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Old 06-17-2013, 08:12 PM   #1
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Jeep 4.2 head swap question

If you put a 4.0 head, fuel rail, etc. on a 4.2 will the max torque be at the same rpm as the 4.2 (I think 1800) or closer to the 4.0 (4000)?

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Old 06-17-2013, 11:34 PM   #2
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No one knows the answer?

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Old 06-18-2013, 10:31 AM   #3
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Well usually it's the other way putting the 4.2 head on the 4.0 engine. Not sure about going the other way on it.
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Old 06-18-2013, 11:26 AM   #4
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no, he has it right, 4.0 head on a 4.2 is a popular mod.
very interesting question, I remember reading posts from a guy who had a built 4.2, he did the head swap and changed back cause he lost low end torque. I would assume that with the 4.2 and the longer stroke it would be somewhere in the middle. now if you changed cams that would probably raise it higher.
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Old 06-18-2013, 11:34 AM   #5
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heres a dyno chart, before and after the swap but they did change the cam
Six Bangin Dyno Graph Photo 8
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Old 06-19-2013, 11:26 AM   #6
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I was thinking it would be somewhere in the middle too, thanks for the chart.
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Old 06-19-2013, 11:34 AM   #7
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seems like a good thing to do?

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