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Rev. Limiter Question


12212005, 11:04 AM  #5 
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I got a bit too agressive and rapped it off the limiter last week...it is exactly like traction control...it just totally loses power...thank god...but I'm sure the engine can handle another 1000 RPM occasionally...it's the overengineered factor.
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12212005, 12:28 PM  #7  
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I'm pretty sure this is correct. I cross checks with a couple of web based calculators out there. It is however, in US units (i.e. MPH). I have put in as an example a 330xi. Use in good health.
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12302005, 01:37 PM  #8 
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Speed versus RPMs
espo89,
Thanks for the spread sheet. I am interested in how you arrived at the numbers. Where do you get the "336" factor? Can you write down the equation for the rolling diameter thtt you arrived at? I have a similar spread sheet that I made up for the full range of tires and rims for the E90. I got the manufacturer's data from their web sites. It calculates the unloaded tire diameter for reference. It then calculates the tire rolling diameterin inches by dividing one mile distance (5280x12=63360 inch) by pi(3.1416) and the manufacturer's revs per mile specification. From this I multiply by the rear end ratio and 6 gear ratio as you did to get the RPMs for 60 MPH. I used the average rolling diameter to establish the calibration for the speedometer that BMW uses. I WAS UNSUCCESSFUL IN UPLOADING SPREADSHEET AS I DID NOT SEE THAT I NEEDED TO ZIP IT FIRST. I AM GETTING WINZIP AND WILL POST THE SPREADSHEET SOON! Last edited by WallyF; 12302005 at 05:13 PM. Reason: Spreadsheet did not upload 
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12302005, 04:07 PM  #10  
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12302005, 07:02 PM  #13  
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http://www.offroaders.com/info/tech...ng/mphrpm.htm (Google is great!! ) I believe that the 336 is the result of all of the conversions to get the end speed in MPH.
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12312005, 03:12 AM  #14 
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Hey guys I'm new to this forum. I've been at dtmpower.net and bimmerforums.com for a while. But yea be careful bouncing off the rev limiter more than once at a time...repeated bounces can cause damage to lifters, valves, and trannies from the rapid stress and unstress.
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12312005, 11:48 AM  #15 
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Speed vs RPMs
espo89,
Thanks for the web link. I was able to get the 336 factor after doing some calculations. However, the rolling diameter of the tire that yields the revolutions per mile is not exactly the same as the calculated value. I have been struggling with this for awhile but believe that you must use the rolling diameter as the tire is rolling uphill slightly (3/8 inch typically). The means that the rolling diameter is the calculated diameter reduced by twice this 3/8 inch or 3/4 inch typically. If you go to a tire manufacturer's web page i.e. (Bridgestone.com) they specify the calculated diameter as well as the revolutions per mile (RPM). For the rolling diameter use the revolutions per mile per the following equation: Rolling Diameter = (5,280*12)/(Pi()*Revolutions per mile) in inches. For the Bridgestone EL 42 run flat tire in the 205/5516 size that is on my BMW 325xi with 6 spd auto, the revs per mile is specified as 837 by Bridgestone. Rolling diameter = 20,168/833 = 24.10 inches compared to the calculated diameter value of 24.88 inches. This is 0.78 inches less. If you use this in your equations the tachometer RPMs at 60 MPH I get 2153 versus your 2086 using the calculated value of 24.88. Any comments? Hope this helps. I will post my spreadsheet as soon as I get some zip software. 
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12312005, 12:20 PM  #16 
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Yes WallyF, the exact diameter of the tire is the difficult part of the calculation. It will change as the speed of the car changes and the tire heats up (ballooning slightly). Using the static rolling diameter (without any deformation from the weight of the car or ballooning at higher speeds) is just a compromise. The calculations are not precise, but ballpark numbers. The difference you show in the calculations is about 75 RPM which is good enough for a back of the envelope number.
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