Hi Everyone;-)
Vic and others are talking transaxles/converters
> The switch
> pitch has been altered by another GMC owner as per specifications supplied
> by Don Brown, which changes the angle of the switch pitch blades to 30
> degrees from 21 degrees. This lowers the stall speed to about 2800 (from
> 3600) and reduces the multiplier gain slightly. Don maintains that it
helps
> to keep the switchpitch running cooler so that he can leave it in the "s
> witched" position going up long hills. He has over 100,000 miles on his
> transmission and has had no trouble with it.
There are many possible modifications to the switch pitch converter- Most
are related to increased durability, and pretty much any modification
possible for the 'standard' unit can be done to the SP, as example using
needle Torrington Bearings in place of the nylon thrust washers in the
converter case and improved fin retention from furnace brazing etc.
The stator hub of the SP and the stator blades themselves can also be
manipulated to achieve different objectives. Changing the angles, length and
curvature of the stator blades can make the converter respond differently.
Poston Buick Parts in Georgia(? I think- they advertise in Hemmings Motor
News and other antique/hot rod car books) offers SP converters modified for
racing, they typically modify them to move the stall speeds APART from each
other, namely, the 'cruise' stall speed is VERY low (~1200 RPM) and the
'performance' stall speed is VERY high (~4000+ rpm)... the modifications
done to achieve this might also be of interest to some people.
Having used off the shelf (well, used actually, from the junkyards) GM
Switch Pitch converters for many years in tow vehicles and regular passenger
cars with no problems at all and well balanced performance, I'd say
modification to the converter is more a peace of mind question than a real
service issue. Certainly, theres improvements possible for extra durability,
but the regular GM parts are perfectly adequate and should last a very long
time.
I personally like the factory selected 'high stall' speed range as it
matches up well with most GM V8 Net Horsepower peaks- just flooring the gas
puts the engine pretty much exactly on the peak of its output, regardless of
the gear or road speed, and means the absolute maximum acceleration is
availible under any circumstances. Below stall, this is further aided by the
torque multiplication of the SP converter. To me, this is more valuable than
extending the duration of time I might use the SP feature before increasing
transmission oil temperatures to an uncomfortable degree- (I get
uncomfortable at 300F)- Normal results from towing with a variety of SP
vehicles using standard components has revealed no significant change in
transmission temperatures, as the SP is so seldom used in 'performance' mode
and for such very short periods that it has no particular effect on
operating temps-aditionally, the 'cruise' mode is such a low stall that
temperatures are extremely modest in normal driving.
I feel its a poor idea to operate a converter for long periods under its
stall speed with a heavy load. Thats what the gears are for, not the
converter;-) I think the solution is to operate using a converter running a
few hundred rpm above stall speed at minimum when cruising along, and use
the switch pitch if so equipped as an aid to acceleration for short periods.
Its especially helpful on starting from a standing start on a steep grade,
or for passing at highway speeds, when you want a short burst of power to
get around a truck on a two lane as example. The switch pitch really shines
for passing especially, its a dramatic improvement, perhaps 20% less
exposure during passing or thereabouts.
> I have had discussions with a number of people about the pros and cons of
> the five Vs the six clutch pack and things like that. I am most interested
> in knowing what others feel makes a HD transmission for the GMC.
The main difference is six clutches at a given line pressure will support
120% the torque that five will before they slip. Five at 120% the pressure
equals six at 100%. The downside is six takes longer to couple on a shift,
and may delay a shift long enough to injure the clutches if a very quick
apply is desired. I think either would work fine in a GMC. I'd lean towards
six and a lower pressure however if I had a free hand, as low pressures are
generally desirable from a seal life perspective. High transmission
pressures also eat fuel.
> Received the following informtion from Bruce Roe, the resident
transmission
> guru on the Oldsmobile site. Paul Bartz gave me his name as an excellent
> reference.
>
> When scouring the salvage yards for 425 transmissions, be aware that the
67
> Eldo transmission is a bit of an orphan. The two bottom engine bolts are
off
> by 3/8 of an inch. They could be filled and redrilled, but the pre mid 69
> cases did not have a case protection ring where the center drive section
goes
> in, so a 70 or later case is the better choice.
>
> FYI courtesy of Bruce
According to ( a friend on the Cadillac Mail List who has looked this up in)
the 1971 GM parts catalog, the transmission case is group 4.103 and the part
number for '67 through '71 Eldorado is 8624986- this said, I also seem to
dimly recall two bolt holes dont work on the '67 Cadillac unit. Vic, we'll
double check- four out of the six line up for certain anyhow. The conflict
arises because the 429 Cadillac engine uses the old 'round bell' type
bellhousing, (a'la Dual Range HydraMatic) not the regular
Buick-Oldsmobile-Pontiac one with two ears at the top- and theres a steel
adapter ring between the back of the 429 and the BOP cases. In 1968, the new
Cadillac 472 engine adopted the conventional BOP pattern. I'll make a paper
template off a Olds or later Cadillac engine here Vic, and we can double
check and see whats needed. One other change is the late (1976?-'78) THM 425
transmissions don't have the passages drilled out for the switch pitch
circuit apparently, so a very late case cannot be used either. Its unclear
which year was the transistion to this style case.
