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E36 BMW Check Coolant Level OBC Message
E30 BMW 3 Series Pre Purchase Checklist
E36/E46 Rear Trailing Arm Bushing (RTAB) Replacement 1
E36/E46 BMW Rear Shock Mount (RSM) Replacement
BMW Control Arm Bushing Replacement Tips
E36 Exhaust Replacement and M3 Exhaust Swap Upgrade
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E36 Warped Rotors and Brake Pad Deposits
E30 318i and 318is Suspension and Tie Rod Replacement
Replacing your BMW's fuel filter
BMW Cooling System Flush/Refill Part 2
BMW Cooling System Flush/Antifreeze/Refill I
E36 BMW Power Window Problems and Repair
BMW Headliner Repair and Replacement
M50 Intake Manifold Swap for 328i/328is and M3
E28 BMW 5 Series History and Information
More Power for BMW 2002 and 2002tii Part 2
More Power for BMW 2002 and 2002tii Part 1
Replacing E36 and E46 BMW Tie Rods
Hints and Tips for Washing and Waxing your BMW
Performance Modifications for E36 M52 328i and 328is
1991 E30 318is Performance Mods
E36 BMW 3 Series Oxygen Sensor Replacement Instructions Part 2
E36 BMW 3 Series Oxygen Sensor Replacement Instructions Part 1
E30 325i and 325is vs E30 M3: The Better Street Car
Suspension and Handling Upgrades for E34 BMW 5 Series Sedans
BMW Differential Repair, Replacement, and Upgrades Part 2
BMW Differential Repair, Replacement, and Upgrades Part 1
E46 3 Series Wear Items Checklist: What To Expect
E30 BMW 3 Series History and Performance
E34/E36/E39 M50/S50/M52/S52 BMW Engine Coil Replacement
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From E12 to E39: BMW M5 and M535i History and Development
My BMW Dream Garage
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E24 BMW 6 Series History and Development: The Shark
Brief History of the M3: From E30 to E46
E36 M3 vs E36 325is and 328is: Performance Comparison
Replacing E30, E36, and E46 Ball Joints
E36 BMW Slip Ring Replacement and Why It Fails
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E36 BMW Oxygen Sensor Replacement Instructions Part II

Continude from the previous article, this will detail the procedure for replacing you E36 BMW's oxygen sensor. The o2 sensor is located in the exhaust system, and senses the oxygen content of the exhaust gases. What does the sensor do? The amount of oxygen in the exhaust varies according to the air/fuel ratio of the fuel injection system. The oxygen sensor produces a small voltage signal that is interpreted by the electronic control unit (ECU) of the fuel injection system. The ECU makes constant adjustments in fuel delivery according to the signal generated by the oxygen sensor in order to maintain the optimum air/fuel ratio.

On most BMWs, the engine's computer will throw the Check Engine Light if the signal received by the computer is out of it's normal range, meaning in all likelihood that you need a new sensor. Chipped cars (those with aftermarket ECU software or for OBDII cars the popular Shark Injector) tend to have shorter replacement intervals. Just a word to the wise.

Some shops recommend a replacement interval as short as 30,000 miles for optimal performance, emissions output, and catalytic converter lifespan. The oxygen sensor is located on exhaust manifold and/or exhaust pipes that lead to the muffler. The exact location changed over model years and E36 model. For example, the M42 and M44 318 models have their sensor located in a difficult place. On my 95 325is, the sensor is located just upstream from the catalytic converter, sitting at an angle on top of the pipe. A large adjustable wrench in thoery fits on there, but you are better off spending $20 to get the right tool for the job, which is an oxygen sensor socket which is a special deep sockety with a slit cut in the side. Unless you have a lift (I know a guy who does, that b@stard!), you will need to jack the car up and put it on jackstands or ramps to get under there.

Using a socket or wrench, simply remove the sensor from the exhaust pipe. The electrical plug for the O2 sensor is removed by simply rotating the plastic retainer counter-clockwise, and the plug should come right off. New O2 sensors should have the same exact plug - ready to attach to your car. On the E36 3-Series, the plug is located towards the right side of the rear of the transmission.

An old worn out 02 sensor will likely be covered with blakc soot. Clean off the mounting bung and use never-seize or some other thread lubricating antiseize compound
on the new oxygen sensor. My new one (for OBD-1 E36 M50 cars ie 325i and 325is is Bosch 134321) came with this good already on it!

