A 400 small block is a V8 engine with a 400 cubic-inch displacement, built on the compact frame that Chevrolet designed in 1955

The term "small block" refers to the physical size of the engine, not its power. Chevrolet's small block design was revolutionary because it packed serious displacement into an engine small enough to fit in regular cars, trucks, and even some sports cars. A 400 cubic-inch displacement means the total volume of air and fuel that all eight cylinders can draw in during one complete engine cycle is 400 cubic inches — roughly 6.6 liters.

The 400 small block was produced primarily from 1970 to 1980, though the basic small block platform continued long after. It sits between the 350 (the most common small block) and the 454 big block in Chevrolet's lineup. The 400 was designed to bridge the gap: more displacement than a 350, but still compact enough to fit in vehicles designed for smaller engines.

Key Takeaways

  • The 400 small block is a V8 engine with 400 cubic inches of displacement, produced mainly from 1970 to 1980 by Chevrolet.
  • It uses the same basic block and mounting points as the 350 and other small blocks, making it a bolt-in replacement in many vehicles.
  • Factory 400 engines produced between 150 and 265 horsepower depending on the year and emissions controls, with later models producing less power.
  • The 400 is heavier and thirstier for fuel than a 350, but offers more torque and pulling power for trucks and performance builds.
  • Used 400 small blocks are common in the used engine market because they were installed in millions of Chevrolet vehicles over a decade.

How the 400 Compares to Other Small Blocks

The small block family includes the 262, 267, 283, 305, 327, 350, and 400 — all built on the same basic architecture. The main difference between them is bore (cylinder width) and stroke (how far the piston travels). A 400 has a longer stroke than a 350, which means the piston travels farther down the cylinder on each cycle. This longer stroke creates more torque — the twisting force that moves a vehicle from a stop or up a hill.

The trade-off is that a longer stroke limits how fast the engine can safely spin. A 350 can rev higher and produce more horsepower at high RPM, while a 400 produces peak power lower in the RPM range. For trucks, towing, and low-end acceleration, the 400 is stronger. For racing and high-RPM performance, the 350 is the better choice.

The 400 is also heavier than a 350 and uses more fuel to produce similar horsepower. This is why the 400 fell out of favor in the 1980s as fuel economy became a selling point. The 350 could do almost everything a 400 could do while weighing less and burning less gas.

Factory Power Ratings and What They Mean

Chevrolet rated 400 small blocks between 150 and 265 horsepower depending on the year and how the engine was equipped. Early 400s (1970–1972) produced the most power, with some versions rated at 265 horsepower. By the mid-1970s, emissions controls and lower-octane fuel reduced power ratings to around 180–210 horsepower. By 1980, the last year of production, a 400 was rated at just 150 horsepower.

These factory ratings are measured at the engine's crankshaft, not at the wheels. Real-world power delivered to the road is lower because the transmission, driveshaft, and differential all consume some energy. A 265-horsepower 400 might deliver 200 horsepower to the wheels in a light truck.

Factory ratings also reflect the engine as it left the assembly line with all original equipment. A used 400 may produce more or less power depending on its condition, modifications, and what fuel it was designed to run. An engine with worn rings or carbon buildup will produce less power than the original rating.

Why 400 Small Blocks Are Still Common in the Used Market

Millions of 400 small blocks were installed in Chevrolet trucks, vans, and passenger cars between 1970 and 1980. Because so many were built, used 400 engines are still available from salvage yards, engine rebuilders, and private sellers. They are cheaper than newer engines and cheaper than some other small blocks because they are plentiful.

The 400 is also popular with people rebuilding or modifying older vehicles. The small block platform is well understood, parts are widely available, and the 400 can be modified to produce significantly more power than it did from the factory. A rebuilt 400 with modern fuel injection and a performance cam can produce 400+ horsepower — double the original rating.

However, availability varies by region and season. In agricultural areas where older trucks are still in use, 400s are easier to find. In urban areas focused on newer vehicles, they may be harder to locate. Prices also fluctuate based on condition, mileage, and whether the engine is complete or missing accessories.

What to Check When Buying a Used 400 Small Block

A used 400 engine is a major purchase, and condition varies widely. Before buying, have the engine inspected by someone familiar with small blocks. Look for oil leaks around the pan gasket, valve covers, and front seal. Leaks are common in older engines but can be fixed during a rebuild.

Check the oil color and smell. Dark, burnt-smelling oil suggests the engine ran hot or was not maintained. Milky or foamy oil suggests coolant is leaking into the oil, which is a serious problem. Ask the seller for maintenance records if available.

Listen to the engine run if it is still in a vehicle or on a test stand. Knocking or pinging under load suggests bearing wear or carbon buildup. A loud valve train noise suggests worn rocker arms or pushrods. These problems are fixable but add cost to a rebuild.

Ask whether the engine is complete — does it include the intake manifold, exhaust manifolds, alternator, water pump, and other accessories? A bare block is cheaper but requires sourcing and installing all these parts. A complete engine is more expensive but ready to install.

Fuel Economy and Operating Costs

A 400 small block typically returns 8 to 12 miles per gallon in a truck or van, depending on driving conditions and transmission type. An automatic transmission reduces fuel economy compared to a manual. Highway driving is more efficient than city driving. A truck loaded with cargo uses more fuel than an empty truck.

These numbers are lower than a modern engine, but they reflect the 400's design for torque rather than efficiency. If fuel economy is a priority, a 350 or newer engine is a better choice. If towing capacity or low-end power is the priority, the 400's fuel consumption may be acceptable.

Operating costs beyond fuel include oil changes, spark plugs, and routine maintenance. A 400 uses the same maintenance schedule as any small block — oil and filter every 3,000 to 5,000 miles, spark plugs every 30,000 miles, and coolant every 30,000 to 50,000 miles. Parts are inexpensive because the small block is so common.

Frequently Asked Questions

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

Yes, in most cases. The 400 uses the same bolt pattern and mounting points as the 350, so it bolts into the same engine bay. You may need to modify the engine mounts or radiator hose routing, but the swap is straightforward. Check with a mechanic familiar with your specific vehicle to confirm clearance.

How much horsepower can a 400 small block make with modifications?

A stock 400 produces 150 to 265 horsepower depending on the year. With a performance cam, intake manifold, carburetor or fuel injection, and headers, a 400 can produce 350 to 450 horsepower. A fully built 400 with a stroker crank and high-compression pistons can exceed 500 horsepower, but cost and complexity increase significantly.

What is the difference between a 400 and a 455 big block?

A 455 is a big block — a completely different engine family with a larger physical size, different block casting, and different mounting points. A 455 produces more power but weighs more and does not fit in vehicles designed for small blocks. The 400 small block is compact; the 455 is not.

Is a 400 small block reliable for daily driving?

Yes, if it is in good condition and properly maintained. The small block design is proven and durable. A used 400 with unknown history carries more risk than a rebuilt engine with a warranty. Have any used engine inspected before purchase and budget for potential repairs during the first year of use.

Where can I find a 400 small block engine?

Salvage yards, engine rebuilders, online marketplaces like eBay Motors and Craigslist, and local mechanics who specialize in older vehicles are common sources. Prices vary widely based on condition, mileage, and completeness. Get multiple quotes and ask about warranty or return policy before committing to a purchase.