If you’re into classic Volvos, you’ve probably heard the engine codes B18 and B20 thrown around. Knowing the difference between a B18 and a B20 is crucial for any owner, restorer, or enthusiast.
These robust four-cylinder engines powered millions of Volvo cars from the 1950s through the 1970s. They’re known for their durability and simplicity. But they are not the same. The choice between them can affect your car’s performance, parts sourcing, and restoration plans.
This guide breaks down everything in simple terms.
B18 and B20
At their core, both the B18 and B20 are inline-four cylinder, overhead valve engines. They share a fundamental design and many components. Think of the B20 as the direct, evolved successor to the B18. Volvo didn’t start from scratch; they improved upon a proven platform.
The most obvious difference is right there in the name: the numbers refer to the engine’s displacement in deciliters. So, a B18 is a 1.8-liter engine, and a B20 is a 2.0-liter engine. This bigger size is the result of specific internal changes, which we’ll get into next.
Here’s a quick overview of where you’ll find each engine:
* B18: Produced from 1961 to 1969. Found in the Amazon/122, P1800, and early 140 series.
* B20: Produced from 1969 to 1975. Found in the later P1800, 140 series, and the 164.
Core Mechanical Differences
The increase from 1.8 to 2.0 liters was achieved through two key modifications. These changes are the heart of the matter.
Bore and Stroke Dimensions
This is the engineering data that explains the capacity change.
* B18: Has a bore (cylinder diameter) of 84.14 mm and a stroke (piston travel distance) of 80.0 mm. This math gives you 1778 cc, or 1.8 liters.
* B20: Has a larger bore of 88.9 mm, while keeping the same 80.0 mm stroke. The wider cylinders increase the total displacement to 1986 cc, or 2.0 liters.
Because the stroke is identical, many internal parts like connecting rods and crankshafts can be interchangeable in some cases. But the larger bore means you must use B20-specific pistons, cylinder liners (if equipped), and a different cylinder head gasket.
Cylinder Block and Crankshaft
The block casting itself is slightly different to accommodate the larger cylinders. While external dimensions are very similar, you cannot simply bore out a B18 block to B20 specifications reliably; the B20 block was designed with more material in the right places.
A common myth is that the B20 always has a stronger crankshaft. Early B18s used a crankshaft with 4 main bearings, which was plenty strong. Later B18s and all B20s used a crankshaft with 5 main bearings for even greater rigidity and smoothness. So, a late B18 and a B20 are very similar in this regard.
Performance and Power Output
You might assume the bigger engine is always more powerful. It usually is, but it’s not that straightforward. Power varied widely based on the carburetion, compression ratio, and intended market (e.g., USA vs Europe).
Typical Horsepower Figures
These numbers are gross (SAE) ratings, common for the era.
* B18: Ranged from about 75 hp (single carb, low compression) to over 100 hp (twin SU carbs, higher compression in the B18B).
* B20: Ranged from around 90 hp (emissions-strangled US spec with low compression) to 130 hp (fuel-injected B20E in the 164E).
The B20 generally has more torque across the rev range due to its larger displacement. This makes for a more relaxed driving feel, with less need to downshift on hills.
Carburetion and Fuel Injection
Both engines used similar carburetor setups, primarily SU or Zenith/Stromberg carbs.
* B18: Commonly seen with a single Solex, twin SUs, or a single Zenith.
* B20: Also used twins SUs, but later models, especially for the US market, used a single Stromberg CD carburetor for emissions. The top-performance variant was the B20E, which featured Bosch D-Jetronic electronic fuel injection.
The fuel-injected B20E is a highly sought-after version for its power and reliability when properly maintained.
Physical Identification and Interchangeability
Telling a B18 and B20 apart by just looking can be tricky. Here’s how to do it.
Location of Engine Codes
The engine serial number is stamped on a flat pad on the block, just behind the distributor.
1. Clean the area near the distributor shaft.
2. Look for a sequence like “B20B 1234.”
3. The first part (B20B) is your code. “B18” or “B20” will be clearly stamped.
Visual Clues and External Differences
While not foolproof, these hints can help:
* Oil Filler Cap: Early B18s had a breather cap on the valve cover. Later B18s and most B20s have a simple twist-off cap.
* Dizzy: The distributor on a B20 is usually a little larger in body.
* Emissions: US-spec B20s (post-1970) will have more vacuum hoses and an air pump.
What Parts Swap Over?
This is key for restorers. Many parts are interchangeable, which is a blessing.
* Interchangeable: Cylinder heads (with attention to combustion chamber size), water pumps, many front covers, manifolds (though jetting may need change), and engine mounts.