You will probably find the supplied cable length is way too long. Use zip to fold the cable over to take up the slack. Keep the cable away the exhaust so you don't melt anything.

If you did everything right, the check engine light should shut off upon restarting when you are done and in most cases the car will run perceptibly better.

E36 BMW Oxygen (O2) Sensor Replacement - Part I Diagnosing and Symptoms

This article pertains to all E36 BMW models, both 4 cylindf (M42 and M44) and 6 cylinder (M50, M50TU, M52, S50, S52) engine powered cars. As these cars are up to 15 years old now, chances are their oxygen sensor (O2 sensor) has been replaced at least once or currently needs replacing. The oxygen sensor is one of the most important elements of your BMW's fuel injection system. A finely tuned fuel injection system with an oxygen sensor can maintain an air/fuel ratio within a close tolerance of .02 percent. Keeping the engine at the stoichiometric level (14.6:1 air/fuel ratio) helps the engine generate the most power with the least amount of emissions. Why is this important, you might ask, if you are not a racer or automotive performance enthusiast? Well, a junk or failing oxygen sensor can affect driveability of your BMW in many ways.

There are a few signs that your oxygen sensor may be failing. In general, it is difficult to diagnose problems with the sensor, unless all of the other components in the fuel injection system have been checked and determined to be operating correctly. Some of the symptoms of a failed oxygen sensor system are:

  • Irregular idle during warm-up
  • Irregular idle with warm engine
  • Engine will not accelerate and backfires
  • Poor engine performance
  • Fuel consumption is high
  • Driving performance is weak
  • CO concentration at idle is too high or too low
  • Check Engine light is illuminated

    In general, if the oxygen sensor is not working, the car can run poorly, and will also be generating harmful emissions. As your bosch fuel injection system has several components, it is best to eliminate unknowns before blindly replacing your o2 sensor. However, if the check engine light comes on, you can run a diagnostic test to verify if indeed it is time to replace.

    Turn the key to the run position (as opposed to the start position), and press the gas pedal to the floor and release 5 times. Then wait. The check engine light should go off, then flash in a pattern of dashes with specific durations. The E36 factory Bentley Service Manual shows the correct number of flashes. If it matches up, then go to your local parts supplier and order a new sensor. I highly recommend getting the bosch or equivalent oem replacement sensor and not a universal one. You don't save much money by getting the universal one, anyways. Next article will detail oxygen sensor troubleshooting, testing, and replacement instructions for E36 BMW models, including 1992 325i, 1992 325is, 1992 318i, 1992 318is, 1993 325i, 1993 325is, 1993 318i, 1993 318is, 1994 325i, 1994 325is, 1994 318i, 1994 318is, 1995 325i, 1995 325is, 1995 318i, 1995 318is, 1995 318ti, 1995 M3, 1996 328i, 1996 328is, 1996 318i, 1996 318is, 1996 318ti, 1996 M3, 1997 328i, 1997 328is, 1997 318i, 1997 318is, 1997 318ti, 1997 M3, 1998 328i, 1998 328is, 1998 323i, 1998 323is, 1996 318ti, 1998 M3 models.

  • E30 325i and 325is VS E30 M3 As Street Cars

    In previous articles, I brielfy mentioned my preference for the E30 325i and 325is cars instead of the legendary E30 M3 when it comes to street driving. This is not some blind devotion to the regular 3 series or believing that the E30 M3's giant killer status is overrated, but rather a logical analysis of purchase cost, ownership costs, insurance, maintenance, and overall value for money spent. The principle advtanges the E30 M3 has in *racing* environments are its 4 valve/cylinder high-revving inline 4, which is basically 2/3 of the block from the M1/M635/M5/M6 S88/S38 engine, its wider track, beefier suspension design, quicker steering rack, and aerodynamic bodywork. However, on the street, the streets where most folks use these cars, the advantages of the M3 are not as clear cut. The simple M20 2 valve/cylinder engine is better suited for cut-and-thrust commuting duty. Let me go as far as to say that the 1987-1991 325i and 325is manual transmission cars are not that far off the pace of the M3 in stock form!

