M6 Which Differential 3.70 or 3.90

[16GoManGoHC2]

Ok, I’ve used both, 3.70 and 3.90 Diffs in my 16 Hellcat. Not using stock ratio trans, 1-5 2.66, 1.90, 1.35, 1.00, .85. I switched from a 3.70 to a 3.90 when I went to mentioned trans ratios mainly to get the 3rd gear acceleration back I had with previous trans that was 2.98, 2.10, 1.43, 1.00, .85 that I ran with the 3.70 rear. My best 60 ft was with the 2.98/3.70 combo, my best 0-60 was also with the 2.98/3.70 combo. I went with the 2.66 ratio trans mostly because a 2.98 version was not available made from 9310 steel, only 4130 which I had just broke after 2+ years of beating on it, I wanted stronger. I’d have to say the higher ration 2.66 trans is a better street geared trans but it lacked the acceleration the 1.43/3.70 3rd gear had in the 2.66 trans so I installed a 3.90 rear to get that 3rd gear acceleration back. 3rd gear is my make time gear, car pulls like hell on 3rd. BUT power has increased about 200 hp from the start of the 2.66/3.90 combo days I’m wondering if I could get better 60-130 performance going back to the 3.70 Diff? Estimating will be 1100-1150 wheel now. My best 60-130 on the street with 20” tires was 5.24 at 1050 hp. I have 75 or so more un street tested HP now and I’ll be changing axles over the winter and that would be the time to put a 3.70 back in. My desire is better 60-130 times, want to get under 5 seconds. My wonder is would I possibly spin less with the higher gear combo going back to the 3.70? I’m on the spin/grip edge at the moment Any known calculators out there that I can input data and it do simulated acceleration Curves with different gearing at the same tq and Hp figures? Question is, Is it worth a try going back to a 3.70 for better 60-130 times?? I have a NIB 3.70 I was saving for if the car ever had to go back to stock time, hate to use it if it not going to do me better then the 3.90.
Opinions??

 

[Speedy!]

My opinion, leave the 3.90 as it's going to make more grunt. Your problem sounds like traction, perhaps around the 4500 to 5000 rpm range where peak torque occurs? I'd personally dial the timing back a hair in that spot to alleviate the spin on your current tires. That way you can always dial it back in if you ever have the ability to use it. I used to do this at the track to go faster and pulling 2-3 deg would often net me a .2-.4 increase in ET if track conditions weren't stellar.

 

[16GoManGoHC2]

My opinion, leave the 3.90 as it's going to make more grunt. Your problem sounds like traction, perhaps around the 4500 to 5000 rpm range where peak torque occurs? I'd personally dial the timing back a hair in that spot to alleviate the spin on your current tires. That way you can always dial it back in if you ever have the ability to use it. I used to do this at the track to go faster and pulling 2-3 deg would often net me a .2-.4 increase in ET if track conditions weren't stellar.

Hi @Speedy! , yes, it’s a traction problem. I do have a tune that will pull timing by mph and it helps. I also leave traction control on as it pulls timing too when it senses wheel spin.
Anyone know how to make traction control more or less sensitive and adjust how much timing it does or doesn’t pull?

 

[BULL]

What is your redline?

 

[BULL]

 

[16GoManGoHC2]

What is your redline?

I shift at 66-6800, been to 7200 at times, especially in 4th.

 

[BULL]

I shift at 66-6800, been to 7200 at times, especially in 4th.

Maybe you already have it, but in case not, and you want it, I can send you the sheet to fiddle with if it's of use.

 

[16GoManGoHC2]

Maybe you already have it, but in case not, and you want it, I can send you the sheet to fiddle with if it's of use.

Thank you, I’m good though, I use an online gear calculator that’s pretty quick and slick. I know what my speeds are per gear already. I just was wondering if a different rear ratio may help tame torque some. I think there’s a Calculator that you could input torque and horsepower curves on along with gear ratios, cars weight, etc I’ll have to look for it.

 

[BULL]

Thank you, I’m good though, I use an online gear calculator that’s pretty quick and slick. I know what my speeds are per gear already. I just was wondering if a different rear ratio may help tame torque some. I think there’s a Calculator that you could input torque and horsepower curves on along with gear ratios, cars weight, etc I’ll have to look for it.

Linkage if you find it.

 

[16GoManGoHC2]

Linkage if you find it.

This the one I use for gearing, haven’t found the other yet

https://www.blocklayer.com/rpm-gear