Maximum Reliability For Harsh Environments and Cheap Fuel
Posted: May 26th, 2024, 2:57 pm
My 1996 OBD-II XJ burns oil and needs more power for steep, sandy hill climbs and passing people on the freeway. It's heavy and even with 4.88's/35's it struggles in loose, sandy soil, which is a problem because I live in a desert. A high desert specifically, where temperatures range from 0F in winter to 110F in summer and altitudes range from 3,500 feet feet to 10,000 feet. I'm usually somewhere around 5-6k feet in elevation, and I'm out year round regardless of temperature, but I'm out more often in the freezing cold. The most readily available fuel here is 85 octane, and I need this Jeep to reliably run 85 octane year round because sometimes small gas stations in the middle of nowhere only have 85.
Most stroker recipes these days call for 4.0 rods and forged pistons, but sometimes it takes 30 minutes for my engine to warm up in cold weather, and sometimes it never warms up because I'm running the heater full blast in freezing cold 40mph wind. Letting it warm up before driving isn't an option, especially when I have 200+ miles between gas stations.
I found a recipe for 4.2 rods and off the shelf pistons in an old forum post, but the quench and compression have me hesitant when it comes to running 85 octane in the middle of summer:
Jeep 4.2L 3.895" stroke crank
Jeep 4.2L 5.875" rods
Sealed Power 677CP standard bore pistons
9.2:1 CR
Stock 4.0 camshaft
Ported HO 1.91"/1.50" cylinder head
Mopar/Victor 0.043" head gasket
0.088" quench height
Ford 24lb/hr injectors with stock 39psi FPR for '87-'95 engines, stock injectors with stock 49psi FPR for '96 and later engines
247hp @ 4900rpm, 300lbft @ 3500rpm
Are there any other recipes out there that aren't terribly expensive? Are there any off the shelf pistons that retain the D shape/squish pad and keep a tight enough quench/low enough compression to run 85 octane in the summer? KB424C sounded good until I realized it's a circular dish. The Silvolite 2229 pistons dished to around 22cc sound viable, but I don't know where to have them dished that won't cost me $750 in machine work, giving me roughly $1,000 pistons to go with my 4.2 rods. Are broken piston skirts with 4.2 rods a genuine concern if I rarely, but do sometimes (ie sandy hill climbs), exceed 4,500 rpm? Will modern forged pistons knock in cold temperatures?
I currently have an extra 1998 short block, 1989 4.2 long block, rebuilt 7120 head, and Comp Cams Xtreme 4x4 197/201 sitting in my garage. I'm getting ready to send out the 4.0 block for machine work and need to know what to tell the shop.
Most stroker recipes these days call for 4.0 rods and forged pistons, but sometimes it takes 30 minutes for my engine to warm up in cold weather, and sometimes it never warms up because I'm running the heater full blast in freezing cold 40mph wind. Letting it warm up before driving isn't an option, especially when I have 200+ miles between gas stations.
I found a recipe for 4.2 rods and off the shelf pistons in an old forum post, but the quench and compression have me hesitant when it comes to running 85 octane in the middle of summer:
Jeep 4.2L 3.895" stroke crank
Jeep 4.2L 5.875" rods
Sealed Power 677CP standard bore pistons
9.2:1 CR
Stock 4.0 camshaft
Ported HO 1.91"/1.50" cylinder head
Mopar/Victor 0.043" head gasket
0.088" quench height
Ford 24lb/hr injectors with stock 39psi FPR for '87-'95 engines, stock injectors with stock 49psi FPR for '96 and later engines
247hp @ 4900rpm, 300lbft @ 3500rpm
Are there any other recipes out there that aren't terribly expensive? Are there any off the shelf pistons that retain the D shape/squish pad and keep a tight enough quench/low enough compression to run 85 octane in the summer? KB424C sounded good until I realized it's a circular dish. The Silvolite 2229 pistons dished to around 22cc sound viable, but I don't know where to have them dished that won't cost me $750 in machine work, giving me roughly $1,000 pistons to go with my 4.2 rods. Are broken piston skirts with 4.2 rods a genuine concern if I rarely, but do sometimes (ie sandy hill climbs), exceed 4,500 rpm? Will modern forged pistons knock in cold temperatures?
I currently have an extra 1998 short block, 1989 4.2 long block, rebuilt 7120 head, and Comp Cams Xtreme 4x4 197/201 sitting in my garage. I'm getting ready to send out the 4.0 block for machine work and need to know what to tell the shop.