727 Transmission Efficiency...

-
7

7milesout

Well-Known Member
Joined
Feb 17, 2017
Messages
407
Reaction score
101
Location
PTC
Guys,

I have a different sort of question. It's regarding a transmission's efficiency, and specifically any transmission such as the 727, when in top / 3rd / drive / 1:1 gear.

I ask this question because I'm doing some "figgerin" as I'm doubting the accuracy of my tachometer. I'm not in love with it, I don't like its style, it hangs up when accelerating quickly, and I don't like where it's mounted. However, I'm a fairly big proponent of not fixing something if it's not broken. As for the hanging up of the tach when accelerating quickly, I consider this minor as when I'm accelerating quickly, I don't need to be staring at the tach anyway. But when cruising along, I'm tending the think the tach is far enough off, that I want to replace it.

So, my question is: When the transmission (727 for example) is in drive, and say I'm cruising along at 3,000 rpm steady, the crankshaft is turning at 3,000 rpm. Due to the 1:1 ratio of drive (3rd) gear, logically one might think the driveshaft is also turning 3,000 rpm.

But is that true? I ask because of the "slushbox" nature of an automatic transmission. How "slushy" is the 727? I know about the "15% drivetrain loss" rule. But that doesn't directly correspond to a 15% loss in driveshaft speed due to slush. I'm not an auto trans expert at all. Barely a novice. But I suspect that in reality, the driveshaft will be turning less than 3,000 rpm, but not significantly.

Here's my math: 3,000 engine rpm = 3,000 driveshaft rpm = (3.23 diff) 928.8 wheel rpm.
My tires are roughly 24" diameter = 6.283 feet per revolution.
6.283 * 928.8 = 5,835 feet = 1.1 miles = 66.3 mph.

My speedometer is accurate, GPS verified frequently. When I'm turning said 3,000 rpm, the Scamp is running 60 mph almost exactly. The math above tells me the tach should actually be reading less than 3,000 rpm, more like 2,700 rpm.

So that brings me back to my question. When in 3rd gear (1:1) assuming a perfectly operating 727, will the output shaft / driveshaft be turning exactly the same as the crank? Or is there a slush factor? And if there's a slush factor, would anyone have a guess as to the slush factor … or "slushiness?" :D

Thanks!


7milesout
 
I wouldn't call it a slush factor, I would think it is torque converter efficiency. Unless you have issues with the transmission, like slipping clutches, I think you are measuring the inefficiencies of your converter
 
Yes, you are referring to inefficiencies in the torque converter. Different torque converters (specifically "looser" performance converters) will have more slippage on the highway. With converters, you get what you pay for. With an off the shelf higher stall converter you can expect more slippage than with a more expensive custom built one. Also, the later (78 or 79 and on) "lock up" 727s (they have a special lock up torque converter) there should be minimal slippage on the highway. This was done for fuel efficiency.

To answer your question, the transmission itself should never slip in any gear. If it does it is toast and needs a rebuild.
 
I just noticed that engine rpm calculators online, are doing the same math as me. The result was within 1 rpm. I guess my math was a waste of time.

But the question about the slush factor remains.


7milesout
 
Catching up to your responses. Mine is running the stock torque converter. Everything seems on the up and up. I recently serviced the trans, and set the adjusters. It worked well before and works well now. So you guys are saying that in my condition, the crankshaft and the driveshaft should essentially be running the same speeds (in drive), right? Even though I have an older transmission and converter and it does not lock up.
 
The tach works off of the distributor.
The transmission has nothing to do with how the tach works. The tach spins at 1/2 of the actual engine rpm.

If suspect your tach is sluggish or off, you could double check it vs a new tach and/or have your OE unit restored.
 
Right, I am familiar with how the tach is signaled. I think I'll just replace it. I want a better unit with a better sweep and mount it in a better location.

I've been PM'ing Valiant 1 about his tach mount. But I can't see to get his attention.


7milesout
 
Valiant1 hasn’t been on since early November. The asked him about a MP guate in October but he never got back to me on it.

I searched his threads that he started but didn’t find anything in ref to him selling what your looking for. I may have missed it. (?)
 
if you had a manual trans or a lockup converter you would be at 2700 rpm at 60. with the slush of your converter you are at 3000 at 60 mph
 
That seems like too much slush to me (however, I said, I'm no expert). At that amount of difference, I would imagine that would generate a lot of (extra / excess) heat in the converter / transmission. But this transmission is a very typical, run of the mill, solid 727.

