Upgrading the 3 speed automatic transmission

This thread reminded me one of my dream projects is putting a modern BMW DSG / double clutch into a 456M GTA using the now recently developed independent/aftermarket transmission controller. The days of a 50k GTA maybe behind us though.

 

Has anyone tried installing a "low gear" planetary kit on the TH400?

The stock TH400 has 2.48/1.48/1 ratios.

There are 2.75/1.57/1 and 2.97/1.57 available.

The 2.97 would bring it close to the manual ratio of 2.838/1.706.

Perhaps this with billet drum and enhanced torque converter is the way to go.





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You can see here how badly geared the TH400 is for the 400/412, there is a huge difference compared to the manual speeds.

The automatic goes 16 km/h in 1st @1000 RPM compared to 10 km/h for the manual - the TH400 is extremely "high geared".

Going to a 2.75 1st gear would mean a change of 9.8%, and with 2.97, 16.5%.

This would mean that in 1st gear @1000RPM 15.1 km/h and 14 km/h for the 2.75 and 2.97 gear.

At redline (6600 RPM) it could reach 99 and 96 km/h, which is more than sufficient.

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You cannot compare the manual/auto ratios based on the gear ratio: the clutch does not offer torque multiplication whereas the converter can slip, so the overall ratio is rear axle ratio + gear ratios + converter "ratio".

I made a post that gave some basic indication on how to calculate actual torque multiplication (flash speed) based on the quoted "stall torque": https://www.ferrarichat.com/forum/threads/recommended-torque-converter-for-400i-th400.601192/#post-146637992

Here are a few things that come to mind:

1) If you want to obtain the same ratio gap as the manual transmission you would need a 3.23 first gear ratio (remember that you do not have an overdrive on the TH400 so the last gear is 1/1 instead of 1/0.8). That's a 30% difference compared to the stock gear. Assuming that you want to compensate for this difference you could just try to find a converter that offers 30% flash slippage at max torque (~4000rpm). In other words a 1800 stall torque converter (versus the 1200 stock converter). If you want to be more aggressive you can decide to achieve this slippage when all the torque is not already available and go for a 2400 converter but you will loose top speed (in this case maybe that a shorter gear would make sense).

2) Closer transmission ratio does offer better acceleration, higher ratio does the opposite (shorter 1st gear would indeed be better for kick-off but worse for mid-range). That's one of the main reason the 700R4 transmission is not as desirable as a th400 for drag racing. Don't know how this logic applies to the 400 that is indeed high geared, but definitively something to be discussed with your shop before changing gears.

 

Thank you, Raemin for your always valuable comments.

Did you change the gears in your car, or only the converter?

What improvements did you notice?



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Regarding the overdrive, with the stock diff and 1:1 gear ratio, the car can do 120 km/h at 3100 RPM which is suitable.

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Only the converter and gearbox. Improvement is night and day...

The 412 does have a shorter rear axle ratio, so the acceleration will be even better. It also suspect it has a (much) better converter: there was much more stall on the two 412 auto that i know compared to my original setup. All in all the 412 is not as sluggish as the earlier models.

 

The 412 Automatic has a 12/39 final gear, and the 412 manual 10/43 (over 7% difference).

 

What do you think about these:

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The problem with the wide gear sets is the engine is going to really fall out of its power curve on each up shift. Sure the stock gearing might be a little slow off the line but it would be better on the shifts than the wide gear ratios. Really the best bet if you’re going to go to the effort is getting a 4 or 5 speed auto and getting a better read end ratio out of a 400 or 365GT4 2+2. These have 4.30 rear gears. The other option would be a 365GT4C, it has a 4.00 ratio. Major problem is not any spare gear sets laying around. The TH400 is designed to take the torque of an American big block V8 and requires a lot of hp to operate. A TH350 would have been a better choice in my opinion.

 

Yes, but considering the stock ratios 2.48/1.48 vs 2.97/1.57, it will surely be in a much better power/torque range for everyday driving.

In 2nd gear with 1.57, @1000 rpm the speed with 24.36 km/h vs. 25.83 for the stock 1.48.

With this gear you would be driving 60 km/h at 2500 RPM which good for city driving.

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I really don’t think there has ever been a transmission swap in one of these cars. There supposedly was one done but I’ve never seen any pictures. I think it would be a great project though.

 

I think for the actual use we give these cars modifying the TH400 is more than enough.

Low gear set, lightweight drum, roller bearing (Torrington) washers, and better matched torque converter is all you need. I read that John Kilgore, an authority with TH400 was able to move 30(!) lbs from the rotational mass of the TH400, that is an insane amount, and would be a world of difference on a high revving engine like the 400/412.

There is really no need for more overdrive than 1:1, at 3000-3500RPM you would be going over 115-130 km/h which is perfect for long trips.

 

Do you know what the stall speed is on the 412 torque converter?



