A few 340 questions

[billbob1936]

Hello. I have a 340 question as to the determination of a factory pop-up engine (pistons above deck). Is the engine block (crank area) casting slightly differently on the early vs. late 340 engines, or is it the original crankshaft stroke itself that makes it a popup engine, or perhaps a combination of the rods and pistons? Also, could my 73 engine be changed with a different crankshaft to make it internally balanced instead of its current external setup, or is that not possible? thanks

 

[marksmopars]

Not certain what you mean by a popup engine but the crankshaft rods and pistons are the heart of it. The block casting varies a bit for applications I’m sure, but the cast versus forged crank goes with the internal versus external balancing.
Get yourself a good set of forged internals, stuff them into your 340 block, balance it the way you want and hang on.

 

[mrmopar340]

Pop ups were for positive deck motors upto 71 I think. After that they were in the hole to cut compression and made to run at about 8.5 to one compression. Mancini had a damn good rotating assembly a while back as a kit. Callies crank, H beam rods and Diamond pistons with bearings and rings. Put one in mine after having it all balanced. World of difference with a good cam. Hughes .576 .593 I think and 4" stroke and 500 on motor. 444ft lbs at the tire.

 

[billbob1936]

Hello. I have a 340 question as to the determination of a factory pop-up engine (pistons above deck). Is the engine block (crank area) casting slightly differently on the early vs. late 340 engines, or is it the original crankshaft stroke itself that makes it a popup engine, or perhaps a combination of the rods and pistons? Also, could my 73 engine be changed with a different crankshaft to make it internally balanced instead of its current external setup, or is that not possible? thanks

Is it possible to take a 1970 crankshaft and put it into a 1973 block?

 

[S Vicious]

Is it possible to take a 1970 crankshaft and put it into a 1973 block?

Absolutely, Changing from cast crank to forged requires rebalancing the lower assembly enabling a change from externally to internally balanced.

 

[JDMopar]

The difference in the 68-71 pistons and the later 72-73 pistons is in the pin to crown height of the pistons. The early pistons have higher crowns and only 2 valve reliefs, where the later pistons have lower crowns and 4 valve reliefs. The rods were the same. I have taken a 340 forged crank and pistons and installed them into a 340 block that came with a cast crank. That was in 1977 when all of that junk was fairly new and not worn slap out....lol. I was a broke kid with little money, and didn't have it balanced, and it was fine because it was all factory parts except rings and bearings. It worked just fine. Now? I would have it balanced because of the need to use aftermarket rods and pistons.

 

[billbob1936]

Hello. I have a 340 question as to the determination of a factory pop-up engine (pistons above deck). Is the engine block (crank area) casting slightly differently on the early vs. late 340 engines, or is it the original crankshaft stroke itself that makes it a popup engine, or perhaps a combination of the rods and pistons? Also, could my 73 engine be changed with a different crankshaft to make it internally balanced instead of its current external setup, or is that not possible? thanks

Great information here folks, thanks. I'm trying to wrap my head around the reason Chrysler made the cast version externally balanced, why not keep it the same? I get the understanding that casting is a cheaper way to do it, but why trade internally balanced to externally balanced, resulting in a bigger chunk of rotating steel on the HB, not to mention the weights added to the torque converter. Would it have anything to do with the ease of milling a forged crank to get it balanced versus not being able to do that on a cast crank?

 

[Xcudame]

68 - 71 340s used forged crankshafts and 10.5:1 compression pistons. 72 - 73 340s used cast crankshafts and 8.5:1 pistons. To realistically use the 68 - 71 pistons and pump gasoline, you need either aluminum heads or iron heads with a gasket that will deliver around 9.5:1 compression. As to why Chrysler ever went to cast crankshafts instead of forged, it was a bean counter (money saved) move.

I have three 340s now collecting dust! The one built up the most has 10.5:1 pistons and a forged crankshafts along with adjustable rocker arms. Anything less is just wasting horse power.