    Exhibit A is the February 1988 issue of Road and Track magazine, which compared the M3 and 325is. The M3 edged out the 325is in the 0 to 60 test, doing it in 7.1 seconds vs 7.5 (some other magazines reported 7.4 for the 325i/is) for the 325. The M3 held a minor advantage through the quarter mile, running it in 15.4 seconds at 91 MPH vis 15.7 seconds at 89 mph for the 325is. Interesting right? Now consider the fact that the M3 already has a 4.10 limited slip differential vs the E30 325i's 3.73 diff, thus giving the M3 a gearing advantage! Put a 4.10 differential in a 325i or 325is, and it will instantly make it faster than a stock M3, top speed aside.

    Now, I do realize the M3 has a 24 hp advantage stock for stock, with 192 horsepower vs 168 hp. What people forget is that the power curve/delivery and gearing play a large role in how quickly a car can accelerate. The peak M3 engine has much smaller usable power range, whereas the fat curve of the M20 325i engine is much less spiky. The area under the power curve of the six-cylinder is greater than the area under the power curve of the four cylinder, for the total RPM range used in accelerating. That's why the 325is is faster when you give it the same gearing.

    A chipped 325i or 325is with a MAF conversion (which is the only way to make a cold air intake effective on these cars) will narrow the horsepower gap considerably. You'll be within 10 or so hp of matching the stock M3 in peak horsepower, and have all that nice low and mid range power. Upgrade to a 2.7 liter conversion from the 1988 only 528e or 325 base model, add a Schrick cam and Mass Air Flow meter and you will be able to put the hurt on even many modified E30 M3's.

    Remember, in the end the M3 is still a superior performance car, with better brake design, suspension geometry, weight balance, braking dynamics, handling, and oh so much cachet and panache. Take a look at the Grassroots Motorsports Magazine's E30 comparison from a few years ago. They pitted a 1991 318is, 1990 325is, and 1988 M3 against each other on the street, autocross course, and on the road course. The high revving M3 was untouchable over 5000 rpm, and all agreed its ultimate performance ceiling was much higher, but the real world of cops, traffic, traffic lights, and sane speeds makes the decision much tougher. The 325is feels faster on the street, and besides, you can always swap in a M50 engine for cheap money. Have you considered how expensive the M3;s S14 engine is to work on or rebuild??

    E34 BMW 5 Series Suspension Upgrades

    Available in the US from 1989-1995, the E34 BMW 5 series is the last of the non-computer assisted design (C.A.D.) cars from BMW. Much more modern looking and driving than the E28 5 series it replaced, the E34 remains today a highly desirable car, for its durability, styling, reliability, and performance. However, even the newest E34 is over a decade old now, and chances are that unless a collector had it tucked away in storage, or was owned by the proverbial little old lady, your 525i, 525it (or 525i touring) 535i, 535it (or 535i Touring), 540i, 540i sport, or M5 has some miles on it. People are usually amazed at the difference new shocks and struts plus upgraded springs and swaybars make. Wanna make your 525i wagon dance in the twisties like an M5? Not that hard. It's all in the sway bar size, spring rates, damping rates, and quality and durometer of the bushings.

    E34 BMW 5 Series Shocks - Choosing the Right Shocks and Struts for Your BMW and Driving Style

    If you like damping rates of stock BMW 5'ers, with the consequent stock ride quality, I recommend Boge or Koni. For a firmer, more overtly sporting ride, go with Bilstein HD or Bilstein Sport, Boge Turbo Gas (which are somewhat softer) or Koni yellow. All out sporty ride, short of coilover suspension? Koni adjustable shocks or KYB's if you are on a budget (warning - KYB shocks are very stiff and not in the same league as Bilstein or Koni with regard to quality and lifespan. However, they are certainly an upgrade over anything worn out!). Some Bilstein shock experts think both the HD and Sport have the same valving, but the stroke is simply shorter on the sport models. The sports are your best bet for use with lowering springs. The stroke length is considered to be unimportant for use with lowering springs according to a few BMW experts. In the past, you were led to believe that using lowering springs with standard stroke length dampers would lead to rapid shock or strut wear, but this may not be true. The throw is shorter, and fully compressed position is the same and that is what counts. The worst thing that would happen is that some of the stroke length potential is taken up and not utilized by using a shorter spring. This comes as a relief to many BMW enthusiasts concerned that using H&R or similar springs with newer stock or Bilstein HD's would have a deleterious affect on their car's suspension components lifespans.

