High RPM at highway speeds

Discussion in 'Engine Topic' started by Sunny Z/28, Nov 15, 2020.

  1. biker

    biker Veteran Member

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    Really interested to see what the outcome is. Great looking car. I had a 77 that was the same colour.

    Since you have a posi, it's a bit easier to check. Put car in neutral, parking brake off. Jack both wheels off the ground (support with jack stands). Crawl under car, mark driveshaft and inside of one wheel. Turn the driveshaft by hand while watching the mark on the wheel. Count how many revolutions of the driveshaft it takes to get one full rotation of the wheel.
    If it takes just under 4 driveshaft turns to get one rotation of the wheel, you are probably still 3.73. If you are over 4 driveshaft turns, somebody charged the ring and pinion at some point.
    Make sure both wheels turn when you turn the driveshaft. If not, you will have to hold one wheel with your foot to keep it still, and double the amount of driveshaft turns to get one rotation of the wheel.
     
    Last edited: Nov 18, 2020
  2. DennisG

    DennisG Member

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    Lets try it another way. If you are unhappy with engine's noise it is not difficult to change gear ratios in the rear end.

    I am very happy with rear axles having removable carriers, they come apart just ahead of the rear axle. It is just a bolt in job and you can make the change in an hour. So if you have that, just get a wrecking yard carrier with a ratio between 3.07 to 3.25 to 1 and it will be a different car [smooth, quiet, fuel efficent but with less pulling power]. It is an easy change in my 1965 Mustang. Try it and see if you like it, your speedometer gears will have to be changed with any axle ratio change.

    I recently sold my 16 ton truck which literally screamed at 62mph but it was for hauling weight and had 24 forward speeds. It depends on what you expect of it.

    I changed an 1800 lb car's ratio from 3.74 to 3.07 and was pleased with the result.
    It was not a removable carrier so I had to set it up using white pigment on the gears for proper contact as I had to change the ring gear.. The gears came out the back of the axle so you see the circle of bolts behind the axle.
     
  3. STUCKINCALI

    STUCKINCALI New Member

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    To add to what pomartin said (sage advice), take a long ziptie and wrap your drive shaft with it at the rear yoke, leaving the tail of the ziptie on as a pointer. The longer the better, it makes it easy to count. You can even do it by yourself if you can see the tail from the rear wheel as you turn it (or, if you can't and you have no help, put your phone under the car directly beneath and hit record video then say start... turn the wheel 10 times then say end... then go back and count the revolutions).

    Good luck!

    BTW, here's a link to this site's '78 Z car info from Chevrolet (build data):
    https://nastyz28.com/camaro/camaro78.html

    Just wondering what trans you have in it... Says a Saginaw 4-speed and I would be interested to see if someone, while rebuilding it, maybe put the incorrect speedo gearset inside.

    Are you using the stock/factory dash tach to get your RPMs? Might want to run a tach/dwell tool on the ignition to see if it matches the tach in the dash if so.

    Best,

    ~Andy
     
  4. Rosster

    Rosster Veteran Member Gold Member

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    unfortunately, these cars don’t Use 3rd member Like ford’s 8” and 9” to do A gear change out is way more complicated. Installing a replacement using a 9” diff is $2500 which is way more than a gear swap but a hell of a lot stronger. That being said. You could likely find a used complete axle in a taller ratio for $500 As the GM 10 bolts are plentiful. But, my money’s on a bad tach.
     
  5. third_edition

    third_edition Member Lifetime Gold Member

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    Hi Sunny,

    Look at the numbers and then get a table or some software to determine what rpm you should be at for the speed you are going...
    1978 Z28 (stock) would have:
    - 185HP small block (max. HP @4000 rpm) - you could rev it higher but to "diminishing returns" so to speak.
    - 4 spd. close ratio transmission: 1 gear= 2.64:1; 2 gear= 1.75:1; 3 gear= 1.34:1; 4 gear= 1.0:1
    - lowest ratio gear set available (PZ code) = 3.73:1
    - Stock Tire (F60 - 15) 225/60R15 = diameter 25.65"

    Yours may be different, but it is easy to plug different numbers into the calculation as follows...

