Explaining the 'wow factor' of 12 bolt and 9" rearends
02-15-2009, 09:22 AM
#1
Explaining the 'wow factor' of 12 bolt and 9" rearends Our F-body OEM rearends are mostly 2.73:1,3.23:1,and 3.42:1 of the 7 5/8" 10 bolt variety.When we upgrade our performance via gearset only,the A4s' typically go 3.73:1 and the M6s' typically go 4.10:1.Gears mods,even though they're of the 7.5 /7.625 variety,do add to the increased acceleration ability.
We always want more power and typically we add more power.When we get that more power,we need to get more traction to utilize that more power.When we get that more traction for that more power that was added,our OEM 10 bolts start to fail,a very common occurance for the M6s'.The next mandatory upgrade is typically a GM 12 bolt 8.8" or a Ford
9",both deserving as upgrades as they are capable of the strenght needed.
Torque (rotational force) is the primary factor that influences vehicle acceleration with all others factors being equal.Two identical f-bods of equal weight,equal horsepower,and equal driver ability,but one has 3.42:1 gears and the other has 4.10:1 gears,the 4.10 vehicle will out accelerate the 3.42:1 vehicle.Why,the 4.10 vehicle is capable is transmitting 20% more torque
to the carrier and axles because of 'torque multiplication' of a 4.10 over a 3.42.
Torque can be increased though a gearset and /or increased leverage.A leverage affect on torque(in this example,reducing torque) would be when we go from our standard 26" dia.tire to a 28" tire to increase starting line traction by reducing the tires' leverage against the pavement.
12 bolt and 9" rearends provide increased leverage torque over the 10 bolt 7 5/8"Where a good gearset contact pattern would be located on the ring gear,the center of that pattern would be 3.244" from the carrier /axles centerline,a 12 bolt would be 3.805,and a 9" would be 3.844".A 12 bolt 8.8" has a 17.3% increase in leverage over the 10 bolt 7 5/8' in the aspect of distance between the ring gear and carrier /axles centerline.A 9" has a 18.5% increase in leverage over the 10 bolt 7 5/8" in the aspect of distance between the ring gear and carrier /axles centerline.
Now taking torque calculations for ratio changes and increased leverages,I've come up with with the following 4 values.
Putting a 12 bolt w3.73s' in makes the vehicle feel like 4.38s' have been put in(granted that there's no such ratio as 4.38 for a 12 bolt,but that's what the calculations go to).
Putting a 9" w3.89s' in makes the vehicle feel like 4.39s' have been put in.
Putting a 12 bolt w4.10s' in makes the vehicle feel like 4.81s' have been put in.
Putting a 9" w 4.11s' in makes the vehicle feel like 4.87s' have been put in.
This is only the acceleration capability that is effected.A 3.73 10 bolt,3.73 12 bolt,and a 9" 3.70 all would have the same rpm vs. mph as driveshaft rpm vs. axle rpm is not affected,but acceleration due to incresed axle torque is affected.
I haven't included the GM 10 bolt 8.2 and 8.5 as we don't use those as aftermarket upgrades and I don't have data /dimensions for the Danas' so I couldn't discuss those.
I anticipate getting posts and feedbacks either disputing or agreeing especially from those that have first went to 3.73s' and 4.10s' into our 7 5/8 10 bolt and then eventually converted over to 12 bolts and /or 9".
We always want more power and typically we add more power.When we get that more power,we need to get more traction to utilize that more power.When we get that more traction for that more power that was added,our OEM 10 bolts start to fail,a very common occurance for the M6s'.The next mandatory upgrade is typically a GM 12 bolt 8.8" or a Ford
9",both deserving as upgrades as they are capable of the strenght needed.
Torque (rotational force) is the primary factor that influences vehicle acceleration with all others factors being equal.Two identical f-bods of equal weight,equal horsepower,and equal driver ability,but one has 3.42:1 gears and the other has 4.10:1 gears,the 4.10 vehicle will out accelerate the 3.42:1 vehicle.Why,the 4.10 vehicle is capable is transmitting 20% more torque
to the carrier and axles because of 'torque multiplication' of a 4.10 over a 3.42.
Torque can be increased though a gearset and /or increased leverage.A leverage affect on torque(in this example,reducing torque) would be when we go from our standard 26" dia.tire to a 28" tire to increase starting line traction by reducing the tires' leverage against the pavement.
12 bolt and 9" rearends provide increased leverage torque over the 10 bolt 7 5/8"Where a good gearset contact pattern would be located on the ring gear,the center of that pattern would be 3.244" from the carrier /axles centerline,a 12 bolt would be 3.805,and a 9" would be 3.844".A 12 bolt 8.8" has a 17.3% increase in leverage over the 10 bolt 7 5/8' in the aspect of distance between the ring gear and carrier /axles centerline.A 9" has a 18.5% increase in leverage over the 10 bolt 7 5/8" in the aspect of distance between the ring gear and carrier /axles centerline.
Now taking torque calculations for ratio changes and increased leverages,I've come up with with the following 4 values.
Putting a 12 bolt w3.73s' in makes the vehicle feel like 4.38s' have been put in(granted that there's no such ratio as 4.38 for a 12 bolt,but that's what the calculations go to).
