Exhaust

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The Exhaust system removes all the waste gasses from your engine, allowing more air and fuel into the cylinder for the next explosion.

Contents


Overview

If this gas wasn't removed, the engine would explode under pressure. Getting the waste gas out quickly and smoothly is an essential part of an efficient engine. It's not just as simple as slapping a chimney on the engine to get rid of the smoke; a well-built exhaust system will provide the best gas velocity while filtering the gasses and providing a satisfying exhaust note .

Don't kid yourself, however. If you think you can go to a certain motor factor and buy a chrome exhaust tip and it'll make your 316 go faster, you're in for a big surprise. The standard BMW exhaust is pretty much perfect for the engine it's fitted to, and the only way to get serious performance improvements is to spend serious money; there are no quick bolt-on upgrades for the E30 exhaust system.

With that said, let's get down to working out how your engine vents itself.

Components

The Exhaust system starts within the Head of your engine, where a series of valves open up to let the waste gas out of the cylinder. The gases from all the cylinders are gathered up into the manifold, and from there they are pushed down into one pipe called, appropriately, the downpipe.

Exhaust Manifold

On M20 engines the exhaust manifold has six branches, ending in a twin-port outlet, with the same design being used on both 320i and 325i models. However, there is a difference between pre- and facelift vehicles; manifolds before 1987 had only 3 studs for the downpipe, while later ones have the four studs for the downpipe. TO make things even more confusing, there are two sizes of 4-stud manifolds; small and large. When building an exhaust system, it is not possible to "mix and match" between these parts.

On M40 and M42 engines the manifold has four branches, ending in a single outlet for the downpipe.

The M21 diesel engine is derived from the M20, and its exhaust manifold is similar to that engine. However, it is designed for use with a turbo.

If you are thinking of fitting a turbo to your M20, then the M21 manifold can be fitted upside down with minor adjustment to the bolt holes. Bear in mind that this will limit your choice of turbo due to the small size of the manifold.

LHD manifolds differ substantially, being a two-piece unit, with each piece scavenging exhaust from three cylinders and reducing down to one outlet each for the downpipe.

Manifold Studs

The manifold is held on with a number of threaded studs that screw into the engine head. The manifold then slides onto these studs, and is held in place with copper nuts. A common cause of exhaust problems, especially blowing gaskets, is caused by these studs breaking over time. Learn more about broken manifold studs.

Heatshield

Since the exhaust manifold sits directly underneath the HT leads on most E30 engines, and kicks out a fair amount of heat, BMW saw fit to fix some overhanging pieces of metalised fabric called Heatshields. The purpose of these is to protect the rubber and cables above the exhaust system from excessive heat, and any failure of the heatshield can lead to melted cables and all sorts of other issues.

Exhaust Pipes

M20 3-piece Exhaust system
M20 3-piece Exhaust system

M20-engined cars were fitted with two basic types of system; a three piece and a two piece.

A two-piece system has the downpipe and centre box in one, and a joint straight after the centre box, and the back box joins onto this.

A three-piece consists of downpipe, then centre box with a length of pipe, before another joint at the back box.

The systems are interchangeable complete, note that you cannot 'pick and mix' between systems, so the centre and rear of a three piece system will not fit the front of a two piece system.

320i and 325i systems are interchangeable as long as they are of similar ages. In the BMW inventory, the centre sections of these two models are the same, and the pipe bore is identical; 45mm. However, on the 325i the rear exit pipes are 60mm compared to 45mm for the 320i; this has no impact on the performance of the system, since all other pipes are 45mm.

Downpipe

The downpipe meets the manifold, and takes the exhaust system under the car. Depending on the system, it will be either a straight pipe, a pipe with a centre box, or a pipe with built-in catalytic converter.

Centre Box

The Centre Box is the first baffle unit, designed to dampen the engine sound to reduce the noise of the vehicle. Depending on the system, the centre box will be either part of the downpipe or its own unit with a pipe leading to the back box.

