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New rear diff - gear selection

4K views 18 replies 11 participants last post by  random84 
#1 ·
So have an M6, comes with the 3.73 rear and cruises at 80mph around 2500 rpm.

I have to find the tr6060 gear ratios for the V but my hunch is to stay with the 3.73 ratio, even though I can pick essentially any ratio with the new rear differential.

Thoughts on 3.50 or 3.89 in a car that already will probably be tire limited?

I guess I'd be curious to know what gear is ideal for the 1/4 mile assuming redline is 7k rpm, power around 850-900 flywheel?

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#3 ·
Looking hard at the GForce 9" diff kit, upgraded driveshaft, axles and carrier for $7k.

Or waiting 3+ months? on a replacement GM unit and a cooler for probably almost $4k, plus the risk of it degrading again, while keeping the OEM axles and drive-shafts.

I figure with my expected power levels, sticky tires are going to get risky with the rear end. So the upgrade might make more sense for peace of mind and less down-time.
 
#4 ·
Going from a 3.73 to either a 3.50 or 3.89 is an almost imperceptible change. Don't overthink it, put a 3.73 back in it. You'll struggle with traction and getting the launch right so much more so than optimizing finish line RPM.

IMO Buy the best Diff possible, since you street drive it more than drag race, something like the detroit truetrac, Wavetrac or strange's equivalent would be the best.
 
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#5 ·
The GForce kit uses Strange internals - and drop-ships the diff from them if I recall my prior conversation correctly. They make the third member, axles and driveshafts to go along with it. I'll have to look at those others but I'm not sure of the compatibility so for an idiot like me buying the "package" has some allure.
 
#6 ·
I tend to agree with Mike.

But here are a couple of things you can play with to keep you occupied.. lol

First you can go to this site:
Link => Stan Weiss' - Automotive Performance Software / Interactive JavaScript to Calculate Rear End Gear Ratio needed

Remember;
You have a larger engine now so the torque value will
be higher at the flywheel, assuming the static compression
ratio is the same. If the static Cr is higher, then the
engine will produce more torque at the flywheel.

Volumetric efficiency (VE%) is the highest at 'Peak Torque'.
So with your new engine I would imagine VE% will also
increase.

Moving either to a shorter tire, or a higher numerical gear
ratio will multiply the torque more.

Add the above to a higher flywheel torque value_lbs / ft
and you could simply be spinning your wheels. . lol

------------------------------------------------------------------

Next is a post I made sometime back on this forum:

Originally Posted by JRockey
Maybe too much emphasis on what should happen on paper here. It may not be making a lot of power at 7200 BUT the advantage of torque multiplication produces more acceleration and outweighs the loss of power. Eventually the slope of the torque curve becomes to (-) steep and cannot support the advantage so shift rpm must be carefully selected in each gear. Just my half cent of opinion.

------------------------------------------------------------------

Yes, eventually the slope of the torque curve turns downward
(because of a decline in %VE) as you indicated above.

The torque curve pretty much shows one the VE% curve
as one follows the other.

So we desire to maintain a high torque curve out to the highest engine rpm that we can.

I have not used this formula for quite some time,
but it might be useful to use it here today, to show
how this works.

((Engine Torque * Trans Ratio * Rear Gear Ratio) / Circumference of the tire.)

The engine produces its maximum force at its torque peak and
its maximum work at its power peak.

Torque does not contain the HP component
but the HP component does include the
torque component.

And finally:
The engine uses more fuel and air, per revolution, at its torque peak.
The engine uses more fuel and air, per unit time, at its horsepower peak.

Cheers,
The Duck
 
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#8 ·
https://www.chevrolet.com/performance/transmissions/mg9-tr6060

With a 7000 RPM redline you're looking at 56 in first, 115 in 3rd gear, and 150 in 4th with the stock tire height and a 3.73. Now, stock tire height is probably an invalid assumption to make here. So, knowing what tire you are running is important and from there figure out how short you really want first to be want where you want to be on the big end.
 
#9 · (Edited)
I actually estimate 6800 as the shift point (really... "clutch in" point), leaving a few hundred RPM for margin. I checked on the DR's I was most likely to use and they came in around 28" even.

I used this link for playing with gear ratios - I sent them an email and they added the CTSV TR-6060 to the drop down menu:

https://www.blocklayer.com/rpm-geareng.aspx


3.50 seems a bit tall for the quarter mile, but admittedly it would be decent on the street for a lower cruising RPM, and the first three gears being all but unusable short of rolling the throttle:





3.70 (aka the OEM 3.73) is a pretty good choice for the quarter mile based on anticipated trap speeds in the mid-to high 140's:




3.89 is the next most realistic option, and is right on the edge of needing to shift into 5th at the end of the quarter. MPG takes a hit on the street and further limits traction. :D


 
#11 ·
I'd personally love to keep the 3.7 and just change 6th gear for fuel economy

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#12 ·
6th is easy to get to FWIW

Forget the vendor I used but he sells the zr1 gear i think it is.

Don’t have to worry about any bearings for 6th swap. Or shimming the case for that matter.
 
#13 ·
I still stick with my recommendation above. But if you’re set on a change, definitely don’t go with more ratio (3.89). The last thing you’ll need with a 427 is more torque multiplication. A 3.50 will kill a little a little torque if you really want a change. I had a 3.50 and a t56 magnum (tr6060) in my GN with a 277” v6 and it was great.

Finish line rpm at the track will be nothing compared to making the car hook. So, maybe less ratio could help in that regard. Although a manual in a 4500+ lb car will be a challenge no matter what.
 
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#18 ·
If you are a drag racer I would base your gear selection on what you would like to trap...
I just went through this. I have a 9" setup (driveshaft shop) and a 4L80. It was pretty straight forward. A 3.50:1 gave me about 167mph with a 28" tire at 7k in 3rd gear, so the 325:1 came out and the 3.5:1 went in.
I did glance at cruising engine speed with the different gear selections but the 1/4 Mile trap made the decision.
 
#19 ·
Did you do the 9" conversion in a V2?

If so, I'd like to know more about what kit you use and any installation difficulties!



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