Author Topic: Rear Axle Modification (Read 15006 times)

Ed Cave · « **on:** September 01, 2004, 21:21:27 »

Someone recently suggested...

"Drive the car a bit first to see if you want a taller gear. The stock axle ratio gives 4600rpm at 80mph.. This can be changed to 3700 at 80 with a different axle."

Obviously this would make the car slower off the line but a lot more tolerable at higher speeds.

Can anyone elaborate on the cost / difficulty to achieve this and their satisfaction with a similar modification?

Ed Cave
Atlanta, GA

1964 356C
1971 280SL

TheEngineer · « **Reply #1 on:** September 01, 2004, 21:50:23 »

I put a 3.27 in my car. Makes a whole new automobile out of it: I cruise at about 3000 RPM on the freeway at 63MPH. I do not feel that the car is slower off the line, but it should be. If I really want to beat that Audi or Porsche, I start in first gear. My cost was $250 for the axle plus new bushings, maybe another 50 bucks. It's quite a bit of work and you have to be careful removing the compensator spring. But if I can do it, so can you. The attached picture shows the new axle ready to install

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RearAxle.JPG
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1969 280SL,Signal Red, A/T, P/S, A/C, Both tops. Ret. engr. 1967 230SL - Sold

TheEngineer · « **Reply #2 on:** September 02, 2004, 11:26:27 »

This car does not have a spedometer. But it has the original speedometer and it now reads 25% slow; i.e. 40MPH is really 50. On the funny side: 50MPH is 62.5MPH which is exactly 100Km/hr. So I made stickers to go on the glass in a few places, like at 30 (is 38) 50 and 60 (is 75). That takes care of our usual posted speed limits. Because of my extensive education I am also able - sometimes - to multiply the indicated speed by 1.25. It is especially easy when the speedometer indicates 100MPH

1969 280SL,Signal Red, A/T, P/S, A/C, Both tops. Ret. engr. 1967 230SL - Sold

Cees Klumper · « **Reply #3 on:** September 02, 2004, 15:36:34 »

I put a 3.69 (was 4.08) in mine. The car was slower off the line, but now that my engine has been rebuilt, it's quite peppy again. Although the RPM's went down by around 10%, the difference is less noticeable than I had hoped for. Still an improvement I would recommend.

Cees Klumper in Amsterdam
'69 white 280 SL automatic

Malc · « **Reply #4 on:** September 03, 2004, 01:29:00 »

Being cheap

Did you get the diff out of another Mercedes, scrap car or what???
Or are these diffs unique to the 230/280 range?
Thanks
Malc

Oh BTW can you get limited slip differentials for a 230??

Ed Cave · « **Reply #5 on:** September 03, 2004, 07:12:41 »

I've got the car completely apart right now, gauges out and everything. Does anyone know if the speedometer could be recalibrated (by a professional)to synchronize with a modified rear end?

If it can be done, now is the time! Also, it seems like half of the professional guage shops, repairers and restorers are in Van Nuys, CA. Anybody have a particular shop reccommendation?

Douglas · « **Reply #6 on:** September 03, 2004, 08:00:50 »

North Hollywood Speedometer is terrific. I sent a speedometer there on a Monday and got it back as-new on a Thursday.

www.nhspeedometer.com

Douglas Kim
New York, NY
280 SL #016220

Cees Klumper · « **Reply #7 on:** September 03, 2004, 10:06:42 »

The differential I had put in came from a sedan car. And yes it was cheap: as I recall something like $400 including labor to put it in, and that took an entire day.

Cees Klumper in Amsterdam
'69 white 280 SL automatic

A Dalton · « **Reply #8 on:** September 03, 2004, 10:22:19 »

The tallest you will find in swing axle will be the 3:27.
Keep a look out for 108 chassis , 280SE 4.5 only ..

The 280SE , 6 banger does not use the 3:27... they figured the v8,3:27 match would be the best for a large sedan with the added power, but you can do the 113 cuz of the lighter weight , but it is more recommended for stick vs auto...
Opionions vary... I've had both ... if you do around town/back road driving , stay stock... but if you are a cruiser , the 3:27 makes it a new car on the highway...
I think it is an American thing....
If one does the math , the cruise of the 3:27 comes close to the RPM/MPH specs of the ZF 5 speed O.D in top gear.........

