The exhaust can be broken down into three discrete sections:
The exhaust valve and port
The exhaust manifold
The exhaust system
The exhaust valve and port are discussed separately in the 'Cylinder Head' section.
For many cars, such as Ford Escorts and Sierras, Vauxhall / Opel Vectras and Astras, Peugeot 205 and 306's, opening a magazine like Car And Car Conversions to any of their advertisement pages is sufficient to show you a multitude of aftermarket companies all vying for you custom. The likes of Janspeed, Peco, Scorpion, Magnex, Jetex, to name a few, all offering a bewildering array of uprated, free flowing, big bore, modified exhaust manifold.
Then you start looking at their listings for Alfas, and.....well, let's not beat around the bush, there's bugger all out there, really. As far as I am aware, the only company that supplies modified exhaust manifolds for Alfa boxers is Ansa, and even then, they appear to have discontinued them.
The above are all the references for Ansa Marmitte's site, and there is nothing at all about manifolds in there.
As mentioned, Ansa used to produce a modified manifold that was quite different to the production item. The standard manifold joins cylinders 1 and 3 on the right hand branch, and 2 and 4 on the left hand and then fed these two secondary pipes into one. This results in the gases pulses being irregularly spaced during running. The pulses would be (where o equals no pulse) 1 - 3 - o - o on the right hand branch, and o - o - 2 - 4 on the left hand. More evenly spaced pulses could be achieved by joining the primary pipes of cylinders 1 and 2, and 3 and 4 together, so one branch would be 1 - o - 2 - o, and the other o - 3 - o - 4. In theory, this would give better pulse extraction characteristics, with the possibility of more power. This is a 'crossover' type manifold.
Ansa used to actually produce a manifold with this layout. It was said to increase power by up to 10%, which is a little difficult to believe. Having said that, it is possible to see that some gains would be possible. The downside to these manifolds were that they were expensive, and that they reduced ground clearance. Since cylinders 1 and 2 were linked, a primary pipe had to pass under the sump to do so, ditto for 3 and 4. They also had different lenth primary pipes. These pipes were not manhole or pothole friendly. The other aspect is that if they smoothed out the power pulse sequence, they may have got rid of the boxer's characteristic exhaust note, which is one of the joys of the boxer for me!
Whatever, you cannot find them now, so we can't say for sure. I personally do not know of anyone who produces an uprated manifold for these engines. If anyone does, I am an interested party, and I would be happy to pass that info on to everyone via this page.
Other engines also have the option of a 4 into 1 manifold, where the four individual primary pipes join at one point directly. 4 into 1 manifolds are generally accepted as being better for higher rpm operation, where as 4 into 2 into 1 items are better for torque. The Ford Pinto and crossflow engines are a good example of what is available. But no such choice exists for us Alfa boxers boys.
So if you can't get a modified one, what is there to do? Well, at least in this respect, the standard manifolds are pretty good. It doesn't appear to matter who produces it, there is not much to get wrong. The one I used on my 108bhp 1500 was a TI Bainbridge item bought cheaply at Partco. I have not gone round with a vernier caliper to check the individual diameters of the manifold on the market, but I would be surprised if there was more than a couple of millimeters between the largest and smallest commonly available. If you were seriously modifying your engine, it may be worth checking this, but I suspect that by this time you would have spent so much that making your own would not be that expensive, relatively.
The 33 manifolds are different to the AlfaSud items as their run rearward is straighter, as the 33's didn't have inboard brakes to worry about. The Sud manifold will fit the 33, but not the other way around.
The 16v manifold is a larger diameter, but before you try to install one on your 8v, the manifold to head flange pattern is different, so it will not be a direct fit. You will have to modify it to make it fit, and as for the 33 manifolds, the run would foul the inboard brakes of the Suds.
Many companies now offer special poly-aramid type fibre high temperature wraps, like Kevlar, that are used to insulate the exhaust manifold. Claimed benefits are lowered under-bonnet temperatures, increased power and protection of engine ancillary equipment and wiring. The protection of the ancillaries against heat damage speaks for itself, but what of the other two claims? Surely the flow of air as the car moves along keeps the manifold cool, and blows the hot air out of the bay? Yes, to a large degree this is correct, but there will be times when the car is moving slowly (tight, twisty roads, traffic) where the reduction would be seen. In theory this could improve power as lower intake charge temperatures would be seen, but a good cold air pickup for the induction system would be far better. What do you way power for in traffic, anyway?
So, the power gained is simply due to the reduction in charge temperature? No, not quite. There are some theoretical gains from insulating the manifold. As discussed, the exhaust gases rushing along the pipe acts as a pump, pulling the gas behind it out. The faster this gas moves, the stronger the pulse effect and the vacuum generated behind it. If you cool this gas, it slows down, and becomes more dense. As it slows down, the pulse effect lowers. As the blast of cold air that rushes under the front of your car is great at cooling things, it stands to reason that the exposed exhaust manifold will be cooled too. So you may lose some of that pulse tuning efficiency. By wrapping this stuff over the manifold, this blast cooling is significantly reduced, and so the pulses remain strong. It is even possible that the engine will come up on the cam slightly earlier due to the increased gas speeds.
In addition to this, those nice eco-friendly chappies amongst us with catalytic converters would be interested to know that their cats would reach light-off temperatures much quicker too, so their engines would run cleaner quicker. One final possible benefit is slightly reduced noise, especially if your exhaust 'rings'.
Before anyone suggests sticking this stuff all over the entire exhaust, a word of warning. Two, actually. The first is you'd need a sultans budget 'cos this stuff is not cheap. The second is that this stuff would become quickly soaked with water in rainy weather, and would stay wet and pressed against warm exhaust for long after the engine was switched off. You would end up with an exhaust that would disintegrate due to rust in no time at all, wrapped up in an expensive non-reusable coat. The manifold gets hot enough to burn off the water, but the exhaust wouldn't.
