E46 BMW Social Directory E46 FAQ 3-Series Discussion Forums BMW Photo Gallery BMW 3-Series Technical Information E46 Fanatics - The Ultimate BMW Resource BMW Vendors General E46 Forum The Tire Rack's Tire Wheel Forum Forced Induction Forum The Off-Topic The E46 BMW Showroom For Sale, For Trade or Wanting to Buy

Welcome to the E46Fanatics forums. E46Fanatics is the premiere website for BMW 3 series owners around the world with interactive forums, a geographical enthusiast directory, photo galleries, and technical information for BMW enthusiasts.

You are currently viewing our boards as a guest which gives you limited access to view most discussions and access our other features. By joining our free community you will have access to post topics, communicate privately with other members (PM), respond to polls, upload content and access many other special features. Registration is fast, simple and absolutely free so please, join our community today!

If you have any problems with the registration process or your account login, please contact contact us.

Go Back   E46Fanatics > E46 BMW > General E46 Forum

General E46 Forum
This is the place to get answers, opinions and everything you need related to your E46 (sedan, coupe, convertible and wagon) BMW!

Reply
 
Thread Tools Search this Thread Rate Thread Display Modes
Old 01-08-2013, 12:10 PM   #1
jay786
Registered User
 
Join Date: Jan 2013
Location: London
Posts: 4
My Ride: 318CI
Exclamation Higher Gear Ratio Diff!

Hi there,

I have a 2002 plate BMW 318CI Manual and my diff needs to be replaced. Can i install a diff with a ratio of 3.73 or higher? What are the benefits and flaws of installing a diff with a higher gear ratio?

What is the ratio for a standard 318CI?

Many Thanks
jay786 is offline   Reply With Quote
Ads by Google

Guests, get your FREE E46Fanatics.com membership to remove this ad.
Old 01-08-2013, 12:19 PM   #2
mwt330i
Registered User
 
Join Date: Nov 2011
Location: Western MA
Posts: 52
My Ride: 2004 330i 6speed
Yours may have been originally 3.45 according to bokchoys web site.....going a bit higher to the 3.73 would improve your low end acceleration but cost you a tiny bit in highway RPM's (higher rpms at speed).
mwt330i is offline   Reply With Quote
Old 01-08-2013, 12:22 PM   #3
jay786
Registered User
 
Join Date: Jan 2013
Location: London
Posts: 4
My Ride: 318CI
the 3.73 diff is from a 1 series but its the same fit ... would it cause any problems?
jay786 is offline   Reply With Quote
Old 01-08-2013, 12:39 PM   #4
white46
Registered User
 
Join Date: Feb 2006
Location: Vancouver Canada
Posts: 1,920
My Ride: e46 323ci
I don't think it's going to cause as problems as long as it's direct fit, and if you can live with higher rpm at higher speed.
__________________
white46 is offline   Reply With Quote
Old 01-08-2013, 12:46 PM   #5
jay786
Registered User
 
Join Date: Jan 2013
Location: London
Posts: 4
My Ride: 318CI
whats the problem with higher rpm at high speeds ... obviously drinks more petrol ... what else?

Thanks
jay786 is offline   Reply With Quote
Old 01-08-2013, 12:48 PM   #6
LeMansteve
Registered User
 
Join Date: Nov 2008
Location: USA - GA
Posts: 1,704
My Ride: '04 330i 6mt ZSP ZPP
Higher ratio differential

Pros:
better acceleration

Cons:
higher fuel consumption
reduced top speed
more shifting
increased wear on drivetrain components
__________________
BMW CCA member - Peachtree Chapter
LeMansteve is offline   Reply With Quote
Old 01-08-2013, 02:04 PM   #7
RayPooley
Registered User
 
Join Date: Jul 2012
Location: Liverpool
Posts: 1,440
My Ride: 320i SE Coupe
Quote:
Originally Posted by jay786 View Post
Hi there,

I have a 2002 plate BMW 318CI Manual and my diff needs to be replaced. Can i install a diff with a ratio of 3.73 or higher? What are the benefits and flaws of installing a diff with a higher gear ratio?

What is the ratio for a standard 318CI?