    E34 BMW 5 Series Control arms (aka aluminum wishbones)

    Aluminum is lighter and stiffer than steel. Aluminum control arms are available for e34 5 series BMWs. E31 8 series arms will work, but they have spherical bearings vs the E34 rubber bushing-equipped ones. If you are just looking to replace the bushings, BMW dealers do not sell them separately, but aftermarket suppliers do.

    Choosing Springs for your E34 BMW 5 Series
    If you choose to upgrade from the stock springs, you are faced with a ton of brand and model choices. The most popular spring manufacturers seem to be H&R, whose sport springs drop the car roughly 1.3" Front and 1" in the rear while providing a moderately stiff ride; Eibach, whose springs are similarly stiff with a similar drop compared to H&R; Racing Dynamics, whose springs are slightly softer, but still stiffer than stock with a mild 1" front and .75" rear drop; Bavarian Auto, whose sport springs offer close-to-stock ride quality with a 1.5" front and 1" rear drop; Dinan, whose sport springs lower the car about as much as OEM 540i Sport (sport package) springs, and OEM M5 springs, the rear s of which will not work without self-levelling suspension (the fronts work just fine).

    Matching Shocks and Springs for Your E34
    In order of increasing stiffness:

  • Original ride: Original springs or Bavauto springs with Boge
  • Slightly stiffer ride: Original springs or Racing Dynamics springs with Bilsteins HD.
  • Moderately stiff ride: H&R or Eibach or Racing Dynamics springs with Boge.
  • Moderately stiffer ride: H&R or Eibach or Racing Dynamics springs with Bilsteins Sports/HD.

    If you want to lower the car and keep the OEM ride quality, go with the Bavauto springs.

    E34 Sway Bars (Also called anti-roll bars)
    Sway bars tie the lower suspension components together accross the front and the back, they affect the car oversteer and understeer. Bigger sway bars improve stability while cornering, the best setting is fully soft for the front and hard for the back. Stock E34 models usually have 23mm Front and 16mm rear bars. Cars equipped with factory sport suspension have 24mm front and 18mm rear sway bars. The M-Technic equipped cars and N. America-spec M5 models have 25mm front and 18mm rear bars. Eibach and Racing Dynamics are popular aftermarket providers for sway bars. The Eibac kit comes with a 26mm front and 13mm rear. The Racing Dynamics setup includes a 27mm and 19mm swaybar setup. The euro spec 3.8l E34 M5 had a 25mm bar, as did the rare M5 touring. 1994 and 1995 european M5's had 19mm rear swaybars, for all you Ebay and shoppers!

    Go with oem rubber bushings with larger sway bar if you want less harshness and vibration. As far as strut bars, also called strut braces or strut tower bars, they are designed to link the two strut towers together. A good strut brace reduces flex in the strut towers when the car is cornering. The tendency to flex is magnified with lowered cars. Good strut bars are a must if ever plan on taking your E34 BMW to the track. They improve chassis stiffness, making the the steering quicker and more responsive. Watch out for junk Ebay strut bars that are adjustable - they are compromised in design and useful for aesthetic purposes only haha!

  • BMW Differentials Part II: Diagnosing, Repairing, Removing, Replacing, Fluid Changes

    If your BMW is exhibiting symptoms of differential problems, you can rebuild your existing differential, and use a new ring and pinion gearset from BMW to change your gear ratio. Those with cars that already have limited slip differentials with the rato they prefer will want to go this route. Be warned: BMW gearsets are not cheap. Figure $500 - 900 at the dealer and a bit less used. The course of rebuilding the differential in a E24, E28, E30, E34, or E36 BMW will also require assorted bearings, seals and other bits. On top of this, you will need to figure a good four to five hours labor with the diff out of the car, and another two hours for removal and installation.

    How To Tell if Your BMW's Differential is Worn Out

    BMW differentials rarely fail as long as the oil level is reasonably well maintained and the oil is changed at least once in a while. Failure is rare even when the oil is not changed, as long as it's in there. The limited slip differentials are fairly bulletproof, but spider gears have been known to break pretty easily on open differentials that are subject to heavy wheelspin-like those that result from autocrossing or becoming stuck in the snow, or being joe cool when accelerating hard from a stop. You will know immediately if the spider gear has failed as the car will not move!

