Cummins SC vs. BC1 vs. BC3 compression ratios

My 1951. Nh743/220 is 15:1.
1972. Nh855/250 is 14:1
1974. NTC855/350 is 13:1

 

I don’t know that the compression ratio followed the series of the engine as much as it did the hp
Generally speaking the higher hp the lower the compression ratio. Probably a few exceptions.

 

When I rebuilt my engine I looked at what pistons were available. I couldn't get the 15:1 pistons for CPL 1280/1256. The 15:1 that were available didn't have cutouts for a high lift cam

14.0:1 3050480, 3053500, 3048808
14.5:1 3045948
15.0:1 3048809
15.5:1 3052794, 3050366
17.0:1 3069212, 3053526,3095064

There may be others but those were the most common

 

One thing I can tell you is the ratios didn't really drop until the NTC’s because of turbo’s. Then they dropped a little more with the after cooled models and slowly came back up a little with some of the BC’s. With it’s high compression the old 743 cid NH220 had probably the best sound and had a race car idle at 500 rpm if tuned right. The 855 cid NH250 was the best sounding 855 ever. Also the loudest straight stacked Cummins besides the two and six bits supercharged NHRS with back drop manifold. Both were normal aspirated.

 

What piston did you decide on? I’m about to tear into a 1256

 

I used 15.5:1 pistons from PAI. Not completely sure that was the best option or not. That's what capitol reman was going to use when I first talked to them

Another guy sent me an ESN for a rebuilt 444 and they used 14:1 pistons. They changed a bunch of other things too so I don't really know how to compare. I had already rebuilt mine at that point also. Wasn't going to go back and change everything

Field Fix 155 uprated the NTC-444 to NTC-450 but I never did find documentation on what that change involved