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The Muscle Car Decoder

A VIN engine decoder that knows what model year it is looking at, and a gross to net horsepower converter built from factory-published pairs instead of a made-up multiplier. Both of those things sound obvious. Almost nothing on the internet does either one correctly.

VIN Engine Code Decoder

Chrysler and Ford stamped the engine into the fifth character of the VIN. The catch is that the letters were reassigned between model years, and most published charts do not say which year they apply to.

Make

Model year

5th VIN character, or paste the full VIN

Enter a code

Type the 5th character of the VIN, or paste the whole VIN.

Why the year matters. In 1968 and 1969 a Chrysler VIN reading H is a 383 four-barrel and J is a 426 Hemi. For 1970 the same letters mean a 340 four-barrel and a 340 Six Pack, and the Hemi moved to R. G is a 383 in 1969 and a 318 in 1970. Several widely reproduced Mopar charts, including ones published by large and otherwise reliable outlets, print the 1968 set under a 1970 heading. Run a 1970 VIN through one of those and it will hand you the wrong engine with total confidence.

Gross to Net Horsepower

Advertised power through 1971 was SAE gross, measured on a stand with open headers and no accessories. From 1972 the published figure was SAE net, measured as installed. Comparing a 1970 brochure against a 1972 one without accounting for that is the single most common mistake in muscle car spec talk.

Look up a documented engine

Pick an engine

Or estimate from any gross figure

Enter a gross figure

Advertised horsepower for 1971 and earlier is a gross figure.

We will not give you one number. Every other converter multiplies by a fixed factor, usually 0.8, and that is simply not how it worked. In the table below the net figure runs from 66% of gross on the 1971 440 four-barrel to 91% on the Pontiac 455 HO. Chrysler over-rated the 318 and under-rated the 340 and the 440 Six Barrel, and the published pairs prove it. A single multiplier hides exactly the thing worth knowing.

The Factory Pairs

Engines whose maker published a gross and a net rating for the same motor. Chrysler printed both in its 1971 Plymouth and Chrysler brochures, which is why that one year carries most of this table. Nothing below is calculated by us except the percentage column.

Year	Engine	Gross	Net	Net as % of gross
1971	Chrysler 318 2bbl	230	155	67%
1971	Chrysler 340 4bbl	275	235	85%
1971	Chrysler 383 2bbl	275	190	69%
1971	Chrysler 383 4bbl	300	250	83%
1971	Chrysler 440 4bbl	335	220	66%
1971	Chrysler 440 Six Barrel	385	330	86%
1971	Chrysler 426 Hemi	425	350	82%
1971	Pontiac 455 HO	335	305	91%
1971	Chevrolet LT-1 350	330	255	77%
1971	Chevrolet 250 six	145	110	76%
1966	Chevrolet 250 six	155	125	81%

What a GM VIN Cannot Tell You

The most useful thing this page can do for a General Motors car is stop you trusting a decoder that pretends to work.

For the classic muscle years, a Chevrolet VIN does not encode the engine, and it does not encode the SS package either. The 1969 and 1970 Chevelle SS was an option code, not a separate series, so there is nothing in the VIN that proves a 396 or a 454 car. Any site that decodes a 1970 Chevelle VIN into an engine is guessing from the model, and a big block swapped into a six cylinder car will decode exactly the same.

What actually proves it, in descending order of weight: the partial VIN stamped on the engine block matching the car, the cowl or trim tag, the build sheet, the Protect-O-Plate, and block casting numbers with date codes that predate the build. Paint codes can help at the margins, since Monaco Orange and Daytona Yellow were SS-only choices on the 1969 Chevelle.

GM VIN formats were only standardised across divisions for 1972, and even then the use of each position varied. Treat any pre-1972 GM engine decode as unsupported.

Sources and Method

VIN engine codes: year-by-year 13 digit VIN breakdowns published by the Mopar Enthusiast Network for 1968, 1969, 1970 and 1971, cross-checked against the Charger Club of WA production tables. Ford Mustang codes from Mustang Reloaded and CJ Pony Parts year decoders. The Ford table is scoped to the Mustang on purpose, because Ford used some of the same letters differently on other lines.

Horsepower pairs: 1971 Plymouth and Chrysler brochures, which printed SAE gross and SAE net side by side, as documented by Paul Niedermeyer at Curbside Classic. Pontiac 455 HO and Chevrolet LT-1 figures corroborated across period specification listings. Chevrolet gross and net pairs also appear in the Vehicle Information Kits held by the GM Heritage Center.

Corrections are welcome and we will publish them. If you hold factory documentation that contradicts anything here, we would rather be corrected than be repeated.

Cite this page: Big Block Magazine, "The Muscle Car Decoder: year-aware VIN engine codes and factory gross to net horsepower pairs", bigblockmagazine.com/decoder/