For those of you who hadn't heard Bruce Roe markets a Switch Pitch
electronic controller of his own design that operates the SP converter with
a high level of sophistication electronically. It sells for less than $200
and is an excellent unit, I would encourage any SP owners to get in touch
with Bruce to get more information regarding this device. It also allows
some tailoring of variables to permit SP operation at any time it would be
desirable, not just "on" at heavy throttle and idle like standard. He may be
reached at;
bcroe
> Subject: GMC: Allison Torque Converter
>
> I just received the following from Dick Paterson. When my transmission
went
> bad this summer the rebuilder put in an Allison torque converter and with
the
> postings on the net I was concerned.
> that I had installed an Allison with most unsatisfactory results. It was
a
> high stall unit with a 3 flange setup in the motorhome it acted like a
city
> bus. the engine would run up to almost 2000 rpm before the coach began to
> creep ; in addition it slipped at low speeds and generated a high amount
of
> heat, coupled with very poor mileage.
This is also a byproduct of having a tall axle ratio in a heavy vehicle. My
454 truck has a ~2200 rpm stall converter and works extremely well, but uses
a 3.73 axle and seldom operates below stall at any time. Overall performance
is quite good, but I'll get a switch pitch in there as soon as I get a
little time.
> Most torque converter rebuilders can use the h.d. components found in the
> allison such as bearings ,seals ,bushings etc. and the front half of the
> converter with the h. d. hub. it already has the anti-balloon plate on
it. I
> spec my rebuilder to use the motorhome or toro 13 inch case and the three
> bolt back half with the allison front half. And all the h. d. mods with
it,
> so far they are proving to be bulletproof They lock up early for a solid
> feel ; ; they run cooler ; and gas mileage does not suffer. Obviously if
you
> are not suffering the same problems that i and some others experienced
then
> you got the right one and it should do a good job and give good life.
Excellent advice, I would only add these modifications are possible on a
switch pitch core also if a person desired them.
Anyhow, enough of my rambling, I'm back to work!
Kind Regards,
Brent Covey
Vancouver
Vic and others are talking transaxles/converters
> The switch
> pitch has been altered by another GMC owner as per specifications supplied
> by Don Brown, which changes the angle of the switch pitch blades to 30
> degrees from 21 degrees. This lowers the stall speed to about 2800 (from
> 3600) and reduces the multiplier gain slightly. Don maintains that it
helps
> to keep the switchpitch running cooler so that he can leave it in the "s
> witched" position going up long hills. He has over 100,000 miles on his
> transmission and has had no trouble with it.
There are many possible modifications to the switch pitch converter- Most
are related to increased durability, and pretty much any modification
possible for the 'standard' unit can be done to the SP, as example using
needle Torrington Bearings in place of the nylon thrust washers in the
converter case and improved fin retention from furnace brazing etc.
The stator hub of the SP and the stator blades themselves can also be
manipulated to achieve different objectives. Changing the angles, length and
curvature of the stator blades can make the converter respond differently.
Poston Buick Parts in Georgia(? I think- they advertise in Hemmings Motor
News and other antique/hot rod car books) offers SP converters modified for
racing, they typically modify them to move the stall speeds APART from each
other, namely, the 'cruise' stall speed is VERY low (~1200 RPM) and the
'performance' stall speed is VERY high (~4000+ rpm)... the modifications
done to achieve this might also be of interest to some people.
Having used off the shelf (well, used actually, from the junkyards) GM
Switch Pitch converters for many years in tow vehicles and regular passenger
cars with no problems at all and well balanced performance, I'd say
modification to the converter is more a peace of mind question than a real
service issue. Certainly, theres improvements possible for extra durability,
but the regular GM parts are perfectly adequate and should last a very long
time.
I personally like the factory selected 'high stall' speed range as it
matches up well with most GM V8 Net Horsepower peaks- just flooring the gas
puts the engine pretty much exactly on the peak of its output, regardless of
the gear or road speed, and means the absolute maximum acceleration is
availible under any circumstances. Below stall, this is further aided by the
torque multiplication of the SP converter. To me, this is more valuable than
extending the duration of time I might use the SP feature before increasing
transmission oil temperatures to an uncomfortable degree- (I get
uncomfortable at 300F)- Normal results from towing with a variety of SP
vehicles using standard components has revealed no significant change in
transmission temperatures, as the SP is so seldom used in 'performance' mode
and for such very short periods that it has no particular effect on
operating temps-aditionally, the 'cruise' mode is such a low stall that
temperatures are extremely modest in normal driving.
