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captainkim

Rear diff for different engine and tranny

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Posted (edited)

I have a 1971 240z with a S50 (1995 BMW M3) engine and tranny.  My refvs are very high at cruising speeds.  Im at 4000 when going 75.  I would imagine I would have to chenge the rear diff aas its the stock one from the 240z.  Question is.....do i have to change it with a BMW rear end (that may not fit) or can i use r200 so i dont have to make major adjustments.   Im unfamiliar with all the gear ratios but I keep reading that R200 4.11 is the most common used....not exactly sure what that does, but i would like to lower my rpms to 2500 when going 75 if possible.....I believe thatll take some torque away but im hoping the engine can handle the extra oomph from the start so its not too slow.. Also not sure if it makes a difference but its a 5 speed manual

 

Can someone advise me of next steps?  I hope i dont have to get a rear diff from BMW...and if i can use a R200 .... anyone have  place to get one used in working condition?

 

thanks in advance

Edited by captainkim

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Here is where to start:

What transmission are you running? How is it geared? 

What rear gear ratio do you currently have?

What size rear tires?

 

Then use this handy calculator 

http://www.tremec.com/calculadora.php

 

The simplest way to adjust cursing RPM is going to be rear gears. You will trade off acceleration, but you gears may already be too short. IDK. 

I believe factory ratios in R180 (and R200 as well) go as numerically low as 3.30ish. 

Here is a list:

 

If you are able to pull of a motor swap, I am certain you can figure this out. 

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Posted (edited)

Get the 6 speed out of a Z4. It bolts up to the S50 (you'll probably need a different slave cylinder to make sure everything works and more than likely have to modify the shifter arm).

 

But right now, you have no overdrive. its' 1:1 gear ration in 5th for the M3 transmission you're using. BMW got around this by running a 3.15 diff in the back of the E36 for the manual versions. You're probably much higher than that because that is low. The 6 speed out of a Z4/330ZHP/a buncha other BMW's (ZF GS6-37BZ is the transmission)

 

While this thread isn't 100% what you'll need to do, it'll outline the basics of what  you'll need and issues that you may come across.

https://forum.e46fanatics.com/showthread.php?t=790149

 

With the ZF GS6-37BZ final drive of .85:1 and rear diff with 3.545 (Subaru made an R180 with this ratio on some years of STI)  you'd be running about 2650 rpms at 65mph.

Edited by Neverdone

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Posted (edited)

Alright I took a long time to type this, and a few people have responded since I started, so take that into account if this seems redundant (and maybe a bit long-winded)

 

 

 

Why do you want to be at 2500 rpm? What's wrong with cruising at 4000? Where was the motor designed to be at those speeds?

 

R200 vs R180 is about the size of the diff, not the ratios of the gears inside.

 

A 4.11 diff is a great option if you want to cruise at 5000 rpm instead of the 4000 you're at right now, but it sounds like you want to go the other way. Your car currently should have a 3.36 rear, very close to where you probably want to be.

 

I've done a bit of research (tell me if any of this seems off) on your donor car and these are the transmission and diff ratios I came up with

 

1st: 4.20

2nd: 2.49

3rd: 1.66

4th: 1.24

5th: 1.00

 

Final Drive: 3.15 (this is the rear diff ratio)

 

The gears themselves are a bit interesting, since there is no overdrive gear (less than 1.0) making your 5th gear more equivalent to most transmissions' 4th gear. Great for performance, not so great for keeping RPM low.

 

We can calculate the wheel rpm by taking engine RPM divided by all the subsequent gears, so 4000/(1*3.36)  1190

Multiply that by your tire circumference (pi*diameter, I'll assume 24.7" for factory spec) and convert from rotations/min to miles/hour

 

1190 rotations * 24.7 in * pi * 60 min * ft * mi

------------------------------------------------------------------       =   87.44 mph

   min * rotation * hour * 12 in * 5280 ft

 

(units included for clarity)

 

In this case, this doesn't match the number you've given, so one of my assumptions about your car was wrong.

 

In order to really know what gear you want (or if you need to change at all), you need to solidify your expectations. You can work this equation backwards and solve for a specific gear ratio or rpm, or you can use one of many gear calculators that can be found online.

 

Remember there are multiple gear reductions in your driveline, and some are easier to change than others. Using larger diameter tires is an easy way to lower your cruising rpm with minimal investment. Otherwise, you're looking for a slightly higher diff (smaller number), though I'm not aware of any gears higher than the one you have.

Edited by ZHoob2004

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Thank you all for your responses.  I will have to go back and take a look at the specs.   ZHoob....you were close in your calculation...i mistyped....at 4000 RPM I was going 80MPH.  Im trying to lower RPM as my car tends to reach the boiling temp and at times i have to slow down to cool off the engine.  Gas saving is secondary but also a plus.  There seems to be many options but i think we all agree that 5th gear really doesnt do much but make me shift...I have 14 " wheels....I need to change them anyway so ill take that route first....Im guessing  i keep the tire same for width and  depth and just the diameter should be changed (unless it starts to rub).

