The 400 small block is a V8 engine Chevrolet made from 1970 to 1980, with a 400 cubic inch displacement and a larger bore than earlier small blocks

The 400 small block Chevy sits between the smaller 350 cubic inch engine and the larger big block engines. It produces between 265 and 375 horsepower depending on the year and carburetor setup, though most versions fell in the 300 to 330 range. The engine was designed to fit into the same engine bay as the 350, which made it popular for people who wanted more power without major modifications to their car or truck frame.

Chevrolet discontinued the 400 small block after 1980 because emissions regulations made it harder to tune for both power and fuel economy. Today, the 400 appears mainly in older Camaros, Novas, Chevelles, and full-size trucks from the 1970s, and in custom builds where people swap one into a car that originally had a smaller engine.

Key Takeaways

  • The 400 small block was produced from 1970 to 1980 and fits in the same engine bay as the 350, making it a popular upgrade for older Chevrolet vehicles.
  • Horsepower ranged from 265 to 375 depending on the year and carburetor type, with most engines producing between 300 and 330 horsepower.
  • The 400 has a larger bore than the 350 but uses the same block casting, so some internal parts are not interchangeable between the two engines.
  • Fuel economy on the 400 is typically 8 to 12 miles per gallon, which is lower than modern engines but typical for 1970s V8s.

How the 400 differs from the 350 small block

Both engines use the same basic block casting, but the 400 has a larger bore — the diameter of the cylinder — which gives it the extra 50 cubic inches of displacement. The stroke, or the distance the piston travels, is the same on both. This means some parts swap between them, but not all: the 400 needs its own pistons, rings, and connecting rods because they are sized for the larger bore.

The 400 also runs a lower compression ratio than many 350s, which means it produces less peak horsepower per cubic inch. A 350 with a high-compression head can outrun a stock 400 in a straight line, but the 400 makes its power lower in the engine's RPM range, which some drivers prefer for street driving and towing. The 400 is also heavier, adding roughly 30 to 50 pounds depending on which accessories are bolted on.

Power output and performance characteristics

Factory 400s ranged from 265 horsepower in the most restrictive versions to 375 in the high-performance L82 variant, which was only made in 1970. Most 400s you will find today produce somewhere between 300 and 330 horsepower at the crankshaft. The engine makes peak torque between 2,400 and 3,600 RPM, which is why it feels responsive in everyday driving rather than only at high RPM.

In a car weighing 3,500 to 4,000 pounds, a 400 with 330 horsepower will run 0 to 60 in roughly 7 to 8 seconds and cover a quarter mile in about 15 to 16 seconds, depending on the transmission and rear axle ratio. These numbers are respectable for a 1970s engine but not competitive with modern performance cars. The engine is also sensitive to fuel octane: running 87-octane gasoline in a 400 tuned for 91 octane will cause pinging and can damage the engine over time.

Fuel consumption and operating costs

A stock 400 small block typically returns 8 to 12 miles per gallon on the highway and 6 to 9 in city driving, depending on the carburetor, ignition timing, and how aggressively you drive. A 400 with a four-barrel carburetor and a performance cam will consume more fuel than a two-barrel version, sometimes dropping to 7 to 10 miles per gallon overall. These numbers are typical for 1970s V8 engines and reflect the era's design priorities, which favored power over efficiency.

Running a 400 regularly means budgeting for higher fuel costs than a modern four-cylinder or six-cylinder engine. If you drive 12,000 miles per year at 10 miles per gallon and pay $3.50 per gallon, fuel alone costs roughly $4,200 annually. Oil changes are also more frequent — every 3,000 miles is standard for older engines — and the engine burns through oil faster than modern designs, so checking the dipstick monthly is necessary.

Finding parts and rebuilding a 400

Parts availability for the 400 is good but not as extensive as for the 350. Pistons, rings, bearings, and gasket sets are made by several manufacturers including Sealed Power, Federal-Mogul, and Clevite, and can be ordered through automotive suppliers or online retailers. Cylinder heads are harder to source: original 400 heads are scarce, so rebuilders often use 350 heads and machine them to work with the larger bore, or use aftermarket aluminum heads designed for the 400.

A basic rebuild — new pistons, rings, bearings, gaskets, and seals — typically costs $1,500 to $2,500 in labor and parts if you use a local machine shop. A performance rebuild with ported heads, a performance cam, and upgraded valve springs can run $3,500 to $6,000 or more. Many people who own 400s today choose to keep them stock or make only minor modifications because the cost of a full rebuild approaches the cost of buying a used crate engine.

Common problems and maintenance needs

The 400 is prone to oil leaks around the valve covers, oil pan gasket, and rear main seal, especially as the engine ages. These leaks are not dangerous but they are messy and require regular cleanup. Overheating is another common issue: the 400 runs hotter than the 350 in the same car, so upgrading to a larger radiator or a mechanical fan with a higher engagement temperature often becomes necessary.

Carburetor tuning is critical on a 400. If the carburetor is not properly jetted for your altitude and fuel octane, the engine will ping, run rough, or stall. Many people who inherit a 400 spend weeks or months dialing in the carburetor before the engine runs smoothly. Ignition timing also matters: the distributor must be set correctly, and the points or electronic ignition must be in good condition. A worn distributor cap or rotor will cause misfires and poor performance.

Whether a 400 makes sense for your project

A 400 is worth considering if you own a 1970s Chevrolet and want more power than the original engine provides, or if you are building a custom car and want a period-correct V8 that fits in a small block engine bay. The engine is affordable on the used market — a running 400 typically costs $800 to $1,500 — and it is straightforward enough for a home mechanic to work on with basic tools.

A 400 is not the right choice if you want fuel economy, low maintenance, or maximum horsepower. Modern crate engines, including modern small blocks and LS engines, offer better reliability, easier tuning, and better fuel economy. If you are building a street rod or a classic car restoration and want the authenticity of a period engine, the 400 is a solid option. If you are building a daily driver, a modern engine swap will save you money and frustration over time.

Frequently Asked Questions

Can I put a 400 in a car that originally had a 350?

Yes. The 400 uses the same engine mounts and bolt pattern as the 350, so it bolts into the same engine bay. You may need to adjust the radiator, fan, or exhaust to clear the slightly larger engine, but major modifications are not required. The transmission and rear axle from a 350 car will work with a 400, though upgrading the transmission cooler is recommended.

What is the difference between a 400 and a 427 big block?

The 427 is a big block engine with a much larger displacement and different block casting. A 427 requires modifications to the engine bay, radiator, and exhaust, and it weighs significantly more. The 400 small block fits in the same space as a 350, while a 427 does not. A 427 produces more horsepower but costs more to buy, rebuild, and operate.

How long does a 400 engine last if I rebuild it?

A properly rebuilt 400 with quality parts should last 100,000 to 150,000 miles if maintained regularly. Life span depends on how hard you drive it, whether you keep the carburetor tuned, and whether you change the oil on schedule. A 400 driven gently and serviced regularly will outlast one that is raced or neglected.

Is a 400 better than an LS engine swap?

An LS engine produces more horsepower, uses less fuel, and requires less tuning, but it is not original to a classic car. A 400 is period-correct and easier to work on with hand tools. The choice depends on whether you prioritize authenticity and simplicity or performance and efficiency.