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I've used one of these universal ones on my Gen3

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MoparMap

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I think the only real difference between that and the factory tool is the angle of the threaded hole for the slide hammer. The factory one has it coming off at a slight able, presumably so you still seat it evenly against the axle cup but not have the hammer in the back of the knuckle. Good to know a generic one functions though.
 
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Steve M

Steve M

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HPTuners Calibration Adjustments (Gen 4/5 Only)​

As the title states, this section only applies to a small subset of the Gen 4 (2008-2010) and Gen 5 (2013-2017) crowd – those that use HPTuners for a tuning solution.

This is not about speedometer calibration – the Gen 3/4/5 cars reference the ABS sensors to back out vehicle speed, so it doesn’t matter what gear ratios you have in your transmission or differential for the speedometer to function properly.

The Gen 4/5 PCM, however, does need to know what gear the transmission is in during normal operation. I’m not sure how this information is used, I just know that it is referenced for specific functions. One example is the deceleration fuel cut-off (DFCO) algorithm. DFCO does as the name implies – it cuts the fuel off during deceleration to allow for engine braking. It is also responsible for how fast the engine RPMs drop between shifts while making sure that the fuel comes back in around the set idle RPMs to keep the engine from dying. DFCO behavior varies whether the car is in gear vs. in neutral, and it can be set to cut in/out at different RPMs by individual gear (1-6). If the PCM doesn’t know which gear the transmission is in, it reverts to the “in neutral” DFCO routine during any type of deceleration. This will be quite obvious because the “in neutral” DFCO routine will cause the car to backfire/pop much more loudly than it would otherwise during engine braking.

Unfortunately, the TR-6060 transmission doesn’t have any sensors that can tell the PCM what gear it’s in. The PCM, therefore, has to rely on the available sensors to back this information out. The PCM uses vehicle speed (provided by the ABS sensors) and engine RPMs (provided by the crankshaft position sensor) to accomplish this task. There is a single look-up table that is referenced – in HPTuners VCM Editor (ver. 4.8.7), it is under Transmission > VSS/RPM Ratios:

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The table has 6 rows (one per gear) and a single column. The units for each table entry are kilometers per hour per engine RPM (kph/RPM). Any changes to the transmission gear ratios or the differential gear ratio will alter this calculation, thus requiring recalibration. Also note the “Allowed Error” box (set at 5%) in the image above – this is used to help account for tire size variation (e.g., normal wear) among other things.

The math to calculate these values is relatively straightforward – the hardest part is keeping track of the units. To keep things simple and repeatable, I baked it into my personal gear ratio calculator spreadsheet:

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The primary inputs are the tire size, transmission, and differential gear ratios. The rev limiter value is used to figure out the max speed in each gear, and the “Cruising Speed” input is used to figure out what RPMs your engine will be turning at whatever speed/gear combination you care about (good for figuring out how to stay out of a known exhaust drone zone).

Note the VSS/RPM ratio values in my spreadsheet do not exactly match the values in the stock calibration (see the HPTuners screen cap above), but they are in the ball park. If you mess around with the spreadsheet, you can get them to match pretty closely if you assume a ~26” rear tire diameter instead of the OEM tire diameter which is ~27”. Keep in mind my tire diameter calculations are based solely on the tire size specifications – not actual measurements. Regardless, the calculated values work as intended for my application.

If someone really wants to dig into the math, I’d be happy to walk them through it, but I’ll spare everyone the details and skip straight to the answers that cover most of the Gen 4 and Gen 5 configurations you’re likely to encounter in the wild.

Gen 4 VSS/RPM Ratios:

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Gen 5 VSS/RPM Ratios:

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MoparMap

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Huh, that's pretty interesting. The Megasquirt on my 67 Dart does gear position calculation based on speed and rpm as well for things like boost by gear and I'm sure some other features. I only use it for datalogging as I don't have boost on my car, but neat to see how it can be used in other applications.
 
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Steve M

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Full story: I ran into this issue many years ago (~2017) when I swapped a Gen 5 differential assembly into my car. I didn't think I'd need to mess with the calibration since the speedo doesn't care, so I left it alone.

During the first few drives, any time I'd let off the gas the backfiring/popping was ridiculous. It took me a little while to realize something wasn't quite right, and that's what sent me digging back through the archives.

