EXHAUST SYSTEMS

Exhaust Manifolds

Regarding manifolds many things are said about the merits of 4-2-1 as compared to 4-1 as to the performance properties each has.

The basic facts are that a 4-2-1 will give more low down power at the expense of a bit of top end and the 4-1 is the opposite, slightly less bottom end but more top. The problem for someone choosing which manifold to buy are not as simple as this, as the above comments are based on the manifold being built to set physics principals, that do not change, no matter who has built it. The principals are too complex to go into fully, so below are the basics.

When you burn the fuel and it escapes from the exhaust valve is has a pressure wave, this wave has energy and it can be harnessed to suck more gas than normal from the cylinder, this is the reason for performance manifolds. These waves have a natural harmonics and pulses and you have to get these right to make them do their job. To their job the exhaust pipe diameter has to be correct and more importantly the length of the pipes between joints.

To be correct, the length of the exhaust manifold before the gasses from the 4 cylinders come together should be about 32 to 34 inches from the head flange, assuming it is a 4-1 manifold. If it is a 4-2-1 manifold the first 2 pipes should join at between 16 to 17 inches, this is called the primary length, the next 2, the secondary pipes are the same length. The distance to the first silencer or Cat, should be the same distance away from the place the pipes all join.

So why are these distances so critical? The pulses of gas move down the pipe until there is a junction and then reverberate back up to the back of the valve, if these lengths are correct the wave pattern exerts a suction behind the valve head. If these lengths are wrong the pressure wave gets distorted and so looses it’s suction power and in some cases it can actually be worse than the standard manifold, as the waves clash against each other ruling out any suction and even providing positive pressure to hold the gas in the cylinder.

I know many will say, how is this possible, the standard manifold is not anything like the lengths you have quoted. Yes, quite true, here comes a bit of the technical stuff again. The harmonics of the gas and pipe lengths are to do with multiples and divisions of these lengths stated. Lets take 32 inches as the correct length. If you made a manifold that the 4 pipes joined at 64 inches, no problem, if you made one that they joined at 16 or 8 no problem. If the first silencer was 8 or 16 or 32 or 64 inches from where the 4 pipes join together, no problem. Therefore the standard exhaust manifold will have been designed by the manufacturers to coincide with one of these lengths, even though it does look crap.

OK, so where is the problem. The problem is that a lot of exhaust manifold manufacturers do not follow these rules often due to space limitations under the bonnet or for ease of manufacture due to complicated bends and junctions, this being particularly applicable to 4-2-1 units.

So yes on paper a 4-2-1 should give you more bottom end power but if it is not built to the correct lengths it will not and may even be worse than the standard one. A particular car that was built that no one could really improve on the standard manifold was the 205 MI16. It was made of course cast iron but was worked perfectly.

Exhaust Systems

The same problems regarding pipe sizes and lengths apply to exhaust systems, this is why all manufacturers have the Cat or before Cat’s came into being, had an expansion box or silencer just about 3 feet away from the manifold (or a multiple distance of same). So taking off your first silencer and putting in a straight pipe is unlikely to do anything for performance, unless you happen to be lucky that the rear silencer position just so happens to be the correct length away. This is most unlikely as rear silencers are fitted anywhere that room allows.

So what about taking off the cat from under the car, the car goes a lot better without it, yes, it does but this is due to getting rid of the restrictions of the Cat, nothing to do with pipe lengths and gas harmonics. So whilst taking off the cat is good it is not as good as taking it off and replacing it with an expansion box in the same place.