Upgrading the 3 speed automatic transmission

My idle is about 1200-1500 RPM when in neutral or park then drops (crashes) to 750 RPM or so when shifted into gear. I hate the loud "clank" and jerk when shifted from park to drive. Does everybody's do this? The shop said it would stall out when put in gear if the idle is set at 750 RPM in park.

I also hate how it takes almost full throttle to have the trans shift from third to second and then too it's a lurch and a huge change in engine speed but not much more acceleration. I had a mid-90's V-6 Chevy Malibu out pull me coming on the freeway on ramp this afternoon, totally embarrassing....

Is the transmission on the manual a torque tube also? As I understand it, the torque tube basically transmits the drive power back up the tube to the engine/trans mounts instead of a "regular" transmission transmitting the drive thru the rear suspension. If so, what are the differences between the two models suspensions? What I'm getting to is to get rid of the torque tube. This would allow a number of other transmission combinations (a ZF24 comes to mind, awesome trans!) if we could eliminate the TT via some sort of suspension changes like in the manual car, if this is the case. The only other "conventional" car I know of with a TT is a Corvette. Anybody loo into that equipment?

 

Your idle is too high which will make it harsh to engage. Mine idles at 1000 RPM and doesn't do that and the kick down is controlled by a micro-switch by the throttle linkage on the engine which is adjustable. They all have torque tubes and if you eliminated the tube I agree a swap would be much easier. The cost and work involved in doing that is a hurdle but so is killing resale value, in my opinion it becomes a double negative.

 

To eliminate the torque tube you’d have to fab a crossmember at the tail end of the transmission to have a mounting point for a transmission mount to stabilize the transmission and engine. You’d also have to have a similar set up for the differential.

 

Fix your idle first, seems like you have a major air leak. Idle should be around 900RPM.

 

Finally have my car back with new rebuilt transmission and a new Hughes GM25 torque converter. Ater a week of driving, the wait was well worth it. Low end acceleration is vastly improved. Not sluggish at all and feels very responsive. The tranny was adjusted for tighter shifting and the shifting is firm but not "banging" My idle was low in the past at 500-600 rpms and now is where it should be at 1000 rpms. Comes off the line very well and the torque curve appears to be paired better with the higher stall Torque Converter. In retrospect, really no need now for changing the gearing, the higher stall torque converter changed everything.

 

Fantastic news. What stall speed did they end up using?

You should do some 0-60 pulls to get a good comparison.

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This is it.

How did you deal with the bolts? 3 vs 6? Image Unavailable, Please Login

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Go to your favorite machinist and ask him to drill three (properly centered) holes in the flexplate. My guy did it for free.

No point in being restricted on the converter sizes due to odd bolt pattern.

 

Correct. The flexplate needs to be redrilled to accommodate the smaller diameter of the Torque Converter about 10 1/2 inches compared to the 12 inches on the original. Typically, as stall speeds rise, requires a smaller diameter torque converter. the shipping weight for the new Hughes TC was 25 pounds versus the original at 36 pounds.

 

The 10 lbs weight difference alone is enough to make a difference.

 

So 0-60 time is 15 seconds now instead of 16? Sorry I couldn't help myself. I'll do a converter swap in the future but skip the gear change.

 

Ha Ha Ha The last time I measured mine with my wrist watch stop watch and my GPS it was between 8.8 and 9. That's when I sent a transmission to Kilgore in March (lower gear ratio, torque converter). Don't have it back yet. I dig these cars at highway speeds. I think when Ferrari set these cars up they wanted off idle 500 rpm starts in first (easy stop and go driving) and they wanted the shift to second under acceleration to match max torque. They achieved both goals. I will report back when I get my transmission back.

 

I have had my transmission for a bit, but work and family have taken priority. This is from Kilgore with 2.75/1.57/1 gearing and an L88 valve body. He provided a 2200 rpm stall converter. In the process of sealing the backend of the motor with the transmission out. Waiting on a bearing and a seal for the Ferrari tail shaft. Will let you know how it goes. Should have it together in a couple of weeks. John said that I would feel more improvement from the torque converter than the gearing.
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I agree about the Torque Converter making a larger difference than the gearing. I used a 2500 RPM TC and the difference is remarkable. A better match for the torque curve of the V12. A new valve shifter made the shifts tighter but not a sharp banging. A good tranny shop will know how to adjust the TH400 to make for very smooth shifts.

 

Did you measure any acceleration runs?

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No I didn't, but what is apparent is how much quicker it is off the line. That's where the old TC and tranny were sluggish.

 

Could you do a 0-60 mp/h test?

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Hello The ruse,

it happens I just recommissioned a 400i and got the feeling the gearbox was coming out of another century.

Following your advice, I just ordered a new Hughes GM25 torque converter but may I ask you what kind/model of valve shifter did you fit to your car
and if you're still happy with it ?
Thanks in advance for your help.

 

Valve shifter depends on the torque converter. You can have very firm shifts with a loose converter, or loose shifts with a firm converter. The later setup being the stock-one.

I have a variable pitch converter with very firm shifts, at low stall this is a bit too much under high load, so whenever I floor the acelerator, I just keep high stall. (~3000 stall).

As for converter a 2400rpm stall is not too loose and gives the car a bit more juice below 50kmh.

 

Sorry I don't know which valve shifter was used. I just requested that I wanted tighter shifts but not the super firm shifts some guys get with their American muscle cars. I am very happy with the results. Most recently and I will post this in a new thread, I had my six sidedraft Webers synchronized with with a 12 mercury column manometer. I thought the car ran well before, but by balancing all six carbs (twelve barrels simultaneously) is the way to go. Beats using a single manometer on each barrel.

 

Have a look here, the article shows where to drill a bigger hole in the aluminium plate.

https://www.v8buick.com/index.php?threads/th400-simple-shift-kit-and-a-few-easy-internal-upgrades.342929/

Also adjusting the vacuum modulator has an impact on how firm is the shift.

But as said, you'd better select the torque converter and then adjust how firm the shifts should be. I've just changed my converter for a lower stall one (I wanted higher stall, but the high stall converter impeler and thrust bearings are not strong enough). So now I would rather have a slightly smoother shift. (It's easier to enlarge a hole for firmer shifts, than reducing it...)

 

Nota : proper name is "separator plate"

 

Thanks for your prompt response the Ruse !

I'm living far away from you, precisely in Switzerland, and there aren't so many specialists here for such American gearboxes like the TH 400 hence the reason I asked you about your valve shifter.
Do you think the guy who made the installation on your car might remember the correct type of valve shifter he fit in period.
Sorry to bother you with that question but it would help (me) for sure.

Have a great afternoon.

Saluti,
Marc

 

Thanks for your precious advices Raemin,

I'm waiting for the new torque converter before going on with any other mods !

In any case I wouldn't fit a new/better valve shifter with reverse pattern as I understand the gearshift travel would be inverted !

Right?

 

Reverse pattern is more or less mandatory if you want to build a "manual" th400. I don't remember the reason, but it's related to the removal of the governor/modulator and their respective location. So nothing related to the ergonomy, just a technical limit.

To be honest, on our cars, I don't see the point in removing the possibility to let the transmission shift on its own. So no reverse pattern.

The capability to downshit to 1 at any speed is a real plus (as long as you do not try to downshift at full speed).