I finally put two and two together based on a conversation I'd had with another member (Jack B) who was also an HPTuners user at the time. He had swapped the guts of his Gen 5 TR-6060 with a Gen 4 TR-6060 so he could have a shorter first gear - he was tired of killing clutches at the drag strip trying to cut a decent 60' with the loooong 2.26 Gen 5 first gear. He'd asked to see my VSS/RPM ratio values so he could get an idea how it all worked...I'm guessing the Gen 5 is more sensitive to this being correct since it has cruise control, traction/stability control, etc.

Once I remembered that conversation, I started digging around. It wasn't until I saw the units on the table values (kph/RPM) that it all finally clicked. I changed the calibration, and the ridiculous popping immediately went away.

My other clue came from Arrow/Prefix, specifically their Gen 5 race control module product page (https://store.prefix.com/products/viper-gen-5-race-control-module):

  • Allows racing approved final drive ratios from 3.07:1 to 4.10:1.

I'm guessing they account for this wide range of axle ratios by picking values that are somewhere in the middle and opening up the allowable error window. I've thought about doing the same, but since I have the tools to set it exactly the way I need it, I just do that instead. For Arrow/Prefix, it makes much more sense...I'd get pretty tired of adjusting that table any time someone changed gear ratios. I'm sure it doesn't cover all the possibilities roaming around out there in the wild, but it probably covers 95%+ of them.
 

Goggles Pizano

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Aaaa, the metric system something that has eluded Americans from the beginning of time.:)

@MoparMap pictures of the Dart when you can, engine bay and all. Sorry to hijack the thread with a Dart pictures but there isn't a subforum for 'other' cars.
 

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Aaaa, the metric system something that has eluded Americans from the beginning of time.:)

@MoparMap pictures of the Dart when you can, engine bay and all. Sorry to hijack the thread with a Dart pictures but there isn't a subforum for 'other' cars.
Not the most recent pictures, but here are a couple I've got handy at least. Outside still pretty much looks the same, but I've since redone the wiring on the engine bay a little bit. The interior is decently different now as well as I went to a set of Genesis Couple seats I got from a coworker instead of the gen 1 Viper seats in the pictures. The Viper seats were neat, but they don't lean forward enough to get in the back seat easily, so I wanted something a little more useful.
 

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Steve M

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I learned many lessons during my first differential rebuild, and I continued to learn many lessons afterward. I'll do my best to capture these in my next few posts.
 
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Lesson 1 - 3.73s Were Not Ideal For Me​

I originally chose 3.73 gears for drag racing. I’d been using 3.55s with a 26” tire (Hoosier 335/30-18 DR2) for a few years, and I always felt like the car was just starting to pull really hard after I went through the traps in the quarter mile (trap speeds usually around 128-131 MPH). I thought the 3.73s would let me take advantage of this perceived seat-of-the-pants feeling and also help get the car off the line a little easier.

I only got one pass in on the 3.73s before I broke an OS Giken output stub, so I clearly can’t draw any firm conclusions, but I wasn’t all that impressed with what I saw from that single pass. Below are two time slips of similar runs under similar conditions (similar DA’s, nearly identical race weight, same tires, same tire pressures, etc.) – one with 3.55s, and one with 3.73s:

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What stuck out to me the most were the trap speeds – they were all noticeably down with the 3.73s, especially in the 1/8th mile. ~100 MPH is usually what my car would trap in the 1/8th when I was running 3.07s. On the positive side, the 3.73s gave me the best 60’ time I’ve ever cut on a first pass of the night, and also the best first pass e.t. I’ve ever had. I really wish I had been able to get some more passes in to get a better idea of the benefits/drawbacks.

60-130 times were also noticeably down (disregard the Dragy reported density altitudes – the DA values listed under the screen caps were from my Kestrel 5100 weather meter, and should be considered the truth source):

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Shift speed looked about the same between the two, but to be fair, the 60-130 time with the 3.55s was on my fourth pass of that evening, which is usually when I do my best driving (and that was my personal best 60-130 time). It still takes the same number of shifts to cover 60-130 with the 3.73s, but as expected, the shift points have clearly moved down in the MPH range, putting the 3-4 shift right in the middle of going through the 1/8th mile traps. This tracks with what my gear ratio calculator told me had I actually paid attention to the numbers:

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Keep in mind that my rev limiter is set to 6,600 RPMs and my shift light flashes at 6,300 RPMs, so I’m estimating that my shifts happen around 6,400-6,500 RPMs factoring in my reaction time from when the shift light flashes until I actually execute the shift.

