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"Millennium" range. Stainless steel front pipes with intermediate resonator to make two-box system. Connects from standard header to rear muffler (see RC40-051, -052, -053, -054, -055, -056.)
Fit RC40-061 to make two-box, stainless system from LCB.
Fit RC40-062 to make two-box, stainless system from catalyst.
The popular RC-40 exhaust system has been updated for the year 2000. All retain the optimum size 1.75" tubing for maximum performance and efficiency, but we now offer a choice of tailpipe size and configuration. In addition, they now have stainless steel pipes for better appearance an rust resistance. And for the first time ever they come with a front pipe to fit right onto the collector of an LCB header with no modification. Front pipes come either "straight through" for use only with the rear muffler, or with a built-in resonator for quieter twin-box systems. Order front and rear pipes seperately to create your own system. All tail pipes are highly polished with rolled edges.
EXHAUSTS - BASIC INFORMATION
Refer to bottom of article for useful part numbers.
The sheer volume and diversity of bolt on goodies available for our cherished Minis underlines the fact that a very large portion of them is modified in some way. Be it just dress up items to personalise it aesthetically, or tuning parts to improve performance. In the latter’s case, either more power or more economy are sought - both being desired by many. Should it also enhance the looks at the same time, so much the better. Unfortunately not many components can achieve this without integrating with a number of others. The exhaust system, however, is one of the few.
Although the exhaust’s efficiency and effectiveness is reliant on a well matched system from the exhaust valve in the cylinder head to the tail pipe, the majority view it as two separate pieces - the exhaust manifold, and the ‘system’. Not entirely mis-placed, as this is how vendors present them - folk buying one or the other, though mostly both, dependent on guidance and or budget. Contrary to popular belief, the system actually has a greater effect on both power and economy than the manifold and generally produces more performance per pound spent. It’s also far easier to fit than the manifold and first to rot and fall off. Consequently, getting to grips with what the exhaust system is all about will help in making a decision as to what to look out for when making a decision on which product to buy.
Let’s start with a quick and basic look at your Mini engine’s basic functions. Ultimately the end product is energy to drive your Mini along. The engine is a four stroke - induction (piston takes in fuel/air mixture), compression (piston squeezes mixture), ignition (mixture burns - the power stroke), and exhaust (piston pushes burnt gases out). Unfortunately only the power stroke produces energy, the others consume it. The more they consume, the less there is to drive your Mini down the road. The main aim in modifying/tuning the engine to produce more power/economy is to reduce the energy consumed by the three ‘wasted’ strokes (pumping losses).
We’re only interested in the exhaust stroke here. Initially it absorbs power by having to physically push the burnt gases out. To minimise this effort, the exhaust system needs to be as un-restrictive - ‘free flowing’ - as possible. Standard, and unfortunately a number of so called ‘performance’ exhausts, cause excessive power losses because of their designs - the restrictive nature causing ‘back pressure’. Backpressure is the result of an exhaust with poor flow efficiency. To compound the problem, backpressure doesn’t stay a constant. The higher the throughput of gas, the greater backpressure becomes. In fact it rises at something like the square of the engine’s gas throughput. So the more power developed/the bigger the engine, the greater backpressure becomes, consequently sapping more and more power.
Take a look at the relevant diagram (right) to see why. The briefest of examinations will enlighten you as to why a straight through exhaust is offered by most Mini specialists. And seemingly the bigger the better - surely this would ensure minimum restriction and be totally free flowing? Many misguided vendors believe so, offering exhausts the size of drainpipes as the ultimate performance exhaust. But they’re wrong. There are two more important features in exhaust gas behavior that need to be considered - speed and sound.
As the piston nears and reaches top dead centre, the inlet valve has started to open leaving both inlet and exhaust valves momentarily open together - called the ‘overlap’ period. This period can greatly reduce power/torque if the out-going exhaust gases contaminate the in-coming fresh fuel/air mixture. The hotter the cam, the greater the overlap period, the more dilution can take place.