And another thing that now makes me think the input and output are close to 1:1 is that when I push the accelerator from cruising along at 3,000 rpm, the acceleration / power / rpm all respond linearly. The rpm doesn't "jump up" above the road speed due to slippage. They stay very linear. With cars like my Camry, if I'm cruising, and then I push the pedal more to accelerate, the rpm increases (initially) more than the speed of the car. It can be heard and witnessed via the gauges. The road speed of the Camry sort of lags and then catches up to the engine rpm. That indicates slippage. This is normal for the Camry.

But the Scamp doesn't do this, at least not in drive, and not when driving along at "drive" speeds. It reacts more linear … as if it were locked (but I know it's not). I've driven lock-up converter cars, and it is very distinct when those converters lock.

So, I'm more inclined to believe this is more tachometer error than slippage.


7milesout
 
7milesout said:
That seems like too much slush to me (however, I said, I'm no expert). At that amount of difference, I would imagine that would generate a lot of (extra / excess) heat in the converter / transmission. But this transmission is a very typical, run of the mill, solid 727.

And another thing that now makes me think the input and output are close to 1:1 is that when I push the accelerator from cruising along at 3,000 rpm, the acceleration / power / rpm all respond linearly. The rpm doesn't "jump up" above the road speed due to slippage. They stay very linear. With cars like my Camry, if I'm cruising, and then I push the pedal more to accelerate, the rpm increases (initially) more than the speed of the car. It can be heard and witnessed via the gauges. The road speed of the Camry sort of lags and then catches up to the engine rpm. That indicates slippage. This is normal for the Camry.

But the Scamp doesn't do this, at least not in drive, and not when driving along at "drive" speeds. It reacts more linear … as if it were locked (but I know it's not). I've driven lock-up converter cars, and it is very distinct when those converters lock.

So, I'm more inclined to believe this is more tachometer error than slippage.


7milesout
Click to expand...
This is because, at 3000 rpm, you are above the "stall" speed of the converter. Stock stall speeds are 2400 to 2600 rpm. Below the stall speed the converter will allow the engine to flash up to the stall rpm. Above it the input and output rpm are linear with a certain percentage of slippage.

Performance converters (some people wrongly call them stall converters, ALL converters have a stall speed) have a higher stall speed to allow the engine to flash up into the powerband rpm.
 
There is another member who made a similar part for the dash to mount the tach. I can not remember who I got it from.
 
Getting back to the original post, it's not unheard of to experience 10% slippage in a vintage stock converter. This is precisely the reason "lock-up" converters were invented, to improve driveline efficiency, and lower transmission fluid temps for better reliability. I believe I read somewhere that the first trans with lock-up converter was the Ultramatic built by Packard in 1949. Chrysler incorporated lock up converters in many vehicles in 1978 due to the fuel crisis. (Interestingly, the Chrysler units used the same principle as the Packard.)
The slippage is not (or at least should not be) inside the transmission, it's in the fluid coupler that connects the crankshaft to the 727's input shaft.
 
d55dave said:
This is because, at 3000 rpm, you are above the "stall" speed of the converter. Stock stall speeds are 2400 to 2600 rpm. Below the stall speed the converter will allow the engine to flash up to the stall rpm. Above it the input and output rpm are linear with a certain percentage of slippage.

Performance converters (some people wrongly call them stall converters, ALL converters have a stall speed) have a higher stall speed to allow the engine to flash up into the powerband rpm.
Click to expand...

That is a really good explanation. I have some vague images of a torque converter in my head from seeing such images on the internet. But I've never taken one apart. To me, the whole converter and automatic transmission is magic and wizardry inside a metal housing.

I'm getting my mind wrapped up into replacing the tachometer. Right now its a vintage Suntach? unit. Most people like these, I guess for the nostalgia. But I'm not really down with it. And it's located above my left knee and I don't like that either. I'm wanting to upgrade the tach and put it and the AFR gauge up on the dash.
 
What about the difference between radial tires and bias ply? They can both measure the same diameter off the car, but the radial rim is much closer to the ground. Would it be more proper to measure radial tire diameter as twice the radius from wheel center to ground? And how much of a percentage would the switch from one to the other effect cruise rpm? If at all.
 
7milesout said:
That is a really good explanation. I have some vague images of a torque converter in my head from seeing such images on the internet. But I've never taken one apart. To me, the whole converter and automatic transmission is magic and wizardry inside a metal housing.

