I'm sending this here as it may be useful for this forum:
I'm investigating a Clifford Performance built Ford 250 six engine. Supposed to have Jahns 10:1 Pistons, Clifford rods, balanced bottom end, Fisher balancer, 1.88 int. valves, Ported and relieved, Offy 3X1, Re-curved distrib., the works...
I've been trying to find out what was done to accomplish the compression ratio. The pistons were not likely not Jahns sinc they have a D shaped sump in the head, I talked to Chris Neilson at VintageCams , who bought up most of the Jahns' s stock when they folded. I found casting numbers on the top of the pistons to be STD - 470NP. What I discovered is the pistons are likely to be TRW/SealedPower # 470NP's. The application is 79-89 AMC 4.2(258).
I've never heard of using the AMC's but the specs seem to bear out the idea:
Ford 250: bore = 3.68 / comp distance = 1.5 / dish volume = 13cc
AMC 258: bore = 3.75 / comp distance = 1.633 / D-Sump = .178(?)
This would put the bore at @ .070 over and raise the piston toward the deck .133 . The specs list the pin diameter of the ford at .9122 and the AMC's at .9310 which could easily be accomodated.
The AMC Pistons do not have a stated relief volume but the depression is much bigger than a 200/250 piston -I'd guesstimate 2 to 3 times the volume. Since the 200/250's range from 07 to 13 cc's - the CR must be lessened significantly and still maintain zero or negative deck height with the .133 additional compression distance.
Powerband 8)
I'm investigating a Clifford Performance built Ford 250 six engine. Supposed to have Jahns 10:1 Pistons, Clifford rods, balanced bottom end, Fisher balancer, 1.88 int. valves, Ported and relieved, Offy 3X1, Re-curved distrib., the works...
I've been trying to find out what was done to accomplish the compression ratio. The pistons were not likely not Jahns sinc they have a D shaped sump in the head, I talked to Chris Neilson at VintageCams , who bought up most of the Jahns' s stock when they folded. I found casting numbers on the top of the pistons to be STD - 470NP. What I discovered is the pistons are likely to be TRW/SealedPower # 470NP's. The application is 79-89 AMC 4.2(258).
I've never heard of using the AMC's but the specs seem to bear out the idea:
Ford 250: bore = 3.68 / comp distance = 1.5 / dish volume = 13cc
AMC 258: bore = 3.75 / comp distance = 1.633 / D-Sump = .178(?)
This would put the bore at @ .070 over and raise the piston toward the deck .133 . The specs list the pin diameter of the ford at .9122 and the AMC's at .9310 which could easily be accomodated.
The AMC Pistons do not have a stated relief volume but the depression is much bigger than a 200/250 piston -I'd guesstimate 2 to 3 times the volume. Since the 200/250's range from 07 to 13 cc's - the CR must be lessened significantly and still maintain zero or negative deck height with the .133 additional compression distance.
Powerband 8)
