M-Block 351M/400 Specifications

Factory Power Ratings

Copyright 2002-2003 Dave Resch
All rights reserved.


Many people wonder, “What is the factory power rating for my M-block (351M/400) engine?”

Of course, like other engines, the answer varies some from year to year, and perhaps even depending on the version or “calibration” of the engine. For example, in the 1973 model year, there were four calibrations for the 400 engine, each with a different advertised power rating (163, 167, 168, and 171 horsepower). On the other hand, sometimes different power ratings are not advertised for different versions of an engine.

There are several reasons for multiple calibrations with different power ratings:

  • Different tuning specifications for different vehicles (such as less ignition timing advance in bigger and heavier cars, or the same engine used in both cars and trucks, or the same engine used in light-duty and heavy-duty trucks)

  • Different tuning specifications for automatic and manual transmissions

  • Regulatory reasons (such as California and 49-state emissions compliance)

  • Mid-year design changes (such as introducing new cylinder heads and retarded cam timing in the 1973 model year)

The following table lists Ford advertised power ratings for M-block engines from MY 1971 through MY 1982. Horsepower figures are not available for some years, and torque figures are not available for many years. Ford did not always advertise M-block torque figures, and I have not yet been able to find reliable horsepower figures for the last two years of M-block production (1981 and 1982).

Model Year

Engine

Horsepower @ RPM
(SAE net, except '71)

Torque (ft/lbs)
@ RPM

1971*

400 (car only)

260 @ 4400

    —

1972

400 (car only)

172 @ 4000

298 @ 2200

1973

400 (car only)

171 @ 4000

    —

1974

400 (car only)

170 @ 4000

    —

1975

351M (car only)

148 @ 3400

    —

1975

400 (car only)

158 @ 3800

    —

1976

351M (car only)

152 @ 3400

    —

1976

400 (car only)

180 @ 3800

    —

1977

351M (car/truck)

161 @ 3400 / —

    —

1977

400 (car/truck)

173 @ 3800 / 158 @ 3800

    —

1978

351M (car/truck)

152 @ 3400 / 156 @ 4000

— / 262 @ 2000

1978

400 (car/truck)

160 @ 3800 / 158 @ 3800

— / 276 @ 2000

1979

351M (car/truck)

151 @ 3400 / 156 @ 4000

— / 262 @ 2000

1979

400 (car/truck)

159 @ 3800 / 159 @ 3800

— / 276 @ 2000

1980

351M (truck only)

138 @ 3400

263 @ 2000

1980

400 (truck only)

163 @ 3600

275 @ 2000

1981

351M (truck only)

    —

    —

1981

400 (truck only)

    —

    —

1982

351M (truck only)

    —

    —

1982

400 (truck only)

    —

    —

How is engine power measured?

Engine power is measured with a device called a dynamometer. Dynamometers are so named because the earliest versions were literally a dynamo (i.e., electrical generator) connected to the engine crankshaft. Engine power output was measured by converting it to electricity and measuring the electrical energy. Most modern dynamometers use a fluid brake to load the engine and measure its power. Dynamometers are sometimes referred to by the nickname “dyno.”

There are two major types of dynamometers used to measure automotive engine power:

  • Chassis dynamometers

  • Engine dynamometers

The difference between a chassis dynamometer and an engine dynamometer is that the chassis dynamometer loads the engine at the drive wheels of the vehicle (through the drivetrain) and the engine dynamometer loads the engine at its flywheel.

The problem with measuring engine power from the drive wheels is that all the drivetrain components (transmission, differential, etc.) consume some of the engine’s power. The power you measure at the wheels is considreably less than the power produced by the engine. To determine the engine’s actual power output with a chassis dynamometer is very difficult because you have to calculate (or estimate) the drivetrain’s parasitic power consumption, and add that to the measured power at the wheels. Drivetrain losses are usually estimated as a percentage of the engine’s net power output.

Drivetrain power losses can vary significantly from one test run to the next. The type, quality, and condition of the drivetrain components, the temperature of the oil in the transmission and differential, and even tire temperature and pressure can all affect drivetrain power loss. If you’re doing serious engine tuning or R&D, you need to know exactly what happens to engine power output when you make small changes, and adding the many drivetrain variables makes that kind of precision almost impossible. With an engine dynamometer, you don’t have to factor in drivetrain losses.

One advantage of a chassis dynamometer is somewhat more “real world” performance measurement, although estimating the inconsistent drivetrain power loss on a not-brand-new vehicle compromises the quality of the information. The biggest advantage of a chassis dyno is that you can get some performance measurement on a running vehicle, without having to remove the engine.

What is brake horsepower?

The term “brake horsepower” simply refers to power measured by a dynamometer. All dynamometers work by applying a brake against the engine to load it, and measuring how much brake force it takes to overcome the engine’s power. Some people mistakenly think this term refers to power measured at the wheels of a vehicle, but brake horsepower can be measured with either an engine dynamometer or a chassis dynamometer.

What is the difference between gross power and net power?

The “gross” power measurement is derived from an engine dynamometer, with the engine under ideal laboratory conditions. This includes using a specialized dynamometer intake and exhaust system, no engine-driven accessories (such as water pump, alternator, power steering pump, etc.), no emission control devices such as AIR or EGR, special fuel formulations, and fuel delivery and ignition tuning that could never be used on a street vehicle. These were the type of engine power ratings that auto manufacturers advertised before the 1972 model year.

There were two main disadvantages of using gross power ratings:

  • They were not representative of the power produced by an engine installed in a vehicle.

  • The testing method was not standardized nor openly documented.

With gross power ratings, you had no way to compare ratings from different engines, even if they were made by the same company. Of course, Ford would try to get the best numbers they could from each engine, but you can bet they put more effort into getting good numbers from a Boss 351C 4V than from a 97ci, 4-cylinder Pinto engine.

Starting in MY 1972, the US Federal Trade Commission (FTC) mandated that automobile manufacturers use “SAE net” power ratings when they advertised claims of engine power. The SAE net power ratings were based on testing standards developed by the Society of Automotive Engineers (SAE). Ultimately, this was a benefit to the entire automotive industry, as the manufacturers all played by the same rules when rating engine power output.

The net power measurement is still derived from an engine dynamometer, but with the engine using a production induction system and air filter, production exhaust manifolds and exhaust system (including mufflers and catalytic converters), production emission controls including AIR and EGR, production fuel delivery and ignition tuning specifications, and all the engine-driven accessories required for a particular application.

The advantage of SAE net power ratings was that advertised power more closely represented the performance of an engine in a real vehicle, and the ratings were comparable between different engines because a standardized testing method was used.

The 1971 and 1972 M-block 400 makes a good example to compare gross (260hp) and net (172hp) power ratings. There was no significant change in the 400’s specification (compression ratio, camshaft profile and timing, etc.) from 1971 to 1972, yet the advertised power rating dropped by about 33%. That lower rating was entirely a result of switching from gross power to net power ratings, and that drop of about one-third is typical of most engines.

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