[RezinTexas]

The new transmission is in! The 2.66 1st gear is amazing! It’s a totally different car and it is a travesty that it did not come this way from the factory.

I recently finished the install of a new RPM Transmissions (from Anderson, IN) “level 6” TR-6060 with modified gear ratios including the 2.66 1st gear. It is so much better to drive, more consistent to launch and roll away from a stop sign, and ultimately more fun around town. I no longer have to think or concentrate on perfect footwork to get the car moving, it just goes. I will update this thread if/when I go to the track with this new setup.

The car is a stock-power 2018 WBHC M6. Minor mods include Barton shifter, MT ET streets, LMI intake, and a few other small cosmetic things. The best pass I ever got on the stock transmission was 11.48 @ 120.8mph.

This is a long thread all about the negative effects of the OEM M6 hellcat gearing, my journey to discovering the solution, and details of how I made the swap. I appreciate any comments or feedback!

For all M6 hellcat drivers, we are intimately familiar with the “crazy tall” 1st gear ratio in the TR-6060 – which may be OK in the Viper (the only other car in the whole industry to use the TR-6060 with the 2.26 1st gear), however for the heavy Hellcat it just plain sucks. Whether you’re puttering around town, or trying to get a decent 60-ft time at the track, the practical effect of the 2.26 1st gear is very low torque at the wheels when the car starts to move from a dead stop. Yeah it’s fun going 60mph in 1st gear, but if you’re constantly bogging or over-revving to get moving, then you’re already disappointed before you even get to 10mph.

I noticed it immediately when driving my new 2018 WBHC off the dealer lot for the first time – why am I constantly over-revving or bogging when driving away from a stop sign? Why can’t I just have smooth and easy starts? I’ve been driving manuals for all of my 23 driving years – German cars, Japanese, Corvettes, small trucks, 2500 trucks, shifting with my left hand in the UK, Nürburgring track car rental, even an 8-seater Mercedes sprinter van in Spain – and never had this issue before driving the M6 hellcat.

At first I thought I needed more seat time with the hellcat, and I would eventually “learn” how to drive it properly. Well… 4 years, 12k miles and 40 track passes later didn’t help – I still struggle to get consistent starts (at the track or on the street) and it really takes away from the driving experience in my opinion.

The issue was never more apparent than when I raced an M6 392 Challenger at the drag strip a couple years back (both cars were stock with 20” drag radials) – we both got a decent launch and were dead-even on the 60-ft times. I obviously pulled away on the top end, but we were evenly matched on the low end. The reason is simple – although the 392 has roughly 30% less HP/TQ, it’s 1st gear has a 30% torque advantage. Of course there are other factors that help the 392 like weight and axle ratio – but that gearing advantage off-the-line is huge.

So why would Dodge put such a crappy 1st gear set in an otherwise perfectly spectacular car? I don’t know for sure, but there are a few theories. I speculate that it’s the same reason you can’t get a M6 with the Redeye challenger – the 650 ft-lb torque output of the “base” hellcat engine matches the maximum rating the TR-6060 in factory form. First gear is the highest load case from a torque perspective on a transmission (not counting the reverse gear, but who’s doing hard launches in reverse??). Tremec does not offer a higher torque rated TR-6060, and that also drives the requirement to pair the most torquey engines with the numerically lowest gearset offering – and Dodge isn’t about to develop their own transmission or modify the Tremec on a relatively low volume option of a mass produced vehicle, it doesn’t make financial sense in my opinion. Forum user @charger440rt summarized it nicely in this thread: “I assume Dodge went with a 2.26 in 1st to increase the torque capacity on the transmission, as the choice of 1st gear ratio is directly proportional to what a trans can ultimately withstand.” That explanation makes sense to me, however it doesn’t explain how the 2.90 reverse gear is OK, or the 2.66 1st gear in the 2013 Mustang GT-500 with a 625 lb-ft engine, so maybe we’ll never know for sure why the M6 Hellcat came with the 2.26 1st gear set. Some people also speculate that Dodge wanted the best 0-60ft time possible for “magazine racing bragging rights”, however that doesn’t make much sense to me because the A8’s can smoke the M6 0-60 time in factory form and they have a much lower (numerically higher) final drive ratio in 1st gear. Maybe Dodge was just lazy and “copy & pasted” the same transmission from the Viper and couldn’t be bothered to research / optimize the gear ratios for the different platform? Perhaps that last theory is the most plausible as they obviously know that manual transmissions are on their way out forever, so why waste any more development time on them?

