I am ready to place an order to Quaife for a custom rear diff with 40 % Lock up.........
I have a 2000 M Coupe with a Twin Screw SC.......something has to be done and done today.

The big question that I am wrestling with is what ratio????

Car has stock 3.23. With the Twin Screw,,,,,,,,1,2 and 3 are way short. Fifth is also annoying on the Highway.hitting close to what.........3000 RPMS at 65 MPH

I have used the 368s.com RPM tool.........most interesting thing is to see what effect Plus 1 and 2 size tires have on gearing and wheell speed!

Doing to 3.15 seems like too small a jump to me.,,,buy going from 17's to 18's I am right back where I would have started witha 3.15.

The AA Tuning cars all run 2.79..now those cars have a ton of RWHP.............something like 600 rwhp.......so i can see the need.

I am looking for comments and feedback.........throw it out there if it adds value to the thread...........

I am leaning toward a 2.90 to 3.00 custom quaife diff.

My objectives are:

1) To be able to use the low end torgue of the Twin Screw
2) To drop highway RPM's in fifth
3) Reduce throws from zero to 100
4) Improve higher end accleration......zero to 60 is less important to me than 40 to 140

Anyone have any experience with longer diff ratio for 5 speed...keep in mind car puts out over 350 rwhp ..and over 285 ft lbs of torgue with most of it as low as 2500 rpm's.

Thanks for reading and your comments....

Wow, 2.90 is really low. You would have some top speed of like 180mph in 5th. I would go with something around 2.93 or that.

I use 427 on my M Coupe. The only way to improve acceleration is to have shorter gears, not longer gears. If you think 323 is too short in 5th then maybe you need a totally different setup.

Didn't the the M coupe come with a 3.15 lsd stock?

Yes

Yes

3.15 was only on the s54 engine. 3.23 was used on the s52 engine.

I believe you're right. And 315 was on the S50B32 engine.

Why not spend the money for some race logic traction control instead of a Quaiffe. A 2.93 with a TS cranking 12 PSI might be about perfect. Since you would have LOADS or torque.

Race Logic is my next mod. I am waiting to get a harness made to make the installation easier

im using a 2.93 now and love it. at highway speeds it pulls forever!

Why not spend the money for some race logic traction control instead of a Quaiffe. A 2.93 with a TS cranking 12 PSI might be about perfect. Since you would have LOADS or torque.

I forgot about the race logic.......with the TS set up I no longer have the DSC or Cruise Control....Cruise control I know I will miss.....the DSC was crap.

Mr Blonde...wow a 427- you must have changed all your gears as well to work with that??? or we something is not translating for me. Maybe the gearbox for Australia is different than US?

For sure the 3.23 is as short as I will go...

CDCJON551- Glad to hear you liek the 2.93.have you noticed a significant change in 1, 2 and 3rd gear acceleration ( for the worse?) You have about 33 to 40% more RWHP than me.maybe you can pul that gear more easily?

I'll google race logic and see what thats all about...maybe a 3.00 with Race logic would be perfect.

TS- comes with 7 lbs of boost....Unless Eurosport decides to make a smaller drive pulley available. I will not mess with it. Car runs to nicely .I need the reliability for track.

Thanks for comments......

Your car really doesn't make enough power to justify those long gears. The guy with a 2.93 has over 200 ft-lbs more tq than you, which is massive.

I'd say go with a 3.15 and learn to control the throttle. Go any longer than that and you will slow the car down.

I've got more torque on my Nissan and shorter gears and want shorter gears if anything. Like Kenny said - shorter gears let you accelerate faster.

Your car really doesn't make enough power to justify those long gears. The guy with a 2.93 has over 200 ft-lbs more tq than you, which is massive.

I'd say go with a 3.15 and learn to control the throttle. Go any longer than that and you will slow the car down.

I agree, stick with 3.15.

"shorter gears let you accelerate faster"

This is not necessarily true, you need to have the right overall gearing that complements your individual set up/need. :)

I agree, stick with 3.15.

