1970 shift points

[mikanystrom]

Hi everyone,

Wondering a bit about the shift points on a 1970 De Ville.  As seen in other threads, I've replaced the camshaft and figured out the ignition timing.  Car runs great, finally!  (Much thanks to suggestions I've received on this board, too!)  Step on the gas and let it downshift, and there's lots of power.  But could it be better?

Can someone tell me if I'm wrong anywhere in the following...

rear end ratio is 2.93:1, gear ratios are 1.48 and 2.48.  Stock tire is L78-15, which is 28.8 inches in diameter (2.4 feet exactly), (5760/2.4/pi=) 700.3 revolutions per mile, or (700.3x2.93=) 2052 driveshaft revolutions per mile (2052/60) = 34.20 rpm per mph (direct drive), 50.61 rpm per mph (intermediate), 84.81 rpm per mph (low).

Book (attached) has full-throttle upshifts at 37-53 mph and 68-88 mph.

37mph x 84.81 rpm/mph = 3,138 rpm   +/- 43 rpm    53mph x 84.81 rpm/mph = 4.495 rpm +/- 43 rpm
68 mph x 50.61 rpm/mph = 3,441 rpm   +/- 26 rpm    88mph x 50.61 rpm/mph = 4,453 rpm +/- 26 rpm

What is odd is that there is no mention of the axle ratios in the book.  My understanding is that the Fleetwood 75 had a 3.15 rear axle, so in theory the lower numbers could be for that car (in which case 3138 would become 3374 and 3441 would become 3699 rpm)?  (There is a special column not for the 75, but for the Eldorado.. which I think was a 3.07?)

The 1300 rpm range for the upshift sounds too big to me?  Also, I've understood that in stock form the engine can't really rev beyond 4200 (giving a top speed of 122.9 mph, BTW), so why is the upper point so high?

In my own car I see shifts at around 43 and 75 mph (per the speedometer)---well within spec---, which would mean 3646 rpm +/- 100 rpm and 3796 rpm +50 rpm/-150 rpm (actually sometimes the 2-3 upshift is at lower speeds, around 73..)  My tires are 235/70R15s, not L78-15s, so a little smaller, by 28.8/27.95-1 = 3% but that shouldn't matter, if I'm reading off the speedometer.

Of course what I'm getting to is that with my cam and new valve springs I should be able to raise the shift points a bit... say the upshifts happen at 3700 rpm, then the 1-2 upshift has the engine at 2210 rpm after the upshift and the 2-3 upshift puts it at 2500 rpm in direct drive.  If I wanted to drag race the car (and who doesn't? :-) ) none of this sounds close to optimal...

The camshaft is a Comp 260H "Hydraulic-Strong torque through low & mid-range, slightly noticeable idle" and an advertised RPM range of 1000-5000.  I have MTS's valve springs.  I'm thinking.. raise the shift points to around 4400-4500 rpm?  (4500 rpm upshifts would yield 2685 and 3040 rpm in the next gear.  Hmm.. to bring it to at least 3000, supposedly the torque peak of the engine, the 1-2 would have to happen at 5,027 rpm!) Lighten the weights in the transmission governor (is it easy to get at?  on the Eldorado it's trivial, barely even requires tools, but looks different here?)  Or is that pushing it with the engine?  Even with an unmodified engine, 3700 sounds like it would be unnecessarily low.

["Cadillac Kid" Greg Surfas 15364]

Mika
What is it you are trying to is? Cadillac motors are torque motors not high rev motors. I doubt seriously if there would be any real difference between manually shifting  and putting the selector in "D" and flooring it as far a quarter mike times goes.
Greg Surfas

[mikanystrom]

Actually a bit more acceleration at high city speeds (~30 mph) would be nice.  Kickdown to 1st instead of in 2nd, I'm thinking?

But for the sake of argument, let's say I wanted best time in the quarter mile.

I do feel that the engine bogs a bit after the automatic 1-2 upshift.  2200 rpm ought not to be in the power band even for this engine, and the top end at 3700 is significantly (20%) below the advertised power peak.  It ought to be noticeably faster with higher shift points, I think.  Especially with the upgraded valve springs.

