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258 upgrade

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Posts: 1
Joined: May 31st, 2016, 1:17 pm
Vehicle Year: 1977
Vehicle Make: AMC
Vehicle Model: Pacer

258 upgrade

Post by Goul6891 » June 16th, 2016, 7:44 pm

Hi guys, thanks for letting me join the site, theres so much information here!

Currently I'm building a new motor to replace the blown up 258 in my AMC Pacer, and am looking for some direction. Basically upgrading to a 4.0L head and a more aggressive cam without breaking the bank. I'm using the 258 core since all the accessories and mounts should bolt right on when its ready. From my research the newer heads flow quite a bit better, have a much better valve cover and valve seal system, and can utilize a factory jeep 'header'.

So far I just have the big pieces. The core I have is an early 80s 258 so its the four weight crank. I've also sourced an 0630 4.0L head. I intend to keep it carbureted for slimplicity. The block needs a 0.020-0.030 overbore since it had been well used.

I'm aware of filling the head coolant ports for the block, and have some Manley 40180 ready. The big questions I have are custom pistons/4.0L rods worth the expense to take advantage of the chamber shape? Will stock 258 or 4.0L pushrods fit correctly? What will be the best cam and intake combination?

Looking to have fun street motor that runs strong to 5500 or so.


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