How does exhaust system size affect speed? Bigger diameter pipes allows the hot exhaust gases to expand, slowing them down. The earlier the exhaust gases encounter a bigger bore, the sooner they slow down and reverse flow. This accentuates the low rpm/light cruising speed problems, making hot cams run even more erratically. Using a system with correctly sized pipe diameters the exhaust gas speed is at least maintained, if not increased, therefore taking longer to slow down and reverse flow at the end of the exhaust stroke. One of the major benefits making ‘hot’ cams come in sooner, and run better at low speed.
Sound or music
This comes in the guise of a negative pressure pulse wave, and hurtles back up the pipe towards the exhaust valve. No, it really does! If this wave gets back to the exhaust valve whilst it’s still open and in the over-lap period, it helps suck out residual exhaust gases. Even better, it can also reduce the combustion chamber pressure to below atmospheric pressure, actually starting the intake charge flowing before the piston starts moving back down the bore on it’s induction stroke. Sort of ‘super charging’ the engine by increasing volumetric efficiency (total amount of induction charge sucked in). The more charge you get in, the more power you get out. These ‘shock waves’ also create the pinging, drumming, thudding noises produced by the pipe.
How does size affect sound? As mentioned earlier, the bigger the diameter the harder it is to silence; more of a space problem than anything else. The bigger diameter the pipe bore is, the proportionally bigger the silencer needs to be to cope. For excessively large pipe diameters there simply isn’t the space under the car to fit a suitably sized silencer. Consequently using a really big-bore system produces horrendous drumming at low rpm/light throttle/cruising speeds, and a right din when the (literally) ‘loud pedal’ is floored.
Pipe diameter has less of a direct effect on shock waves as they’re more dependent on temperature and pressure - so it’s speed is pretty much a fixed quantity. ‘Tuning’ the shock waves is therefore done by altering the pipe’s overall length - the reason why in some circumstances a pipe going straight down the middle of the car will give different results to one exiting in the more normal position - the former being shorter than the latter. Unfortunately it rarely gives more performance. Having said that, shock waves only really has an effect over a very narrow rpm range - around 3-400 rpm. Optimising lengths to maximise potential performance increases are mainly affected by the manifold - for more information see 'Exhausts - Manifolds, use'. However, careful development of the system shape can be used to good effect. More on this later. For now, exhaust systems are generally fixed in one of two lengths as mentioned above.
Little or Large?
Size-wise, a bore diameter of 1.625” is more than ample for practically any road engine. Right from 850 up to 1400+, where power outputs don’t exceed 125bhp or so. Even then, going to a bigger size - say 1.75” or 1.875” bore - will only gain a few extra horses right at the top end of the rev range - OK on a racer where the engine is flat-out all the time, but on a road car the benefits of increased gas speed and getting a hot cam pulling power in earlier far outweigh the slight hp gain. Conversely, fitting a 2” system to a small-bore engine (850/998/1098 based) will actually loose power because of it’s relatively small through-put of exhaust - the huge bore increase prematurely slowing the exhaust gases, bringing about the unwanted effects outlined earlier. Using a 1.5" bore size instead of the more common 1.625" can have marginal gain on a small-bore engine. But I stress these are small. The uncommon pipe size being reflected in its price. Given the choice of the two different lengths generally available, the longer side-exit type almost always gives the best over-all results.
Silence of the Lambs
Many believe that ‘straight through’ means ‘more noise’. This just isn’t the case, so let’s get this straight - just because a system is quiet doesn’t mean it’s ineffective. A properly designed ‘straight through’ system with correctly proportioned silencer boxes will give both maximum flow and civilised noise levels. Conversely, just because a system is noisy doesn’t mean it’s efficient. This is more of a feature of its size than silencer type. The only silencers on the market of a seemingly correct size that make quite a row, and folk think they’re a ‘performance’ exhaust, use the constriction method. From earlier comments, we know that the larger the bore, the more difficult it is to silence adequately.
Useful part numbers:
This article written by Keith Calver.