12 Sept 2023
It has been quite some time since there has been an update. Things have been happening, but progress has been slow. There are two experimental aspects of this latest round of the project:
1. The rear end
2. The transmission

Lets talk about #1. The rear end is a HAG215, from a 2008 Dodge Challenger SRT-8 6.1L Hemi. It has a 3.06 gear ratio. It is essentially the same as the HAG215 found in a number of MBs, but it is laid out somewhat similarly to the Crossfire's rear end, with the exception of the dogleg rear cover, found in later MBs. The front mount of it is very similar to the Crossfire. As you may have read in previous posts, using it required quite a bit of fabrication. If it were not for the fact that I already had the HAG215 and a Wavetrac for it in hand, there is no way I would have gone this route. Instead, I would have gone with a HAG210, found in the W210 and other cars, and I would have gotten a Quaife LSD for it (no Wavetrac for the HAG210). What's done is done, and we moved on. The mechanical aspects of the HAG215 have been worked out, and it is in the car. What was not reported in the earlier parts of the thread is that the tool that was constructed for measuring the shimming actually broke during reassembly. This invalidated all of the readings that were taken. Given that, best guess on shimming was made, and a wear pattern check was taken. In short, there is too much clearance (backlash), and now the rear end whines quite a bit. There is still light at the end of the tunnel, though. There is a plan to fix the whine! The 3.06 gears were always a temporary solution, until a set of 2.65 gears could be found. These will work with the 7-speed transmission more properly. Right now, 1st gear is not especially useful, and just leaving the transmission in Comfort mode, leaving in 2nd gear is ok. So let's look at a chart, that is part of a larger spreadsheet. This sheet was developed by me, and is nothing magical, but surprisingly accurate.

What needs to be pointed out are 1st and top gear for each transmission. Notice the spread in gears of the 7G vs the stock Crossfire 5G. First is much lower (numerically higher) and top gear (7th/5th) is much lower. In other words a much wider range of ratios in the 7G. For now, let's look at Final Drive. Final Drive (FD) is the product of whatever gear you are in multiplied by the differential ratio. So, on a stock SRT6, 5th gear is 0.83 and the differential ratio (3.07) is 2.5481. Now let's take a look at 1st gear. Stock SRT6 would be 11.0213. Actually, a 1st gear multiplier of between 11-12 in a street car is considered ideal, so it is very well matched. Even a Limited with its 3.27 ratio is 11.74. Great! Now this car is technically using a 3.06 ratio, but for all intents and purposes, directly comparable to the SRT6. So, we know what first gear looks like on the 5G already, how about that 3.06 against the 4.3772 of the 7G 1st gear? 13.394. Yikes! Way too high! And this is in a car that makes a ton of torque? Can we say tyre-fryer? So, now we step off to a 2.65 differential ratio, and now we arrive at a much more usable 11.5995 multiplier. Will the car still light up the tires? Well, yes. But now let's look at the chart, and see what the highway RPM looks like. The chart has 75MPH entered, and this car's rear tire size. If you look at 7th gear, and follow it across to the 3.06 ratio, you'll find 2181 rpm, which is about dead-on for what it really is doing on the highway. Now follow that over to the 2.65, and you find 1889 rpm. That sounds low, but remember, there's a big V8 pulling this along. For additional reference, look at the 5G trans and 3.06 that was in the car previously: 2488. So between the 2.65 gears and the 7G, this engine will turn 599 LESS RPM going down the highway. That will reduce cabin noise substantially, increase economy, and result in a little less stress on the powertrain. Wins all around!

(For those wondering about all of the differential ratios, this spreadsheet was originally started while researching gearing on my old Mustang SVO, and there are gear ratios in the table to cover those, along with several other transmissions found in other models.)

Now, onto #2. The transmission. This is the big thing that is different in this round of changes. Some pretty significant challenges with the physical fitment were apparent from the start, requiring a modified driveshaft, fabrication on the transmission crossmember, modified transmission fluid lines, and several other items. That portion all worked out great! Now, on to the transmission electronics, fluid, valve body and more. This transmission is actually a 2014 722.931, which makes it to where it originally ran as a start/stop ECO transmission, on far later electronics (PTCAN FBS4, for the nerds), and the later 236.15 "blue" fluid. Not going to get into it all, but a lot of mistakes in this regard, but that happens every time new ground is broken. Someone has to blaze the trail!

This is a 2004 Crossfire, which means it is using 'leftover' R170 SLK320 ESP (ESPMk203 ABS) module and stability sensors. Very quickly it was realized that the 7G would not talk to the ESPMk203 properly, and an upgrade to the SRT6 ESPMk25 would be required. They look about identical, and the brake lines go to the same places, so no problem, right? Um, no. This required the main connector to be swapped, as they are different (soldering almost 50 wires), different stability sensors (in center console, behind the shifter), and a few changes to even how the brake lamps are wired. All of that was done in early August, and that made the 7G happy again. This is required, because the 722.930/931 does not use a torque converter, but rather a wet clutch (called NAK in MB parlance). This is pretty much what it sounds like. A clutch, just like in a manual car, except that it is controlled by computer and hydraulics. It does not engage when the car is stopped, just like you don't have a manual car in gear, and clutch engaged when stopped. So, if the computer cannot see wheel speed and brake system status, bad things happen. In short, if you decide to put a MCT transmission in your car, start with a 2005+ Crossfire, or plan on swapping the ESP. @Dave2302 is working on swapping a 7G into an R170, and his tests so far show that this may not be a problem for a "regular" 722.9, which uses a conventional torque converter, just like your stock Crossfire does.

About two weeks ago, we had a breakthrough (I say we, because there is a small team of absolute MB diehards with a ton of knowledge contributing to this), and the car began driving almost normally. A couple more breakthroughs later, and it can now be reported that there are about 200 driving miles on the 7G transmission, including highway and heavy stop and go traffic. It is a different experience driving the NAK equipped transmission. Somewhat like a manual, you can feel engagement as the brakes are released and the NAK engages. Kind of cool, actually, but not as smooth as a torque converter. However, once engaged, the NAK does not release unless there is something wrong, or the car is stopped again. Much more like a manual. The gear changes are very precise feeling. More confidence has to be built into the whole package, but it is truly beginning to come together!

A lot more to do. The 7G uses a different protocol to talk to the mode selected switch (W/S switch on the shifter), and work is ongoing for the electronic device that will translate that message. Once done, there will be four modes, Comfort, Sport, Sport+ and Manual. Yes, swapping in a different steering wheel with shift paddles is in the works, but that is likely not to happen until winter time.

There have been complications with the Killer Chiller. (DO NOT BUY ONE OF THESE! Get a system from Chemcool, FI, or just about anywhere else. There are plenty of vendors out there.) Still working through those challenges, but the solution is a known item that just requires time to get to it.

As the car is not 100%, it was decided to leave it home for the Fall Crossfire Rally, and instead I will ride up in @QuadPrism (William) SRT6, and just enjoy the event without worrying about any potential teething issues.

One last thing for now, in case you are wondering. There has not yet been a single full WOT run with the transmission. I am working up to the power very gradually, and need to be very measured in this, as I do not like broken parts! That said, the throttle has been rolled into a few times, and the feeling is very good. Much more direct and powerful feeling over the 722.6, if for no other reason than that the NAK is 100% engaged, like a manual. There is no lockup clutch turning off, and running the driveline off of the torque converter itself. Acceleration feels instantaneous, as do heavy decels when manually shifting the trans. Basically, the benefits of a manual, but no 3rd pedal.