Many Thanks
You all seem to be ignoring an obvious constraint. Its not all about ratio. Its about power throughput and heat dissipation. For example, if you took the diff from a 1.6L and strapped it to a 3.0L I wouldn't be surprised if the thing glowed in the dark. If you have a 1.8L then pick a diff from a 1.8L or above and then consider the ratio.
RayPooley is offline   Reply With Quote
Old 01-08-2013, 03:03 PM   #8
Alex323Ci
Registered User
 
Join Date: Jan 2005
Location: Huntington Beach -Surf City USA
Posts: 17,044
My Ride: M3 & 323Ci (sold)
you should have a 3.38 ratio now. im pretty sure that's the small pumpkin diff, so you could go to the 3.91 for a jump. the 3.45 would be so slight i would not do it unless old diff was blown.
if the 3.73 fits then that would be a ~10.4% change.
the 3.91 would be ~15.7% change
__________________
OE CSL WHEELS - OE CSL BRAKES - OE AUTOFLD MIRRORS - CSL TRUNK - CSL DIFFUSER - AA TUNE - AA PULLEYS - AS 40% SSK - BILSTEIN PSS9s
CF CSL LIP - NAVI - BMWPERF CF STRUT BAR - BMW GROUP N MNTS - GRUPPE M CF INTAKE - SUPERSPRINT - UUC BRKLINES, TRANS, RTABs - H&R SWAYS - 4.10 M-SPORT DIFF
Alex323Ci is offline   Reply With Quote
Old 01-08-2013, 03:14 PM   #9
Alex323Ci
Registered User
 
Join Date: Jan 2005
Location: Huntington Beach -Surf City USA
Posts: 17,044
My Ride: M3 & 323Ci (sold)
Quote:
Originally Posted by RayPooley View Post
You all seem to be ignoring an obvious constraint. Its not all about ratio. Its about power throughput and heat dissipation. For example, if you took the diff from a 1.6L and strapped it to a 3.0L I wouldn't be surprised if the thing glowed in the dark. If you have a 1.8L then pick a diff from a 1.8L or above and then consider the ratio.
no it won't be that issue of heat dissipation. besides OP has a 318Ci's, that use 3.38 and 3.45 ratios. He's not going the other way around.

In your example of a 330 retrofitting a 318Ci diff, the issue would be they use a smaller diff and not as strong on launch. Not about heat like you espress. Race teams often use a smaller diff for less weight
__________________
OE CSL WHEELS - OE CSL BRAKES - OE AUTOFLD MIRRORS - CSL TRUNK - CSL DIFFUSER - AA TUNE - AA PULLEYS - AS 40% SSK - BILSTEIN PSS9s
CF CSL LIP - NAVI - BMWPERF CF STRUT BAR - BMW GROUP N MNTS - GRUPPE M CF INTAKE - SUPERSPRINT - UUC BRKLINES, TRANS, RTABs - H&R SWAYS - 4.10 M-SPORT DIFF
Alex323Ci is offline   Reply With Quote
Old 01-08-2013, 04:12 PM   #10
RayPooley
Registered User
 
Join Date: Jul 2012
Location: Liverpool
Posts: 1,440
My Ride: 320i SE Coupe
Quote:
Originally Posted by Alex323Ci View Post
no it won't be that issue of heat dissipation. besides OP has a 318Ci's, that use 3.38 and 3.45 ratios. He's not going the other way around.

In your example of a 330 retrofitting a 318Ci diff, the issue would be they use a smaller diff and not as strong on launch. Not about heat like you espress. Race teams often use a smaller diff for less weight
I disagree. I have seen diffs explode through overheating. Seals burst, bearings fail, oil escapes and boom. Gears shatter. Gears are just like bearings. You overload them and they will fail. Its not something you can just ignore theoretically speaking.

Last edited by RayPooley; 01-08-2013 at 04:13 PM.
RayPooley is offline   Reply With Quote
Old 01-08-2013, 04:47 PM   #11
E46 Coupe
Registered User
 
E46 Coupe's Avatar
 
Join Date: May 2008
Location: Australia
Posts: 5,019
My Ride: Rules1 & 2: S T F U!
tmk the 318 and 320 diffs have smaller output flanges...
Came across this on my search for specific ratios and a 320 diffs output flanges did not mate up to my 323.
__________________
E46 Coupe is offline   Reply With Quote
Old 01-08-2013, 06:12 PM   #12
Alex323Ci
Registered User
 
Join Date: Jan 2005
Location: Huntington Beach -Surf City USA
Posts: 17,044
My Ride: M3 & 323Ci (sold)
Quote:
Originally Posted by RayPooley View Post
I disagree. I have seen diffs explode through overheating. Seals burst, bearings fail, oil escapes and boom. Gears shatter. Gears are just like bearings. You overload them and they will fail. Its not something you can just ignore theoretically speaking.
yes diffs do fail, just like any other mechanical component. your example is of a failure of a seal and loss of fluid running dry, not that the diff was unable to overcome it's duty. or that it wouldn't be able to handle the 3L engine and glow from getting hot. any differential, be it out of the smallest engined 4 cylinder E46 to 3.2L if run extremely low or dry will fail.