So why go through all the hassle of changing the transmission gearing when traditional wisdom says you can just swap out the rear axle ratio to get better gearing? That doesn’t help you much on the M6 hellcat for a couple reasons. Firstly, the M6 hellcat already comes with a 3.70 axle ratio, and the only other option for stock replacement is 3.90 – that’s not going to move the needle far enough. Sure you can go custom 9” setup, but that’s not a good or cheap option for those of us who want to keep the car “stockish”. Even if you manage to get a 4.3 or higher rear end, cruising at 80mph in 6th gear would result in 2500+ RPMs because the 6th gear ratio is also poorly spec’d in the M6 hellcat in my opinion (it should be numerically lower). Of course we’re not trying to optimize gas mileage in these cars, however I would like to be able to drive on the freeway for long distances without unnecessarily running that big powerplant at 2500+ RPMs continuously. So that’s why the only realistic option to improve the driving experience is changing the transmission gearing itself.

And about that 0.63 6th gear ratio in the M6 hellcat… it is a very strange choice when you consider that the M6 392 cars come with the 0.50 6th gear ratio. The 0.50 is better for cruising on the freeway, why put a numerically lower 6th gear in a car with more than 200hp less output? Shouldn’t that be the other-way-around? More evidence of why Dodge perhaps just slapped in the Viper trans into the M6 hellcat without even thinking about the gearing. To be clear, these gearing choices make perfect sense for the Viper – a close ratio trans optimized for high-speed road racing in a lightweight sports car – what could be better? It just doesn’t make any sense for the heavy muscle car hellcat.

In the below charts you can see the numbers – OEM gearing and shift RPMs for the Hellcat M6, Hellcat A8, and Scat Pack M6. On the A8 Hellcat and M6 Scat Pack, the final drive numbers are comparable – around 12 for 1st gear, and under 2.0 for the top gear. This gives the best spread for a heavy muscle car – lots of torque off the line, and low RPMs for highway cruising. The M6 hellcat numbers are much different – final drives of 8.36 and 2.33 for lowest and highest gears respectively. For a practical comparison, try driving and launching an A8 hellcat or M6 scat pack while always starting in 2nd gear… that’s what the M6 hellcat acts like in 1st gear!

For my new transmission from RPM, you can see the chart at below left. The new 1st gear final drive is much lower (numerically higher), and the top gear final drive ratio is closer to the A8 cars. The chart on the right shows how I can get just a little more by going to a 3.9 rear end – I’m thinking that this might be the perfect sweet spot for mostly street driving.

I found inspiration for this project based on reading the experiences of forum users @catdodgehell and @16GoManGoHC , among others who blazed this trail and showed us it's possible to lay down low 10s and high 9s, bringing parity with similarly-modded A8 cars. I don’t know if I’ll spend much time at the track with the new tranny, I opted for a new factory clutch (instead of aftermarket) and I don’t really want to be upgrading axles and the driveshaft. With the new built trans and 2.66 first gear, I image the weak point in the system is now the driveshaft? Not sure about that.

I initially wanted to build the transmission myself with upgraded components. I ended up ruling out this option because of how hard it is to find detailed instructions or writeups online. I taught myself how to build LS racing engines with the vast information available online, however the same level of detail is clearly not available or publicized for performance TR-6060 transmissions. In the few instances where I could find apparently reliable information, it was for Corvette or Camaro versions of the TR-6060. For this reason, I ended up ordering a new “level 6” built transmission from RPM transmissions in Anderson, IN. I paid full price and plan to keep my core transmission as backup, sacrificing the $2500 core charge in the process. Here is where my factory transmission will live long term:

 

[RezinTexas]

Here are the choices I made regarding components:

