BMW M20 Engine Guide

The BMW M20 is yet another engine on the long list of BMW’s legendary inline-6 motors. As BMW’s replacement for the older and larger M30 engine, the M20 was specifically designed with compactness and efficiency in mind. The smaller form was meant to fit into BMW’s more compact vehicles developed in the late 70s and early 80s. These included the E30 and E28 platforms primarily, though the M20 was used in earlier and later models as well. 

BMW clearly felt that they had cracked the straight-6 code, at least for the tail end of the 20th-century, with the M20 remaining in production for a ludicrous 16-year run between 1977 and 1993. Most manufacturers in the present day develop engines every 5 years for incoming models, so the M20’s staying power was clearly a testament to its great design.

The M20 is a fan favorite among BMW enthusiasts for a few reasons. Primarily, they are extremely dependable if properly cared for. They’re also extremely susceptible to modification and have huge aftermarket support decades down the line. Finally, they are simple. Without as many moving parts or complicated electrical components, M20s are great engines to learn engine maintenance skills for beginners.

In this guide, we’ll cover some of the main things that make the M20 such a staple piece in BMW’s straight-6 catalog. We’ll also cover the M20’s specs and technical developments over the years, and some common issues that M20s face approaching their 45th birthday. 

BMW M20 Engine History

By the late 1970s, BMW had thoroughly recovered from their post-WWII slump and made a name for themselves internationally through their performance street cars and racing endeavors. As the Neue Klasse of road cars had reached its pinnacle, BMW sought to modernize their line once again. 

Through the 60s, BMW relied heavily on two engines to power their road cars. One was the strong yet underpowered M10 4-cylinder found in cars like the 2000c and 2002. The other was the M30 straight-6, which was an unquestionable powerhouse, yet as bulky as a three-story house. BMW wanted to develop a new, smaller line of straight-6 engines to fit in new, smaller chassis’. The result was the M20.

The M20 shared much of its design and construction with its bodybuilder older brother. The primary differences between the two are the timing belt, versus timing chain on the M30, lesser displacement in the M20 resulting in smaller bore spacing, and a lesser slant angle. Overall, the two are quite similar.

BMW was successful in their endeavor of creating a “Baby Six” engine for their new line with the M20. Weighing significantly less, with the M30 weighing 315lbs versus the M20’s 258lbs, the M20 is able to produce comparable power to the M30 without straining front suspension. 

BMW M20 Engine Variants

Over the M20’s 16-year production run, BMW developed four variants. Each variant was designed with a different application in mind, with multiple of the variants featuring their own upgrades through the years. 

BMW M20 B20

The first M20B20 variant had the smallest displacement and lacked fuel injection for most of its production run. Initially, the B20 featured a Solex 4A1 carburetor for air/fueling needs. In 1981, fuel injection was introduced to the M20B20 through a K-Jetronic system, which was later upgraded to an LE-Jetronic system in 1982. 1982 M20B20s also featured a lighter block, new manifolds, and a “731” cylinder head. The M20B20 was found in the lower-spec European-only E21 and E12 models until 1982. 

BMW M20 B23

The second M20B23 variant featured some modern engine technology that the B20 lacked from its initial release. Displacement was increased significantly and all M20B23 engines prior to 1981 featured K-Jetronic fuel injection. In 1982, fuel injection was upgraded to a LE-Jetronic injection system and featured a higher flowing 731 head, also adjusting the compression ratio to 9.8:1. The final M20B23 iteration came in 1983, with a few more performance upgrades, including adjustments to fuel injection, exhaust, and camshaft. The M20B23 was available in mid-range E21 and E30 chassis, also unavailable in the US.

BMW M20 B25

The third iteration of the M20 was released 8 years into the M20 production cycle and was called the M20B25. It was also the direct replacement for the M20B23. Displacement, and consequently power and torque, were increased again for the B25 due to an upgraded “885” cylinder head and a Motronic 1.1 engine management system. In 1987, the M20B25 received a newly upgraded Motronic 1.3 engine management system. While compression was decreased, power stayed almost the same. The M20B25 was used in 325i/is E30s and 525i E34s, which were both available worldwide.

BMW M20 B27 

The fourth and final iteration of the M20 came in the form of the M20B27 variant, often nicknamed the ‘Eta’. The B27 debuted in 1982 with the goal of maximizing fuel efficiency. The M20B27 featured a longer 81mm stroke and higher displacement than the B25. While stroke and displacement increased significantly, the B27’s power output was significantly less than the B25, mainly due to a dramatically subdued compression ratio. The M20B27 features a “200” cylinder head with smaller ports, a different camshaft, and softer valve springs. This dramatically reduced power and dramatically increased fuel economy.