    ((1/rear end gear ratio) x (3.14 x tire diameter) x ((rpm / trans ratio) x 60))) / 63360 = speed (mph)

    Eg. at max rpm in 1st gear you would be going... ((1/3.73)x(3.14x25.65)x((4000/2.64)x60))/63360 = app. 31mph
    at max rpm in 4th gear you would be going... ((1/3.73)x(3.14x25.65)x((4000/1)x60))/63360 = app. 81mph
    at 6000 rpm in 4th gear you would be going... ((1/3.73)x(3.14x25.65)x((6000/1)x60))/63360 = app. 122mph

    Logically, look at the variables: the rear end is a fixed value (3.73) and cannot change, the tire size is a fixed value and cannot change (25.65" diameter), the transmission ratios are fixed and cannot change. Engine speed changes, but if you plug it into the equation, what you read on your tach should equate to what you read on your speedo.

    The only weakness in all of the above would be the clutch. If the clutch were not firmly attached to the engine (slipping), you could run into a new variable which "may" (and I mean MAY as I have not seen your car) cause you to rev higher than the rpm being transferred to the gearbox.

    As it happens, most clutch slippage happens in high gear at higher rpm as the mechanical advantage of the driveline is under its highest load.

    If this situation has begun recently and you are noticing it more and more, a worn clutch could be the problem.

    Good Luck
    George
     
  6. mrluckies

    mrluckies Veteran Member Lifetime Gold Member

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    I highly doubt your cruising 5000 rpm on that engine. Especially if it is mostly stock as the valvetrain and heads would likely cause the engine to be done pulling. And you definitely would not be able to go much faster than that, it would be maxed out.

    Since you said that your gps has verified your speedometer (and you haven’t gotten crazy speeding tickets), most likely the tach is wrong.

    I’m not familiar with the factory tach, but most aftermarket tachs are set up to work on 4, 6 and 8 cylinder engines. You need to move a switch or connect a wire differently depending on which engine you have. Based on what everyone else is saying that it’s double as high, I’ll bet it’s currently set up for a 4 cyl (half as many ignition pulses in one revolution) Or if someone replaced the factory tach in your dash, maybe they put the wrong one in.

    What ignition are you running? If you have a msd box or other aftermarket, some of those need the tach hooked up to a different wire, not the coil, to work correctly.

    I agree with everyone that I would jack up the car and count the revolutions to determine what gear you have, just so you know, but it’s highly unlikely that’s the issue.

    also, my BBC with a Muncie and 3.42’s tachs about 3000 rpm at 70 mph with my tire size. And yes, while cruising, it is loud and shaking. While accelerating, it feels fine, but cruising more than that speed it doesn’t like.

    eventually when I get more money I’ll get tko. Cruising the engine at elevated speeds adds extra wear and tear on the engine that it doesn’t need. It won’t kill it (or shouldn’t of everything is ok), but will shorten its life over the long hall.
     
  7. mrluckies

    mrluckies Veteran Member Lifetime Gold Member

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    Oh yeah, before I forget, see if one of your friends has a tach you can temporarily wire up in the engine compartment and have someone rev the engine in neutral to verify. Or, if you know someone with an innova timing light that has the built in tach.

    also, idling at 1000 rpm is high for a mostly stock engine and should be slowed down. If you have a big cam, you would need to be up there. I have a 228/238 @ 050 cam and idle at 700 rpm with 16 in-hg vacuum. This is another reason I think your tach reading is wrong for your engine and set up for a 4 or 6 cylinder engine.
     
  8. COPO

    COPO Veteran Member Lifetime Gold Member

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    If you had a separate Tach then these would need to be opened up to set for 4, 6 or 8 cyl.
     
  9. Postmech

    Postmech Member

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    When I bought my 78, I remember it came with G70-15 tires on special order aluminum wheels. It ran 70mph at 3500 rpm. I would run it at 100 to 110 for long periods . Did yall know that 2 reckless driving tickets (too fast) and they take your license for a while. lol I use an old Garmin to check my speed.
     
  10. COPO

    COPO Veteran Member Lifetime Gold Member

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    Yup when I ordered a new 1977 and 1978 Z28 4 speed it revved 3500 at 70 mph in 4th with stock tires and 3.73 rear. My 71 Z28 was the same with the F60-15 poly glass tires.
     

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