• 1st gear ratio: 2.66… This is the biggest choice to make, either the 2.66 or 2.98. I was very tempted to get the 2.98, however I opted for the 2.66 based on feedback from @16GoManGoHC in this thread.
• 6th gear ratio: 0.50… As discussed above, I wanted a tall overdrive gear for freeway driving. The factory 6th gear is now right in the middle between the new 5th and 6th gears. I can’t see a situation where I would ever want the old 6th gear ratio.
• All 6 ratios are shown in the chart below. You can see how I “spread out” the ratios, shorter on the low end, taller on the high end, moving away from the close-ratio ethos of the Viper gearing.

• Clutch – I opted to go with a new stock replacement clutch, for a few reasons. Firstly, to change it “while I was in there” seemed logical – however see photos below of the factory clutch upon removal – it looked basically brand new, only very minor material removed. Pretty impressive for having 40 track passes! Secondly, a new OEM clutch seemed like the best fit for my needs: I read about how multiple forum members used them hard at the track pass after pass without issue, and also I didn’t want to deal with the risk of issues regarding fitment, shimming, pedal feel, daily drivability, NVH, etc. that I was reading about with some aftermarket clutch situations. Thirdly, I scored a new OEM clutch from a forum member on here shipped to my house for about a third of what a new one cost, he even included a new flywheel with it. The clutch was new, never used, and just sat on the shelf in his California garage for a few years. See photo of my “new” clutch below.

Condition of factory clutch upon removal:

Replacement "new" factory clutch:

• Flywheel – After seeing the excellent condition of the factory clutch, I opted to leave the factory flywheel in place in the car. There were heat marks on it, but it was smooth and free of any scratches or rough edges, etc. See photo below.

• Clutch alignment tool – I chose the Mantic Clutch 1-1/8”x26 tool for $45 + $50 shipping (which is absolutely nuts) for $95 delivered to my door. I didn’t want to trust a plastic tool, so I shelled out the money for this steel version. It seemed a little strange that it had painters tape wrapped around the tip to properly match the ID of the pilot bearing, however in the end it didn’t matter. Here is a photo, other than the ridiculous shipping cost I’m happy with it.

I did the swap myself in my garage using tall jack-stands and a “low lift” transmission jack from Harbor Freight. Obviously, a lift would be ideal, however I took my time and made sure everything was safe and secure and was able to get it done without any real difficulty. Honesty the hardest part was removing the exhaust hangers – those suckers are tight! I found the following YouTube links which were enormously helpful:

Here is the sequence I followed when removing the factory transmission:

• Before parking and lifting up the car, be sure you have adequate clearance & access around the car to remove the exhaust system – it is quite large and cumbersome as one piece. Consider where you put the jack stands to enable this access. Pulling the exhaust assembly out from the back end of the car is easiest if you have the room.
• Disconnect the battery – important!!
• Remove shifter assembly from inside the car, especially if you have a Barton shifter.
• Remove exhaust tips – this helps with clearance when maneuvering the exhaust assembly and avoids scratching/damaging the tips.
• Disconnect the oxygen sensor connectors on the catalytic converters. Disconnect the active exhaust valve electrical connectors. There is also an electrical grounding strap on the passenger side, on the exhaust tip hanger.
• Remove exhaust as one piece, use prybars and lubricant to remove rubber hangers. 16mm stud bolts on the cats. Alternatively, the exhaust system can come out in 3 smaller pieces if necessary.

• Remove driveshaft, mark the flanges, using T-50 socket
• Remove trans cooler lines at junction box under engine, with 20mm wrench (I got a cheap one on Amazon) and plug the lines.
• Remove starter (this is why disconnecting the battery is critical).
• Disconnect all electrical connectors from the transmission and remove the clutch line (cap the clutch line otherwise it will leak brake fluid), I used a little rubber cap that came in an assortment pack from Autozone.
• Support bellhousing with jack and remove the transmission support & cross member.
• Remove tranny to bellhousing bolts (8x 15mm), very long socket extensions work well here! See picture below.