That could be fixed pretty easily, however, by switching back to a 28” tire (e.g., a 345/35-18 Mickey Thompson ET Street S/S):

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As with anything, there are pros and cons for each.

26” Hoosier DR2
Pros:
- One of the lightest weight Viper-sized drag radials out there (26 pounds per tire)
- Very sticky tire (once properly heated)
Cons:
- Short sidewall has little to no flex (more drivetrain shock)
- Takes a solid 2nd gear burnout to really heat the tires up
- Not DOT street legal (no tread whatsoever)
- Looks goofy on a Gen 3/4 Viper (huge rear wheel gap)

28” Mickey Thompson ET Street S/S
Pros:
- Requires only a light burnout to make them sticky
- DOT street legal
Cons:
- Sidewall flex makes the car feel a bit unstable at times
- Heavy (38 pounds per tire)

If you can tolerate the extra 24 pounds of rotating mass, 3.73 gears with a 345/35-18 drag radial might be a nice choice for drag racing. The extra sidewall would certainly be welcome for keeping the drivetrain shock to tolerable levels during the launch. Would it have kept me from breaking an OS Giken output stub? Who knows, but probably not.

But for my setup and my goals with the car, the 3.73s weren’t the right choice. They are also way too much gear on the street unless you want to run a drag radial at all times. If you like spinning tires and going nowhere fast, however, 3.73s might be for you.
 

MoparMap

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A CARBURETOR!
Funny story, but it's actually not, lol. It was a carb when I started, but what is in that picture is a 4 barrel throttle body that looks like a carb. Got it in a package deal with the Megasquirt and a bunch of other stuff. The setup I bought was originally installed on a 340 and the guy took everything off to swap back to a carb, so I got an intake, fuel rails, throttle body, O2 sensor, fuel pump, computer, etc. Sold all the parts I didn't need and ended up with a really good deal.
 
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Lesson 2 - Check the Dates​

Moving forward, I knew two things for certain:

1. I was not going to be putting 3.73s back in my Viper
2. I wanted to make sure nothing else broke inside the case when I sheared off the OS Giken output stub

So with a grand total of 523 miles since its last rebuild (and with much sadness), out came the differential. I bolted it to my home-made fixture, hoisted it back up on to my work bench, and prepared myself for yet another whiff of that awful smelling OS-250R 80W250 gear oil. On the positive side, that stuff sticks to everything, so it provides about as much protection as you are going to find from a gear oil. On the negative side, that stuff sticks to everything, so you’ll be spending a lot of time cleaning it off, and waste a lot of paper towels during the process.

I popped off the differential cover and made an unexpected discovery – portions of the Permatex Ultra Grey RTV silicone sealant were still wet like I had just applied it (which was clearly not the case). There were never any leaks, and the portions that were applied to the carrier bearing caps had thankfully cured properly, but the sealant in some of the threaded and unthreaded blind holes in the housing had not.

I know I cleaned everything thoroughly with brake cleaner prior to applying the sealant, and I know I let it cure for 24+ hours before exposing it to any type of oil, so I was at a loss as to why this might have happened. A quick Google search netted a few anecdotes from various folks scattered around the globe that complained of similar issues, but most used something different than Permatex Ultra Grey. I did, however, find one nugget of information that was eye opening, and which led me to my next lesson learned:

RTV Silicone Sealant has a limited shelf life.

Permatex doesn’t come right out and say it in any of their open literature (at least not that I could easily find), but The Drive published an interesting article back on September 8, 2022 titled “PSA: RTV Expires”. Link: https://www.thedrive.com/guides-and-gear/psa-rtv-expires

Pertinent quote from that article:

“Frustratingly, Permatex doesn’t readily list shelf life for any of its automotive-grade RTV sealant. Nor is there a general consensus on the shelf life of any RTV with some touting as short as six months, while others list as long as two years, depending on storage conditions. We reached out to Permatex for clarification on expiration.

“According to Don Sutliff, Senior Product Support Specialist, ‘Silicone compounds have an approximately 24-month shelf life from the date of manufacture unopened.’”


