DirtyComanche Posted September 30, 2005 Share Posted September 30, 2005 I know that the HO head differs from the Renix and 4.2 head. Are the 4.2 and Renix ones the same? Logically, I'd think they are but I've not thought to look at the casting numbers in the junkyard. The HO head is obviously different from the Renix/4.2 in that the intake ports are higher. The higher intake port leads to better flow, which lead to throwing HO heads on 4.2s. I also read somewhere that the HO head has a smaller combustion chamber, leading to a slightly higher CR. Could anybody verify that? Everything I've read, and personal experience indicates HOs are missing a bunch of low end torque in comparison with a Renix. I assume this is because the HO has a cam with a bit more lift and a bit more duration, but I've never been able to track down the numbers for each cam. Am I correct in assuming that the cam change was to blame for this lack of low end? If they even did change the cam? The only other thing I could think of is the HO FI system just being worthless. So, anybody here an expert on the 258, Renix and HO? If you're wondering why I'm asking such questions, I'm looking to put the HO head on my Renix. Then I'll use the 99+(?) better intake manifold and the HO TB but adapt it to work with the Renix computer. Eventually I might track down a better header too. Of course, at this stage it's really just "I'm asking because I'm sick and can't wrench on the 'manche". Link to comment Share on other sites More sharing options...
DirtyComanche Posted October 5, 2005 Author Share Posted October 5, 2005 No answers, eh? No suprise either. Time to ask on Pirate! Or to just try it out. Link to comment Share on other sites More sharing options...
pingpong Posted October 5, 2005 Share Posted October 5, 2005 The 258 and the 4.0 head are different. Mostly it is a compression difference, and where the wter jackets are. The HO head mostly just flows better the the regular 4.0 head. That being said. If you go to use the HO head you will need a special valve cover gasket. Personally I would just port my stock intake to minimize headaches down the road with adapting parts. Keep in mind that the early heads have an EGR valve on a smog tube going to the exhaust. Patrick Link to comment Share on other sites More sharing options...
DirtyComanche Posted October 5, 2005 Author Share Posted October 5, 2005 Don't the water jackets still line up? I've read that the guys with the 258 can throw a HO head on without much issues, other than they have to plug a few extra jackets, which mysteriously don't line up with anything on the 4.0 block anyways... Compression difference? Meaning combustion chamber difference? Know either of the sizes? Maybe I should just go find all that out myself. Or thinking, I could try that for a change. Link to comment Share on other sites More sharing options...
mjeff87 Posted October 6, 2005 Share Posted October 6, 2005 The changeover from Renix to HO eliminated the EGR valve. Valve overlap was increased, which may be a *tiny* reason for reduced torque (?) There's a really good renix group on Yahoo (created by a guy over on NAXJA)...you might wanna check that out for any renix specific questions, like the ones you are asking about. Do a search on yahoo groups for "renix" and it'll pop up. HTH, Jeff Link to comment Share on other sites More sharing options...
DirtyComanche Posted October 6, 2005 Author Share Posted October 6, 2005 Thanks. Valve overlap. Hmmm, yes, that would do it. Link to comment Share on other sites More sharing options...
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