"shorter gears let you accelerate faster"

This is not necessarily true, you need to have the right overall gearing that complements your individual set up/need. :)

If you can still grip going to shorter gears then you'll likely be faster. Of course there is a practical limit, but the statement is pretty much true.

I've never driven a FI E36 chassis, but I really don't see 285 ft-lbs of TQ being a problem with the stock gearing.

..
Mr Blonde...wow a 427- you must have changed all your gears as well to work with that??? or we something is not translating for me. Maybe the gearbox for Australia is different than US?

For sure the 3.23 is as short as I will go...
..

As far as I know my English translates fairly well. All M Coupes in the world used the same gearbox, even for the US market.

The 427 was my favourite ratio, but I must admit I didn't try the 445 which I suspect might be even better. Currently we run 391 gears, but only because the car won't keep the rubber underneath it with 427s.

There are a large number of choices between 273 and 445 that you should look into, always figuring the different wheel/tyre choices and your engine's redline to determine your speeds in gears. Then you can match your R&P ratio with the intended purpose of the car.

Regarding Racelogic or any other traction control, it can only optimse the traction you have, not create any more traction. I cannot see you having any traction problems with 323 gears.

Although I appreciate your comments- ( except for the learn throtle control- that s bit obnoxious ) Maybe we are not talking about or looking for the same things from our cars.

Longer gearing allows a FI car to use the boost to accelerate. I don't know about 1/4mi cars..and would have no idea what it takes to make a fast car faster in the 1/4 mi. Sure you can gear a car to accelerate faster ..but always trade offs.

My car is a road circuit car that gets taken out occassionally for bonzai mountain runs. I need long gears for top end and being able to carry speed in and out of corners. If I put a 3.46 in my rear with the current gearing - car would be fast to 100 and fall on its nose at 130mph.

The S 52 engine with the M coupe achieves optimum shift point at around 6200 RPM. By adding more teeth to the diff..you reduce top end and essentially rob Peter to pay Paul.

If I was running the S 54 with the E 46 6 Speed.I would agree that shorter can work...and it does. An 8000 RPM redline coupled with long 5th and 6th gears lets the driver maximze torgue to suit his needs. The S 52 is not so flexible.

I bet these high HP cars would be even faster with longer diff ratios assuming stock BMW transmissions.

Turbos also benefit from longer gears as the turbo works well with increased load on the motor.

My M Coupe comes with a 3.23. I went Plus 1 in wheel and tire to 255-30-18's for the street. That makes the car think it has a 3.15 diff. And that is still too short. I am banging rev limiter in 1st and 2nd as fast as I can pull them.

I still think longer is the way to go. Maybe 3.00 if not 2.93. I do agree that 2.79 is too long.

[quote=MrBlonde]As far as I know my English translates fairly well.
quote]

I am talking about a stock transmission and changing the rear diff from 3.23 to 2.93.

If I ran a 4.27 with my stock transmsission I would not go faser than 90 mph redlined in 5th.

I am not questionning your English..what I mean by lost in translation is that we have two totally different applications....and I am completely unfamilair with how your car is set up.

Is that right..you changed out the gears in your transmisision as well so you could run the 4.27? What are yor gears cut at......I can check it out on 368s.com using the RPM tool/

I am talking about a stock transmission and changing the rear diff from 3.23 to 2.93.

If I ran a 4.27 with my stock transmsission I would not go faser than 90 mph redlined in 5th.

I am not questionning your English..what I mean by lost in translation is that we have two totally different applications....and I am completely unfamilair with how your car is set up.

Is that right..you changed out the gears in your transmisision as well so you could run the 4.27? What are yor gears cut at......I can check it out on 368s.com using the RPM tool/
Not sure how I can make it any clearer, NO I DID NOT CHANGE THE TRANSMISSION GEARING.

However the S50B32 redline is 7600 RPM which might make a bit of difference.

