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Jeep Wrangler YJ
YJ General Discussion
Time to rebuild my 258 /4.2 engine and I'm wondering those of you here who have done it what would you do again what not to bother with?
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<blockquote data-quote="Smellypiratehooker" data-source="post: 435398" data-attributes="member: 20406"><p>I was recently in the same position with the daughters jeep and I ended up sticking with the 4.2. Now we already had the 4.0 HO head and header with a Motorcraft 2100 carb just for background. </p><p>Things I can tell you about the 4.2;</p><p>It dumps its torque so low that putting it into a jeep that had a 2.5 you feel like you've lowered the gear ratio one notch. Our example had 33" and 4:56 gearing. It needed 1st gear and fell on its face in 5th. The 2.5 was in perfect operating order. A simple swap to a 4.2 (before HO head) made it so 1st gear was an option and it would pull 5th for days. Keeping in mind I believe the ax15 trans 1st gear is actually higher the the ax5!. The 4.2 runs like a diesel as its all low end and nothing past it. I like that, some don't. All things similar, the 4.2 CAN achieve better fuel mileage then the 4.0. Before you jump in with one being injection and the other not, I did say all things similar! Just like the Chevy 305, the 4.2 has tiny pistons that shroud the valves. AKA hurt breathing, which means less air/fuel in. We beat the 2.5 milage significantly with the 4.2 with and without the 4.0 head swap. Now for the bad parts.....the 4.2 isn't cheap to rebuild. Machine work is all the same but parts not so much. And there ain't much of them. Even piston selection is tough, I ended up using pistons ment for the smaller 60's amc inline and decking the block to achieve 0 deck. And paid much more for them. Add maybe 500 extra for parts compared to the 4.0 depending on what you are doing. If you are looking for an HP build go for a 4.0 or Chevy. All three end up being the same weight which was the cap I set for any swap. The 4.0 can make power for sure, but the small block with obviously make more and probably do it with better mileage. Infact I kind of regret not going a 305 virtec build only because the rebuild would have been cheaper and I would have had the option of much more power if ever needed. 305 vortec were getting 18-19+ mpg in full size 4x4 trucks. Let's get back on track......so if you want a stockish build the 4.2 is a great engine. Ours has about 9.3:1 compression, stock cam with HO head and a sniper injection now after rebuild. We honestly don't need any more power as the torque is nasty. We've managed 17-18 mpg and its still breaking in. It will spin the 33" tires with 4:56 gears if you so choose on asphalt. With nothing more then a bump in compression, an HO head and attention to detail in the build. If you need more then that, certainly go the 4.0 or the small block.</p></blockquote><p></p>
[QUOTE="Smellypiratehooker, post: 435398, member: 20406"] I was recently in the same position with the daughters jeep and I ended up sticking with the 4.2. Now we already had the 4.0 HO head and header with a Motorcraft 2100 carb just for background. Things I can tell you about the 4.2; It dumps its torque so low that putting it into a jeep that had a 2.5 you feel like you've lowered the gear ratio one notch. Our example had 33" and 4:56 gearing. It needed 1st gear and fell on its face in 5th. The 2.5 was in perfect operating order. A simple swap to a 4.2 (before HO head) made it so 1st gear was an option and it would pull 5th for days. Keeping in mind I believe the ax15 trans 1st gear is actually higher the the ax5!. The 4.2 runs like a diesel as its all low end and nothing past it. I like that, some don't. All things similar, the 4.2 CAN achieve better fuel mileage then the 4.0. Before you jump in with one being injection and the other not, I did say all things similar! Just like the Chevy 305, the 4.2 has tiny pistons that shroud the valves. AKA hurt breathing, which means less air/fuel in. We beat the 2.5 milage significantly with the 4.2 with and without the 4.0 head swap. Now for the bad parts.....the 4.2 isn't cheap to rebuild. Machine work is all the same but parts not so much. And there ain't much of them. Even piston selection is tough, I ended up using pistons ment for the smaller 60's amc inline and decking the block to achieve 0 deck. And paid much more for them. Add maybe 500 extra for parts compared to the 4.0 depending on what you are doing. If you are looking for an HP build go for a 4.0 or Chevy. All three end up being the same weight which was the cap I set for any swap. The 4.0 can make power for sure, but the small block with obviously make more and probably do it with better mileage. Infact I kind of regret not going a 305 virtec build only because the rebuild would have been cheaper and I would have had the option of much more power if ever needed. 305 vortec were getting 18-19+ mpg in full size 4x4 trucks. Let's get back on track......so if you want a stockish build the 4.2 is a great engine. Ours has about 9.3:1 compression, stock cam with HO head and a sniper injection now after rebuild. We honestly don't need any more power as the torque is nasty. We've managed 17-18 mpg and its still breaking in. It will spin the 33" tires with 4:56 gears if you so choose on asphalt. With nothing more then a bump in compression, an HO head and attention to detail in the build. If you need more then that, certainly go the 4.0 or the small block. [/QUOTE]
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Jeep Wrangler YJ
YJ General Discussion
Time to rebuild my 258 /4.2 engine and I'm wondering those of you here who have done it what would you do again what not to bother with?
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