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I've seen ~8 years ago an ebay advert for a twin turbo (Norwood) 400 with 700r4 transmission. You could tell the transmission was there as the transmission tunnel had been butchered in order to fit the wide transmission. Based on the German (?) seller description ("car is a rocket: slow take-off then insane speed"),I would say the tranny swap was not so much of a success. I wish I had kept a record of that listing...

I personally could not find a cost effective way of swapping the transmission, so decided to keep the TH400. But even with he TH400 as a basis I ended up at roughly 7.000€~10.000€ when factoring for trial and error. To name a few initial converter destroyed with major damage everywhere, not fully happy with the second converter, etc... Another transmission would incur even more fine-tuning.

I honestly believe my next upgrade would be to work on the weight of the direct drum and reaction carrier. That's how they are supposed to rotate (as a % of motor rpm):

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A 10pound lighter direct drum would make a huge difference at 6000rpm.
If you want to go wild, the 3 above bits can be sourced in titanium for approx $3500 (that's an overall 24pound weight reduction)

 

@jacques said 1800rpm in a previous thread, mine was a 1200rpm according to the transmission shop. God knows what BorgWarner actually supplied to Ferrari...

 

The forward and direct can be found inexpensively in billet aluminum.

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Are these the weights of the stock parts?

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14pound : these are the stock weight.

The titanium ones do look much more skinny than the billet aluminium ones you've posted . You can have a look at kilgoretrans or jakesperformance to get an idea. These are the only two suppliers that I know. Kilgore invented them.

https://shop.jakesperformance.com/shop/ols/products/billet-titanium-w-super-sprag

 

Did you ever measure the acceleration after the transmission upgrades? The 400i is quoted as having a 0-100 time of 8.5 seconds, the 412 only marginally faster at 8.2.

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Would never argue that the automatic doesn't give a more "sluggish" behavior, and couldn't use improvement , but a calculated table like this can be a little misleading because they often don't take into account that the (old style) torque converter is not a constant "speed ratio" device. At lower RPM, it acts like another gear set with about a ~2:1 reduction, while at higher RPM it gets closer to a 1:1 ratio. For example, the value in the table of 10.43 mph per 1000 RPM for the automatic in 1st gear is probably perfectly accurate when the engine is at 6000 RPM, but, when idling at 1000 RPM in 1st gear, the coachwork speed would be more like ~5 mph.

 

Conclusive proof that the gearing of the automatic is extremely mismatched is in the terrible 0-60 time. For the 400i it was so bad in fact, that Ferrari does not even mention it on their website.

8.2 sec vs 6.7 sec is clearly due to gearing mismatch, not drivetrain losses in the torque converter, the most it can lose is 10%.

Judging by the "over-all" gear ratios, it would be nearly the same if the manual started off in second gear, then went to forth.


Automatic/manual over-all ratios are:

1st: 8.06 /12.20 (!)
2nd: 4.81 / 7.338
3rd:. 3.25 / 5.394

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Here is the torque/power chart of the 412 engine.

You can see here that from 2000 to 5000 RPM you have nearly 90% of the maximum torque.

A stall speed of 1800 and 2.97 1st gear would cover most city driving.

@ 5000 RPM would be 70 km/h.




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There seem to be several types of “Stall” revs being discussed and they are sometimes used together and sometimes separate. To confuse people like me further, not all manufacturers use the same description for their “Stall Speed” on their converters and sometimes they just state “Stall”.

“Flash Stall” seems to always be 4-600 rpm’s higher than “Brake Stall” according to TCI.

Member “raemin” wrote in a very informative post (as always) above that stock converter was Stall 1200 rpm (from info from his shop). Maybe the stated “Brake Stall” at 1200 rpm which would imply “Flash Stall” at about 17-1800 rpm and in line with what “jaques” wrote in a different thread.

From earlier posts here on FC I have understood that a 10-inch converter would be the preferred choice for replacing the factory converter. But I’m still at loss when it comes to stall speed.

TCI offers a 10-inch converter with dual mounting patterns. They state a Flash Stall of about 2200-2400 rpm on a small block (probably like our engines) and 2400-2600 rpm on a big block (more torque)

Breakaway Converter, GM, 1965-91 TH350/400 w/ Dual Bolt Pattern (tciauto.com)

Would that be a suitable converter for a 400i TH400 upgrade? If not, why?

With dual bolt patterns would it be a straight bolt on?

I do have a B&M shift kit going in and also bought a Raybestos full rebuild kit do to that at the same time. The gearbox has not left the car for over 42 years now so it is time. I'm not sure I will do it myself or get it to a shop as the rear axle need to be moved backwards to get the gearbox out.

Kind regards

//Rob

 

I would suggest to get an aluminum direct and forward drum and also consider changing the gear ratios (by changing the planetary and sun gear).



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