U.S.-spec M20B27s featured a different compression ratio to the others developed for other markets. This was due to different fuel requirements to meet American standards. The M20B27 saw an increase in horsepower in 1985 and an upgraded Motronic 1.3 fuel injection system in 1987. 1987 M20B27s also featured an upgraded “885” cylinder head and increased redline. These significant performance-increasing upgrades led enthusiasts to deem ‘87 M20B27 engines the ‘Super Eta.’

BMW M20 Engine Specs

Due to the fact that there are 4 variants of the M20 motor, each with its own specifications and dimensions, we’ll break them down in a table. It is also important to note that each of the variants has its own sub-variants, with minor changes taking place over their life cycles. 

BMW M20 Model Applications

Most M20 variants were used in either the 3-Series or 5-Series chassis. In the M20’s infancy, it was used in the later antiquated E21 and E12 chassis. These models were then replaced by the incoming E30 and E28 chassis. The M20 also had one-off uses in the Z1, which was BMWs early-90s attempt at a small sports coupe, and models made for motorsport, like the South African 325iS.

BMW M20 B20 – All Non-US:

• 1977–1981 E12 5 Series 520/6 (carburettor)
• 1977–1982 E21 3 Series 320/6 (carburettor)
• 1981–1982 E28 5 Series 520i (K-Jetronic)
• 1982–1984 E28 5 Series 520i (L-Jetronic)
• 1982–1984 E30 3 Series 320i (L-Jetronic)
• 1984–1987 E28 5 Series 520i (LE-Jetronic)
• 1984–1987 E30 3 Series 320i (LE-Jetronic)
• 1986–1987 E28 5 Series 520i (Motronic)
• 1987–1992 E30 3 Series 320i (Motronic)
• 1988–1990 E34 5 Series 520i (Motronic)

BMW M20 B23 – All Non-US:

• 1977–1982 E21 3 Series 323i (K-Jetronic)
• 1982–1984 E30 3 Series 323i (L-Jetronic)
• 1984–1987 E30 3 Series 323i (LE-Jetronic)

BMW M20 B25:

• 1985–1993 E30 3 Series 325i
• 1989–1990 E34 5 Series 525i
• 1988–1991 Z1

BMW M20 B27:

• 1982–1987 E30 3 Series 325e, 325e
• 1982–1988 E28 5 Series 525e (called 528e in North America)
• 1989–1992 E30 3 Series 325iS (only available in South Africa)

BMW M20 Reliability & Problems

By most accounts, the BMW M20 is an extremely dependable engine when taken care of properly. This is even more true for later M20 variants that feature more modern fuel injection and Motronic engine management systems. The combination of a cast-iron block with an aluminum cylinder head is a tried and true recipe for a sturdy engine and a combination that the M20 employs.

With that being said, neglected M20s can be a world of perpetual headaches and pricey repair costs. Luckily, as the M20 has been around for such a long time, most of the common issues that the engines often face are well documented and outlined. 

BMW M20 Timing Belt Maintenance

One of the most finicky and high-maintenance items on the M20 is its timing belt. As we covered earlier, BMW switched from a timing chain, which was found on the M30 straight-6, to a timing belt on the M20. While this helped keep the form-factor small with the M20, the trade-off was durability.

The BMW M20 is notorious for its need for fastidious timing belt service performed every 50,000 miles, or every 5 years. This advice comes contrary to BMW’s own recommended timing belt interval for the M20, which they say is every 60,000 miles. However, there have been many reports throughout the years of M20 belts snapping before the recommended 60,000-mile interval.

It is extremely important to stay up to code with an M20’s timing belt, as they are interference engines, meaning that serious damage can be done if the timing belt snaps. At the very least, a snapped M20 timing belt can result in bent valves caused by the pistons smashing into them. At higher RPMs, a snapped timing belt can result in a damaged cylinder head or, in truly severe cases, a cracked block.

If you are unsure of when the last timing belt service was performed on your M20, it is always a good idea to replace it straight away. In comparison to a full top-end engine rebuild, a timing belt service is a walk in the park.

BMW M20 Oil Leaks

Even when new, the M20 has always been a leaky engine. As BMWs with M20s age, the problem only gets worse due to part degradation. There are a few extremely common sources of leaks on the M20, and almost every M20 that is still around today will likely have at least one. 