tell me how much turning does a 3.38 diff do in a 318Ci compared to a 330Ci ?

but all this still does not matter in the topic at hand where the Op would be going with a larger diff compared to smaller.
__________________
OE CSL WHEELS - OE CSL BRAKES - OE AUTOFLD MIRRORS - CSL TRUNK - CSL DIFFUSER - AA TUNE - AA PULLEYS - AS 40% SSK - BILSTEIN PSS9s
CF CSL LIP - NAVI - BMWPERF CF STRUT BAR - BMW GROUP N MNTS - GRUPPE M CF INTAKE - SUPERSPRINT - UUC BRKLINES, TRANS, RTABs - H&R SWAYS - 4.10 M-SPORT DIFF
Alex323Ci is offline   Reply With Quote
Old 01-08-2013, 06:41 PM   #13
wildirish317
Registered User
 
Join Date: Sep 2012
Location: Louisville, KY
Posts: 2,676
My Ride: 2004 325Ci vert
Quote:
Originally Posted by jay786 View Post
whats the problem with higher rpm at high speeds ... obviously drinks more petrol ... what else?

Thanks
Lower top speed, as you'll hit your rev limiter at a lower speed. If that doesn't bother you, and the higher fuel consumption doesn't bother you, then go for it.
__________________


“The test of the machine is the satisfaction it gives you. There isn't any other test. If the machine produces tranquility it's right. If it disturbs you it's wrong until either the machine or your mind is changed.”
wildirish317 is offline   Reply With Quote
Old 01-08-2013, 08:08 PM   #14
Alex323Ci
Registered User
 
Join Date: Jan 2005
Location: Huntington Beach -Surf City USA
Posts: 17,044
My Ride: M3 & 323Ci (sold)
Quote:
Originally Posted by wildirish317 View Post
Lower top speed, as you'll hit your rev limiter at a lower speed. If that doesn't bother you, and the higher fuel consumption doesn't bother you, then go for it.

"theoretical" top speed.
i don't know of anyone who has run out of revs for their car before running out of engine power to overcome wind, air and rolling resistence. when this happens is the actual "top speed" of any given car.


but the car will absolutely rev higher in it's final drive gear. but it's not a given that one's top speed will suffer. on a 318Ci which factory spec does 135mph i seriously doubt it.
__________________
OE CSL WHEELS - OE CSL BRAKES - OE AUTOFLD MIRRORS - CSL TRUNK - CSL DIFFUSER - AA TUNE - AA PULLEYS - AS 40% SSK - BILSTEIN PSS9s
CF CSL LIP - NAVI - BMWPERF CF STRUT BAR - BMW GROUP N MNTS - GRUPPE M CF INTAKE - SUPERSPRINT - UUC BRKLINES, TRANS, RTABs - H&R SWAYS - 4.10 M-SPORT DIFF
Alex323Ci is offline   Reply With Quote
Old 01-08-2013, 08:28 PM   #15
wildirish317
Registered User
 
Join Date: Sep 2012
Location: Louisville, KY
Posts: 2,676
My Ride: 2004 325Ci vert
Quote:
Originally Posted by Alex323Ci View Post
but it's not a given that one's top speed will suffer. on a 318Ci which factory spec does 135mph i seriously doubt it.
You're probably right. Going from a 3.38 to 3.73 will increase his speed in any gear by about 10%. This means that he'll be at 6,600 rpm at the same speed he used to run 6,000 rpm.

I have to confess, I've never pushed my car anywhere close to 6,000 rpm in 5th gear (5 speed).
__________________


“The test of the machine is the satisfaction it gives you. There isn't any other test. If the machine produces tranquility it's right. If it disturbs you it's wrong until either the machine or your mind is changed.”
wildirish317 is offline   Reply With Quote
Old 01-09-2013, 10:09 PM   #16
Alex323Ci
Registered User
 
Join Date: Jan 2005
Location: Huntington Beach -Surf City USA
Posts: 17,044
My Ride: M3 & 323Ci (sold)
Quote:
Originally Posted by wildirish317 View Post
You're probably right. Going from a 3.38 to 3.73 will increase his speed in any gear by about 10%. This means that he'll be at 6,600 rpm at the same speed he used to run 6,000 rpm.