Catalyzer

Catalytic converters are a ceramic honeycomb covered in precious metals; the idea is that exhaust gas flows through this honeycomb and all the nasty pollutants are magically converted into safe, harmless and peace-loving hippy gasses.

Most E30 owners instead find the catalyst to be a very expensive potato rammed into the exhaust system, plugging up the airflow and strangling the engine; something especially true on the 325i. While catalyzers were installed in 1989, UK MOT requirements only demand that cars built after 09/1992 meet certain exhaust requirements, which makes it ok to rip out your catalyzer if it's broken or annoying.

If you do remove your catalyzer during an exhaust swap, DON'T let the pikey exhaust guys keep it. Those precious metals inside are worth some serious cash, and your friendly exhaust man "taking it off your hands for you" will be weighing it in for quite a few notes, so make sure you sell him the catalyzer rather than giving it to him for free.

Back Box

It's no fun cruising smoothly in your E30 if the exhaust sounds like a tin can full of bees. But in the same way, people will make all sorts of hand gestures at you if your car sounds like a jet engine but can't overtake a Ford Fiesta. So getting the right noise from your exhaust to do your engine justice is what the back box is for.

Aside from the noise, there's also the image. As well as the number of pipes leading in, there's also the pipes leading out; how many, straight or angled, flat-cut or chisel cut. All of these are issues for people who think other people care what your car's arsehole looks like, but to each their own.

Despite these differences, all M20 back boxes are interchangeable as long as they come from the same (2- or 3-piece) system.

BMW back boxes are supported by one of two methods: either two straps around the box, supported by a rubber hanger each side (one on each), or a clamp that goes around the two tail pipes, supported by a single rubber hanger.

Lambda Sensor

Although mounted in the exhaust system, the Lambda sensor is an essential part of the Intake system, as it sends an oxygen reading back to the ECU to help control the Fuel mixture. This is especially pertinent for late-generation LPG systems.

These oxygen sensors were fitted in line with the catalytic converter in 1989 to help reduce emissions. However, vehicles produced before 09/1992 are not required to meet emissions levels, and therefore the catalyzer and lamba can be removed to free up power.

If you wish to keep your lambda, then any Bosch unit fitted to the E30 can be used, with the only difference being the length of cable fitted to the sensor. This is especially pertinent if you have changed your manifold to an aftermarket unit but wish to keep your lambda sensor.

Bosch Lambda Sensors
Part Number Fitted To Cable Length
258005313 E34/E32 152cm
258005322 M50 525i 145cm
258005324 E30 325i 57cm
258005325 E30 318i 94cm

Mounting

The exhaust pipes are held to the body of the car using three mounting points. The first is at the Gearbox, and holds the downpipe using a bracket. The second mounting point is on the middle to rear pipe, while the third mount holds the rear box to the body of the car using one (M40) or two (M20) clamps.

Common Problems

Ticking Noise

It's not uncommon for engines to develop a ticking noise over time, and this is usually attributed to badly-adjusted rockers in the head. However, after playing around with feeler gauges and changing the oil, the noise is still there; this usually means one of the manifold studs has broken. It may not be visible, but to test, try...

If you have identified a broken stud, learn more about removing a broken stud.

Broken Studs

Broken studs are normally caused by a warping of the manifold due to excessive heat. While the manifold can be fixed fairly easily by skimming it, the stud will have snapped inside the head of the engine, leaving you with an annoying noise and an expensive repair.

There are a few things you can try to remove a broken stud before you give up the will to live:

Depending on where the stud has snapped, you can just unscrew the stud if it snaps just below the nut. But if it snaps below the surface it has to be drilled out and possibly the hole will need to be re-threaded (heli coil or a bigger bolt). This is even more of a problem if it's the rear ports because the engine or just the head will have to be removed from the car.

If the stud still has a few mm poking out of the head, it is recommended trying to weld something to stud. A simple blob of weld will allow you to get mole grips onto the stud to try twisting it out, while a penny washer welded in sideways will give you a much bigger purchase to wind it out.