66andBlue · « **Reply #9 on:** September 04, 2005, 19:55:52 »

In another topic the effects of high gas prices (and more philosophical and social aspects) are being discussed.
I was wondering whether any of you know how the gear ratio affects the gasoline consumption (in mpg or ltr/100 km, I don't care which unit we use). In theory I'd say that a lower ratio (lower rpm at same speed) should improve gas mileage. But is that true in reality?
Perhaps Theengineer could tell us whether his gas mileage got better after the exchange. Ed, have you driven your car often enough to report your's? What gear ratio did you finally settle on?

Alfred
1966 blue 230SL automatic

Ed Cave · « **Reply #10 on:** September 04, 2005, 22:35:22 »

Gas mileage / rear end question:
Probably depends on how and where you drive your car; city or highway. I did not change the rear end in my car and have not found driving at 70 - 75 mph to be annoying at all (as I thought it might be from some of what I had read).

Ed Cave
Atlanta, GA

1971 280SL
1973 911S
2004 A4 3.0
2006 GS430

Cees Klumper · « **Reply #11 on:** September 04, 2005, 23:02:14 »

I did not notice a drop in fuel consumption after going from 4.08 to 3.69.

Cees ("Case") Klumper in Amsterdam
'69 white 280 SL automatic

Tom · « **Reply #12 on:** September 05, 2005, 20:42:09 »

I went from a 4.08 to a 3.27 rear-end. Too many overall changes to isolate any gas mileage benefit from the 3.27. I can say that while cruising with the top down at 60 mph, I have to listen hard for the sound of the engine. Driving around town is not bad, and if you need the extra push, you can use the shifter (automatic).

Joe Alexander, Pete Lesler and I did some back of the envelope calculations at the Ohio tech session and determined that the 3.27 had a slight rpm advantage over the 5 speed ZF. That was after several libations, so maybe the calculations were off.

1971 280sl Tunis Beige Metallic (restored & enhanced)
1971 280sl Tobacco Brown (low mileage stock)
1970 280sl Deep Red (Project Car)

A Dalton · « **Reply #13 on:** September 05, 2005, 23:23:41 »

<<determined that the 3.27 had a slight rpm advantage over the 5 speed ZF.>>

The 3:27 would have quite the advantage on RPM.
A ZF520 in 5th gear with a diff ratio of 4:08 would equate to the same RPM as a 4 speed in top gear with a 3:46 ratio differental.

JamesL · « **Reply #14 on:** September 06, 2005, 03:16:48 »

I must have a different back end to you lot already

I figure 50rpm per mph in top gear (280, Auto). It sits at
4000rpm/80mph or
3500rpm/70mph or
3000rpm/60mph

very happily.

Only having ever driven the one car, I have no basis for comparison

mdsalemi · « **Reply #15 on:** September 06, 2005, 08:33:15 »

Mike Halleck ("Norton") is supposed to be putting a Mustang T5 transmission in his 113. It will be interesting to see if this has the intended result, and if it gives an improvement over the ZF and the effect of a shorter rear axle.

One thing I can tell you is that T5 transmissions should not be too hard to find.

Michael Salemi
1969 280SL
Signal Red w/Black Leather
Restored

Tom · « **Reply #16 on:** September 06, 2005, 09:46:15 »

Tosh,

I am at about 2800 rpm at 60 MPH. I don't hit 4000 RPM until over 90 MPH. I had my speedo recalibrated to the new rear-end and is very accurate. Sounds like you have a rear-end between the 4.08 and the 3.27 (I guess that would be the 3.46). I think Cees has a mid-range rear end in his 280sl.

1971 280sl Tunis Beige Metallic (restored & enhanced)
1971 280sl Tobacco Brown (low mileage stock)
1970 280sl Deep Red (Project Car)

norton · « **Reply #17 on:** September 06, 2005, 10:47:34 »

"One thing I can tell you is that T5 transmissions should not be too hard to find" Right you are Mike, The T5's are falling out of tree's on my side of town.

