89 XJ WITH A 4.2 FROM A 90 WRANGLER WITH A 4.0 FUEL INJECTED HEAD, THEY SAID THIS MAKES 50 MORE HORSE THAN STOCK IS THIS TRUE? WHAT WOULD THE C.R BE ON THIS? IS THIS A COMMON BUILD.  

IDK bout the horsepower/cr, many have done it.  

The 258 is an undersquare design, meaning bore is smaller than stroke. Undersquare motors develop great low end torque, have low HP because they have low redlines. The 4.0L is oversquare so it has a higher RPM.

It takes RPM to produce hp so that makes it hard to just add things up.

The 258 had 150 hp stock. A HO 4.0L has 190 hp. A RENIX injected 4.0L had 177 hp.

So here's my guess. Your Jeep came with the RENIX injection system stock. So, my guess is that you have the RENIX on your 258 and not the Chrysler HO injection.

The RENIX head is not as free flowing as the HO head.

The HP gains between a 258 carbed motor and a 4.0L injected motor would have more to do with the cam taking advantage of the head and the computer controlled injection than the head or injection in of them selves.

So, if it also has the 4.0L cam profile, then I think you probably have a real nice running 258, with a little more HP than the stock 258, but with more torque than a 4.0L.

My guess, is 20-30 hp with a cam profile matching the 4.0L and maybe only 10 hp for a head and injection alone. Remember it takes rpm to develop hp so the hp numbers might suffer just because of the lower rpm capabilities of the 258.

As for compression, that's impossible to tell without knowing more about the engine. The head could have bumped it up, and the piston choice could have brought it back down. If the compression is higher, then that equals hp for sure.

Does it knock?  

So if a renix 4.0 has 177 hp shoudlnt a renix 4.2 have more hp. Bigger discplacment 4.2 + smaller combustion chamber 4.0 head= more compression which almost always =more hp and at the very least more torque.  Im going to estimate 180-185 hp.  HOw hard would it be to convert to the chrysler ho fuel injection.  

Remember the oversquare, undersquare explanation. The 258 will probably not develop as much hp because of it's lower rpm capabilities.

But, it will develop more torque. And that is power that you can feel down low for sure.

People use the 258 crank in the 4.0L block for a 4.6L stroker all the time. You see some that claim 260 hp. But most people that build them end up with 220-240 hp on the dyno.

Not to dis your setup, but I think that a 4.6 based on the 4.0L block is the way go.

Then you get a motor that is nearly square which will produce good torque and good hp.

Sometimes putting heads with the wrong shape/sized combustion chamber, and the way that it affects the "squish", can have a negative affect too.

Again there is a lot to building a good power producing motor, and I don't know if what you have has had all the propper work done to it to produce good power.

The big thing is having fuel injection. Beats a carb any day. But, fuel injection by it's self doesn't produce power. Knowing more about the cam in your motor would tell more of the story.

And, don't discount the balanced approach to engine building. Many people make this error. Well matched components produce more power than a mismatch of parts.  

im not going to *** with the motor, unles i blow it or something, as it runs great right now, maybe if i come across a cheap HO 4.0 ill do some swapping, for right now ill do some bolt-ons, underdrive pulleys, some good plugs and wires, maybe a throttle body spacer, stuff like that, once i go to 4.88 gears it will feel like i have 100 more horses.  

I'm not advocating getting rid of your motor. Enjoy it. I'm sure it runs great. Just sharing some of the things that people are doing.

Underdrive pulleys are always a bad idea. They can lead to hot spots in your engine and overheating. It's not a good idea to slow down your water pump. They also cause under charging.

Throttle body spacers are for TBI injection not for Port injection. They do absolutely nothing.  Click here for more info.  

thanks that will save me some money for some other goodies, like gears and locker.  

I don't think your 258 produces as much hp as the RENIX 4.0 because the 4.0 head has a larger combustion chamber. That means less compression than the stock 258.

Plus, the fact that the circumference of the combustion chamber is larger than the bore diameter.

This would create a little shelf area and would affect the flame during combustion. People who do this swap do some block work to taper out the difference. They claim 5-10 hp, but pay a grand to get it done.