I realize it's not a high rev motor... that's why I'm considering a goal of 4500..

Or am I talking crazy?

[bcroe]

You can move the shift points by modifying the TH400 governor.  My
procedure is to get a spare governor to play with, and weight/spring kit
for the governor.  Keep track of changes, it may take a while to get the
desired results. 

The vac modulator has influence as well; after market units are easily
adjusted.  I regard this as primarily for setting shift harshness. 
Bruce Roe

[Scot Minesinger]

2,200 rpm should be in peak torque band.  My 1970 Caddys seems to have nice power throughout the range of shifting.  Fastest ever drove it was 90 mph on rt 95 going along East coast following a pack of cars for a 100 miles or so in NC.  In normal driving say 0-70mph it is more than adequate.  If you are trying to get it to super car status, that may be tough.  In any event as Bruce wrote mods/adjustments to vacuum modulator and governor will change shift points. 

To answer your question the shift points are controlled by vacuum modulator and governor which receive inputs from vacuum and throttle position.  There is also a downshift that is directly controlled by a switch in response to throttle position mounted on intake on driver side of carb-this switch is completely adjustable (manual states 60% of throttle position).  This same switch was used for a number of years and often the plastic arm breaks off when unknowledgeable people don't re-install it correctly after carb was re-built. 

When I first bought my 1970 Caddy in 2005 and before I came to the realization that have to do everything myself to be right, told the mechanic downshift did not work.  He told me he adjusted it, and it still did not work.  Years later I noticed the arm was broken off and gone - so he lied.

["Cadillac Kid" Greg Surfas 15364]

Mika,
That cam is a torque cam, not a HP cam.  Your HP itself (according to the cam card specs) essentially does not change, but your peak torque goes from a maximum (net) 250 lb.ft at 2000 to 406 at 2000, with the bottom end increasing to 327lb/ft at 1500 and maintaining 310 lb/ft at 4000 RPM.
The cam is intended for a stock geared vehicle. Keeping the motor between 1500 and 4000 RPM wil result in the best performance from an essentially stock motor with this cam.
Greg Surfas

[bcroe]

If you converted to a switch pitch trans the lower gears could be used less. 
And it would be smoother.  Bruce Roe

[mikanystrom]

Bruce, yeah I am waiting for my transmissions to go out to have an excuse to pull them and convert to switch pitch.  And definitely want to get some of your control boxes when that happens, so don't sell them all, will you..!

It sounds like you guys think that running through the gears from 2200 to 3700 rpm is OK.

But the book has max torque 525 gross @ 3000 and max power 375 gross @ 4400.  Are these numbers completely off, then (assuming stiff enough valve springs)?

I observe that 4400/3000 = 1.467 and the intermediate ratio on the Turbo 400 is 1.48 so if the book numbers are accurate, upshifting 2-3 at the power peak of 4400 would put you basically exactly at the torque peak of 3000 in 3rd after the shift, and I don't see why you'd want to be going any slower than that.  If 3000 *is* the torque peak (525.2 lb-ft @ 3000 is exactly 300 hp, BTW), the power falls off pretty sharply at 2500, by at least 50 gross hp...

Now if the torque peaks at a lower RPM (2000, say), it's obviously a different story and maybe there isn't that much to gain by fiddling with the shift points.  I hear there's a good dyno place not too far from here...

Greg, where did you get those numbers from?

[bcroe]

You want a Transmission Governor Recalibration Kit For TH-400 B&M
Part 20248.  This will save constantly having to shift the trans by
hand (like I used to do).  I end up modifying standard parts and
putting the kit back for next time.  I use a long cotter pin for the
weight pivots. 

