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Mopar 340 Small Block

Updated on September 9, 2014

340 - Mopar's Hotrod Small Block

Find the best parts for your Mopar 340 - 340 race blocks, Mopar restoration blocks, racing and high performance cylinder heads, pistons, connecting rods, forged crankshaft, and more The 340 Mopar is the best choice for building a high performance or race SB Mopar. Find the best information on building a 340 for the street or the track Building a 340 isn't cheap so make sure you get it right the first time with the best parts and correct information.

This page will be updated regularly so be sure to check back often.

Mopar 340 - The Basics

Overview and brief history...

The Mopar 340 is the best engine to build if you want a high performance small block Mopar. Why? The 340 Mopar is better than the 360 because it has a forged crankshaft, shorter stroke, and smaller main journal diameter - all of which help it out-rev and out-last the 360. It's better than the 273 and 318 because of the bigger cubes and better flowing heads. The Mopar 340 was introduced in late 1967 and instantly became the performance flagship of Mopar's small block line. It came from the factory with 10.5:1 compression, forged steel crankshaft, 4 barrel carburetor, high flow cylinder heads (2.02" intake / 1.60" exhaust valves), dual exhaust, and a factory rating of 275 horsepower. Performance-wise, it was easily the equal of the 383 in a straight line and better in the corners due to the lighter weight. In 1972, smog laws and bean counters started to take a toll on the automotive industry, and the 340 suffered. Compression dropped to 8.5:1, intake valves were reduced to 1.88", and the forged crank was replaced with a cast version. Power was down to 240 horsepower. The last year of production was 1973.

Why Build a 340 Mopar?

For high performance or racing, it's the best factory Mopar small block

Without question, the 340 Mopar was Chrysler's premier performance small block. Unfortunately, it was produced in limited numbers, and a good rebuildable core is very difficult to find. Most people these days wanting to build a high performance small block Mopar go for a 360. The Mopar 360 is plentiful, and with modern aftermarket parts can be built into a very decent performance piece. However, I believe the smaller main journal size of the 340 Mopar block is a big advantage over the 360 if you are building an all out performance or racing engine (if you can find an original or don't mind paying big bucks for a NEW retro 340 block).

Building a Mopar 340 is a lot different than building a small block Chevy, so if you've never built a small block Mopar make sure you have the right info before you start. The Mopar 340 is a fairly rare engine so you don't want to mess it up by building it wrong. How to Rebuild the Small-Block Mopar from SA Design has all the info you'll need to build your 340 Mopar or any small block Mopar, including the later Magnum series small blocks.If you don't need the Magnum info, IMHO How to Rebuild Small-Block Mopar Engines is a slightly better, but it's can be hard to find one at a reasonable price.

Mopar 340 Resto and Race Blocks

Things you need to know...

Original Mopar 340 blocks are almost as rare as hen's teeth, so if you can find one in rebuildable condition for a reasonable price, count yourself lucky. For the rest of us, there are the Mopar Performance 340 blocks. These come in several flavors and are quite expensive - from around $2500 to over $3000. What do you get for your money? A much stronger block that can be safely bored to 4.200", a thicker deck surface, choice of a full water jacket or siamesed bores, and in the case of the R3 race block an extra row of head bolts. You might be tempted to run the R3 block, instead of the resto block since it's about $500 cheaper. Unfortunately, the race blocks require custom pistons which will likely eat up any cost savings over the resto block. Why? Deck height on a stock Mopar 340 is 9.58", same as the 340 resto block. The R3 race blocks have a deck height of 9.025" or 9.200" - you can't run stock pistons and rods in the race blocks, and the short deck height also means the stock intake won't fit. In addition, the R3 block uses a 48° lifter angle with offset push rods, so you'll need special lifters, push rods, and rocker arms. Both resto and R3 blocks are available from several vendors. My favorite is Mancini Racing but it's a good idea to check with all of them to see who as the best price at any given time.

For 1970 only, Chrysler produced a special Six Pack version of the 340 for the AAR Cuda and Challenger T/A. The 340 Six Pack had a special block with thicker main bearing webs, specially machined cylinder heads and offset rocker arms that allowed the intake ports to be opened up a lot more than the standard cylinder heads, and 3 2-bbl Holley Carburetors mounted on an Edelbrock aluminum intake manifold. Even though the Six Pack was only offered in the AAR Cuda and Challenger T/A, all the parts are available to build your own and it makes an excellent combination for any small block powered Mopar. The Mopar Six-Pack Engine Handbook has all the info you need to build your own 340 Six Pack.

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The Mopar 340 Engine Block

The block is the foundation of any high performance engine

When the 340 Mopar was introduced, it shared the short stroke (3.31") of the 273 and 318, along with the journal sizes of those engines (2.4997" main, 2.125" rod). Combined with a bore of 4.04" and a set of free flowing heads, the resulting combination was an engine that could rev freely. The 360, by contrast, had a smaller bore than the 340 (4.00"), a longer stroke (3.58"), and worst of all the main journal size was increased to 2.81". Increasing the journal diameter results in higher bearing speeds for a given RPM, increased heat, and may have been one of the factors contributing to the 360's reputation for spinning bearings (at least in stock form). Although the 360 can be built into a good performance engine, especially with modern parts and building techniques, the 340 block is a better foundation to build on - if you can find one.

There are 2 reasons that 340 blocks are so hard to find. First of all, they were only produced for 6 years during a time when most people preferred a 318 for economy or a big block for performance. In a sense, you could say they were ahead of their time. Second, because of their reputation, they tended to get used and abused pretty hard, and therefore worn out. If you can find a good rebuildable 340 block today at a reasonable price, count yourself lucky.

If you can't find one, you don't need to worry too much, as the Mopar 360 will serve you well as a pretty potent street engine. If you want to go racing or you're restoring an original 340 car that "has" to have the original displacement you can get new blocks from Indy Cylinder Heads (317-862-3724; I'm not posting a link to their web site because Google reports it as hosting malicious software). They're pricey at around $2500, but they're capable of producing much more power than any "factory" small block Mopar.

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      anonymous 4 years ago

      I paid 700 for my 69 340 block the older ones are more and hard too find. The guy I bought mine from had two, but it took me two yrs of bugging him too sell it too me there like gold in the mopar world.

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      anonymous 5 years ago

      What should a guy expect to pay (fair price) for a 340 block used?


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      dream1983 5 years ago

      Very nice and useful lens, well done!

    • profile image

      anonymous 5 years ago

      The 340 in a light car (A body} is by far the best combo to build imho. light weight high RPM gear...can't beat it

    • dzautobody profile image

      dzautobody 5 years ago

      Worthy...Make nice nitrous rotating assembly!

    • profile image

      Pangionedevelopers 5 years ago

      bet you used a lot of gojo growing up

    • KustomsandChopp profile image

      KustomsandChopp 6 years ago

      I want to build an old style 50s Hot Rod using the 340.

    • javr profile image

      javr 6 years ago from British Columbia, Canada

      @javr: Returned to give this lens a Squid Angel blessing.

    • javr profile image

      javr 7 years ago from British Columbia, Canada

      My 1969 Barracuda was fast with the stock 318. When I put in a 340, it became one of the hottest cars around. Gas mileage plunged to 10mpg when the 340 was put to good use. The 318 could get 23mpg regularly.