The most common M20 oil leak location is the oil pan gasket. The factory cork oil pan gaskets are prone to failure after decades of service and should be replaced if there are leaks present. In many cases, the M20’s oil pan is itself the overall biggest contributor to accruing stains on your garage floor. As the M20 oil pan sits so low to the ground, it is extremely common for M20 oil pans to develop hairline cracks from bottoming out or running over road debris. 

Leaks stemming from the M20’s valve cover gasket are also extremely common, mainly resulting from BMW’s chosen material for the gaskets. Typical aftermarket valve cover gaskets are made from either a soft silicone material or a hard composite material. It is generally stated that the softer silicone gaskets seal much better than the factory, or composite, gaskets. Regardless, replacing your M20’s valve cover gasket can resolve pesky oil leaks.

BMW M20 Rough Idle

A rough idle is another common issue that plagues unmaintained M20 engines quite frequently. A rough idle can be caused by quite a few different factors and is, therefore, a pain to diagnose most of the time. There are a few places to check first, though, as some parts are easier to rule out than others.

One common source of a rough M20 idle is the idle control valve or ICV. This one is pretty self-explanatory, as the ICV is responsible for regulating the M20s idle. Over years of use, the ICV can get gummed up by leaking fluid, causing it to malfunction. Before replacing it, you can attempt to clean it with brake cleaner first to try to eliminate some of the internal buildup. If that doesn’t work, you can purchase a new or used one for around $200. While the part is pricey, it is relatively easy to replace on your own if you have rudimentary skills.

Another common cause of a rough idle is cracked or deteriorating vacuum lines. This one’s a bit more difficult to diagnose, as a smoke test is typically required to determine where the leak is. Rubbing soapy water on the vacuum hoses and looking for bubbles is another way to test for vacuum leaks. If you notice that your M20-powered BMW is also down on power under acceleration, that is another indicator that vacuum lines might be your issue.

Finally, a failing crank position sensor is another, less common, cause of erratic M20 idle. The blue CPS is the one that might cause this issue, as it is responsible for delivering engine temperatures to the vehicle’s ECU. An incorrect reading could interfere with the M20’s idle.

BMW M20 Cracked Cylinder Head

While M20 engines are typically considered bulletproof, overheating issues can spell serious issues for the M20’s cylinder head. This is especially prevalent in early-model, pre-1987, cars due to their underperforming cooling systems. 

Early M20 cooling systems were notoriously bad due to weak radiator fan clutches and ineffective radiators. If you, or the previous owner of your M20-powered car, allowed the engine to overheat, it is common for hairline cracks to form on the head, effectively rendering it scrap. If you can see a creamy, ‘mayonnaise’-like residue collecting in the coolant expansion tank, the oil filler cap, or under the rocker cover, chances are that there is a crack in the M20’s head. The substance is a combination of coolant and oil, meaning that coolant was allowed to seep into the oil through the head. Unfortunately, the only solution to this is sourcing a new cylinder head.

Best BMW M20 Performance Upgrades

Another reason that the M20 is loved in the BMW community is its modification potential. With generally sound construction and a low risk for damage when maintained to a proper degree, the M20 has a lot of performance potential. In this section, we’ll briefly discuss some of the most common M20 engine upgrades and cover their benefits. For a more comprehensive understanding of some of the mods listed below, check out our M20 Performance Upgrades article.

M20 High-Flow Cylinder Head Upgrade

As the M20 is an antiquated engine at this point, it is somewhat difficult to squeeze high performance out of it without doing some significant upgrades. Much of the untapped performance potential comes from either changing displacement or increasing the breathability of the cylinder head.

Many M20 enthusiasts will opt for the second option, often going with widely available cylinder head upgrade kits sold by reputable machine shops. Oftentimes, these companies will take factory M20 ‘885’ or ‘200’ cylinder heads and make significant upgrades to make them more breathable. This typically includes porting the head, replacing the valves, springs, and retainers with better-performing ones, replacing the cams with more aggressive ones, and resurfacing the mating surface.

A remanufactured head will likely yield double-digit performance increases to horsepower and torque for an M20 engine. In addition, the refreshed parts will ensure durability for years to come. It is important to keep in mind which head your M20 came with from the factory. On ‘200’ head-equipped M20B27s, it might be a worthy investment to look into a refreshed ‘885’ head and related modifications. An ‘885’ head on an M20B27 block is a cost-effective stroker option for those looking for more power at a lower premium.