I have 3.46 and I am thinking about going back to 3.15, good thing I still kept 3.15. :)

I had the stock 3.23 rear end in my car and went to a 3.46 with a CF supercharger. I can tell you on the road courses it was the perfect set up for my car. I was seeing top speeds increase by 10mph at the end of long straights. I really doubt there are more top speed road courses in America outside Road America and that gearing worked perfect. The added boost out of the corners was fantastic. Go on the racing forum and ask them most of the people run 3.46 gears or even 3.64. Neil has a twin screw and I think he also runs 3.46 gears. Believe me it is a sweet set up for these cars. You never use first or second on a road course anyway.

Neil has a twin screw and I think he also runs 3.46 gears. Believe me it is a sweet set up for these cars. You never use first or second on a road course anyway.

Niels has a 3.15 in his 99.

Curious if you are comparing your 10 mph increase in straightaway speed to car with CF and 3.23 to car with CF and 3.46. Could you or something else improved to attribute that increase in speed. Maybe you are carrying more mid corner and exit speeddo to improved lines or traction? Or is it the power of the supercharger? Point being..maybe you would be even faster with a taller diff?
What was your top speed at Road America before and after by the way? A 3.46 would redline 5th gear at Watkins Glen and run out at Pocono.

Ideally, it would be awsome to have a specific diff and gear box for each track. Every road circuit I race in shifter I run a different final drive ratio. Sometimes a different gear combo for qualifying and the heats.

I would be curious to see what the racer forum guys have to say........but I bet many of them who are racing do not have the horsepower that your car has...would you agree?

In the end..looking for the best compromise....

Taller diff = less torque to the wheels

If you can gear the car shorter and still reach whatever speed you need to get to(which it seems you can with the 3.23) - you generally go slower with longer gears since you'll likely suffer the disadvantages of holding a gear longer and accelerating slower the whole time holding it vs. having that very brief period where you hold that gear longer(i.e. all things being equal you have a greater chance of reaching a speed where shorter gears give you more advantages than disadvantages).

You're also confusing applying more load to the engine to get a larger turbo to spool earlier, thus usually evening out the negatives of having less torque multiplication. Your SC is engine driven, you don't need to "load" the engine up to produce boost.

Even with a 2.93 you will probably have traction issues if you just plant your foot in the lower gears. You have more power than stock, so you can't drive it like a stock car. High HP cars are supposed to be able to spin the tires in lower gears if you really goose it - that's the only way you'll have enough power to accelerate decently in higher gears. Just spend a bit of time with it and train your right foot to keep it right on the brink of spinning the rear wheels and you'll go just as fast now as you would with a longer diff since you're not going to get more traction from a diff swap... only lower your drivewheel torque.

I had 3.73 gears on my car.. and while I do feel the car felt more powerful in the first three gears with the shorter gearing... I was having major issues with traction.

So, much so that if I was on a tight, undivided two way traffic street/highway.. I was afraid to "punch it" because I was afraid I'd drift into the other lane!

Now, while that may be fun to write about.. it's detracts from MY fun... so, much so I went out and purchased a racelogic control system..

I now have the 3.15 back in the car, and this seems like the perfect gear for what I have. I still have some traction issues (esp. in the cold northeast weather right now).. but the earlier gears are at least usable right now.

Gonna put in the racelogic.. I know how some people feel about electronics.. but hell.. I do street driving.. and for that.. the racelogic and 3.15 gears should increase the fun factor for me.

I'm also thinking about running slightly wider rear tires... I have stock 245/17s out back now.. and running 255s should effectively lower the gearing some.. and should help with traction issues.. the racelogic won't give you more traction.. it just controls it..

If you can still grip going to shorter gears then you'll likely be faster. Of course there is a practical limit, but the statement is pretty much true.

"If" is a strong word here...as I said before it depends on the car's setup. Going shorter is not better always, that is why gearing is changed in racing cars depending on the set up and road course.:)

gear it to run just short of the rev limiter in 5th for the estimated fastest speed you will see on all race tracks you plan to run at

In my car I didn't change my power set up at all, I had the same power with different gearing on the same road course I always go to. The increase in speeds was definitely due to the gearing. At the exit I had more power available to me because the car was always in the sweet spot. My trap speeds at the end of the straights were right at the rev limiter at the top of 5th getting me to 150mph with the correction for tire size. I also removed 3 gear changes on the track since the car was in the right torque area with the new gearing no need to downshift. 3.46 was perfect for me.

gear it to run just short of the rev limiter in 5th for the estimated fastest speed you will see on all race tracks you plan to run at
Exactly (as long as 5th gear is 1:1).

