Here are some of the most common "codes" that you will see. The code is often stamped on the timing gear sprocket end...
BL Cams: 731 (800), 544, 649
These were produced by BL for the rally minis and some of the cooper models. The 731 is quite a tame cam, the 544 is a sporty fast-road cam, and the 649 is really for race-only applications. The 731 is a pin drive for the oil pump, the 800 is spider drive. Both have 1/2" lobes and three rings.
MG Metro Cam:
This is similar to the 731 BL cam, but has been shown to give better bottom and top end power. Highly recommended for fast-road 998 engines, especially those with big valve heads.
Piper Cams: 270, 285
These both have the same fast-road duration (288') but the 285 cam has more lift. The 270 can be used in a 998 engine, but gives best results with a smaller valve head. The power won't really come in until about 4krpm though.
Kent Cams: 266, 276, 286, 296
These are good cams and are very popular. The 266 is a mild street cam, the 276 and 286 are both fast road cams, and the 296 is a rally/race cam. The 276 and 286 are very similar performance-wise, as both have similar power integrals (area under the power/RPM graph). The 286's higher peak power is matched by the 276's longer power span. These are mainly suited to tuned 1275+ engines, although the 266 can be used in 998s. It is not recommended to use the 276 or higher in a 998 unless small valves are used. Large valved 998s will have too slower gas speed for these cams to work.
So if I get a cam kit (e.g. 286 cam, springs and followers) do I need to uprate the pushrods?
Not really. Just inspect the ends for wear to see if the case hardening has worn away (the ends will be pitted) as this could lead to rapid wear
Differences between cams for A and A+ blocks
When fitting a slot drive A+ cam, you should find the end comes flush with the oil pump rebate. Using an A series cam, it may be 1/4" short. The BMC ST C-AEA 731 has a pin drive for the oil pump, 1/2" lobes, three rings. The C-AEA 800 is spider drive, 1/2" lobes, three rings.
The Haynes manual says:
"Now refit the camshaft locating plate and tighten the three retaining bolts. Check the camshaft endfloat, referring to the figures given in the specifications."
This is wrong. You must fit the cam sprocket before measuring the end float; the camgear sets the endfloat, not the lock plate.
Cam bearing removal, drift diameters
Steel cylinders with a step half way along the length are best to remove cam bearings. These are the sizes of the drifts needed:
| Drift | First diameter | Second diameter |
| 1 | 34.5 | 36.5 |
| 2 | 40.5 | 42.5 |
| 3 | 42 | 44 |
Changing a cam in situ
Ryan Parle was kind enough to give us this step-by-step guide to changing a cam with the engine still in the car.
"On a 998 or cooper s engine with tappet chest covers you can change the cam with the engine in situ.
07 June 2003
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