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Thread: Help me I need some Info

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  1. #1
    gzweber's Avatar
    gzweber is offline Registered User Visit my Photo Gallery
    Join Date
    Aug 2003
    Yankton, SD
    Car Year, Make, Model: 1992 Dodge B250 "Shorty Van"

    Exclamation Help me I need some Info


    Hey just to establish this I am not a ricer. I just wanted to do something different with my Jeep. Ok well here is my problem. I have an inline six 242 (4.0) and I wish to stroke it out to a 4.7, but to do this I am using the crank out of a 258(4.2) and I need to know the right combo for the rest of my setup. Ok well origanally I was going to use the 258 rods and crank and 242 pistons and then get myself a lobey cam for better top end torque and hp, but If I could I would rather use the 242 rods because they provide more stroke (qurter inch more). Well I know that some guys use the 242 rods and from what I can tell a stock 242 cam but the problem is that I dont know if I should do this and use a bigger cam and if so will I have valve/piston contact. On the other hand I want as much power as possible but basically I dont know whether that quarter of an inch less stroke with the 258 rods would make much of a difference or if the combo of 242 rods and bigger cam would end up being just as powerful.

    Ok so the other thing I will be using Nitrous Oxide and willl either be super or turbocharging it.

    So to beter explain my self here is some info to clairify what I have already said.

    Crank: 258
    rods: 258
    Cam: custom Fair idle, daily performance usage, good mid-range HP, 3400-3800 cruise RPM, 10.0 to 11.5 compression ratio advised. Basic RPM 3000-6500 aka lobey cam
    This combo works aaccording to several people
    Question would the shorter stroke outway the bigger cam?

    rods: 242
    pistons: 242
    cam: 242
    I know this combo works also but would it have less power than the above? Also could I have this combo and a Bigger cam w/out valve/piston contact?

    So the other thing I think that I am covered in low end torque but at about 85mph it lags in stock config so If anyone knows what the best combo for me would be I would greatly appreciate the help.

  2. #2
    Don Meyer is offline Moderator Visit my Photo Gallery
    Join Date
    Apr 2001
    St Cloud
    Car Year, Make, Model: 48 GMC trk & a 66 Rolls Royce

    The new Chev str 6 is an other option - great motor but, my choice would be a SBC. Probably cheaper than modifying your motor.
    Don Meyer, PhD-Mech Engr(48 GMC Trk/chopped/cab extended/caddy fins & a GM converted Rolls Royce Silver Shadow).

  3. #3
    offyman is offline Registered User Visit my Photo Gallery
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    Apr 2001

    IF both engines use the same block and you put the 242 rods and pistons on the 256 crank my guess would be that you will have major piston to head interference.

  4. #4
    lakehunter5's Avatar
    lakehunter5 is offline Registered User Visit my Photo Gallery
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    Jul 2003
    Vancouver, WA
    Car Year, Make, Model: 53 Bel Air, among others

    Well gzweber,

    if you have access to both sets of parts mock up the engine and apply a thin layer of modeling clay to the tops of the pistons and crank it by hand to look for piston to valve contact ..... OR you can take Dons suggestion and put one of these bad boys in the jeep



  5. #5
    drg84's Avatar
    drg84 is offline CHR Member Visit my Photo Gallery
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    Sep 2003
    Car Year, Make, Model: 1996 Aurora Autobahn edition



    OK, I think (but im not sure) that the combo can be done, but it may take some machining. The 4.2 crank with the 4.0 rods will clear, but not by much. The high lift cam will open the valves too far, and it will make contact with the pistons for sure. I would suggest boring it out, and keeping the factory crank. If you have your heart set on the 4.2 crank, recess the valves or get a long duration cam with a low lift. In a little while I can get more info, because a friend of mine bored his out to 5.1L!
    Right engine, Wrong Wheels

  6. #6
    screamer63_1979's Avatar
    screamer63_1979 is offline CHR Member Visit my Photo Gallery
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    Jul 2003
    Car Year, Make, Model: 90 Jeep Wrangler

    Don't know alot about the 258. I understand teh 242 is a 3 main bearing motor - yes only 3!!!!!!!!! This makes for a whippy crank and more HP may not be adviseable.

    The early ones made had a lower peak torque number (1800 RPM) versus the later ones (chrysler engineers got in there, moved peak torque up toward 3,000 rom if i recall)

    The head on that mnotor probably can't be made to breathe as it is not a cross flow hear (cross flow - intake on one side, exhaust on the other). There is just no room for bigger ports, and they have to be small to start with to even fit.

    Go see Jeep sites and see what those looneys are up to. I know a V8 is probably heavier than the 6, but it may be shorter in height and more rearwardly mounted to help weight distribution.

    what is your ultimate goal? 1.4 mile, rock crawling, or F150 lightning beater?
    Only the dead fish go with the flow.

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