 

Thanks again everyone....i hope the wheel change will be enough...Im also going to try the royal purple coolant..supposed to drop temp up to 25 degrees

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28 minutes ago, captainkim said:

its not overheating per say, but unless the gauge is wrong, im nearing 250 degrees quite often

 

Yes.  Make sure your thermostat is working, your radiator is in good condition and your radiator and reservoir are full.  It's easier to keep the engine cool at speed due to the air flowing through the radiator.  If you have a coolant flow issue, it will tend to overheat when your engine is working, at speed.

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Cruising near 4000 isn't necessarily a bad thing depending on where your motor is designed to operate. My daily Honda cruises 4200+ at around 75, but is happy to go much higher so I don't worry about it (besides the noise).

 

I ran a few more numbers on your donor vs your car and the 240z has slightly taller tires than the m3 that almost completely cancel out the difference in final drive ratios. At 75mph, the M3 should have been turning 3250 rpm, while with a 3.36 rear end and 24.7" tires that number rises slightly to 3460, hardly a remarkable difference.

 

As was said by others, engine cooling should be performing at its best while cruising on the highway, so something isn't right if you're not sitting at thermostat temperature. What have you got as far as fans go? Could they actually be blocking the airflow, instead of helping? (air needs a way to bypass the fan because it will want to flow faster than your fan can pull it.) Check for air bubbles in the system. Verify your thermostat opening with hot water and a thermometer. Verify your gauge.

 

You shouldn't need anything other than standard coolant to maintain your temperatures. If you do, something in the system is inadequate for the heat load you are producing, or the system is operating inefficiently because of other problems.

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Might want to verify your rear end ratio before you go changing things. Stock 240Z rear end is 3.36, going to a 4:11 is only going to make the problem worse. If you do indeed have the stock 3.36 rear end, I would look at changing the transmission, since a taller gear isn't going to be easy to find. Sort of a common problem for 24V swaps into E30s as well, since the stock rear end usually ends up being geared too low.

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I ended up changing the thermostat and the waterpump and now the temp is within reason.   Thanks again for the input.   Now i have to deal with the dreaded clunk in the rear.   

 

Question...off topic...Im on the fence of moving forward with this build as it will end up costing more $$.   Im thinking getting rid of this and starting from a purist side with original matching engine and hopefully a rustfree car which is also$$.   At the end of the day, I may end up spending the same amount but who knows how long the s50 will last me and then id be stuck as im clueless to the S50 engine.  Thoughts?  suggestions?  Really need the groups advice.

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Regarding the clunk, do you know if you have an upgraded diff mount in the rear? I put in a Ron Tyler style mount (from Technoversions) as a replacement for the factory strap and it almost completely eliminated the clunk with no noticeable change in noise and vibration otherwise.

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How long will an L24 last you? If you're an expert in those engines, and that's what you want, then I'd sell what you have and go that route. 

 

But if you aren't an expert in either engine, then I'd do some research on what you have now. Learn how to replace Vanos seals, learn how to replace the plastic thermostat housing with a metal one, learn how to replace your water pump with one that has a metal impeller instead of the shitty plastic one that some early E36's came with, learn how to replace your mechnical fan with an electric one so that the nut that holds it on doesn't come loose and explode.

 

Other those those failure points, it's pretty similar to any other engine. You need to change the oil. You need to make sure its gaskets aren't leaking oil. There's nothing magical about it, or nothing that can't be learned with a good service manual and reading up more on the failure points I brought up earlier. I guarantee you without even looking, there's at least 3 youtube videos of guys replacing every single one of the things I brought up that basically holds your hand all the way through it.

 

While BMW parts aren't nearly as cheap as Toyota Camry parts, they're probably on par on cheaper than 50 year old Datsun parts.

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I ended up changing the thermostat and the waterpump and now the temp is within reason.   Thanks again for the input.   Now i have to deal with the dreaded clunk in the rear.   I did get the metal housing and the metal impeller....mechanical is not the problem.....its the wiring im afraid of.   Im clueless in both for the wire and electricity.   More so on this since I didnt set it up 

 

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Even a stock 240z has wires and electrical systems.

 

You're just trading an engine ECU for balancing carbs...which involves casting black magic spells to make work 100% correctly.

 

Also, post pics of your engine bay. There's not exactly a lot of BMW powered Z's running around.

 

The rear end clunk is solved here. It replaces the strap that goes over the top of the front end of your diff. 

https://www.technoversions.com/DiffMountHome.html

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Clunked has been fixed.  The bolts connecting the diff bolts connecting to the bar was hand loose....Ive stripped the inside and rustproofed the entire interior.   The PO used sound deadener to mask some of the rust.  Had to sand it all out and seal it.  Waiting for the new carpet and liners to install.

 

Car runs strong...and Im in the process of trying to restore but my hearts not into it....I think I may just buy one that is stock and work on that.   Ill put the interior in when I get it and then sell it for someone else to care  for it.  Still on the fence

IMG_7330.JPG

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