• I removed the trans with the 800-lb rated “low lift” transmission jack from Harbor Freight. The jack worked OK, however it wasn’t tall enough because I had the car pretty high in the air. So I had to use two 4x4 wood blocks (see image below) under the transmission. Once I got the transmission free of the engine, the wood blocks had to be removed or else the trans would not clear the underside of the car while pulling out the jack! This was pretty difficult to do without rolling the transmission off the jack – I strongly recommend a friend to help with this step.

(don’t worry, those wooden Jenga blocks are all held tightly together by dozens of 2-1/2” 16-ga nail-brads)

• With the trans removed, I was able to remove the jack supporting the bellhousing by placing some wood supports between the engine and the sub-frame – to prevent the engine from rocking backwards. I also placed another jack on the engine oil pan (with very light pressure) to prevent the engine from rocking forwards. Not sure if that was totally necessary, but better safe than sorry.
• Remove the bellhousing with 13mm nuts on the top to hold the wiring conduit bracket, 14mm stud bolts on the top, 15mm bolts on the bottom. Again, use the long extensions here and it’s easy. No need to access these bolts from under the windshield cowl (that’s too much extra work).
• Remove pressure plate with E12 “inverse-torx” bit (6 bolts).

Installation of the new trans is basically a reversal of the above steps. Here are a few random tips for the installation:

• I filled the new trans with 3.75 qt of new fluid, while it was still on the bench. I also flushed out the fluid in the trans cooler and lines using a small hand-operated fluid transfer pump.
• I used Royal Purple manual transmission fluid, per recommendation from RPM.
• I opted for a new throw-out bearing, and 2 new bolts from Mopar. I’m not sure this was necessary, given the low mileage and excellent condition of the factory clutch.
• To bleed the clutch line, I depressed the clutch pedal about 50 times, then it was as-good-as-new. I topped off the fluid in the reservoir (it was down slightly from max fill) with new DOT-4 fluid.
• On the OEM driveshaft bolts, there is a green type of sealant on the threads which seems to seal and/or centralize the bolts inside the rubber bushing assembly. This green sealant isn’t actually applied on the male to female thread engagement area. For re-assembly, I stripped off the green substance, added black RTV in it’s place, and added blue loctite to the thread-to-thread engagement area. I’m not 100% this was the right approach, however it’s what I did. I plan to check these bolts at each oil change to ensure they aren’t backing out.
• Torque values I found online for re-assembly:
• Bellhousing bolts & studs: 33 ft-lb
• Driveline flange bolts: 49 ft-lb
• Pressure plate to flywheel bolts: 55 ft-lb

Thanks for reading! Let me know what you think.

 

[OldMoparGuy]

Great write up. Interesting you went with the oem clutch. I imagine with that better 1st gear ratio it might even last longer. Love to upgrade mine eventually, care to discuss price? Btw-I’d keep my original tranny as well.

 

[RezinTexas]

Great write up. Interesting you went with the oem clutch. I imagine with that better 1st gear ratio im might even last longer. Love to upgrade mine eventually, care to discuss price? Btw-I’d keep my original tranny as well.

Thanks man, $6,634 for the new trans, including shipping to my local FedEx freight facility. I could probably sell the factory tranny for $3kish, but definitely planning to keep it on the shelf for a while.

 

[OldMoparGuy]

So that 6.6k included the 2.5k you would have received in credit had you turned in you tranny core, is that right? I guess there is the option to rebuild yours to keep matching numbers tranny and save the 2.5k core charge?

 

[RezinTexas]

So that 6.6k included the 2.5k you would have received in credit had you turned in you tranny core, is that right?

correct

 

[16GoManGoHC]

NICE write up, glad you LOVE the new feel of the car. I ended up going to a 3.90 rear to get 3rd gear final drive ratio more close to the 1.43 in the 2.98 version. Car pulls like mad in third now, will generate .84 G of acceleration just stomping on it in third at 50 mph and 6 seconds later your up past 130, LOVE it!! Glad you found happiness finally!! 👍
I haven’t had my cat to the track yet this year but street performance tells me it should be into the 9’s at the track no problem. These are all street times on 315/35-20 Nitto 555R2 rears.

