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1971 250C 70mph at 4,000rpm?
Having replaced the Zenith carbs with Webers, this car runs great. One thing that remains the same is the feeling that it's reving too high (approx 70mph at 4,000rpm) on the freeway. Is this to be expected from a car with an automatic transmission built for the autobahn, or is it likely 4th gear is not engaging?
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Normal. These cars were geared pretty low and were built to rev high, all day long.
You're most likely starting off in 2nd, again, as is normal. |
Yep, normal. You can install a lower ratio diff it it really bothers you. Will lose some around town power but could gain some highway mpg's.
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Thanks. I'd like to convert to manual, but in lieu of that I am interested in a lower ratio diff. Any ideas as to what ratio I currently have, might consider changing to, & where to get one?
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I think it's a 3.69, and you can swap to a 2.88. I do believe there is one in the classifieds now. And on swapping to a manual, the ratios are nearly identical and 4th is still 1:1. 5 Speeds with an overdrive are priced their weight in gold.
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I don't like it either
Lucky for me I rarely go on highway First gear is when you tough or really steep hill |
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Nope, First gear start. Light throttle will upshift to second quickly. And, yes, it's spinning merrily along at freeway speeds. Doesn't seem to bother it any, very smooth. Jim |
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Later MB developed systems to shift back to first gear upon take-off. The transmission would stand in second, but shift back to first gear. On some models it required the accelerator pedal to be pressed deeply, on others only a slight touch would produce the shift back. MB used the bowden cable, shift rod or vacuum to shift back, depending on model/engine. |
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On this forum in the parts for sale area. Also check Benzworld as they have a strong W123 community and their diffs should work as well.
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The car in question IS a first gear start car. It does not have a torque converter, it has a fluid a coupling. The second gear roll off start you are describing became common in the eighties. Yes, while standing, the transmission could select second gear to reduce the "pull", but upon any movement, it shifts to first. Even under light throttle, it will start in first. How quickly it upshifts depends upon the amount of throttle given when accelerating. I've owned dozens of W114/W115 cars over the last 30+ years, and all of them exhibited first gear start. I didn't experience the second gear start until I bought my first early W124. Late W124s have first gear start. Jim |
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http://i185.photobucket.com/albums/x...ssionstart.jpg The W114/115 had indeed a fluid coupling until 1973. Diesels with a torque converter transmission had a second-gear start. Most 123 petrol cars had second-gear start, diesels got a first-gear start from 1980 and turbodiesels from the start in 1978. High-power models (V8s) had second-gear start. |
To return to the OP’s opening query, we can begin by calculating engine speed at 70 MPH for the car in question (’70 114C), which, if a US spec car will have a final drive ratio of 3.92:1, and tires of 76.5” circumference.
70 MPH = 6160 ft/min 6160/(76.5/12) = 966 wheel RPM, 966x3.92 = 3788 engine RPM If the car were non-US, with a 3.69 final drive, engine speed would be 3565 RPM At 4000 RPM, vehicle speed would be 74 MPH, and 79 MPH, for the two final drive ratios respectively. |
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