I'm getting my mind wrapped up into replacing the tachometer. Right now its a vintage Suntach? unit. Most people like these, I guess for the nostalgia. But I'm not really down with it. And it's located above my left knee and I don't like that either. I'm wanting to upgrade the tach and put it and the AFR gauge up on the dash.
Click to expand...

I run a 42RH overdrive trans with converter lockup in my Dart and can tell you that when the converter locks up the RPM's drop 300 rpm's pretty much on the nose.
I can also tell you that when it is in lockup it feels just like a stick in high gear, as it has that same direct drive feel through the drivetrain.
In the summer I also notice a drop even in the engine temps when in lockup since that is where they cool best as already mentioned.
 
CudaFactHackJob said:
What about the difference between radial tires and bias ply? They can both measure the same diameter off the car, but the radial rim is much closer to the ground. Would it be more proper to measure radial tire diameter as twice the radius from wheel center to ground? And how much of a percentage would the switch from one to the other effect cruise rpm? If at all.
Click to expand...

CFHJ - I believe the proper method is to hand measure the tire O.D. mounted, and sitting on the ground. Then it wouldn't matter if it were radial or bias ply (I think).
 
TrailBeast said:
I run a 42RH overdrive trans with converter lockup in my Dart and can tell you that when the converter locks up the RPM's drop 300 rpm's pretty much on the nose.
I can also tell you that when it is in lockup it feels just like a stick in high gear, as it has that same direct drive feel through the drivetrain.
In the summer I also notice a drop even in the engine temps when in lockup since that is where they cool best as already mentioned.
Click to expand...

Got any spare 42RH's you'd like to give me? :D

The potential flaw in what you're saying is, that a 42RH may be set up for more slushiness through all the gears, including O.D., for increased comfort … due to the fact that the lock-up feature is available. Meaning, if it has the ability to lock up and make things more efficient and cooler (when locked up), why not make the other gears more comfortable? The reduction of shift shock is important to OEMs. I know, I worked for a couple OEMs in design engineering for hundreds of years. And shift shock was always an issue that was fought to remove or reduce.

Not saying your wrong, but the 300 rpm drop you see (which I know you're right about that), might be more of a drop than I would see in my 727 if my drive gear (1:1) magically had the ability to lock-up (no other changes to the trans).

I'm probably as clear as mud.


7milesout
 
7milesout said:
Got any spare 42RH's you'd like to give me? :D

The potential flaw in what you're saying is, that a 42RH may be set up for more slushiness through all the gears, including O.D., for increased comfort … due to the fact that the lock-up feature is available. Meaning, if it has the ability to lock up and make things more efficient and cooler (when locked up), why not make the other gears more comfortable? The reduction of shift shock is important to OEMs. I know, I worked for a couple OEMs in design engineering for hundreds of years. And shift shock was always an issue that was fought to remove or reduce.

Not saying your wrong, but the 300 rpm drop you see (which I know you're right about that), might be more of a drop than I would see in my 727 if my drive gear (1:1) magically had the ability to lock-up (no other changes to the trans).

I'm probably as clear as mud.


7milesout
Click to expand...

The stage 2 Transgo kit took care of the sissy shifts.:D
The RPM drop is when the converter has zero slip, so one would think it would be fairly universal assuming the converter stall is about the same.
This one is the stock Dakota converter and stalls at about 1,800 rpm.

I got my trans out of a 96 Dakota in the local pick a part for $150 and that included the converter and large yoke.
The worst part of getting it was that they wouldn't let me take a jack in and I was by myself.
It took me a couple of hours to get the sucker out on the ground.
They wouldn't even help me put it in the trunk.:rolleyes:
 
us racers calculate diameter by measuring rollout. and the 42rh is not set up for more slushiness, shift firmness may vary, and converter efficiency may vary, but as has been said - a healthy transmission will not have any slushy loss .
 
From what I've seen, manufacturers have tailored vehicle horsepower to the stall they can get away with when using lockups. My 94 Aerostar with the 3.0 has an extremely high stall when compared to vehicles of the same size with more power. They use tiny converters compared to say a Dart with a 318/904; and the Dart is probably lighter. You get used to the slipping because it eventually locks up. It needs the overdrive, high stall and the lockup because it needs a 4:10 gear to get going. This is the same type of thinking used by Dodge Truck when they put a 4:10 gear behind a 727. They used high stall converters because the rpms would be way over stall at cruising, but they didn't want a harsh initial engagement caused by the low gear.
 
You must log in or register to reply here.
-

Back
Top