M20 Cam Upgrades

One of the most common M20 upgrades, especially if the engine needs to be torn down for additional repairs/upgrades, is swapping the stock camshaft for a performance one. While this does fall in line with the above-mentioned high-flow head upgrade, a cam upgrade alone is a significantly less-pricey option for a bit of additional performance. 

Even today, many prominent camshaft manufacturers offer performance M20 cams with a high degree of customizability. Schrick, for example, provides five different cam durations for different uses. The least aggressive 272/272 grind angle cams are the best for daily driver applications with a power increase at all RPMs. For race applications, a more aggressive 308/308 grind is available for high RPM gains.

Upgraded cams change the characteristics of the M20’s valve responses, with certain cam shapes/grinds being better for different applications. If you are considering upgrading your M20 cams, it is important to do extensive research to find the right cam for your application. Generally speaking, the lower the grind angle, the better low-RPM performance you’ll have. The opposite can be said for cams with a more aggressive grind. It is important to find the middle-ground for your own individual application.

M20 Performance Chip or ECU Tune

One of the most cost-effective and dramatic performance mods for any engine is a performance chip or an ECU tune. The same can be said for the BMW M20. However, it is important to note that these kinds of upgrades are much easier on M20B25 and M20B27 engines with a Motronic engine management system, as some of the older M20 models lacked the computational power to truly benefit from ECU tuning.

One route to bump M20 horsepower and torque extremely easily is to install a performance chip directly onto the engine’s ECU. Many performance part manufacturers still manufacture M20 performance chips to this day. Many of them, including the dyno-tested and highly rated M20 Turner performance chip, are rated for a 15+ horsepower and torque boost over stock, with further benefits depending on other installed performance parts.

For more involved M20 builds, a beefier engine management system will likely be needed. Options like the M20 PnP MegaSquirt are offered for enthusiasts looking to customize engine timing and fueling to a high degree. In general, a MegaSquirt, or comparable EMS, is only really necessary for turbo applications or if you have made extensive modifications to the fuel system of your M20.

M20 Exhaust Upgrade

Yet another common performance-enhancing modification for almost any engine under the sun is a high-performance exhaust setup. The BMW M20 can also see a solid increase in performance from an upgraded exhaust system, especially if the upgrade includes a better flowing exhaust manifold and cold air intake.

While the M20’s exhaust isn’t as restrictive as some other BMW factory exhausts, they can still benefit from a slightly increased diameter and higher flow catalytic converter. This is especially true for the M20B27, which utilizes two heavy runner pipes instead of a single, larger diameter one found on M20B25s. As with all of the performance upgrades listed here, there are a ton of reputable aftermarket manufacturers producing performance M20 exhausts today. 

Noise is also a factor when upgrading an M20 exhaust system. While exhaust tone might not be directly related to engine performance, it is an important factor to note as you’ll be listening to it all the time. With that being said, it is hard to make an M20 sound bad. The design of the ‘baby’ straight-6 is conducive to producing lovely sounds regardless of your setup. However, it is always a good idea to listen to sound clips of your prospective system to make sure it is a good fit for you.

BMW M20 Engine Summary

The BMW M20 is a legendary engine found in some of BMW’s most iconic 80s and 90s chassis. Its impressive 16-year lifespan is a testament to the engine’s superior engineering and strength. Over that period, the M20 evolved to include variants for multiple applications in BMW’s 3-Series and 5-Series lineups. 

While the M20 is a sturdy engine if maintained properly, problems are common, and sometimes headache-inducing, if the engine is neglected. The most common issues include frequent timing belt maintenance, common oil leaks, rough idle, and cracked cylinder heads from overheating. While most of those issues have straightforward solutions, they can be costly, especially if you don’t have any engine maintenance skills. Fortunately, due to the M20’s rudimentary design and use of mainly analog parts, it is a very good engine to learn some basic handiwork.

The BMW M20, especially the M20B25, is a fantastic engine for modification, with many options and routes to follow in order to gain more power. Displacement modifications and cylinder head upgrades are the most effective methods of doing so, however, there are less expensive options, like installing a performance chip, that will improve performance marginally.

If you found the information in this article useful, check out some of our other classic BMW content. We have a similar M10 Engine Guide, as well as more information on M20-powered BMWs, like our BMW E30 Engine Swap Guide. As always, safe driving!