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-   -   who can help with gear/diff ratios relating to rpms! (http://www.maxbimmer.com/forums/showthread.php?t=124333)

m60e30 08-26-2009 10:27 AM

who can help with gear/diff ratios relating to rpms!
 
ok guys, need to get some imput here, something just isnt adding up.

ok, got me a 5 speed gear box in my e30 ratios are
1- 4.20
2- 2.49
3- 1.67
4- 1.24
5- 1.00

those are pretty low gears, but im using a 2.93 diff, (or i thought i was anyway), but this thing revs like nobodies business.
so i guess my question is, does anyone know how to calculate what my revs should be using this combo?? cuz i thought i would have revs to spare.. the only thing i could see being the problem is its not a 2.93 diff?

the trans is out of a e34 530i, and apparently they used a 3.07 diff in the manual cars...so my taller diff should not be rev city. i can just throw it in second gear for all take offs and its like the perfect first gear.

anyone? anyone?
oh my wheels are 22.5 inch diameter

BigD 08-26-2009 10:50 AM

I use this site:

http://www.unixnerd.demon.co.uk/revs.html

gervs 08-26-2009 10:55 AM

Hah, funny you post this now, i'm actually looking at the exact thing atm to see if the getrag260 will do well with the 4:10 diff.

http://www.diffsonline.com/techinfo/...Calculator.xls

everlast 08-26-2009 11:46 AM

I've been using this one:

http://ctny.audiworld.com/mark/s4/gears/gear_calc.html

I calc that in fifth you should be turning 2760 RPM at 100kph with a 2.93. With a 3.73 it would be 3500 RPM (ouch).


(BTW, those are quite undersized tires. E30 should be around 24". )

m60e30 08-26-2009 11:58 AM

ya thats what i calculated as well. hmm, well i better check my tach first see if its acurate, cuz those are the numbers the tach is telling me, about 3400ish at 100km. and it feels that high to. ill hvae to double check this diff. i took a 3.73 out of it!

everlast 08-26-2009 12:01 PM

Sounds like a 3.73 to me. Maybe you put the wrong one back in.

:D

m60e30 08-26-2009 12:22 PM

and i dont know it my tach can be off that muhc. i mean the e34 530 with that exact same trans used a 3.08 diff, and on a 5 series i dont know if bmw would hvae made a first gear that rediculously low, maybe its for hill climbing?
hmm, ok thanks for the help. ill check diff and rpms tonight.
sure is lively though with this gearing! haha

Duct_Taper 08-26-2009 01:41 PM

If you swapped from a 6-cyl to a V8 and didn't do anything to the tach then it's entirely possible that it's way off. Pretty sure you need to get a coding plug for a 4-cyl and rig up some sort of resistor setup to make it read right.

Mister 08-27-2009 08:35 AM

calculator
 
http://wahiduddin.net/calc/calc_speed_rpm.htm

Mister


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