If the broken stud is really flush with the head surface, or even recessed a little, your last resort is to drill it out.

To drill out a broken stud, start with a centre punch and create an indent just off-centre of the broken stud. Then, using a small drill bit (2-3mm), make a pilot hole and work your way into the stud. Try to be as straight as possible, and go slowly; you don't want to drill too deep or you risk penetrating the Head itself.

Once you've got into the stud, repeat again with bigger drill bits (4-5-6mm), working your way up until you've broken through the side of the stud into the thread of the hole. You will inevitably do some damage to the thread, but hopefully not enough to cause lasting problems.

When you've removed the shredded remnants of the stud, you can use a Heli-coil to repair the thread of the head, or try using a fresh stud if the existing threads are in good condition.

Upgrades

Stainless Systems

A brilliant way to get rid of hundreds of pounds is to invest in a stainless steel exhaust systems. These systems are available from a variety of manufacturers at wildly differing prices, with all sorts of magical improvements offered, including huge BHP increases which in the E30s case simply aren't true.

Many people buy stainless for the rust factor; stainless doesn't rust, and in the UK's wet environment, that's quite an important characteristic to have.

On the other side, while the outside may last for ever the internal baffles usually blow out just like normal exhausts; it is therefore questionable whether a stainless system really "lasts longer" since blown baffles render the exhaust useless.

Manifolds

Stainless or Ported manifolds with better gas flow are available from a number of aftermarket suppliers including BTB and Racing Dynamics. However, the biggest number of these manifolds are designed for LHD engines for the European and American markets, and are therefore incompatible with UK systems.

We've compiled a list of manifolds so that you can see which manifolds will help your engine, and which ones are just for show.

Note that for some cars there may be clearance issues if you are already using a modified steering rack such as the E36, E46 or Z3 racks. In that case you may want to fit a different alternative steering linkages to ensure you have enough clearance.

Supplier Comment Rating Picture
Alpina Commissioned by Sytners and built by Janspeed, the Alpina manifold is a RHD-specific six-branch. Yields modest gains (5hp) but can go further with surrounding mods. Recommended
BTB Over the years have developed a range of impressive manifolds, all of which yield proven power increases of up to 15bhp. Current model is BTB3

BTB3 specifically designed to work with Z3/E36 steering rack upgrade Dyno chart

Highly recommended
Fritz's Bitz Despite the impressive sales pitch, dyno results yield no improvement over standard Not recommended
Griffin Occasionally requires minor modification to fit properly Recommended
Hartge The Hartge H26 used little more than a cam, a chip and one of these manifolds to get +20bhp from the M20. Recommended
Hottuning Small power increase and an increase of midrange torque

Incompatible with the E36/E46 steering rack upgrade

Dyno chart Red line shows pre hottuning, purple with hottuning

Budget recommendation
Magnex Proven modest increase (similar to Alpina) from dyno reports Recommended
Quicksilver Claimed to be a perfect copy of the Alpina unit
Racing Dynamics High quality manifold with guaranteed performance increase, but exceptionally rare. Possibly superseded by the BTB v3 manifold. Recommended
Sebring Good kit, but unfortunately LHD only Not recommended for RHD cars
Supersprint LHD only Not recommended for RHD cars
Hottuning COPY An unbelievably cheap lump of metal advertised on a popular online auction site

Problems fitting the unit due to bad stud placement, plus poor welding quality. Purely cosmetic

Not recommended

BTB2 alongside a rare Racing Dynamics Manifold Image:4DEB58E0-403C-4F83-88BF-5AC969561DFD-8416-0000052B4BB4094C.jpg Image:5EA3166E-84E0-4B11-B330-E26ADC880189-8416-0000052B52BBCDC4.jpg Image:BA4CAA0B-5543-4B25-9A6D-9EC05273E11B-8416-0000052B5BBA5672.jpg