Mercedes W114 250 Coupe - help!

[Charles Morgan]

Plan of attack is to check the compression on the existing engine and if good, put that in (after mounting CPS on back plate). If not good, we will rebuild head I had refurbed (or use bits to help). We'll do compression check using starter motor.

Finish off suspension work (drop links and ARBs at rear), fit driveshafts, refit propshaft and box (after a small overhaul), overhaul and fit new bits to fuel system, then drop engine in when above work done. Do whatever work required to fuel rail (at machine shop).

 

[Charles Morgan]

Spent three hours on the M25 yesterday to do 35 miles (half term plus rush hour). As I was in the Alpina I was on and off the clutch for that entire time. Crikey it was hard work. I'm rapidly inclining to the idea of fitting the auto. I live in entirely the wrong place for a manual...

 

[Bellow]

Think of the longer trips and it pulling its rangey and relaxing fifth gear...

 

[Charles Morgan]

Round here a long trip is a succession of short trips from jam to jam.

I wonder if a five speed auto could be made to fit...(cue Simeon being dragged away by the men in white coats in Bicester).

 

[Bellow]

Taller diff with the 4sp auto and force it to step-off in first? (Or does the 4sp start off in first anyway?)

 

[grober]

W114-W115-250CE-Getriebe/en ? Mercedes-Benz Classic Wiki - Online-Lexikon rund um Mercedes-Benz Oldtimer

http://mbturbo.com/how-to-change-final-drive-w115/

 

[Charles Morgan]

I'm a bit alarmed to see the first link state that the rear diff ratio on the 5 speed manual is 3.92, rather than 3.69 of the 4spd (auto and manual). This would completely bugger up the point of having the top gear ratio of .87.

 

[Bellow]

Not entirely.
3.92 x 0.87 = 3.41. Some 8.2% higher than a 4sp and 3.69 diff.

edit PS.
Have you not upped the wheel diameter by one inch also? If so, that is worth in the order of 4% (assuming tyre aspect ratio remains as before).

 

[Charles Morgan]

I've adjusted the aspect ratio - but by how much I don't know. The originals were 175 14 changed to 195 65 15.

 

[grober]

see Car Bibles : The Wheel and Tyre Bible

Assuming 175/70R14 TO 195/65R15----????

gives an increase in circumference of 102.24mm or 5.64%
meaning when your car speedo[ gearbox take off ?] is reading 70mph you are doing 73.95mph

 

[Bellow]

If the 175s were 80% AR as I suspect - then the move to 15'' is negated by a 28mm reduction in sidewall height.
Still though, the overall gearing in 5th with the lower diff is higher than in 4th with the taller diff.

Can anyone say that the 4sp auto steps-off in 2nd? (Most autos with TCs do, but the MB 4sp has a fluid coupling I think).

edit PS. My figures are unchecked - if critical, I'll run them again.

 

[Charles Morgan]

I have one old wheel remaining so can measure the sidewall (when it's not pouring outside).

I honestly don't know about starting in second, the box has far less known about it than later ones.

 

[grober]

Certainly using the wheel and tyre bible's calculator
175/80R14 to 195/65R15 rEsults in a negligible difference in rolling circumference.

 

[grober]

Why 2nd Gear Start? | Mercedes-Benz Club of America

NO DEFINITIVE ANSWER I'M AFRAID

 

[Bellow]

Certainly using the wheel and tyre bible's calculator
175/80R14 to 195/65R15 rEsults in a negligible difference in rolling circumference.

0.4% Negligible, agreed.

I honestly don't know about starting in second, the box has far less known about it than later ones.

Car (magazine) may have the answer. I'll look later.
My thinking here is a higher diff (which you already have?) with a 4sp auto would give relaxed top gear performance but compromised take off. Unless the auto steps-off in 2nd but can be forced to start off in 1st using the stick.

 

[grober]

Interesting video??
[YOUTUBE HD]RfA4FWWaOCk[/YOUTUBE HD]

 

[Bellow]

Car (magazine) may have the answer. I'll look later..

In 1970 Car said that 'a recent change to start off in 1st not 2nd as previously'.

Must be down to the age of the box and its valving (double presumption).

 

[Charles Morgan]

The diff I have is just way too high - 2.47 would really need a much torquier engine to cope. I doubt acceleration would be greatly compromised with the 3.68 diff - if you look at the first 4 ratios on the 5 speed box they are pretty much the same as the 4 speed manual. WTF apart from some theological addiction to unnecessarily high revs did MB stick such a ratio diff in the 5 speed? Makes no sense otherwise.

 

[Charles Morgan]

The video - spot on, the ratio from the 450 116 is 3.06, which is just leagues better.

 

[Bellow]

The answer probably lies in what was termed tractability - much was made of accelerative ability in top gear. For some reason, back then. cars were expected to pull a single ratio with the zeal of a steam engine.
That, and BMW looming in their rearview mirror keen to steal a performance advantage. Adding a fifth ratio and closing them all up with a lower FD ratio would have helped the road test statistics which, at the time, had began to form the backbone of advertising campaigns. Adults and their number questions...