340 Compression

[FlagCraig]

I am rebuilding the 340 motor in my 72 Challlenger Rallye. It is the original motor. Does anyone know what compression it came with from the factory?

Also, if I go with a 10.5 to 1 compression with a slightly beefed up cam will I be able to run on today's 93 Octane gas without issues?

Thanks for any help. It is appreciated. This is a great website.

 

[moparleo]

Are you personally rebuilding this engine or having a shop build it ? As far as running on todays pump gas, there are many factors that come into play. Cylinder head material, iron or aluminum? Combustion chamber shape and type, open or closed chamber, high quench ? Camshaft duration, ignition timing, advance curve, fuel system type, carb or F.I.? Altitude you live at if in the mountain areas ?

Factory C.R. 68-70 10.5:1
1971 10.25:1
1972 8.5:1
.

 

[challenger6pak]

10.5:1 and todays fuel. With an all cast iron, carbureted motor, I would say no. With aluminum heads and programmable fuel injection, yes.

 

[FlagCraig]

Are you personally rebuilding this engine or having a shop build it ? As far as running on todays pump gas, there are many factors that come into play. Cylinder head material, iron or aluminum? Combustion chamber shape and type, open or closed chamber, high quench ? Camshaft duration, ignition timing, advance curve, fuel system type, carb or F.I.? Altitude you live at if in the mountain areas ?

Factory C.R. 68-70 10.5:1
1971 10.25:1
1972 8.5:1
.

Shop is doing the rebuild but haven't started yet. No mountains....Nashville, TN. The goal is to try and keep it close to stock but give it more muscle while still using today's pump gas. In addition to the new cam I will be putting on a new carb and intake. Would prefer to keep the same heads but would consider aluminum. When we start the rebuild I just didn't want to go in blind to what my options are. I'm not a deep in detailed engine guy. Thanks again.

 

[Chryco Psycho]

You may be able to get away with the 10>2 with Iron heads If you use the later 308 swirl castings & get proper quench & select the cam carefully

 

[moper]

There's advertised compression and actual compression. None of the factory ratios were close to advertised. So if you have a '72, it's probably closer to 7.8-8:1 as delievered from the factory. You can run earlier pistons and get some back - but only about a point unless you address the other issues and blueprint it. If you want more snap, run the Kb hypereutectic pistons that mimic the old TRW forged ones. they are lighter, cheaper, and light years ahead of the old stuff and they will get back some static compression. With a factory head and common head gasket it will be around 9.8:1. Then even if you want a modest camshaft, it will be fine on pump 89.