Pondering differential switch - PeachParts Mercedes-Benz Forum

guyonabuffalo · #1 03-09-2016, 02:47 AM

This question is specifically for anyone that has swapped a 3.07 diff from a 1980+ 300D turbo onto an earlier Non-Turbo 300D.

-Has anyone actually tried this?

--If so, whats the effect/outcome in terms of acceleration, top speed and day to day driving in both traffic and highways?

-Is this a terrible idea? Should I shut up immediately and go to sleep?

My car is a 1978 non turbo; therefore it has the older transmission with only shift control adjustment rods and no vacuum. The gear ratios are also different for gears 1,2,3 compared to the post 1983 non turbos.

I drive in the city quite a bit during the week with occasional highway runs so being able to move around with ease in city traffic is crucial; however a slightly lower revving car on the highway and modest fuel economy boost does seem quite enticing.

Will this ruin the car?

On a slightly different hypothetical note, would any of you consider putting in a more modern 5 or 6 speed automatic transmission that can be programmed to fit the engines personality? (eg. change gears accordingly to maximize engine torque usage etc.)

Your thoughts and or factual wisdom is greatly appreciated.

vstech · #2 03-09-2016, 10:43 AM

I think you will be ok with this plan. going from 3.46 to 3.07 isn't huge, and first gear on the na's is steep, so you'll still have decent off the line acceleration (as long as you still floor it off the line to engage 1st) 2nd to 3rd might be an issue, as will 3rd to 4th on the highway... when I drove my 79, as long as I was up to speed, it was no issue, but if for any reason on a hill I had to drop speed, recovering it was taxing. mostly due to not being able to drop to 3rd above 45mph... perhaps with the taller diff, dropping to 3rd may work ok. if that's the case, it may be fantastic for the highway. it'll certainly give you better economy cruising on the freeway, but don't expect higher speeds... the 80ish HP you likely have will limit that.

as for a modern transmission... unless you have mad machine skills... it's not going to happen. the starter is on the wrong side for any newer mb trans. your best bet is a manual 5 speed ($$$) and rare...

t walgamuth · #3 03-09-2016, 10:45 AM

I did that. Its fine but taking off headed up hill is a little sketchy.
Cruising speed is much reduced in engine noise.

charmalu · #4 03-09-2016, 01:45 PM

Going to a 3:07 Differential from a 3:46 is around a 12% difference.
You may see a 2 mpg increase. City driving you should be ok, but hills...well the folks behind you will just have to chill out.

5-spd manual? good luck finding one. Best bet is swapping in a 4-spd out of a 240D if you go with the 3:07..
You basically have a 240D with one extra cylinder, so that gives you about 17 HP advantage.

Charlie

guyonabuffalo · #5 03-09-2016, 06:04 PM

Thanks for all the great advice.

vstech - I wish a 5 speed manual would fall from the sky. Maybe theres a retired w123 driving producer out there who will trade me their 5 speed transmission for some audio work.

t walgamuth - Unfortunately I live in a city with numerous hill streets and sections of highway that are relatively steep for our old cars.

Charlie - I ve been keeping an eye out for a 4 speed or a car with one that I can try and test out. I did find a few 4 speeds that have been removed already, if the price was right I would consider one but not being able to test one out before buying just feels wrong. Greazzer suggested the same thing that a 4 speed from a 240D and the 3.07 diff would be worthwhile, otherwise things would get too sluggish.

I have nursed my car back to what I think is a respectable standard; however I wish rolling roads / dyno tests were cheaper and I could really find out how many of my arthritic 79 horses are still alive; the result would most likely remove any of these ideas from my head.

Thanks everyone. I will think about it some more and fantasize etc.

t walgamuth · #6 03-09-2016, 06:45 PM

At least you are not in Frisco.

Prolly ok in LA from what I saw of it.

mannys9130 · #7 03-09-2016, 07:42 PM

Mölyapina had a bunch of 5 speed 717.400 transmissions. You might PM him and ask if everything is sold.

123mike · #8 03-10-2016, 06:05 AM

I would not do it. Going from slow to slower sounds dull and dangerous. I often forget which car I'm driving and think my 300d is the turbo Saab and pull into fast traffic. It scares me when I forget to drop it in S so I can accelerate . Having cars on my bumper stresses me out.

t walgamuth · #9 03-10-2016, 06:30 AM

The only place you'll notice the taller gear is on take off and the lovely quiet cruise on the highway. In between you just hold your gears to a higher speed and shift.

I ran a 307 with a euro NA 83 motor and it was lovely. It would run 100 mph in either fourth or fifth gear.

The motor was fresh though and the euros seem to have a little more power but I would not hesitate to do it with a us spec na 300 either.

We also ran a 300cd turbo with a four speed. The issue taking off was equal to the na but at speed it had more suds.

With the na five speed with the 307 we got over 40 mph on a few occasions when driving roads that kept you to about 60. With the 300 turbo and a four speed it ran in the mid to high twenties.

charmalu · #10 03-10-2016, 12:00 PM

The five speed is the same as a four speed first thru fourth gear.
The fifth gear is an over drive.

Yeah San Francisco has some hills, a good portion is more or less flat, and remember, it is at sea level.
With the heavy traffic you are not going any where fast anyway.

Here in NV we live at the 5200+ ft elevation, the Carson valley is 45 -4600ft elevation give or take.
We drive the 240D everywhere, and she does pretty good. Now I would not want to drive a 240D automatic.

I have never driven a 300D na, so don`t have any experience there.

A few days ago I got pulled over by the local Sheriff, he said I was going so slow he thought I was on something or drinking.
I told his I was going slow so I didn`t scare him and wake him up form his sound sleep.

Charlie

t walgamuth · #11 03-10-2016, 05:49 PM

Were you in the 240D?

A 300 na drives like a 240 but with a very noticeable boost in torque.

guyonabuffalo · #12 03-10-2016, 06:52 PM

A friend with a manual 240D drove my car and said the same thing. My 300D NA drives kind of like his 240D except a little more torque.

Thanks for the suggestion for the 5 speed manuals. I will check it out.

I think I d like a 5 speed manual more since the 5th gear would act as an overdrive hence lower engine rpms at highway speeds and I could still change gears at lower speeds to utilize better torque.

I was only really considering the diff swap because its easier to tackle as a job than a tranny switch. Cheaper too.

bigpanda16 · #13 03-10-2016, 07:56 PM

I am about to do this task on also a 78 300D, albeit turboed with extra throttle tension and a backed out ada.
I got an 84 300sd chunk last weekend and am looking forward to the cruising rpm at 80mph around Atlanta.
I can see times when going up hills at around only 41mph in top gear(auto) when this car is agonizingly slow, only able to gain 5/6 extra mph over the corse of the long hill. It is only at 5/6/7 psi of boost at those low rpm's. Once it is wound up however it pulls like a freight train.
I will swap out my 205/75/14 for 195/70/14 rear tires when I do the gear ratio change and when I fix the oil filter housing I'll turn the max rpm, torque capsule and rack limiter a wee bit to help with the lack of power.

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