Will a 5 speed (W108 / W113) manual transmission fit in a 4 speed W108? - Page 2 - Mercedes-Benz Forum

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post #11 of 15 (permalink) Old 06-13-2013, 06:34 AM
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Let's get something straight about this whole "5 speed lowers the RPM" theory.

Yes, it WILL lower the RPM's but ONLY if the final gear ratio is lower than the stock transmission (usually already at 1-1). If the final ratio is still at 1-1 then all you've accomplished is having more gears to shift through. While this is an improvement on "performance" cars it does not mean the same in a cruiser unless you're pedal to the metal through the "twisties" constantly.

Before you put in the added gear, check to see what the final TRANSMISSION ratio is going to be, Anything less than a 1-1 will give you a gain (.97-1 etc.). This is what is considered an overdrive gear and comes with its own problems. Lack of usable power in mid range and the need for keeping the RPM's up before shifting. This results in you doing your "in-town" driving in 3rd or 4th anyway.

A slightly taller differential ratio will probably be more beneficial.

Last edited by Mike D; 06-13-2013 at 06:36 AM.
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post #12 of 15 (permalink) Old 06-13-2013, 06:36 AM Thread Starter
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Yeah. This is what I've been reading most places. Anyways if the engine is fine with cruising at 50 mp/h at 4000 RPM, then this is fine. I was purely afraid of damaging the engines lifespan (today) by running it at those RPM's and not being able to do a lot about it. I love the sound and all, it was purely mechanical.
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post #13 of 15 (permalink) Old 06-14-2013, 12:05 PM
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Hi jackpody. I am currently in the process of doing what you are contemplating. I have a 1967 Fintail, 2.3L straight six engine, with a 4.1:1 differential ratio, and 4 speed gearbox. First gear is almost useless, and the engine seems to over rev at highway speeds.

I have purchased an entire rear end from a 1972 W108 that had a 4.5L engine. The ratio on this diff is 3.27:1. There was a W108 L6 car (similar to yours I think) at the junk yard, and both the L6 and V8 W108 rear ends looked exactly the same.

The external differences I see between the W108 and W110 rear ends are:
(1) The W110 has drum brakes only, while the W108 has discs and emergency drum brakes.
(2) The W110 has a horizontal spring above the diff while the W108 has a horizontal shock absorber.
(3) the W108 has longer rubber brake lines.

I am not sure if the hand brake cables are the same between the W108 and the W110. Also, the W108 discs require the use of 14" rims.

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post #14 of 15 (permalink) Old 06-14-2013, 12:41 PM
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Note Coolvibes that you will have to do something for your rear brakes to work properly. A drum brake system retains some residual pressure in the rear lines which, when hooked up to discs, will see them slightly on all the time. So you need to look in the line to the rear for a valve that retains this pressure. Remove it. If there is no valve evident, you may have to look in the master cylinder, or change the M/C.
Handbrake cables should be fine. They are different on air suspension rear ends like the 6.3
Cheers, Drew

Last edited by drew56cus; 06-14-2013 at 12:42 PM. Reason: spelling
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post #15 of 15 (permalink) Old 06-21-2013, 03:28 AM
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A 280SE with the normal european axle ratio of 3,92 has 3300 RPM at 60 mp/h in fourth gear. This is no problem for the M130, then he has the highest torque at 4250 RPM.

If you find a 5-gear you need also a other propeller shaft an different switching rods. And you will never find spare parts for this gearbox.

A better way is to change the rear axle. I'll put a 3,46 from a W108 3,5 in my coupé. It reduces the RPM by 11 percent. The reason for the change was not the anxiety for a motor damage, but the noise reduction, because here in Germany I often drive 100 MPH.

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