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  • gear box and final drive q's

    hey guys! I'm Josh and new to the forums
    I just went and had a look at an 87 diamond schwarz M3 the other day. Im in New Zealand, and this one was recently imported. stupidly i didnt ask where from, but i think its euro spec. it has kms on the dash, but had the standard H pattern gearbox instead of the dogleg pattern... ?

    looked in nice overall condition, but was a bit let down when i took it for a drive. knowing it wouldnt be 'fast' by todays standards, I was still really quite let down by its seemingly poor responsiveness.
    It only has around 60k miles on the clock. The engine seemed tight and reasonably rev-happy while driving in the lower gears (though not as much as i'd expected), but was very much 'non sportscar-like' above 75km/h - much much less responsive than my regular car!
    i think the problem may lie in the gearing
    what revs should the engine be doing at 100km/h (60mph) in fourth or 5th?
    This one was revving at below 3000 at 100, in fourth! i only briefly tried it in 5th but found the engine was revving too low to not need me to put my foot flat to go anywhere!

    Is it true that those with the H pattern gearbox (same ratios as the 325, yes?) were fitted with a shorter final drive to compensate for the taller gearing?
    do you think it's possible that someone replaced the gearbox or gearbox internals with those from the 325 (cheaper?) or changed the diff somewhere along the line, making them mis-matched? or are those revs at that speed normal?

    sorry for waffling on, but your help would be greatly appreciated- the car was in really nice condition otherwise!
    cheers
    Josh

  • #2
    3000 RPM at 100 MPH in fourth gear is not normal. I can only speak for experience in a US configured car (stock 4.10 final drive), but in fourth gear I'm at about 125 MPH around 7500 RPM. Unless I'm having a brain cramp, that would put you at about 6000 RPM at 100 MPH.

    Now I do know that the euro cars have both a different transmission and a different final drive, but I believe the combination of the two puts you with similar output ratios... You could do some searching on the site to find out more on how the euro/US trans/diff ratios compare, but I'm sure someone else with a euro car will probably chime in here as well.

    Good luck!
    Dominic

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    • #3
      Yea sounds like a BIG gearing problem. At 80mph in 5th I'm at like 4500 I think....I haven't driven it in a while.

      The euro setup is supposed to be a dogleg box with a 1:1 5th gear and a 3.73 diff. The US setup is H-pattern with an over driven 5th (something like .83) and a 4:10 diff. It sounds like you have the overdriven H-pattern box with the taller 3.73 diff.
      "It is needless to say that self-propelling vehicles, like other machines, will never do as much for one who does not understand them as for one who does."

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      • #4
        That is crazy low... check the tag on the differential and if possible the transmission. It sounds more like a 2.93 diff. the 3.73 only yields @ 9% difference in rpm drop.
        Rich!

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        • #5
          The original post observed <3000rpm in 4th at 100 km/h, not 100 mph.

          Using the factory tire diameter . . .

          (US) A gear ratio of 0.83 and an axle ratio of 4.10 yields 3953rpm @ 80 mph (130 km/h), and 4942rpm @ 100mph (161 km/h).

          (EURO) A gear ratio of 1.00 and an axle ratio of 3.73 yields 4333rpm @ 80 mph (130km/h) and 5417rpm @ 100mph (161 km/h).

          The original post says that 100km/h (60 mph) occurred at less than 3000rpm with the close ratio box in the car he drove. If someone knows the 4th gear ratio of a close ratio box, we can guess the axle ratio of the car.
          Last edited by Ted B; 12-26-2007, 06:48 AM.
          2003 Mitsu EVO VIII - 2.0L / 600+whp
          1988 BMW M3 turbo - Work in progress. . .
          1986 SVO Mustang - Work in progress. . .

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          • #6
            It sounds normal at 100km/h or 62 mph. I felt the same, when I first bought mine. A tune up and finding out that she hates to run below 4000 rpm really helps out. With a chip, she will wind up to 8000 rpm's smoothly. When my car is warmed up, I never drive below 4000 rpm now. But with a euro diff, she may be a little slower.

            my two cents.

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            • #7
              FWIW, at 100km/h (62 mph), a car with a 3.73 axle ratio would see 3358 rpm with a gear ratio of 1.00.

              Since 5th is 1.00 with a close ratio box, I don't see how <3000 rpm could be observed at 100km/h (62 mph) while in 4th gear. . . unless the diff or diff gears have been changed.
              2003 Mitsu EVO VIII - 2.0L / 600+whp
              1988 BMW M3 turbo - Work in progress. . .
              1986 SVO Mustang - Work in progress. . .

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              • #8
                If it has the US Getrag 265, then the diff must be a 3.6x yielding 60 mph @ 3000 rpm.
                Anders

                "Objects in mirror are losing..."

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                • #9
                  Actually, it would be more like 3.25, IF 4th gear is 1.00 (?).
                  2003 Mitsu EVO VIII - 2.0L / 600+whp
                  1988 BMW M3 turbo - Work in progress. . .
                  1986 SVO Mustang - Work in progress. . .

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                  • #10
                    My old '87 euro had the US spec tranny in it with the 3.25 LSD when I first bought it. It sounds like this is the case here. The US spec Getrag should be matched to the 4.10 LSD and the CR Getrag to the 3.25 LSD in stock form. My track car is has the US spec tranny matched to a 4.45 LSD....and is a lot of fun!

                    Damon in STL
                    Damon in STL
                    '99 e39 540 Sport 6spd (DD), '88 e30 M3 - GTS2 #72 - Motorcraft Ignition, Volvo Injectors, Thrush Turbo Muffler, Open Source ECU, Aerospace Connectors, Lowes' Polycarbonate, Alumacore Front Splitter and Rear Diffuser, Racer's Tape.

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                    • #11
                      Originally posted by Ted B View Post
                      The original post observed <3000rpm in 4th at 100 km/h, not 100 mph.
                      Oops! I did indeed have a brain cramp... Thanks for pointing that out.
                      Dominic

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                      • #12
                        Wow, thanks for the great responses

                        ill try and get another look at it in the next few days. where do i find the tags or serial numbers on the gearbox and diff to find out which ones it has? please tell me i can do so without taking too much apart - he wasnt too enthused when i asked to lift the carpets to look for rust... :P
                        What numbers should i expect to see if they are the original items?
                        Thanks for all your help guys

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