In my car I didn't change my power set up at all, I had the same power with different gearing on the same road course I always go to. The increase in speeds was definitely due to the gearing. At the exit I had more power available to me because the car was always in the sweet spot. My trap speeds at the end of the straights were right at the rev limiter at the top of 5th getting me to 150mph with the correction for tire size. I also removed 3 gear changes on the track since the car was in the right torque area with the new gearing no need to downshift. 3.46 was perfect for me.

Great. Now add on 200 more torque and like 250 more hp on top of your set up and run the same road course and notice how soon you run out of gear and feel the need for taller gearing to hit 180 on these straights...:)

3.46 was perfect for me.

Can't dispute that logic.....

Does anyone know the difference between the Gear boxes and Diff that come stock in the E 36 M 3 and the M Z3's?

M Z3
Diff 3.23
1st 4.21
2nd 2.49
3rd 1.66
4th 1.24
5th 1

Great. Now add on 200 more torque and like 250 more hp on top of your set up and run the same road course and notice how soon you run out of gear and feel the need for taller gearing to hit 180 on these straights...:)

I hear you I don't have 600hp though. In that case you would want longer gears. Hitting 150 already scares me because the car has so little downforce on it. I wish they made rear aerodynamics for M3/4's it just doesn't exist though.

I hear you I don't have 600hp though. In that case you would want longer gears. Hitting 150 already scares me because the car has so little downforce on it. I wish they made rear aerodynamics for M3/4's it just doesn't exist though.

Doesn't the rear spoiler help at least some? or is it just for cosmetics?

I doubt it does much the surface is so small. It needs a wing on the back that is functional.

Jim,

Sounds like you need a big-ol Hooptie wing on the back of your car!

How bout we discuss that instead of centrif vs twin screw!

Also, I would think doing something to the front to reduce front end lift (splitters or other) would probably be needed...

I didn't think it was about CF vs TS it was about final drive ratios. As for front down force I agree, I will probably but the combo underpanel splitter from Max Velocity on my car. I don't think I'll be installing the rear rice wing though. Very little has been done for 4 door aerodynamics.

What are you talking about? Put on a LTW wing from a coupe.

What kind of top speed have you seen with that 2.79 and 600 plus horsepower.

South Florida has that Juicy 5 lane wide Route 95 and those Causeways were you can probably haul @$$ with little fear of getting caught..

Do you find the car unstable at speed? Digi Tec makes a great wing that can ass a ton of rear down force if you need it. Its expensive, looks liek a rice wing..but will keep the ass planted.

What kind of top speed have you seen with that 2.79 and 600 plus horsepower.

South Florida has that Juicy 5 lane wide Route 95 and those Causeways were you can probably haul @$$ with little fear of getting caught..

Do you find the car unstable at speed? Digi Tec makes a great wing that can ass a ton of rear down force if you need it. Its expensive, looks liek a rice wing..but will keep the ass planted.


I ran out of speedo a few times...car felt very solid, I have 550/700lbs springs front/rear. My worry in the Mcoupe is lifting the front at high speeds:)

What are you talking about? Put on a LTW wing from a coupe.

They don't fit, 4 door trunk it completely different.

[quote=OZ3M]I ran out of speedo a few times...car felt very solid, I have 550/700lbs springs front/rear. My worry in the Mcoupe is lifting the front at high speeds

NICE

I have driven all of the twinscrew cars that have had the install done in Utah.


If it was my car, and I was using it for what I do (daily driver, autox, track days), I would run a 3:15 for an all around gear. The 2:93 seems just a little too tall, unless it was a mostly freeway car, and the 3:45 is short enough that second comes very fast, and unless you are autoxing, and need to run in that second gear sweet spot, would make for a busy street car.