I feel its a poor idea to operate a converter for long periods under its
stall speed with a heavy load. Thats what the gears are for, not the
converter;-) I think the solution is to operate using a converter running a
few hundred rpm above stall speed at minimum when cruising along, and use
the switch pitch if so equipped as an aid to acceleration for short periods.
Its especially helpful on starting from a standing start on a steep grade,
or for passing at highway speeds, when you want a short burst of power to
get around a truck on a two lane as example. The switch pitch really shines
for passing especially, its a dramatic improvement, perhaps 20% less
exposure during passing or thereabouts.
> I have had discussions with a number of people about the pros and cons of
> the five Vs the six clutch pack and things like that. I am most interested
> in knowing what others feel makes a HD transmission for the GMC.
The main difference is six clutches at a given line pressure will support
120% the torque that five will before they slip. Five at 120% the pressure
equals six at 100%. The downside is six takes longer to couple on a shift,
and may delay a shift long enough to injure the clutches if a very quick
apply is desired. I think either would work fine in a GMC. I'd lean towards
six and a lower pressure however if I had a free hand, as low pressures are
generally desirable from a seal life perspective. High transmission
pressures also eat fuel.
> Received the following informtion from Bruce Roe, the resident
transmission
> guru on the Oldsmobile site. Paul Bartz gave me his name as an excellent
> reference.
>
> When scouring the salvage yards for 425 transmissions, be aware that the
67
> Eldo transmission is a bit of an orphan. The two bottom engine bolts are
off
> by 3/8 of an inch. They could be filled and redrilled, but the pre mid 69
> cases did not have a case protection ring where the center drive section
goes
> in, so a 70 or later case is the better choice.
>
> FYI courtesy of Bruce
According to ( a friend on the Cadillac Mail List who has looked this up in)
the 1971 GM parts catalog, the transmission case is group 4.103 and the part
number for '67 through '71 Eldorado is 8624986- this said, I also seem to
dimly recall two bolt holes dont work on the '67 Cadillac unit. Vic, we'll
double check- four out of the six line up for certain anyhow. The conflict
arises because the 429 Cadillac engine uses the old 'round bell' type
bellhousing, (a'la Dual Range HydraMatic) not the regular
Buick-Oldsmobile-Pontiac one with two ears at the top- and theres a steel
adapter ring between the back of the 429 and the BOP cases. In 1968, the new
Cadillac 472 engine adopted the conventional BOP pattern. I'll make a paper
template off a Olds or later Cadillac engine here Vic, and we can double
check and see whats needed. One other change is the late (1976?-'78) THM 425
transmissions don't have the passages drilled out for the switch pitch
circuit apparently, so a very late case cannot be used either. Its unclear
which year was the transistion to this style case.
For those of you who hadn't heard Bruce Roe markets a Switch Pitch
electronic controller of his own design that operates the SP converter with
a high level of sophistication electronically. It sells for less than $200
and is an excellent unit, I would encourage any SP owners to get in touch
with Bruce to get more information regarding this device. It also allows
some tailoring of variables to permit SP operation at any time it would be
desirable, not just "on" at heavy throttle and idle like standard. He may be
reached at;
bcroe
> Subject: GMC: Allison Torque Converter
>
> I just received the following from Dick Paterson. When my transmission
went
> bad this summer the rebuilder put in an Allison torque converter and with
the
> postings on the net I was concerned.
> that I had installed an Allison with most unsatisfactory results. It was
a
> high stall unit with a 3 flange setup in the motorhome it acted like a
city
> bus. the engine would run up to almost 2000 rpm before the coach began to
> creep ; in addition it slipped at low speeds and generated a high amount
of
> heat, coupled with very poor mileage.
This is also a byproduct of having a tall axle ratio in a heavy vehicle. My
454 truck has a ~2200 rpm stall converter and works extremely well, but uses
a 3.73 axle and seldom operates below stall at any time. Overall performance
is quite good, but I'll get a switch pitch in there as soon as I get a
little time.
> Most torque converter rebuilders can use the h.d. components found in the
> allison such as bearings ,seals ,bushings etc. and the front half of the
> converter with the h. d. hub. it already has the anti-balloon plate on
it. I
> spec my rebuilder to use the motorhome or toro 13 inch case and the three
> bolt back half with the allison front half. And all the h. d. mods with
it,
> so far they are proving to be bulletproof They lock up early for a solid
> feel ; ; they run cooler ; and gas mileage does not suffer. Obviously if
you
> are not suffering the same problems that i and some others experienced
then
> you got the right one and it should do a good job and give good life.
Excellent advice, I would only add these modifications are possible on a
switch pitch core also if a person desired them.
Anyhow, enough of my rambling, I'm back to work!
Kind Regards,
Brent Covey
Vancouver
