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304 Changes Through The Years

Discussion in 'Intermediate CJ-5 and CJ-6 Tech' started by Chilly, Nov 20, 2018.

  1. Nov 20, 2018
    Chilly

    Chilly Active Member

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    What changed through the 304 production years that resulted in lower power? Combustion chamber size? Cam profile? Distributor setup and module? I have a 73 engine and what I think is an early 80's engine. What parts are different?
     
  2. Nov 20, 2018
    mrtii

    mrtii 1972 cj51986 cj7 Sponsor

    gilroy, California
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    Are either engine a California model as they would have had different internal cam lift duration especially on the 73, not sure of the 1980, but suspect so
     
  3. Nov 20, 2018
    Chilly

    Chilly Active Member

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    Nether are CA engines. Did some more digging into engine build tags:

    #1 is June 1977, casting 3220515 (106H08)
    #2 is May 1973, casting 3216086? (605H17)

    It seems 1973 was a transition year where the bridged rocker appeared. Both engines have bridged rockers. I'll confirm actual casting numbers tonight.
     
    Last edited: Nov 20, 2018
  4. Nov 20, 2018
    timgr

    timgr Jeepin' Nerd Sponsor

    Medford Mass USA
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    The story says that the 304 was discontinued because it had been strangled until it wasn't a significant step up from the 258. I would imagine that there was not one thing that reduced power, as much as simply boosting those aspects that were already there - later timing, more exhaust gas, and less fuel from shorter cam duration. I could look up the part numbers, but that would not tell me what had changed to make less power. AFAIK, there were no significant internal changes to the engines that could not be reversed by simple parts swapping. The valve size was the same, cylinder heads did not change, AFAIK the combustion chamber size did not change, so compression would be the same with the same pistons. Compression seems not to have changed, at least from '72 through '79. The change to stamped and bridged rockers seems only to relate to production cost, not performance.

    If you look at the later 360s, there are a lot of vacuum devices on the engine top. Seems there's nothing particularly new, but just more of the same and more gadgets to control the effects of the devices so the engines are still driveable.
     
    Last edited: Nov 20, 2018
  5. Nov 21, 2018
    Keys5a

    Keys5a Sponsor Sponsor

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    I've built a couple 304's, and if emission controls aren't required, an aftermarket torque cam and aluminum 4bbl intake really wake up these engines. A friend couldn't believe it was his same engine. I did minor cleaning up of the intake and exhaust runners, and just rings and bearings, and priority oiling. Night and day difference!
    -Donny
     
  6. Nov 22, 2018
    mrtii

    mrtii 1972 cj51986 cj7 Sponsor

    gilroy, California
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    The necessity to meet ever more stringent emission standards thru the 70's and 80's with the decision to retain a carburetor, meant that power and fuel efficiency would have to be sacrificed. I believe that cost of production played a large roll in the decision to use older less advanced tech to control emissions. Control of Vacuum and mechanical advance, EGR Exhaust Gas recirculation, and a very leaned out fuel air mixture that caused a sacrifice of performance greatly.
     
  7. Nov 22, 2018
    Chilly

    Chilly Active Member

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    If thats the case then the source of performance degradation lies in peripherals: distributor springs, advance vacuum diaphragm and spring, vacuum advance source (ported vs manifold), and carb jetting. Since no difference in heads, pistons, or cams then one of my engines is as good as the other. All of my peripherals are from the 73, not the 77.

    My EGR is plugged, I dont run an air pump, CTCSS is disabled, vac advance comes off manifold vacuum. I am running factory carb, probably with factory jets. Distributor is probably factory setup too, and my timing light grew legs a long time ago. Need a replacement.
     
  8. Nov 22, 2018
    mrtii

    mrtii 1972 cj51986 cj7 Sponsor

    gilroy, California
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    I do know the cam was difference on the California models, specifically the exhaust side duration of overlap was greater than the 49 state models, but yes as you say much had to do with the all the hung on devices to including ever leaning of the fuel mixture I have seen that when throttle body injection has been installed on this engine both performance, driveability and most impressive to me is fuel economy improvement of this engine
     
    heavychevy likes this.

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