Choices, choices....what would you do?

[ajmorell]

I have a few options here, and need some advice.

Option 1:
258 crank
4.0 rods
3.895" bore (+0.020")
-0.0005 deck clearance (decked 0.030")
0.051 head gasket (stock)
21.73cc piston dish vol. (Keith Black K944 forged pistons)
6.123" connecting rods (stock 4.0)
Comp 4250H-13 cam (250 duration/113 lobe separation)
278.5 cubic in.
Static CR: 9.44:1
Dynamic CR: 8.15:1
Quench: 0.0505

Option 2:
258 crank
4.2 rods
3.905 bore (+0.030")
0.0205 deck clearance (decked 0.025")
0.051 head gasket (stock)
26cc piston dish vol. (Sealed Power 677C cast pistons, dish machined to 0.195")
5.875 connecting rods (stock 4.2)
Comp 4250H-13 cam (250 duration/113 lobe seperation)
279.9 cubic in.
Static CR: 8.76:1
Dynamic CR: 7.59:1
Quench: 0.0715

Option 3:
4.2 crank
4.0 rods
3.905 bore (+0.030")
0.0075 deck clearance (not decked)
0.051 head gasket (stock)
24cc piston dish vol. (Diamond Forged piston from Hesco)
6.123 connecting rods (stock 4.0)
Comp 4250H-13 cam (250 duration/113 lobe seperation)
279.9 cubic in.
Static CR: 8.97:1
Dynamic CR: 7.74:1
Quench: 0.0585


I would like to go with option 2 for a number of reasons including cost and the fact that I already have the 4.2 rods (could easily sell though) however I'm just not sure if will be as reliable as 1 or 3. 3 is probably the most expensive but the fact that I would have to have the block decked might make it about equal with option 1. What do you think, I like everything about option 3 except the price, but I am willing to spend the money if it is really 'worth it' and going to leave me with a trouble free motor.

[PolloLoco]

The COMP cam 4250H-13 is more commonly known as the 68-232-4. Some comparisons of cams: http://www.angelfire.com/my/fan/Jeep4.0Camshafts.htm

Negative deck clearance = bad idea? Which ties into a question I've been wondering myself is with such high valve lifts how high in the cylinder can the piston go without hitting the valves? That will limit how much you can deck the block. I don't know enough about the geometry of the engine to answer that yet.

[gradon]

All of your options are nice. -.0005" is fine for a deck clearance(I contemplated a -.002/3" until I found out the KBs couldn't go on the 6.150" eagles), as long as quench is at least .040". You also can make them better with the .044" mopar performance gasket(esp the #2), so make that an option, if not a preference. 1 will probably require 93 octane, but will get you the most power with the higher compression, and 2 and 3 will probably need midgrade. Basically you have some good potential builds, just bring #2's quench down with the HG and maybe some more decking.

[jsawduste]

Option 1 will give you a better bore/stroke to rod angle.

Plus the longer rod, which means a higher wrist pin will not pull the piston so far out of the bore at BDC.

Prefer to see quench in the low to mid 40`s. Also consider the valve train geometry when decking. I do not believe in letting the lifter take up the "slack". Shorter pushrods or adjustable rockers to maintain factory design.

[ajmorell]

Option 1 will give you a better bore/stroke to rod angle.

Plus the longer rod, which means a higher wrist pin will not pull the piston so far out of the bore at BDC.

Prefer to see quench in the low to mid 40`s. Also consider the valve train geometry when decking. I do not believe in letting the lifter take up the "slack". Shorter pushrods or adjustable rockers to maintain factory design.

All good points. I thought about the rod angle but not the valve train geometry. Assuming you deck the head 0.030, do you just need 0.030" shorter pushrods (Assuming so since they are parallel to the motion & block) but just checking. This is a new issue, what potential issues exist if you were to use stock pushrods (just out of curiosity, not really a consideration)