So what about making your own manifold? The flanges that join to the head are easily available, just cut them off an old rusty manifold, or make them from bits of 6mm mild steel plate, they are not rocket science. Use the manifold gaskets as a pattern. Then get some suitable tubing and cut and weld to achieve the shape you require. Demon Tweeks, amongst others, do a large range of tubes and pre bent sections of pipe that would be perfect for this. But before you rush off to get the catalogue, consider this; If you take a stroll down to your local quick fit exhaust and tyre agency, in a corner, you'll see a whopping great skip full of old exhausts that have been replaced due to cracks, broken hangers, rusty silencers etc. Take a good look at all those sections of pipe with preformed bends of every available angle, diameter and sections that are about to be malted down or pressed flat at your local scrap metal merchant. With the aid of a couple of strategically placed beer tokens to the boss man, they can be your new manifold parts.
So what shape should it be? Clearly you have three sensible options.
1) A standard layout but bigger bore manifold
2) A 'crossover' type manifold
3) a 4 into 1 manifold
At this point, I have to tell you that exhaust manifold design is not just a simple job of banging a load of pipes together. Due to pulse tuning effects, different lengths of primary pipes may affect the characteristics of one engine positively, and another with a different cam negatively. F1 cars have exhaust manifolds that are works of art, with different lengths and curves all done to get the best out of the engine, but when they change something in the engine, the exhaust may have to be thrown away and a new design fitted. This is an extreme case, as our engines are nowhere near as highly strung as that.
But what you may find is that a certain shape and length of manifold improves power, except at a certain point where you get a flat spot in the rev range. Trial and error is the way you will find out, but if you have made the manifold yourself, at least you are not paying lots of money to do so. Think of the exhaust as a trombone, where you can change the note by changing the length of the pipework!
Exhaust manifold shapes
One starting point is to look at the lengths of the primary pipes that you have on your existing manifold. If you make sure that you don't make them shorter than this you should be relatively safe. Also, the secondary pipes should more or less the same length as the original. What you must do is try to keep the primary pipes all the same length (at least within 10 cm), no matter what their shape, other wise weird pulse effects can be the result. The trouble is that the crossover type plays merry hell with that, as you would have to either meet the pipes in the middle, or else make a lengthening loop in the 'short' side. In fact, even the standard manifold has to compromise a little on this. Whatever you do, use a piece of string to check the lengths easily, instead of trying to calculate, at least to begin with..
All the joins must be done in such a way as to not interfere with the flow, and the less the number of sharp bends the better. Any bends should have smooth profiles on the inner and outer radii. Cheap exhausts often have wrinkles on the inner part of the bend, or the pipe is narrowed considerably, which are not good for flow.
Other points of note. Where the exhaust diameter suddenly gets bigger (or smaller, but you don't really want it to get smaller), the sudden change in section acts to make a secondary pulse, which can travel back up the manifold and may help or hinder flow depending on the rpm. This phenomenon can be used to pulse tune the manifold, with a bit of experimentation. If you have an engine that has a point at which it suddenly comes up on the cam, changing the distance between the engine and the step may soften that point, bringing it down the rev range slightly.
At low speeds, the exhaust may want to try and push back into the cylinders, due to pulses being reflected back up the exhaust. This is exhaust reversion, and can be controlled to some extent by use of a controlled vortex manifold and careful exhaust port design. The exhaust port is dealt with in the 'cylinder head' section. However, whether or not you are making your own exhaust manifold, a controlled vortex feature can easily be included. Basically, all it is is a conical pipe that is joined to the inlet to the manifold in such a way that is discourages reverse flow. It can either be welded into the manifold, or attached to a thin flange that fits between the manifold and the head. Have a look at the diagram for more detail. This can be used with any engine with no detriment if done correctly. This aids off cam running, and helps to pull the engine up onto the cam a little earlier.
CV adapter to fit between manifold and head
CV tube mounted directly in manifold
The situation regarding exhaust systems is a little more open. A number of companies do systems for the various Boxer engined cars. This following is a (short) list of companies that I know do some parts for our exhaust systems, but just because they are not on this list doesn't mean that they don't do boxer exhaust equipment.
OMP, Ansa, BTB, Bainbridge, Alfa Romeo (obviously!)
I have tried the Bainbridge system and it was ok. When I went the the Ansa, it did appear to give me maybe 4 or 5 bhp more on my 1500. It was easy to fit, well finished in a black crackle finish with a chromed tailpipe. The quality of construction was excellent and the shape was correct. It was expensive though, and the rear tail pipe is a bit XR3i for my liking. The other point of note is that the centre section is some 6 mm smaller diameter than the tail pipe section, and now my 16v manifold. That it gave the goods even though this apparent restiction is there is a sign of how good it is. However, now I have switched to a 16v engine, I may have to reconsider keeping this.
Now, before anyone asks me what it sounds like, I like my car, and apart from one rollng road session, it nhas never been driven by anyone but me. That means that I don't know what it sounds like from the outside, and because I have a ram-pipe induction roar fetish, the sound of the webers (and now the 45mm throttle bodies) drown out the exhaust completely. This indicates to me that if you are after a loud exhaust, Ansa is not for you.
Powerflow is a company that has been mentioned in the Alfa 33 Discussion Listing, and seems to be attracting good comments. You can view their site at
It seems that these boys basically don't produce a system for any particular car, they actually make a system up to your specifications. That to me sounds a good deal, and I shall be investigating further, and will report back on this site.