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Thread: Safe Cruising speed for an 87' 4x4 5 speed?

  1. #1

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    Safe Cruising speed for an 87' 4x4 5 speed?

    I've searched around a bit but haven't found a solid answer so i thought i would ask. I took my 87' Ram 50 4x4 on a trip to school about two hours away which was mostly freeway driving at 65mph. I don't really drive at this speed very often but whenever i do the engine is wound up fairly high but i don't know how fast since my truck doesn't have a tach. my question is is what is a safe speed to run one of these trucks at for trips. I think my truck has the 3.909 rear end if i remember correctly. I was thinking about taking this truck on a trip to Montana this summer and all their roads have speed limits of 70-80 mph which im sure 80 is a bit much for these trucks so i wont be going that fast. also if i do go on a trip what are some good spare parts to bring along, i know a fuel filter is a must and ive got a new bosch coil but what other parts are known to randomly fail on these 30+ year old trucks?

  2. #2

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    I'm sure some more experienced guys will comment, but one comment I could make is if an engine is relatively healthy mechanicals wise, lugging it at low RPMs is harder on it than humming down the road at higher RPMs...you know what I'm trying to say? Which motor does your 87' have? Mine is also a 4x4 5speed but it is a year earlier so it has the 2.6 liter. My truck did come with a tach and at 6,000 RPMs it is shaded red. That indicates to me that my motor should be happy running at pretty high RPMs. We also know these 2.6l have the twin balancing shafts, also suggesting the engineers expected these motors to be able to handle high RPMs....Sorry for the long-winded reply now if someone else comes along with contrary ideas to what I'm saying ignore my post they'll probably know best! Bill

  3. #3

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    It's got the 2.6 in it. I actually took it to college again thinking hey it did fine before right! Well when I got off the freeway it barely went into first and second gear then when I got to the parking lot it just flat out refused to go into second and is very noisy in first. Looks like I'm gonna need to find a new tranny at least I have AAA with towing

  4. #4

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    bummer I've towed with my 2wd auto. about 3400rpm for 3 hours straight (out of o.d.). Felt buzzy at first, but you get used to it. I wonder if your transmission was low on oil? There's no pump in a manual transmission. If the level gets low the bearings and gears starve for oil.
    I routinely commute in my truck at 75mph. I have 3.5ish gears I think. That speed shouldn't have cooked your tranny, I suspect it was weak to begin with.

  5. #5

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    Yea it was definitely showing signs of wear. It had a noisy 5th gear and first gear was a bit tricky to engage. I did change the trans oil along with the transfer case and diffs about 7000 miles ago I think and there wasn't much leakage then but now when I looked under it it looks like both the ends may be leaking

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    If you look at the the numbers on these engines you'll see why they chose the Mikuni carb and what the intended max operating rpm is under load. Works out to be approx 5,500 rpm allowing for CFM for a stock head and intake - pushing it past this offers little purpose. The band for useable torque is around the 3-3,500 rpm ball park. That being said the rule of thumb for longetivity is 2,000 rpm which is the minimum stress threshold for all the reciprocating parts and bearings (ever wondered why most 4 cylinder gas engines sit about 2k on normal road speeds?). 4x4's aren't designed for highway cruising, they're intended for torque and towing. Add to that how old our trucks are getting. There's a lot of thrust load put through a transmission during it's life and you only have to have off a couple o' thous of metal from thrust plates, gear sets and synchros for the internals to get stressed. Did you get any improvement from changing trans oil?
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    I would imagine you want to use brass friendly oil in them as well. The new stuff eats it away

  8. #8

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    I kinda figured 2000 was a good rpm that's what I was told by my dad when learning to drive in his 87 4runner sr5 but I had wondered if it could rest safely at say 2500 to 3000 rpm for freeway driving. I assume that the hearing between the two vehicles were similar in that they were from the same time period running similarly sized engines. Of course I have forgotten what rpm the 4runner sat at at 55 mph and its engine was so tired it wouldn't even go past 55-60. I also wondered about the silent shafts in the g54b engine whether they increased its cruising rpm. I did notice some improvement after changing out the tranny oil.

  9. #9

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    Oops meant to say gearing not hearing

  10. #10

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    Just saw the post about brass friendly oil. What is the new stuff that eats away brass? I think I used a synthetic oil in my tranny and diffs if I remember correctly maybe not though. I do remember it being Castrol brand. Of course this truck has lived a pretty rough life before I got it, i suspect it may have towed some heavy stuff. It's got holes drilled in it possibly from a 5th wheel hitch.