 

[RezinTexas]

NICE write up, glad you LOVE the new feel of the car. I ended up going to a 3.90 rear to get 3rd gear final drive ratio more close to the 1.43 in the 2.98 version. Car pulls like mad in third now, will generate .84 G of acceleration just stomping on it in third at 50 mph and 6 seconds later your up past 130, LOVE it!! Glad you found happiness finally!! 👍
I haven’t had my cat to the track yet this year but street performance tells me it should be into the 9’s at the track no problem. These are all street times on 315/35-20 Nitto 555R2 rears. View attachment 594251 View attachment 594252 View attachment 594253 View attachment 594254 View attachment 594255 View attachment 594256 View attachment 594257 View attachment 594258 View attachment 594259 View attachment 594260

Dang! 11.3s on the street seems insane with an M6 cat!
So you're running the 2.66 gear set now?

 

[RezinTexas]

So that 6.6k included the 2.5k you would have received in credit had you turned in you tranny core, is that right? I guess there is the option to rebuild yours to keep matching numbers tranny and save the 2.5k core charge?

Yes, I believe RPM has the option to rebuild your trans (to keep matching numbers), and I know that Tranzilla and Tick Performance also do it that way. I actually ended up choosing RPM precisely because they had the option to build me a new unit, so I could keep my factory trans as-is and untouched.

 

[16GoManGoHC]

Dang! 11.3s on the street seems insane with an M6 cat!
So you're running the 2.66 gear set now?

Yes, street times, no burn out just launch like from a red light.
Yes, I went with the 2.66 Tranzilla after breaking the 2.98 one late last fall. I would have went with another 2.98 but it’s only available with 4340 material gears and I needed stronger, the 2.66 was available with 9310 material and I needed stronger so went with the 2.66 because of that. Switched rear diff to a 3.90 unit to get 3rd gear ratio deeper.

Do you know the gear material steel type RPM used in your build for that price? That’s a good price for complete trans with no core return.

 

[RezinTexas]

Yes, street times, no burn out just launch like from a red light.
Yes, I went with the 2.66 Tranzilla after breaking the 2.98 one late last fall. I would have went with another 2.98 but it’s only available with 4340 material gears and I needed stronger, the 2.66 was available with 9310 material and I needed stronger so went with the 2.66 because of that. Switched rear diff to a 3.90 unit to get 3rd gear ratio deeper.

Do you know the gear material steel type RPM used in your build for that price? That’s a good price for complete trans with no core return.

I don't know what gear set material I have, I sent a query to RPM to find out. On the phone, they told me that this "level 6" trans is rated for 1,000 ft-lb.

Where did you get your diff? Any mods to the diff required to handle the torque? The build sheet for my hellcat says "Anti Spin Differential", does the 3.9 also include that? Sorry for all the questions, thanks man

 

[16GoManGoHC]

I don't know what gear set material I have, I sent a query to RPM to find out. On the phone, they told me that this "level 6" trans is rated for 1,000 ft-lb.

Where did you get your diff? Any mods to the diff required to handle the torque? The build sheet for my hellcat says "Anti Spin Differential", does the 3.9 also include that? Sorry for all the questions, thanks man

Picked up a few 3.70 and 3.90 diffs from a shop selling out, Mopar out of the box 3.90, I use the P4D on the rear and their race brace on the front. It’s holding up to 950 wheel better then the 2.98 Tranzilla did lol. They start whining after you beat on them but that’s pretty normal with all these diffs from my experience at least. Yes please find out on gear material as if that’s 9310 that’s an excellent price and probably I’d be going to them next if I break another trans

 

[RezinTexas]

Picked up a few 3.70 and 3.90 diffs from a shop selling out, Mopar out of the box 3.90, I use the P4D on the rear and their race brace on the front. It’s holding up to 950 wheel better then the 2.98 Tranzilla did lol. They start whining after you beat on them but that’s pretty normal with all these diffs from my experience at least. Yes please find out on gear material as if that’s 9310 that’s an excellent price and probably I’d be going to them next if I break another trans

Do you have a spare 3.9 you'd sell me?