    A BMW differential's most common wear item are the output bearings. Output bearing wear is characterized by a hum or whine from the back of the car that will become audible between 3000-4000 rpm, usually more noticealbe in fifth gear at a constant cruising speed. The noise will stop or change when you lift your foot from the accelerator pedal and may also change as the suspension rebounds from dips and valleys in the roadway. The perceptability of the noise is dependent on the load on the bearing The noise may also stop entirely above a given rpm. Also, many cars with differentials showing this wear have gone another 50,000 miles with this noise and no further, so it's not necessarily something that needs to be addressed right away. But the longer you let it go, the more likely it is that other parts, such as the expensive ring and pinion gears, may wear out as well....

    It is easy to mistake noise emanating from front wheel bearings for differential whine. But, if you turn the steering wheel, front wheel bearing whine changes or goes away, and also be unaffected by throttle position.

    Should You Get a Used Differential or Rebuild Your Existing One?

    Buying used parts - often sight unseen over the internet these days- is always a gamble, especially when you are dealing with a BMW! If you buy a used diff from a reputable source and it makes the dread whining noise, take it back. However, they might now have another, a problem that occurs when you have one of the rarer BMW differentials, ring and pinions, or gear sets. A good example is a side loader 3.07 differential on the early E12-based 6 series coupes. When looking at a used diff, cleanliness of the unit is a pretty good indicator of quality or wear or lack thereof.

    When looking at used ones, bring a 3/8-in. ratchet, a 17mm socket and a 10mm allen socket along with you. Drain the differential oil, remove the cover and have a look-see. Differential oil is supposed to be changed approximately every 30,000 miles. Is the inside of the housing clean and gray? Do the gears and internals look clean? What about the oil that you just drained out? Is it reasonably clean, or does it look really dark? Another clue to diff maintenance is the drain plug itself. After being removed and replaced a few times, the drain plug will bear some scars. If it looks like it's never been out, it probably hasn't. Run away! On the other hand, don't summarily reject a diff just because it's not pristine inside, especially if the price is right.

    Assessing the quality of a rebuilt diff is trickier. Make sure any rebuilt one was done using factory/genuine BMW tools like bearing pullers and drifts, and according to the factory service specs. Here's a good test: Ask the guy who rebuilt it what oil he recommends. If he says something like Redline 75W-90 gear oil, thumbs up.
    At rebuild time, check the differential mount bushing. A BMW Motorsport solid bushing made of extremely hard rubber is available for those of you who track your car or otherwise drive it hard.

    Removing and Replacing an E30 BMW Differential - Instructions are similar for E24/E28/E30/E34/E36 BMW Models

  • If there's a lot of dirt and grease under the back of your car, do yourself a favor and pressure wash under there before taking on the job. You'll cut the grief and aggravation factor by at least 75 percent.
  • Jack up the rear of the car as high as possible. Don't jack on the differential; use the rear suspension carrier. Place jack stands under the rear suspension carrier mounts.
  • Drain the differential oil. Reinstallation torque on the drain and fill plugs is 50 Nm (36 ft-lb). If you've removed the cover, reinstallation torque is 50 Nm (36 ft-lb), and you must use new wave washers.
  • Cut the safety wire with a pair of side cutters and gently remove the speed sensor cable connector.
  • Using a long-handled 3/8-in. ratchet and an 8mm allen socket (preferably a long one), remove the inner CV joint retaining bolts on both sides. You can use the parking brake to lock the drivetrain. Reinstallation torque is 58 Nm (42 ft-lb).
  • Using a 17mm short open end wrench and the park brake, remove the four driveshaft to differential flange nuts and slip out the special bolts. It's impossible to get a torque wrench on these nuts. Just make 'em real tight. They're lock nuts, so if they've been off more than once or twice buy new ones-part no. 07 12 9 964 672.
  • Using a 22mm box wrench and a 1/2-in. drive ratchet with a 22mm socket, remove the 12mm bolt that toes through the rubber differential mount. The diff will drop about 2 or 3 inches now. Reinstallation torque is 87 Nm (63 ft-lb).
  • Place a floor jack with a piece of plywood under the differential. Using a 19mm short box wrench and a the 1/2-in. ratchet with a 19mm socket and a 1-in. extension, remove the differential housing to axle carrier retaining bolts. Reinstallation torque is 123 Nm (89 ft-lb), if you can get a torque wrench on the fasteners.
  • Carefully lower the floor jack and say hello to your diff. Follow these instructions in reverse to re-assemble. Voila!