Permatex does at least tell you how to read their date/batch codes here (scroll to the bottom): https://www.permatex.com/faqs/

“All Permatex® products are stamped with a date/batch code number indicating when the product was made. Here’s an example of the date code and how to read it (Example: 9AB1234A01) the first digit is the year 1=2011, 0=2000, 9=2009 etc. and the first letter is the month of the year A=Jan. B=Feb. C=Mar. etc. the next letters and digits is the batch number and the last two digits in the day of the week the product was manufactured 01=1st-31=31st. The example: 9AB1234A01= 9=2009 A=January 01=1st day of month.”

Permatex appears to have deviated slightly from this scheme, but the same basic rules should still apply. For example, this tube appears to have been manufactured in September of 2022 (2 = 2022, I = September)?

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And this one in October of 2022 (2 = 2022, J = October)?

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I reached out to Permatex customer support through their website to verify the date of manufacture of the second one since it was slightly newer:

“I wanted to verify the date of manufacture of the product I just bought - the code at the bottom of the tube reads "2JZ2304". I'm assuming that means it was made in October of 2022? Thanks!”

None other than Don Sutliff himself replied:

Steven,

You are correct.

Don Sutliff

ITW Engine Repair | Senior Product Support Specialist


Unfortunately, I never took a close-up picture of the original tube of RTV that came with the rebuild kit from Jon B, so that tube’s date of manufacture is known only to God and maybe a few critters at the local landfill at this point. The two tubes pictured above were purchased after that first rebuild from two different local stores (O’Reilly and Menards). Given the results, I suspect that the original tube of RTV was expired, but I’ll never know for sure.

Regardless, when it comes to RTV silicone sealant, buy it just before you need it, and check the date code. If it doesn’t have one, find one that does. Do not use expired RTV sealant…the cost is too little and the consequences are too great, especially considering the time and effort required to fix it after the fact.

If the sealant on the carrier bearing caps had not set up properly, that would have been a BIG mess.
 

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Lesson 2 - Check the Dates​



portions of the Permatex Ultra Grey RTV silicone sealant were still wet like I had just applied it

This is excellent advice. What's interesting is I had the almost the exact same experience 40 years ago. On a Chrysler products' rear end I built, the gear oil was seeping out. The RTV was still wet. It was a small tube of RTV that came in a Dana 60 bearing kit. Probably on the warehouse shelf for years.

Ever since then I've purchased RTV from a store that sells a lot of it, like Walmart, so the chances of it being old were reduced. These date codes are a much better way to do it.
 
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Steve M

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This is excellent advice. What's interesting is I had the almost the exact same experience 40 years ago. On a Chrysler products' rear end I built, the gear oil was seeping out. The RTV was still wet. It was a small tube of RTV that came in a Dana 60 bearing kit. Probably on the warehouse shelf for years.

Ever since then I've purchased RTV from a store that sells a lot of it, like Walmart, so the chances of it being old were reduced. These date codes are a much better way to do it.

I was always under the impression that as long as a tube of RTV sealant was factory sealed/unopened, it was still good. It was an eye opening experience for sure...hopefully I can keep others from making the same mistake.
 

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Thank you for posting this info, no shop wants to go near a Viper diff and I might have to be undertaking this very project My diff clunks and I suspect the spiders are shot.
 
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Steve M

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Thank you for posting this info, no shop wants to go near a Viper diff and I might have to be undertaking this very project My diff clunks and I suspect the spiders are shot.
That was precisely my intent with this thread...as time goes on, the number of shops that have the knowledge/skills/abilities/willingness to work on Vipers seems to keep going down, which is a shame because as cars go, Vipers are pretty easy to work on mechanically.

There aren't many folks on the forums anymore, but I still believe it is the best format for retaining/maintaining the knowledge base required to keep these things on the road. I still have more to add to this thread (more gear pattern examples, etc.), but for the most part, everything you should need to know is already in here when it comes to a differential rebuild. If you run into a snag, post it up and we'll all learn together.
 

MoparMap

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Thank you for posting this info, no shop wants to go near a Viper diff and I might have to be undertaking this very project My diff clunks and I suspect the spiders are shot.
Viper diffs are known to clunk when brand new. Have you had the car for a while and noticed it getting worse or is it new to you? Just curious as that's one of the number one comments that people have when the drive a Viper.
 

Goggles Pizano

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Thank you for posting this info, no shop wants to go near a Viper diff and I might have to be undertaking this very project My diff clunks and I suspect the spiders are shot.

4x4 shops will be about to do it. Vehicle it is for doesn't matter.
 

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