Putting a 9" w3.89s' in makes the vehicle feel like 4.39s' have been put in.
Putting a 12 bolt w4.10s' in makes the vehicle feel like 4.81s' have been put in.
Putting a 9" w 4.11s' in makes the vehicle feel like 4.87s' have been put in.
This is only the acceleration capability that is effected.A 3.73 10 bolt,3.73 12 bolt,and a 9" 3.70 all would have the same rpm vs. mph as driveshaft rpm vs. axle rpm is not affected,but acceleration due to incresed axle torque is affected.
I haven't included the GM 10 bolt 8.2 and 8.5 as we don't use those as aftermarket upgrades and I don't have data /dimensions for the Danas' so I couldn't discuss those.
I anticipate getting posts and feedbacks either disputing or agreeing especially from those that have first went to 3.73s' and 4.10s' into our 7 5/8 10 bolt and then eventually converted over to 12 bolts and /or 9".
02-15-2009, 04:07 PM
#2
TECH Fanatic
Join Date: Jul 2002
Location: MI
Posts: 1,218
Likes: 0
Received 0 Likes
on
0 Posts
Sweet post!
Everyone should keep in mind though that although torque may increase at the rear wheels, HP will most likely go down due to turning more rotational mass. And the more horsepower you make, the more you'll lose since it is a percentage function.
Everyone should keep in mind though that although torque may increase at the rear wheels, HP will most likely go down due to turning more rotational mass. And the more horsepower you make, the more you'll lose since it is a percentage function.
02-15-2009, 07:18 PM
#7
I also did calculations for 2.80/9",3.00/9",3.08/12 bolt,3.90s',and 4.56s',but didn't add them to the discussion(even though they produced interesting results) because 3.73s' and 4.10s' seem to be the predominant gear ratios used to upgrade.
Trending Topics
02-15-2009, 08:14 PM
#9
That's what I believe and now I'm waiting for input from those that maintained a ratio but upgraded rearend size.
The entire thought process started when I laid a 7 5/8 ring gear on a 12 bolt 8.8" ring gear and noticed the increased distance that the ring gear teeth were from carrier/axle centerline,increased leverage=increased torque,but how much,time to make some measurements and break out the calculator.
The entire thought process started when I laid a 7 5/8 ring gear on a 12 bolt 8.8" ring gear and noticed the increased distance that the ring gear teeth were from carrier/axle centerline,increased leverage=increased torque,but how much,time to make some measurements and break out the calculator.
02-15-2009, 08:18 PM
#10
On The Tree
Join Date: Sep 2008
Location: Roanoke Rapids, NC
Posts: 142
Likes: 0
Received 0 Likes
on
0 Posts
well i just put 4.10s in my 10 bolt but with my driving style i know its on the way out the door! lol so sometime soon i might can confirm that for ya. but i completely understand your theory and it makes perfect sense!
02-16-2009, 11:23 AM
#14
11 Second Club
iTrader: (16)
Join Date: Feb 2006
Location: Cape Girardeau, MO
Posts: 1,865
Likes: 0
Received 0 Likes
on
0 Posts
I went from a 10bolt with 4.10s to a 9" with 4.11s and suffered no ET lose and gained 2mph I beleive, but tires were different so I cant really confirm your thoughts...but agree
02-22-2009, 09:00 AM
#16
Good write up. Will know soon enough.
Wish my set up was the same as last trip to the dyno....i've changed to a twin disk clutch, 15's instead of 18's....and will have a 3' ds this trip so it's not accurate. The thing that really matters will tell.....track times. 10 bolt w/ 4.10's to a fab 9' with 4.11's alumn center section etc....
Wish my set up was the same as last trip to the dyno....i've changed to a twin disk clutch, 15's instead of 18's....and will have a 3' ds this trip so it's not accurate. The thing that really matters will tell.....track times. 10 bolt w/ 4.10's to a fab 9' with 4.11's alumn center section etc....
02-22-2009, 10:36 AM
#17
Dyno figures should not change,maybe slighty,but that's not the concern of starting this thread.Dragstrip times SHOULD improve overall,but I'm more interested in the 1/8(1st half of the 1/4) improvement times and street 1st and 2nd gear 'seat of the pants' feel difference.As you go thru the gears,even though the increase in torque percentage stays the same,acceleration(G force) goes down.Acceleration rate is what I'm after.The ultimate answer would come by use of an accelerometer(G meter) under both situations.
02-22-2009, 11:18 AM
#18
TECH Fanatic
iTrader: (45)
Join Date: Sep 2006
Location: Clarksville, TN
Posts: 1,122
Likes: 0
Received 0 Likes
on
0 Posts
Very interesting, thanks for posting this.
I always thought that installing a 12 bolt or especially a 9 inch would hurt horsepower numbers, and they do, but you are saying that even though RWHP numbers are down, that the RWTQ numbers should be up. And, acceleration off the line should be better because of more torque.
I always thought that installing a 12 bolt or especially a 9 inch would hurt horsepower numbers, and they do, but you are saying that even though RWHP numbers are down, that the RWTQ numbers should be up. And, acceleration off the line should be better because of more torque.