Mike Halleck
Chesterfield Mi
71 280SL
68 250SL (parts car)
94 E320 Coupe

mdsalemi · « **Reply #18 on:** September 06, 2005, 10:58:42 »

quote:
Originally posted by norton

"One thing I can tell you is that T5 transmissions should not be too hard to find" Right you are Mike, The T5's are falling out of tree's on my side of town.

Mike Halleck
Chesterfield Mi
71 280SL
68 250SL (parts car)
94 E320 Coupe

Well of course we are waiting to see your results. Don't work under a tree if they are falling out of them on the East Side.

Michael Salemi
1969 280SL
Signal Red w/Black Leather
Restored

Raymond · « **Reply #19 on:** September 09, 2005, 17:23:32 »

Mike, if you can document what you learn with photos, you will have my undying gratitude. I have been looking at T-5s and there are a several different top shifters for them. One of the combinations should be right for our cars. A T-5 is my 2006 target project.

Will you be using the MB bell housing?

Ray
'68 280SL 4-spd Coupe

rwmastel · « **Reply #20 on:** September 16, 2005, 07:52:51 »

quote:
Originally posted by A Dalton

<<determined that the 3.27 had a slight rpm advantage over the 5 speed ZF.>>

The 3:27 would have quite the advantage on RPM.
A ZF520 in 5th gear with a diff ratio of 4:08 would equate to the same RPM as a 4 speed in top gear with a 3:46 ratio differental.

Most of the original 5-speeds were 230SL/250SL and were sold in the European market, so they would probably have the 3.75 rear end, bringing it's RPM levels closed to the 4-speed with 3.27 rear end.

Rodd
Powell, Ohio, USA
1966 230SL, Euro, Auto, Leather, both tops
1994 E420

Chad · « **Reply #21 on:** September 16, 2005, 08:10:15 »

A pack of Marlboros fell out of a tree on me when I lived off Washington Square North for five years. I remember it seeming not very out of place on that particular day. Gave them to the next free spirit I played chess with at the SW corner of the park.... Never got hit by a T5, just not that lucky.

-CD-
1967 230SL, 113.042 10
1983 300TDT, 123.193

A Dalton · « **Reply #22 on:** September 16, 2005, 08:53:40 »

quote:
Originally posted by rwmastel

Most of the original 5-speeds were 230SL/250SL and were sold in the European market, so they would probably have the 3.75 rear end, bringing it's RPM levels closed to the 4-speed with 3.27 rear end.

Rodd
Powell, Ohio, USA
1966 230SL, Euro, Auto, Leather, both tops
1994 E420

5 speed Euro never came with a 3.75 differential.
A 3:27 w/4 speed would equate to the 5ZF with the 3:92 in top gear.
That is a common swap.

TR · « **Reply #23 on:** September 16, 2005, 09:47:32 »

Dalton -- Re swapping to a 3:27 w/4-spd; can you please comment on the effect this has on acceleration? My question relates to Ed's observation, "obviously that would make it slower off the line".

I'm wondering if it really is much of an issue in terms of giving up some off-the-line performance. 1st gear has always felt a bit low to me anyway...so I have my doubts, but am not sure.

BTW, we are going to do something in this arena over the coming winter...Just not yet 100% certain what that will be. But, naturally, our objective will be to have it all. Meaning strong acceleration, good closeness, as well as comfortable, quiet cruising. We'll also do our best to take the HP & torque curves into account.

Thanks.

Tom in Boise
'71 280SL 4-spd, signal red w/lt. tan interior, restored/enhanced

A Dalton · « **Reply #24 on:** September 16, 2005, 15:36:08 »

TR

Your chassis is a perfect candidate for the swap b/c you have a manual trans coupled with more HP/Torque than the .042/043 for low end.
You also already have the rear disc, so you do not have to worry about brake pressure mods.

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Author Topic: Rear Axle Modification (Read 15006 times)

Ed Cave

Malc

Ed Cave

Douglas

A Dalton

Ed Cave

A Dalton

norton

Chad

A Dalton

TR

A Dalton