Re: GM400 Torque Converter mistake

["Kirbert" <palmk@mailer.gadcomm.net>]

armsco@primenet.com (Michael):
> After reading Kirby's book I just assumed all GM high stall converters
> would have a stall speed of 26-2800 rpm. So I installed a V3 high stall
> converter and was disappointed with the results. If anything the car seemed
> slower. After the fact checking revealed that the V3 high stall converter
> has a stall speed of 16-1750 rpm.

Great!  It appears "high stall" is a relative term!

I presume that your "checking" involved actually performing a stall test on 
the car?  This may involve a whole bevvy of problems.  For example, anything 
-- ANYTHING -- wrong with the engine will cause it to stall at a lower speed.

This, of course, might point to the problem at hand.  If the convertor 
installed was designed to stall at 2600 with some engine with oodles of 
low-end torque, it will stall at a much lower RPM with the Jag V12 with its 
lousy low-end torque.

Please forward any info you can on the torque convertor you used -- make, 
model, intended application, etc.  Even though I haven't fiddled with this 
change myself (hey -- I have a 5-speed!), it appears it'd be a good idea for 
me to become more acquainted with the products available and their 
application to the Jag in order to make things clearer in the book.

> Now I am really confused. It is my understanding that under acceleration the
> engine should quickly rev up to the stall speed, then stay there until road
> speed tells the trans to shift. As I mentioned in my last post, mine rev's
> to 22-2300 rpm before shifting.  Shouldn't it be shifting at 16-1750 rpm
> with this converter? 

No no no -- the stall speed is unrelated to shift points, although it is true 
that it would probably be wise to consider shift points when selecting a 
convertor.

The convertor goes between the engine and tranny.  When operating at high RPM 
and low load, it essentially serves as a fluid coupling.  At high load, there 
is a third impeller within that can convert some of the RPM of the engine 
into additional torque -- a similar effect to having a lower gear.  A typical 
torque convertor can operate continuously between a 2:1 reduction and 1:1 
(direct drive).

The shifting of an automatic is all done behind the torque convertor.  If 
matched properly, the torque convertor should reach nearly 1:1 in each gear, 
and then when the tranny upshifts the torque convertor drops back nearer to 
the 2:1 reduction to provide a nearly continuous application of power as the 
car accelerates.

Contrary to the impression given in that earlier message, the GM 400 doesn't 
have specific "shift points".  It decides when to shift based on driveshaft 
RPM and manifold vacuum.  At high manifold vacuum (light throttle), it will 
indeed shift entirely too early -- but the stall speed of the torque 
convertor isn't an issue here since that number only has meaning at full 
throttle.  At full throttle, the GM 400 shouldn't be upshifting until 4000 
RPM or more, well past the stall speed of any convertor.

I must add that I am not at all an expert on this, and am hoping that others 
on this list will add to or correct this as required.
 
> It looks like I must now live with sluggish acceleration, or pull the trans
> again and install the correct converter.

Unfortunately, I must agree with this conclusion.

> Does anyone have any thoughts on what problems this low stall converter
> might cause other than slower acceleration?

Oh, I suspect that'll be the only problem you have with it.

 -- Kirbert      |     If anything is to be accomplished,
                 |     some rules must be broken.
                 |          - Palm's Postulate

---

References:

• GM400 Torque Converter mistake
• From: armsco@primenet.com (Michael)

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