Was talking to a local Dyno Tune Specialist and he recommended a different Rear Axle Differential Assembly to "Really Pep Up the Car more than any Tune could." He said just keep it under 180 mph otherwise the axle may snap. Any one swap out their stock rear diff on a scat pack to the below? Was it worth $1200 plus labor? 68427798AA - Genuine Mopar Rear Axle (moparpartsgiant.com)
Yes, it will wake your car up FOR CERTAIN. It's a 3.90 axle ratio https://www.tascaparts.com/oem-parts/mopar-rear-axle-differential-68427798aa That means that at 6200 RPM your Hellcat, for example, will be going, in top gear, 171mph versus 203 as a top speed. There is no doubt this would wake up ANY Challenger, but MOST especially the 2.62-gear automatic cars, as you could tune this for: A) higher RPM with just a tune on the engine and transmission, and some valve spring changes. B) A cam that REALLY makes peaky power C) Shifts that target the new, higher RPM and peakier power curve. Net result: more of your time spent near peak power, less time spent waiting FOR peak power. Potential downsides: A) higher revs on the freeway B) You will need stickier, wider tires, STAT. Oh, just noticed it's a Scat Pack. It will definitely put more pep in your step.
The differential the shop is proposing is the one used in the manual trans scat. It's ratio is. 3.90, while the ratio in the auto trans scat is 3.09. You should further research as I don't know whether the auto axles will fit the manual diff. I purchased the manual diff for my 2010 auto which requires an axle change. The change provides the potential for quite a performance difference provided you can get sufficient traction. The change will negatively affect your fuel mileage.
"Mable" is an Auto with Nitto 555 R2 185s in the rear (G2 145 in the front), but I do want to go to 305 in the rear on wider rims one day. I didn't want to have to put in a new cam shaft in order to get the new Rear Axle Differential. Do I need to get a new cam shaft? Do I need new valve springs then too? If so and I'm already knee deep into the opening engine block, is it economical to then just get bigger cylinder bores and the like? Just don't know how deep this rabbit hole goes. Assuming the Rear Axle Differential brace isn't need as well. Welcome all opinions!!
Engine changes are not required to make your diff change. The diff change will have to be communicated to the vehicles PCM which in your case might be more difficult as Dodge started locking them around your model year (your Tune specialist will know whether your PCM needs to be changed). I have an HP Tuner which also allowed me to turn off the MDS through my unlocked PCM. Some of the cam and header kits will bring your crank hp number to the rear wheels (15-20% increase).
It's a nice rabbit hole to be in, but you don't "need" any further modifications. However, if the engine is open already and you have the money, why not just go whole hog? If you are patient, Magnuson has JUST started production on the 3100 TVS-X supercharger, but has not really put it on the market yet. This is a 17 percent jump over the 2650, I believe, and that would be if it only had four-lobe rotors, but Eaton upgraded to three -lobe rotors for more airspace per rotation and less rotor. They already produce a 2650 for the Hellcats and a 2300 for the lesser Dodges. It is al dependent on how much you want to spend. What a higher-RPM-emphasis cam and lifters do is take better advantage of the now-elevated RPM by putting more power at the same (wheel) RPM as you formerly got with the (evidently) 3.09, assuming you have an automatic. Bigger displacement? Yes, if you are keeping it naturally aspirated, that will be advantageous. Here is a package ready to go from MMX: https://www.modernmusclextreme.com/...stroker-kit-by-modern-muscle-performance.aspx 454 cubic inches (7.4L) is a plentitude of cubes with which to work. 15 percent more size gives you 15 percent more of everything with no great expense of brainpower, so 561hp and obviously more torque. The rabbit hole just keeps going further. How long do you plan to keep the car? BTW, the pistons for the above kit are rated for a 250 shot of nitrous, also. The best thing to do is set your hp versus money goals and then pick the route to get to them, and take any suggestions from any no-accountability wags on the internet for exactly what they are: ONLY suggestions based on knowledge of physics, etc. What you NEED is absolutely nothing. How much power you want to step up to, long or short-term, will determine which route you take. If you increase torque via a larger engine displacement or supercharging, then your clutch and transmission will likely want to be upgraded, also. If you increase power by a cam and spring package that largely shifts power up the RPM band without heaping on a great deal of additional peak torque, then having valve springs and retainers suited to higher revs will be in order. Your idle will be sexier. Long-tube headers with high-flow catalysts if you use them and will help, also. What I would do if I had limitless cash is just start out with a Hellcat, wait for Magnuson's 3100 and go whole hog. Custom headers to my design, excessively large exhaust system, monstrous catalytic converters just because I like clean exhaust smell and the challenge of catalytic converter inclusion appeals to me. CNC-ported heads, Redeye driveshaft and half-shafts, cradle lockout bushings, possibly custom differential mount, stiffer springs preferably at stock height, the list just goes on and on. But, for your case, to leave off needing a big increase in trans capacity I would SUGGEST a relatively inexpensive cam and spring package, so the old-school route, in a nutshell: rear end gear, cam, and springs. You may be happy at that, or you just may turn into a vein-tapping addict. I don't know.