My Delta 88 (403) has a 70 Cad TH400, converted.  I just drove 
it back home, and honestly its a joy to drive.  A trans with modest
miles might only need some "soft" parts; the seal kit is only
around $30.  If you wait for failure, all kinds of damage may happen
and it will take lots more to fix it.  I have taken apart a lot of 60s and
70s transmissions including every one in service here.  If you saw the
stuff I have seen, you wouldn't wait to get it apart.  Bruce Roe

[wbdeford]

Can someone tell me if I'm wrong anywhere in the following...

rear end ratio is 2.93:1, gear ratios are 1.48 and 2.48.  Stock tire is L78-15, which is 28.8 inches in diameter (2.4 feet exactly), (5760/2.4/pi=) 700.3 revolutions per mile, or (700.3x2.93=) 2052 driveshaft revolutions per mile (2052/60) = 34.20 rpm per mph (direct drive), 50.61 rpm per mph (intermediate), 84.81 rpm per mph (low).

This whole discussion is way out of my league, so one of you may know something that makes this irrelevant, but:

In your calculations, you take into account the gear ratios, but not the torque converter.  Looking at my '71 manual, so the numbers are different, but '70 should be similar.  At stall, you have a 5:1 ratio coming out of the transmission due to a 2:1 ratio at the torque converter.  Obviously, the converter ratio decreases from there, but it never reaches 1:1 since there is no converter clutch.

[mikanystrom]

Yeah there's some converter slip... so the engine RPMs are always a bit higher than working off the speedo, true... good point.

[mikanystrom]

You want a Transmission Governor Recalibration Kit For TH-400 B&M
Part 20248.  This will save constantly having to shift the trans by
hand (like I used to do).  I end up modifying standard parts and
putting the kit back for next time.  I use a long cotter pin for the
weight pivots. 

My Delta 88 (403) has a 70 Cad TH400, converted.  I just drove 
it back home, and honestly its a joy to drive.  A trans with modest
miles might only need some "soft" parts; the seal kit is only
around $30.  If you wait for failure, all kinds of damage may happen
and it will take lots more to fix it.  I have taken apart a lot of 60s and
70s transmissions including every one in service here.  If you saw the
stuff I have seen, you wouldn't wait to get it apart.  Bruce Roe

Thanks for the pointer on the kit.  And the idea of using cotter pins on the weights!  I do have a spare TH400 governor lying around.  No spare 425 though.

About converting to switch pitch... my Eldorado seems to be leaking pretty profusely, so the transmission needs to come out soon anyhow, but as I'm sure you know that's a huge job.  I was going to take it to Manny Travao in San Jose one of these days.  (Just drive it there and let him do the work.)  But that would probably involve installing a GMC transmission and returning mine as a core. 

On the De Ville I also don't know anybody locally who knows precisely what to do.  There's a Buick outfit somewhere in the midwest that ships rebuilt transmissions.

Do you have any recommendations on finding a shop to just do a conversion and "rejuvenation" of the transmissions (rather than a full rebuild)?  The De Ville probably has 20K miles on it since the last rebuild (by Aamco, but it's holding up well) and I don't think the Eldorado has ever been rebuilt in its 50K miles since new.

[mikanystrom]

Not entirely relevant but cool anyhow!

http://www.cadillacpower.com/forum/download/file.php?id=19269&mode=view

["Cadillac Kid" Greg Surfas 15364]

Mika,
Shift points seem to make less of a difference with these torque motors.  On the "way up" I tried various shift points.  There was little difference in ET and trap speeds when varying the shift speeds between 4800 and 5300 RPM.  The torque curve in a decently set up Cadillac is pretty flat, and when pulling a full size car it is a matter of pulling (like a tug boat) rather than pushing.
New cam, new heads, new carburetor My current motor makes 550 lb ft at 2000 RPM (MAX 603 lb/ft.) and does not drop below 500 until after 5600 RPM.
Horsepower peaks 550 at 5500 RPM.
Shift points are 1-2:5800, 2-3:5600, Quarter mile traps at 5300 RPM.
Again irrelevant as far as your situation goes, but the torque curve is flat.
Greg Surfas

[savemy67]

Hello Mika,

There are 5280 feet in a mile so you will have to recalculate the revolutions per mile.  If the tire diameter was measured with the tire off the car, the radius will change when the car loads (and slightly flattens) the tires.

Christopher Winter

[bcroe]

Thanks for the pointer on the kit.  And the idea of using cotter pins on the weights!  I do have a spare TH400 governor lying around.  No spare 425 though.