 

[16GoManGoHC]

Do you have a spare 3.9 you'd sell me?

Unfortunately I sold them all off last year and only kept one 390 and one 370 as spares. The 390s not that expensive new they were around $800 last I checked the 370s are like 1400 I don’t know why such a difference other than maybe the quantity that they make of them is much less as there’s nothing special or different about them verse the 390

 

[RezinTexas]

I feel that my writeup is wholly insufficient without mentioning Chad F. who has managed to "crack" the code to drag strip success with the 2.26 1st gear. He recently posted his 9-sec pass timeslip on Facebook with his M6 hellcat with basic mods and an aftermarket clutch. Clearly it's possible to make the car fly at the track - even with the 2.26 1st gear! Well done!

 

[charger440rt]

Forum user @charger440rt summarized it nicely in this thread: “I assume Dodge went with a 2.26 in 1st to increase the torque capacity on the transmission, as the choice of 1st gear ratio is directly proportional to what a trans can ultimately withstand.” That explanation makes sense to me, however it doesn’t explain how the 2.90 reverse gear is OK, or the 2.66 1st gear in the 2013 Mustang GT-500 with a 625 lb-ft engine, so maybe we’ll never know for sure why the M6 Hellcat came with the 2.26 1st gear set.

I have not posted or even come to this forum in a very long time (years). It's ironic on the day I came back I had an alert you mentioned me. I still have my 2015 Hellcat but went back to my 60s/70s Mopars. I recently bought a 70 Challenger 440 RT which has my interest currently. Anyway, I think the reason the reverse is 2.90 is because Dodge doesn't expect anyone to do hard launches in reverse, so it does not matter. As to the 2.66 in the Mustang, Ford was probably being less conservative. The 2.26 stock Hellcat 1st gear is exactly the same ratio as the then production Viper transmission if I recall so I assume they repurposed it for the added strength. Take care and good luck with your car.

 

[Frosty]

Unfortunately I sold them all off last year and only kept one 390 and one 370 as spares. The 390s not that expensive new they were around $800 last I checked the 370s are like 1400 I don’t know why such a difference other than maybe the quantity that they make of them is much less as there’s nothing special or different about them verse the 390

The 3.90's have unfortunately gone up quite a bit. I just put mine in a month ago and best I could find was $1300.

You and RezinTexas really suck. I was super happy with my 3.90's but now you got me wanting this trans too. I don't race but I'd love even better 1st gear and would die to have a better 6th gear. I drive my car at highway speeds a lot so that change in 6th is not my favorite but I love the rest of the gears.

RezinTexas - Where are you located at?

 

[shademaker]

Just curious, aside from personal feelings on the matter, are our cars really number matching? I was told that when the trans was swapped out under warranty once that the trans is not tied to the car. I think that can be said for everything. I mean, they might want the chassis vin number to make sure you have a red eye for example before selling a red eye part, but 10 years from now, could any one actually show that the facotry put part serial 1111 and part serial 2222 together in the same car?

 

[charger440rt]

Just curious, aside from personal feelings on the matter, are our cars really number matching? I was told that when the trans was swapped out under warranty once that the trans is not tied to the car. I think that can be said for everything. I mean, they might want the chassis vin number to make sure you have a red eye for example before selling a red eye part, but 10 years from now, could any one actually show that the facotry put part serial 1111 and part serial 2222 together in the same car?

.
This thread leads me to believe major parts are serial numbered:

Location of Engine Serial No. Charger HC

Hello I am looking for the location of the engine serial number as the customs need it to release the car . Anybody have an idea about its location ?

www.hellcat.org

 

[16GoManGoHC]

Just curious, aside from personal feelings on the matter, are our cars really number matching? I was told that when the trans was swapped out under warranty once that the trans is not tied to the car. I think that can be said for everything. I mean, they might want the chassis vin number to make sure you have a red eye for example before selling a red eye part, but 10 years from now, could any one actually show that the facotry put part serial 1111 and part serial 2222 together in the same car?

I was told no, no vin matching of engine or trans or anything besides part number installed.