Appreciate all the advice! You know how it is. Every year you throw 2k or so at your Hotrod so every year you feel like you have a new car. She currently has 1 7/8" Kooks Long tube headers with the Boral Attack exhaust, front and back strut tower braces, 87mm throttle body with spacer, Hellcat Air box, oil catch can, Fasty's Inlet lights, Front Splitter with 6" rods, Nitto 555 R2 185s in rear, smoked reflectors, tinted windows, LED fog lights, 20" Skar Woofer with Amplifier in the trunk, and her name just before the rear tires with a black on black look. Nothing Flashy. Considering the following: (Feel free to shoot these down.) Speedlogix Rear Tension rods (to prevent hopping.) 305s on the rear with custom rims (because the stock rims are too small, supposedly) Viking Adjustable Shocks in the front (to prevent a push in the turns) I tried the Diablo Trinity 2 console and custom web tune, but I keep getting 'Too Lean' errors and the author won't fix it. (Not really sure if all the above engine mods are in the web tune formula.) This is my mid-life crisis car and don't plan on selling it...ever. The new Rear Axle Differential Assembly with matching dyno tune may be a good improvement for 2023. And maybe the stroker kit for 2024. (Another dyno tune required?) The Scatpack doesn't have the internals to support a Supercharger, so I understand, so a Supercharger\Twin Turbo isn't on the list. This is a summer daily driver and will never see the drag strip or an MN winter. Just need something to wash on Saturday mornings and drive the lady around on Saturday Date Nights. Again, Appreciate Everyone's Advice. Excited to hear all the Great Improvement Ideas.!
I quote you and comment directly: "Speedlogix Rear Tension rods (to prevent hopping.)" GOOD IDEA "305s on the rear with custom rims (because the stock rims are too small, supposedly)" GREAT IDEA. These cars NEED more rear tire. 11" wide with 22-25mm of offset (more offset puts tire further into the middle of the car, but can be adjusted with spacers in a more exact fashion.) BTW Nittos/Toyos in 315 are the same width as other 305's but they may offer more sidewall, one can hope. It is hard to have too much sidewall in these cars. If you go with Li'l Devil fender lips, you can get 325-345's in there. How greedy for traction are you? Never buy Pirellis, EVER. "Viking Adjustable Shocks in the front (to prevent a push in the turns)" Adjustable is good. "I tried the Diablo Trinity 2 console and custom web tune, but I keep getting 'Too Lean' errors and the author won't fix it. (Not really sure if all the above engine mods are in the web tune formula.)" Time for a new "custom tuner guy." I have heard absolutely nothing but good things about Curt Dusterhoff. Some of us don't play much as yet, but we sure can coach. Besides, research is my THING, as you can see here: https://www.challengerforum.com/threads/clutch-and-transmission-research-results-thus-far.7076/
The Magnuson supercharger pkg ($6895 not incl installation) at 6 psi boost is safe for your application. I'm on the 5.7 version.
You have more than enough power for the street already.. leave it be before that rabbit hole turns into a cave! Maybe a 3.73 rear differential.
