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 03-12-2014, 04:31 PM   #21
Cavi Mike
Banned User
 
Join Date: Aug 2007
Location: Rochester, NY
Posts: 2,308
My Ride: 2000 328Ci
Send a message via AIM to Cavi Mike Send a message via MSN to Cavi Mike Send a message via Yahoo to Cavi Mike
No because depends on the VE of the engine at each RPM and more torque is required at lower RPM to do the same work.
Cavi Mike is offline   Reply With Quote
Old 03-12-2014, 04:32 PM   #22
Stinger9
Registered User
 
Join Date: Sep 2011
Location: New York
Posts: 9,474
My Ride: '04 330Ci
It's probably a curve that goes up as you decrease the diff number, but then at some point curves back down. Because as you reach the extreme, the torque required to move the car would be enormous.
Stinger9 is offline   Reply With Quote
Old 03-12-2014, 04:41 PM   #23
WDE46
Registered User
 
Join Date: Sep 2012
Location: Old Greg's Cavern
Posts: 12,605
My Ride: '13 128i, '04 330Ci
Quote:
Originally Posted by HLG600 View Post
Let's take two of the same car, going 80 mph. One is at 3000 rpm while the other is at 2500 rpm. If the only delta is the engine speed, would the car at a lower engine speed not achieve a better fuel efficiency in most cases?

There are plenty of documented cases where switching to a less aggressive gear ratio increased fuel economy, and vice a versa. I'm just trying to understand in more detail why Daniel is claiming the inverse.
Quote:
Originally Posted by Cavi Mike View Post
No because depends on the VE of the engine at each RPM and more torque is required at lower RPM to do the same work.
Pretty much this. The throttle position is not the same between those two scenarios.

In a theoretically perfect engine (flat torque curve, 100% VE@WOT), you would burn fuel at the same rate in both scenarios, since the power required to maintain speed is the same regardless of what gearing you have. So, the throttle position of your engine geared for 3000 rpm will be less than the throttle position for 2500 rpm gearing. So, say it requires 50% throttle at 3000 RPM, well at 2500 RPM (in this theoretical engine), it's going to require 20% more throttle, so 0.5*1.2= 60%.

Let's say your throttle position is equal to your VE (which is basically how a throttle body works), so one gearing runs at 50% VE and the other at 60% VE, but output exactly the same power.

Once you add in the bits that ruin the theoretical model I made up, you could go either way. Maybe the engine likes 50% throttle at 3000 RPM more than 60% at 2500 RPM. Whatever non-linearities and inefficiencies the engine design brings, determines what gearing is most efficient and Stinger9 brought up a good point that there is a sweet spot where fuel economy is going to suffer by going up or down.

In other words, there is no RULE that says changing the final drive ratio one way or the other will do anything consistent on any given car at a given speed. It could hurt it, it could improve it, and you can test that by implementing extreme situations:

1. 6500 RPM at min throttle
2. 700 RPM at max throttle

Both of these situations are going to give you **** fuel economy compared to some middle ground simply due to the characteristics of the engine design.
__________________

I have a dream that one day our children will live in a
nation where they will not be judged by the color of
their skin but by the color of their coolant.

Last edited by WDE46; 03-12-2014 at 04:46 PM.
WDE46 is offline   Reply With Quote
Old 03-12-2014, 04:43 PM   #24
Knight
Registered User
 
Join Date: Jan 2008
Location: NNJ
Posts: 3,370
My Ride: '05 330Ci | '01 325i
Makes sense, thanks for the clarification.
Knight is offline   Reply With Quote
Old 03-12-2014, 05:31 PM   #25
Alex323Ci
Registered User
 
Join Date: Jan 2005
Location: Huntington Beach -Surf City USA
Posts: 20,205
My Ride: M3 & 323Ci (sold)
3.38 ratio will be fine in a 330Ci

it'll be about a 10% change
__________________
OE CSL WHEELS - OE CSL BRAKES - OE AUTOFLD MIRRORS - CSL RACK - CSL TRUNK - CSL DIFFUSER - AA TUNE - AA PULLEYS - AS 40% SSK - BILSTEIN PSS9s
CF CSL LIP - NAV- BMWP CF STRUT BAR - BMW GROUP N MNTS - GRUPPE M CF INTAKE - SUPERSPRINT - UUC S/S LINES, RTABs - H&R SWAYS - 4.10 M-SPORT DIFF

Last edited by Alex323Ci; 03-12-2014 at 05:33 PM.
Alex323Ci is offline   Reply With Quote
Old 03-12-2014, 08:29 PM   #26
Archbid
Registered User
 
Join Date: Jun 2013
Location: Marin, CA
Posts: 1,379
My Ride: 2003 330cic
Quote:
Originally Posted by WDE46 View Post
Careful with the 3.38. You may end up at a high RPM on the interstate.

It SHOULD work. The input and output flanges should match up with everything on your car if you purchase one from a 330. I purchased a 3.38 to replace my 3.38 that died (automatic) and got a good one for <$300.

According to my calculator, you'll be 3218 RPM@70 mph and 3677@80 mph with the 3.38 vs 2923 RPM and 3340 RPM respectively for the 3.07. Seems okay to me.
Checked today on the freeway. 3000 RPM on freeway at 70MPH (not scientific). I would be totally fine with 3200 RPM at 70. I have 2 points on my license this year, so there will be no 90MPH driving... I will take solace in acceleration.
Archbid is offline   Reply With Quote
Old 03-12-2014, 08:32 PM   #27
Archbid
Registered User
 
Join Date: Jun 2013
Location: Marin, CA
Posts: 1,379
My Ride: 2003 330cic
Quote:
Originally Posted by Cavi Mike View Post
He has a 5-speed manual. No torque converter.

Archbid, please put in your first post that you have a 5-speed manual. there's no reason why we should have to scour posts to find that vital piece of info when you should have put it there in the first place.
My apologies. And thank you all!
Archbid is offline   Reply With Quote
Reply

Tags
differential

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 01:13 AM.


Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2017, vBulletin Solutions, Inc.
(c) 1999 - 2016 performanceIX Inc - privacy policy - terms of use