* Not Interchangeable: Pistons, cylinder liners (in sleeved blocks), and the head gasket (due to bore size).
You can, with the right combination of parts, put a B20 cylinder head on a B18 block, or vice versa. This is often done to alter compression.
Which Engine is Better for Your Project?
There’s no single “best” engine. The right choice depends entirely on your goals, your car, and your budget.
For a Pure Restoration
You should match the engine to the car’s original model year. A numbers-matching B18 in a 1966 Amazon is more authentic and valuable to a purist than a swapped-in B20, even if the B20 is more powerful.
For a Daily Driver or Touring Car
The B20 is often the preferred choice here. Its extra torque makes modern driving less busy, and it copes better with air conditioning or overdrive units. Parts are still very plentiful. The fuel-injected B20E is a fantastic upgrade if you can find one.
For Performance and Tuning
Both engines respond well to tuning, but the B20 gives you a head start with more displacement. Common upgrades for both include:
1. Installing a performance camshaft.
2. Upgrading to twin SU or Weber carburetors.
3. Adding a free-flowing exhaust manifold and system.
4. Improving ignition with an electronic system.
5. Carefully milling the cylinder head to raise compression.
The B20 block, with its larger bore, also has more potential for further boring and stroking kits to reach 2.1, 2.2, or even 2.3 liters.
Availability and Cost of Parts
As of now, parts for both engines are widely available from specialty suppliers. B20-specific parts like pistons and gaskets are no more expensive than B18 parts. In some cases, B20 parts might be more common because it was produced during higher-volume years.
Common Issues and Reliability
Both engines are famously durable, often reaching 300,000 miles with proper care. They do have some characteristic quirks.
Known B18 Issues
* Early Crankshafts: The 4-main-bearing cranks in very early B18s can develop cracks under extreme use. It’s a rare but known issue for highly tuned engines.
* Timing Gear Wear: The fiber timing gears can wear out and should be inspected or replaced with an aluminum gear.
Known B20 Issues
* Emission Controls: US-spec B20s with low compression and a single Stromberg carb can feel sluggish. Many owners swap to twin SUs or a Weber for better performance and simplicity.
* Fuel Injection (B20E): The old D-Jetronic system is reliable but requires specialist knowledge to diagnose. Faulty coolant temperature sensors or air flow meters are common failure points.
Shared Maintenance Tips
These steps will keep either engine running for decades:
1. Oil Changes: Use a high-ZDDP classic car oil every 3,000 miles. These engines need the zinc for flat-tappet camshaft lubrication.
2. Valve Adjustment: Check and adjust the valve lash (clearance) every 10,000 miles. It’s a simple job that keeps the engine quiet and efficient.
3. Cooling System: Flush the system every two years. These engines have iron blocks and alloy heads, so proper coolant mix is key to prevent corrosion.
4. Fuel System: Keep the carburetors clean and rebuild them every few years. Modern ethanol fuel can damage old rubber diaphragms and seals.
FAQ: B18 and B20 Engines
Can I put a B20 in my car that came with a B18?
Yes, it is a very common and straightforward swap. The engine mounts, transmission bellhousing pattern, and major dimensions are the same. You may need to swap some ancillary items like engine brackets or the distributor for your specific car.
Is the B20 more reliable than the B18?
They are equally reliable in principle. The later B20s benefit from minor refinements in design and the use of a 5-main-bearing crank as standard, but a well-maintained B18 is just as capable of legendary mileage.
Which engine has better fuel economy?
Generally, the smaller B18 might have a slight edge if both are in similar states of tune and driving the same car. However, driving style, gearing (overdrive!), and carburetor condition have a much larger impact than the 200cc difference.
Can I use a B18 cylinder head on a B20 block?
Yes, this is a common swap. The B18 head typically has smaller combustion chambers, which raises the compression ratio on the B20 block, leading to more power. You must ensure you use the correct head gasket for the B20 block’s bore size.
What does the suffix letter mean (e.g., B18B, B20F)?
The suffix letter denotes the specific version.
* B: High-performance twin-carb version (e.g., B18B).
* E: Electronic fuel injection (e.g., B20E).
* F: Low-compression, often single-carb US emission spec.
Are parts getting hard to find?
Not at all. A strong network of classic Volvo specialists worldwide manufactures and stocks nearly every part for both engines, from gasket sets to full rebuild kits.
Choosing between a B18 and a B20 comes down to your specific project. For originality, follow the car’s build plate. For a blend of classic feel and modern driveability, the B20 is an excellent choice. Both are stellar engines that represent a peak of simple, robust engineering. With basic maintenance, either one will provide decades of faithful service, proving that Volvo’s reputation for durability was built on this very foundation.