Gear ratios - BMW Forum - BimmerWerkz.com
3-Series (E36) Chat relating to the BMW 3-Series from 1992-1999. Autodoodad Specific models include: BMW 316i, BMW 318i, BMW 318iS/ti, BMW 320, BMW 323, BMW 320, BMW 324, BMW 325, BMW 328.

 
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#1 (permalink) Old 03-15-2004, 07:28 PM
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can someone link me to a good site that explains how close and wide gear ratios work and how you can benifit from them on a performance level? every site ive seen is either A too confusing or B the info is to vauge.

i would also accept any explanations if anyone personally knows. thanks much in advance guys.

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#2 (permalink) Old 03-16-2004, 10:46 PM
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Generally speaking, close ratio gear boxes are best for engines with lower torque outputs because they allow the use of the narrower high end horsepower bands by allowing the engine to stay in it's power band after each shift. The main downside is loss of top speed or the addition of extra gears which increases the amount of time spent not accellerating (i.e. you're wasting more time shifting because you have to do it more often)

taller gearing is generally best for torquier motors that have wider powerbands. They are also better for fuel economy because all engines are more economical at lower RPMs. That's why economy cars like civics have tall gearing despite their lack of torque.


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#3 (permalink) Old 03-16-2004, 10:51 PM
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so what do you think is the perfect blend? a 6 speed at 1:1? or keep the 5 speed and close the gears just slightly?

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#4 (permalink) Old 03-17-2004, 12:46 AM
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6spd with a slightly shorter ratio diff


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#5 (permalink) Old 03-17-2004, 01:22 AM
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are you more concerned about the closer transmission ratios or the final drive ratio? it really isnt worth your trouble to change the actual transmission ratios unless you plan on competitively tracking the car with a moderately high budget, the final drive, differential, ratio is what most people go after. without the influences of air resistance and power/weight ratios a shorter diff ratio will allow you to accelerate quicker but sacrifice top speed where as a taller diff (smaller number) will gain top speed but slow down acceleration. and like jason said, a torquier more powerful motor is better suited to a taller diff, for example (i'm not sure if these numbers are 100% accurate but theyre close) a 330ci runs a 2.93:1 final drive ratio and a 328ci runs a 3.46:1 ratio, which makes sense given the smaller engine with the same transmission needs shorter gearing to compensate for less power.

changing the diff will just shift the engine to road speed relation into a shorter or taller(longer) distance. so ideally a good upgrade would be to go with a shorter diff, but add another gear (pretty much just for fuel economy reasons as you may not even be able to top out 5th gear, much less 6th)
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#6 (permalink) Old 03-17-2004, 02:17 AM
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I agree with the above. One of the most common changes people do to their transmissions is to swap their top gear for one that is wider to lower their highway speed rpms. For instance older miatas would swap their 5th gear with the 5th gear out of an N/A Rx-7 for the wider gear ratio. Basically unless you swap to a much more or less(?) powerful motor your best bet is to leave the ratios alone. As said before you go for a tighter ratio you will lose time because you will always be shifting to keep the car with in it's powerband. If you go to a wider you lose acceleration but gain potential top speed, if the car has the power to over come wind resistance ect ect.
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