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Unread 01-03-2002, 12:24 PM   #1
88xj
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Join Date: Oct 2001
Location: Portland OR
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Compression and emissions

Hello,

I am rebuilding my 4.0L and wanted to raise the compression just a tad. It's a 1988 with a lot of miles. What is the stock compression? Is it 8.8:1? If so would I pass emissions and or have problems if I raise the compression to 9.5:1. AND,.. would I have to get a new head due to the higher compression pistons? Thanks man!!

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Unread 01-03-2002, 12:25 PM   #2
88xj
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Me again, sorry, this is sorta the same question I asked earlier but yet different.
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Unread 01-13-2002, 08:54 AM   #3
eltj
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You shouldnt have any problems with the 9.5:1 pistons. Talk with who ever you are getting the pistons from and make sure the have valve reliefes in the top which most do. You dont need a new head to run the higher compression but its a good idea to clean the heads up with a valve job and have the valve springs checked. You will what the head milled to make sure its strait and at the same time have the machine shop measure the cc size so you can get the proper size piston for the compression that your after. Also check your cam for wear. To many people run old cams without checking for worn spots on the lobs and end up with poor performance. For what the cost its a good idea to replace it.
After all you dont want to do this again at 60k-100k do you.
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Unread 01-14-2002, 08:32 AM   #4
88xj
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Thanks! Your right, I don't want to have to do this again!
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