5 Speed swap
- Thread starter ronw
- Start date
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MopaR&D
Nerd Member
I'm not trying to disagree with you and am seriously considering swapping in the AX-15 I have sitting on my garage floor out of my old Jeep XJ, but... the key factor I think you're missing is the torque converter. Even a stock converter allows for 1000 RPM or more of slippage when the engine is at WOT during a gear change. So the actual RPM drop from a 904 with those gear ratios is quite a bit less than a manual transmission with those ratios.
@Kern Dog I fantasize about having a TKX in my Duster but $5,000+ on a kit from American Powertrain is just not feasible for me now. They apparently offer financing options now but idk how I feel about that, I'll have to look into it a little deeper. Do I continue running my 904 with 2800-stall TC and no overdrive and swap 3rd-members between a 3.55:1 and 2.94:1 depending on what I want to do with the car? Do I swap in an AX-15 manual with wide ratios intended for a truck or 4x4 that may break behind a 450HP 360? Do I take out a small loan to put a performance manual transmission into my classic car? It's a frustrating situation and you gotta compromise somewhere.
I put a truck 3:42 input into a super T10 and run a 2:50 diff gear in a 68 firebird 400. It takes off like a truck but that pontiac torque moves it out pretty decent for a street car.
Putting 3:00 in it is on the list.
Putting 3:00 in it is on the list.
DionR
Well-Known Member
While I don't completely disagree, remember that above stall the TC isn't going to slip. It might allow for a 1000 RPM "flair" when gently rolling through an intersection, but it isn't going to do that when it shifts at WOT. Unless I am ignorant of something, at best a TC is only going to slip maybe 5% above stall, if it a loose TC.
And a clutch can be slipped, too. If anything, I would expect a clutch to have more of an RPM flair at WOT that a TC will unless the stall is really high.
I've got to add that for most people how an AX15 with a tall gear might drive is an opinion based on theory (myself included), but for @75slant6 it is an opinion based on experience. Doesn't mean that for someone else it might be less than they hoped, but he has at least done it and can say how it feels to him rather than what he thinks it might feel like.
Now, to be fair, I’ve never driven a car with a t56, tr6060, tkx, etc. so I really can’t compare in that aspect. I did have a mustang with a T5 at one point, but that’s been 11 years ago. I also don’t have much experience driving old cars with automatics either. My only 904/727 experience was my Scamp with a slant 6. I do have a few extra small blocks and a sb 904 that I might throw in my Scamp sometime. Maybe I should replace the 7.25 with a 4.10 8.8 at that point so I can do a heads up comparison between shift points on the two? That’ll be a few years I’m sure. Lol
DionR
Well-Known Member
But you have driven an AX15 with a tall rear gear, which I for one have not. So you are far better equipped to actually speak to how it drives regardless of what you compare it to.
Frankly, every trans has compromises. For example, a narrow ratio trans needs a deeper rear gear to make up for the tall 1st gear, and usually doesn't have enough of an OD gear to make up for it. Or it has a "huge rpm drop" into OD. Or the rear gear isn't as deep and 1st overall is compromised, but OD is better. Never a perfect solution, always a compromise.
MopaR&D
Nerd Member
The slippage from a torque converter gets pretty complicated (or I guess, more complicated) when the output side is spinning. The stall speed indicates the max RPM difference between the input and output when the output is stopped but that doesn't mean there isn't still slippage between the two sides once the output starts rotating. The slippage rate is proportional to the difference in RPM and load/torque between the input and output. The slippage is at its max when the car launches then decreases as the car accelerates but never fully goes away; once the transmission shifts to the next gear and the output side drops in speed and increases load the slippage increases again. So basically that difference in percentage of slippage between the top of the first gear and bottom of second gear will reduce the RPM drop somewhat compared to the drop in a manual transmission. Also the momentary spike in load on the output side as the gear change occurs reduces the initial engine RPM drop a bit.
With a manual there is zero slippage through the clutch during a gear change, IF it's performed correctly. There can be some slippage due to driver technique (back on throttle before clutch pedal is fully released), if the clutch is too weak to handle the rotating inertia of the engine or if the engine spins down too slowly and doesn't match the trans input shaft RPM before the shift is completed. I figured out how to get rubber on the 1-2 shift in my Jeep by flooring it to redline, shifting quickly into second gear then dumping the clutch without needing to use any throttle because that 4.0L was so slow to spin down and had a lot of rotating mass with that 7-main-bearing crank. The factory clutch in those is also very "grabby" and doesn't like to be slipped much.
DionR
Well-Known Member
Agreed that a TC (without LU) will never be 100%, but it still seems to me that any RPM above stall isn't going to slip beyond that. And if stall is at max resistance, WOT is never going to see the same load since the car is moving.
I'm no TC expert, much more of a manual guy, so I am just guessing.
Yeah, I thought about that wrong. Even if there is clutch slip and the motor isn't freewheeling, it would only result in the RPM's dropping slower than they would without slip. No RPM flair like I was thinking.
MopaR&D
Nerd Member
I got you, I'm not an expert either but from what I do know... The engine RPM won't increase above stall when the car is stopped but the slippage doesn't disappear once the car is moving. The slippage is still there all the way to redline which is why drag racers sometimes use slippage percentage to determine how efficient their torque converter is. That's done by taking the engine RPM at the end of a pass and dividing it by the RPM of the transmission input shaft which is calculated based on trap speed, tire diameter and final drive ratio (as well as top gear ratio if it isn't 1:1). Converter slippage is generally around 5-10% at the end of a pass when calculated that way. By datalogging engine RPM and vehicle speed over the full run it's possible to find slippage percentage at other points during acceleration.
dartslantsix
Well-Known Member
I had an AX15 in my Barracuda. (Just pulled it this weekend and it's for sale) I have 3.83 gears and was running a Magnum 5.2L.
First gear was pretty much a granny, so when racing I'd start in 2nd. There was a big jump between 2-3 and it felt like the car didn't pick up speed until 3rd. It always shifted smooth, if a little heavy. (I shortened the shifter) I agree with the above. If my rear gears were higher, I think it would better pair with the trans. I ended up doing autocross in 3rd because that engine did not like to rev.
I'm swapping in a Gen3 hemi and debated using the AX15, but have decided to go TKX. Reasons are: not sure what flywheel to use with the AX15, smaller bellhousing for more exhaust clearance, gear spacing and unsure of the strength of the AX15.
First gear was pretty much a granny, so when racing I'd start in 2nd. There was a big jump between 2-3 and it felt like the car didn't pick up speed until 3rd. It always shifted smooth, if a little heavy. (I shortened the shifter) I agree with the above. If my rear gears were higher, I think it would better pair with the trans. I ended up doing autocross in 3rd because that engine did not like to rev.
I'm swapping in a Gen3 hemi and debated using the AX15, but have decided to go TKX. Reasons are: not sure what flywheel to use with the AX15, smaller bellhousing for more exhaust clearance, gear spacing and unsure of the strength of the AX15.
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