About converting to switch pitch... my Eldorado seems to be leaking pretty profusely, so the transmission needs to come out soon anyhow, but as I'm sure you know that's a huge job.  I was going to take it to Manny Travao in San Jose one of these days.  (Just drive it there and let him do the work.)  But that would probably involve installing a GMC transmission and returning mine as a core. 

On the De Ville I also don't know anybody locally who knows precisely what to do.  There's a Buick outfit somewhere in the midwest that ships rebuilt transmissions.

Do you have any recommendations on finding a shop to just do a conversion and "rejuvenation" of the transmissions (rather than a full rebuild)?  The De Ville probably has 20K miles on it since the last rebuild (by Aamco, but it's holding up well) and I don't think the Eldorado has ever been rebuilt in its 50K miles since new.   

Yes I know about the trans in a FWD.  Mine has been out and in a few times.  The
TH425 has some leak points, esp at the bottom of the chain cover; these don't in
themselves mean failure is imminent.  But tough to fix.  I was taught to glue the
chain cover back on; the original cork is useless.  A TH425 is difficult to convert
to Switch Pitch; you really need to find a 66/67 TH425 to get everything needed. 
Manny is a source I have recommended, and one guy who did it was/is very happy
with the results. 

The governor of a TH425 is sealed in a can so you can't work on it.  I'd like to make
a can that comes apart, another project  (I should live so long). 

The "rejuvenation" is probably only an option for DIY; I learned TH400s because I
was tired of being held hostage by the trans shop. 

There is a lot of internet info on converting a RWD trans to SWP, I have done many. 
Any shop should be able to do it if the info and parts are provided.  I take a 60s
SWP trans and a somewhat improved 70s trans, and combine them into one that
is superior to either original.  Some info on my PHOTOBUCKET. 

http://s93.photobucket.com/albums/L71/bcroe/

  click on one of 2 trans Albums
  click on a picture to enlarge + description

Bruce Roe

[mikanystrom]

Hello Mika,

There are 5280 feet in a mile so you will have to recalculate the revolutions per mile.  If the tire diameter was measured with the tire off the car, the radius will change when the car loads (and slightly flattens) the tires.

Christopher Winter

I actually did the calculations with 5280 but forgot to update that in my post.. if you double-check them you'll see :-)

Does it matter that the tire gets flatter?  If the tread doesn't stretch or contract lengthwise... I think of it like a tracked vehicle, the shape of the track doesn't matter, the length does...!

[bcroe]

I actually did the calculations with 5280 but forgot to update that in my post.. if you double-check them you'll see :-)

Does it matter that the tire gets flatter?  If the tread doesn't stretch or
contract lengthwise... I think of it like a tracked vehicle, the shape of
the track doesn't matter, the length does...!   

You are right, and the track isn't the outside tread, its the inner belt the
tread is attached to.  I managed to calibrate my odometer to an extra decimal
place, but at first I didn't understand why wear didn't affect it.  All the calculations
of gears and tire dia (assuming a perfect outer circle) will always be a little off.  To
get it exactly right, measure the odometer error, determine the ratio of the
speedo gears in the trans, and adjust that ratio by the error.  Bruce Roe

[savemy67]

Hello Mika,

Good point!  The circumference is the circumference, and while it does increase where the rubber meets the road, the amount is insignificant.

Christopher Winter

[mikanystrom]

 

You are right, and the track isn't the outside tread, its the inner belt the
tread is attached to.  I managed to calibrate my odometer to an extra decimal
place, but at first I didn't understand why wear didn't affect it.  All the calculations
of gears and tire dia (assuming a perfect outer circle) will always be a little off.  To
get it exactly right, measure the odometer error, determine the ratio of the
speedo gears in the trans, and adjust that ratio by the error.  Bruce Roe

Hah!  Amazing!  I always wondered why that was.  I also noticed that my 1993 Fleetwood speedometer calibration doesn't seem to change at all as the tires wear, and I know it well enough that it should be measurable. 

What you are saying, I think, is that the tread compresses (lengthwise) as it flattens out.

I wonder if the same thing would happen with slicks.  I bet not, I would think you could tell they are wearing.  Maybe not 100% of the expected effect...

Interesting!