    More BMW Differential Notes
    BMW differentials need a fluid change around every 30,000 miles. If filling a limited slip unit be sure to use the correct fluid with friction modifiers. Undo the filler plug first, this will prevent the embarrassment of having a drained diff and no way to fill it if the plug is stuck!

    A limited slip differential is desirable as it provides improved traction, if one wheel slips the other will continue to turn and get the car moving on ice, mud or snow. More modern BMWs have ASC+T electronic traction control and don't need LSDs so much although some cars did have both.

    The best lubricants (& change intervals) for BMW manual gearboxes and differentials: It's generally accepted wisdom that BMW's have special needs for the oil used in the manual gearboxes. Most of us know that you can't just go down to the local autoparts store and buy gear-oil off the shelf that's compatible with your BMW. There are a few gear-oils that are OK for your BMW, but to simplify this issue as, let's go directly to the best solution.

    Since you should be using a premium synthetic gear-oil, Redline makes one that is appropriate for your BMW. Virtually all of us who race BMW's use Redline gear-oil in our gearboxes and differentials. Why? A premium synthetic gear-oil will make the parts inside your gearbox and differential last longer. Reduced "hydraulic drag" will allow more of your engine's horsepower to get from your engine to the rear wheels. Synthetic gear-oils themselves last much longer than conventional oils, so you can reduce maintenance.

    Which type of Redline or other synthetic oil should you use in your bimmer? In the gearbox (manual transmission), some BMW's made between 1986 and 1992 had "labeled" gearboxes with a red or green label on the passenger side of the bellhousing. These had 17mm EXTERNAL wrenching oil plugs. If the label is green, use Redline MT90. If the label is red, use Redline D4 ATF. If there was no label, the gearbox will have 17mm INTERNAL wrench oil plugs, and should use Redline MTL GL-4 70W-80. Virtually all BMW manual gearboxes used before 1986 should use the same Redline MTL GL-4 70W-80 as you would use in the no-label / internal plug 1986-1992 gearboxes. For BMW's 1993 and newer, Redline recommends D4 ATF for all gearboxes, manual OR automatic.

    Now, for differential oil, it is easier. ALL BMW differentials use Redline 75W-90 Gearoil. This is ideal for open or factory limited-slip units, as well as the torque-sensing type if you are lucky enough to have one of those.

    How often should you change the tranny or diff oil?

    In racing, back in the days when people used non-synthetic gear-oils, the useful life of gear-oil in the gearbox and differential under summer racing conditions was just a few hundred miles, or just a few hours. Synthetics now can go an entire racing season, so racer's just make changing them part of the off-season winter maintenance, just once a year. For street cars, you can get away with never changing a synthetic, although I certainly would recommend doing it at least every two or three years for optimal gearbox and diff life.

  • BMW Differentials: What Cars Came With Plus Swaps and Upgrades, Part 1

    BMW enthusiasts, especially those with older BMW cars, have a wide variety of gearing and drive ratios at their disposable as BMW differentials have a wide variety of ratios and offer broad interchangeability across multiple generations of cars and various platforms.

    People talk a lot about the most 'bang for the buck' modifications they can do to improve their car's performance (and by performance they are usually referring to acceleration), and a differential gearset swap or change to a lower (numerically higher) ring and pinion gearset is the single most effective way to increase acceleration. Many call it the 'poor man's supercharger'. It's as bolt-on of a mod as you can get, and has nothing to do with the engine, emissions, or EPA regulations.

    BMW used to intentionally undergear its cars to give them slightly better fuel economy so government corporate average fuel economy (CAFE) requirements would be met. The ironic thing about that is the real world trade off that makes you drive harder in order to accelerate faster, thereby offsetting any gains anyway! While this article focuses mostly on E30 BMW 3 series cars, those with E24 6 series, E28 and E34 5 series, and E36 3 series should also pay close attention. I will recommend some general guidelines for each platform and generation at the end that will make choosing a differential easier.

