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Arch 93

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I'm trying to figure out rpms for a BN-1. Not being an Engineer, I'm not having any luck. Specifically the rpms at 70 mph in 3rd gear with OD disengaged and in 3rd gear with OD engaged. I got the tire height, but gear ratios are the stumbling block. Any help would be appreciated.
Thanks
 
Arch 93 said:
I'm trying to figure out rpms for a BN-1. Not being an Engineer, I'm not having any luck. Specifically the rpms at 70 mph in 3rd gear with OD disengaged and in 3rd gear with OD engaged. I got the tire height, but gear ratios are the stumbling block. Any help would be appreciated.
Thanks
Click to expand...

The link below is a link to a calculator which should help.

My research indicates the BN1 ratios should be as follows, hopefully someone familiar with the BN1's will verify the numbers!


gear ratio bn1.JPG



https://www.angelfire.com/fl/procrastination/rear.html
 
Arch 93 said:
I'm trying to figure out rpms for a BN-1. Not being an Engineer, I'm not having any luck. Specifically the rpms at 70 mph in 3rd gear with OD disengaged and in 3rd gear with OD engaged. I got the tire height, but gear ratios are the stumbling block. Any help would be appreciated.
Thanks
Click to expand...

Arch93,

As near as I can determine...

If you have a 3.66 differential and in 3rd gear without overdrive engaged (Final Drive Ratio of 4.125) your RPM at 70MPH would be 3405.
If you have a 3.66 differential and in 3rd gear with overdrive engaged (Final Drive Ratio of 3.12) your RPM at 70MPH would be 2575.

This is with 25.3 inch diameter tires. If you put the same numbers in the calculator I linked to in my previous post you get the same numbers!

I cannot attached an excel file here but I'll email you the Scott J. Morris file used in the calculations, you can contact me at:

Name.JPG


third.jpg


thirdod.jpg
 
I'm looking for rpm. I think my confusion is about gear ratios. I have been looking at a shop manual which lists gear ratios for the rear axle and another list for the gear box, so which one should be used in the calculation?
Thanks for your help.
 
Duane:
I don't know if the differential is a 3.66, but the shop manual lists Rear Axle in 3rd gear without O.D. as 4.125 gear ratio, which matches the number you listed for final drive ratio. If these two are the same thing, then it looks like the 3405 rpm at 70 mph is the answer. My car is a BN-1,which had a 3 speed tranny, while the BN-2 had a 4 speed. I don't know if that matters. Also, I 'm not sure if tire ht. and tire diameter are the same thing. One web page wanted tire height, and they gave the formula to use. It yielded a ht. of 25.39. The rim diameter is 15 ".
Thanks again.
 
Arch 93 said:
Duane:
I don't know if the differential is a 3.66, but the shop manual lists Rear Axle in 3rd gear without O.D. as 4.125 gear ratio, which matches the number you listed for final drive ratio. If these two are the same thing, then it looks like the 3405 rpm at 70 mph is the answer. My car is a BN-1,which had a 3 speed tranny, while the BN-2 had a 4 speed. I don't know if that matters. Also, I 'm not sure if tire ht. and tire diameter are the same thing. One web page wanted tire height, and they gave the formula to use. It yielded a ht. of 25.39. The rim diameter is 15 ".
Thanks again.
Click to expand...


Arch 93,

If things have not been changed out on your car, the BN1's came with the 3.66 differential when equipped with overdrive. (According to the documentation I have.) Tire height and diameter are the same for these calculations.

Thnaks,
Duane
 
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