  11. #11

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    Quote Originally Posted by geezer101 View Post
    If you look at the the numbers on these engines you'll see why they chose the Mikuni carb and what the intended max operating rpm is under load. Works out to be approx 5,500 rpm allowing for CFM for a stock head and intake - pushing it past this offers little purpose. The band for useable torque is around the 3-3,500 rpm ball park. That being said the rule of thumb for longetivity is 2,000 rpm which is the minimum stress threshold for all the reciprocating parts and bearings (ever wondered why most 4 cylinder gas engines sit about 2k on normal road speeds?). 4x4's aren't designed for highway cruising, they're intended for torque and towing. Add to that how old our trucks are getting. There's a lot of thrust load put through a transmission during it's life and you only have to have off a couple o' thous of metal from thrust plates, gear sets and synchros for the internals to get stressed. Did you get any improvement from changing trans oil?
    mmm--- no ---
    1s't Mikuni was developed and chosen to 'piggy' back on Honda's 3 barrel ---skate in under the then current emissions constraints ---
    ???As to the rest of Your post???
    I guess 'The Mule' is an exception to the rule (retired at 394,002 miles).
    At 239.110K , 301 mile round trip at (average speed)59 MPH.

    Safe Cruising Speed, 80% over build design !!! @ 100% duty cycle

  12. #12

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    https://www.machinerylubrication.com...itives-effects
    Sulphur and phosphorus additives attack the copper in the brass.
    You want to use GL4 and below

  13. #13




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    never use GL5 oil - it will eat the brass synchro's in these trannys - use gl4 and below
    Pennyman1
    The best Dodge that Dodge never made
    Living the D-50 lifestyle since 1980

  14. #14

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    ...when my lecturer did a class on internal masses and redline variations on different engines he gave our class an example of what is actually going on inside of an engine using an old 6 cylinder 202 ci that was in production for years here in Australia (the GM/Holden 6 'red' motor... it's a pushrod dinosaur lol) anyhoo the observation was made that, as the engine reached peak rpm, each one of those pistons was generating one metric ton of mass at the end of it's stroke and if the only thing that was holding it all together was 2 bolts and a bearing (go on, somebody say "ok, what is the crank doing?"). This is a genuinely crap motor that was down on power when you compared it to, well, an Astron 2.6 (another 202 ci engine minus 2 cylinders and up on HP and made about the same torque, but used more fuel as there isn't a free meal when making power) - but was stupid reliable due to how restrictive the overall engine design was. Engineers came to a conclusion that one way to prolong the life of a combustion engine was find a balance between HP, torque and fuel consumption. The magic number seem to be around the 2,000 rpm mark for 4 stroke 4 cylinder gas engine (the start of it's torque curve) A 6 has a lower rpm starting point and as you add more cylinders, the rpm points drop accordingly. Diesels have inherently much lower rpm torque starting points due to the fuel having lousy atomisation and requiring higher compression to assist the fuel combustion cycle. Chuck a turbo in to fudge the numbers. Class dismissed!
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  15. #15

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    Thanks for all the replies, I ended up having to drive back home again late last night i guess my AAA towing expired so I couldn't get a free tow and i tried to take the top of the transmission where the shifter goes in to add some oil but my little tool set didn't have an extension so i just said heck with it and hit the road. I just skipped 2nd gear and lugged 3rd before getting up to speed and kept it at 55 on the freeway. once i got off the freeway i noticed it would go into second but as soon as i let out the clutch it would just kick it right out of gear. so i guess its time to hunt down another km145 transmission

  16. #16

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    I found an empty bottle of what i used. It is valvoline part number vv975 75w-90 gear oil. it says its compatible with differentials where gl4 and 5 are required but it then says its for all manual transmissions where gl5, mt-1 is requested which probably means my syncros are shot

  17. #17

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    i just went and took a look at a generic brand synthetic i was going to use in my tercel just out of curiosity to see what it was classified as and it is gl5 also that says it can be used in everything gl2-gl5 regardless of application which sounds like BS to me!

  18. #18

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    The oil thing is really ambiguous. I would look at the thrust plate before writing off the synchros (you'll know if the synchros are junk the minute you see them...) Gently get the tip of a decent size screwdriver in there and see how much lateral movement is in the main gear set. Also look for obvious signs of wear on the thrust plate. In Mitsubishi transmissions there are 2 types of thrust plates and one of them is made from nitrided/case hardened steel.
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  19. #19

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    ok ill have to try and check it out tomorrow, is that something that can be checked by draining the oil and dropping the square plate off the trans? ive never been inside a transmission before so this is all new to me.

  20. #20

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    I know from personal experience on the 2WD transmissions it's not that terrifying to look at the overall condition of the gear sets and synchros without pulling it all apart. Do it somewhere that will allow you to drain the oil without dumping it everywhere, not have dust or debris blow up off the ground into the trans case and maybe have a can or 2 of engine degreaser, an air compressor with a blow down gun and an inspection light so you can get a better look at it's innards. I would also get a sheet of gasket cork and a couple of hole punches so you can make a new sump gasket before taking the sump cover off just in case the existing (or in some cases non-existing) gasket is shot.
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