    Keep in mind that a (numerically higher) lower ring and pinion/differential/gearset will make your engine turn higher revs, run faster, at any given MPH. This means that when taking highway driving and fuel economy into consideration, there is a point of diminishing returns when going lower (higher numerically). Some BMW's have an overdriven 5th gear that helps mitigate the buzziness of higher revving during highway driving. For example, if your fifth gear is an overdriven 0.81:1 (for example, E30 BMWs) going to a 4.10 diff or gearset means you'll still have a final ratio of only 3.32 in fifth gear (4.10 x .91 =3.321). This translates to just under 75 mph at 3500 rpm in fifth gear using 205/55-15 tires. A 4.45 gearset would raise the revs only to about 3750. So it's not like you'll be getting buzzed out of the cockpit on the highway.

    Believe it or not, the top speed of your BMW is usually affected less by a lower gearset (aka higher numerically differential) because not many cars have enough to pin the tach in top gear. Wind resistance, aerodynamics, enigne horsepower and torque output etc conspire to stop you from reaching to attain maximum rpm in an overdriven fifth gear. This is why your 318is won't do 150 mph. Now, of course your car will have a theoretically lower maximum speed, but improving the gear ratios can help you get to a higher real world top speed by giving you more leverage vs wind and drag by optimizing the usable power curve with the gear ratios. Make sense?

    If you do have a BMW that was available in both 4 and 6 cylinder guise, remember that differentials will not be interchangeable. Don't believe me? Compare the diff housing, casing, and rear axle carriers of a M42 powered 91 318is and a 91 325ic. The only exception to this is the E30 M3, which has a 'large case' 6 cylinder style differential. This 4.10 LSD is really nice on a 87-91 325i or 325is. In fact, give one of these cars a 4.10 limited slip diff and a chip and you'll find the car accelerates at least as hard as the vaunted E30 M3. Be warned: the 4.10 differential is considered too peaky for an M30-powered 535i, 633csi, or 635csi.

    An important thing to bear in mind about swapping is that six- and four-cylinder E30 BMW differentials are different and will not interchange. The housings and rear axle carriers are totally different. Interchangeability only exists among four-cylinder cars and six-cylinder cars separately. The sole exception is the E30 M3, which uses a six-cylinder-type differential, thus providing a source of 4.10 limited slip diffs for the 325i. In addition, differentials from the E28 5 Series BMWs (1982-88) will also interchange with the E30 six-cylinder 3 Series cars, and this becomes key to modifying the early eta engine E30s.

    Have an E28 5 series or E24 6 series? Check this out. E28 models came with one of four differentials: 2.93 open in the 528e; 3.15 open in the 524td (diesel); 3.25 in the 533I and 535i/is; and 3.91 in the M5. Limited slip differentials are identified by a white "S" painted on the outside of the housing, usually on top. You can also turn the input shaft and note what happens: If you're looking at an LSD, both flanges will rotate in the same direction. If it's an open diff, either only one will turn or they'll turn in opposite directions. The ring and pinion gearset ratio is identified on a small metal tag on one of the cover bolts. If the tag is gone, you'll have to remove the cover and count the teeth on the ring and pinion gears. The 3.73 325i/is differential is a really nice setup on 82-88 5 series and 1982-88 6 series. Some people give these cars the M5 3.91 (some M6 models had this or the 3.73), but the M5 and M6 cars have the M88/1 aka S38 DOHC engine that revs higher to support this low gearing, whereas the SOHC M30-powered cars are much milder (comparatively speaking, of course).

    There are a ton of 533I and 535is models in junkyards, and this means easy picking for owners of E30 cars that have the (lame in stock form) "eta" engine and a 2.79 or 2.93 diff. A swap to the 3.25 differential is super cheap and makes the car really come to life...til you hit that 4500 RPM wall in stock form - but we can cure that easily, just see my recommendations for E30 E/ETA performance setups. Junkyards practically give these away. The 1991-only E30 318is already comes with a 4.10 differential (note; not all 91 318is models have a limited slip diff, check the differential to be sure, this in spite of the fact that it was an IS or sports package car!). Thee rare 4.27 unit from 1991 318i Cabriolets is really only good for short track racing or for a 91 318is with aftermarket camshafts and porting. Also, not many of the 4.27 differentials were limited slip. A lot of people also like a 3.91 differential in the 3.91 IS. Time 0-60 acceleration suffers with this swap, but usable acceleration is improved and it allows you to stay in the same gear longer. Great for mildly modified cars.

    Part II of this article will cover BMW Differential Repairing, Maintaining, and Instructions for swapping. Hwo to tell if your differential is worn, how to drain and refill the transmission and differential oil/lubricant etc.

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