I have to confess, I've never pushed my car anywhere close to 6,000 rpm in 5th gear (5 speed).
thanks for reading and using information given to reassess and answer.
all too often on this forum people will just argue a point when reasonably one can see it's just not so.
__________________
OE CSL WHEELS - OE CSL BRAKES - OE AUTOFLD MIRRORS - CSL TRUNK - CSL DIFFUSER - AA TUNE - AA PULLEYS - AS 40% SSK - BILSTEIN PSS9s
CF CSL LIP - NAVI - BMWPERF CF STRUT BAR - BMW GROUP N MNTS - GRUPPE M CF INTAKE - SUPERSPRINT - UUC BRKLINES, TRANS, RTABs - H&R SWAYS - 4.10 M-SPORT DIFF
Alex323Ci is offline   Reply With Quote
Old 01-10-2013, 05:30 AM   #17
RayPooley
Registered User
 
Join Date: Jul 2012
Location: Liverpool
Posts: 1,440
My Ride: 320i SE Coupe
Quote:
Originally Posted by wildirish317 View Post
You're probably right. Going from a 3.38 to 3.73 will increase his speed in any gear by about 10%. This means that he'll be at 6,600 rpm at the same speed he used to run 6,000 rpm.

I have to confess, I've never pushed my car anywhere close to 6,000 rpm in 5th gear (5 speed).

May I just clarify that? The change will effectively move the car further for one rev of the engine in any gear. So what that means is it will be doing a greater linear velocity at 6000 rpm than it used to. 10% is it? Its not a matter of greater rpm for a given linear velocity. Its the other way round. Its lower rpm for a given linear velocity. Fewer turns of the crankshaft per unit of distance covered. This does, of course, raise an issue regarding the speedometer. Will the speedo still reflect the true linear velocity of the vehicle after the change? In essence, at 2500 rpm (56mph?) you might be travelling faster than your speedo tells you and if there's a cop with his speed gun aimed at you that might cost you some dollars.
RayPooley is offline   Reply With Quote
Old 01-10-2013, 06:26 AM   #18
NOVAbimmer
Registered User
 
Join Date: Aug 2006
Location: VA
Posts: 12,396
My Ride: 14 Impala FXST M796
Quote:
Originally Posted by RayPooley View Post
May I just clarify that? The change will effectively move the car further for one rev of the engine in any gear. So what that means is it will be doing a greater linear velocity at 6000 rpm than it used to. 10% is it? Its not a matter of greater rpm for a given linear velocity. Its the other way round. Its lower rpm for a given linear velocity. Fewer turns of the crankshaft per unit of distance covered. This does, of course, raise an issue regarding the speedometer. Will the speedo still reflect the true linear velocity of the vehicle after the change? In essence, at 2500 rpm (56mph?) you might be travelling faster than your speedo tells you and if there's a cop with his speed gun aimed at you that might cost you some dollars.
Your overly-complicated explanation aside, it would only matter for speed if the speed sensor were on the output flange of the transmission.
__________________
NOVAbimmer is online now   Reply With Quote
Old 01-10-2013, 06:29 AM   #19
NOVAbimmer
Registered User
 
Join Date: Aug 2006
Location: VA
Posts: 12,396
My Ride: 14 Impala FXST M796
And increasing your differential ratio will decrease your road speed for a given engine speed in a gear.
__________________
NOVAbimmer is online now   Reply With Quote
Old 01-10-2013, 06:32 AM   #20
RayPooley
Registered User
 
Join Date: Jul 2012
Location: Liverpool
Posts: 1,440
My Ride: 320i SE Coupe
Quote:
Originally Posted by NOVAbimmer View Post
And increasing your differential ratio will decrease your road speed for a given engine speed in a gear.
So when we are talking about diff ratios the ratio is given as output : input rather than input : output. Is that right?

EDIT:
"It's the ratio of how many times the drive shaft turns to how many times the axle turns and is set up by choosing how many teeth are on the ring and pinion gears."

So it is the ratio of input to ouput. But its based on rotations : rotations not teeth : teeth. I see. I am assuming the ratio is teeth : teeth which, in the case of diffs, is incorrect.

Last edited by RayPooley; 01-10-2013 at 06:43 AM.
RayPooley is offline   Reply With Quote
Reply

Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes Rate This Thread
Rate This Thread:

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is On
Censor is ON





All times are GMT -5. The time now is 02:47 PM.


Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2014, vBulletin Solutions, Inc.
(c) 1999 - 2011 performanceIX Inc - privacy policy - terms of use