727 Gains RPM Issue

Trying to help out a friend.
NSS/MW car,10.70s 4.10gear. 493in wedge
Last year it would occasionally gain RPM about the 1,000ft mark.
Jump from 6,200 to nearly 7,000...car would slow 5 to 8 hundredths.
Had trans gone thru by a GOOD trans guy and fellow MW racer.
Only thing he found was some slippage on 3rd clutch.
Refreshed and back in.
Got worse.
Ordered a new converter 9.5".
Car picked up 60ft....but now gains RPM even more frequently.
Pulled the trans again...teardown...no damage or evidence whatsoever.
Is totally unpredictable, but he says it seems more frequent as it gains temperature...so installed bigger cooler and a gauge.
Never gets past 170.
I thought it sounded like it is not keeping the converter full..IE. emptying the pan.
Been through all the usual things..fuel pressure/volume ..ignition...shifter.
Ideas???

What transmission fluid is he using?




Joe

Not sure.
Probably Type F

Is this per data recording?

If you have had the trans looked at a couple of times with nothing wrong, start looking elsewhere

No...tach goes up ...car noses over.

Type F from NAPA

Agreed...but what else will gain RPM and lose acceleration together?

Check your line pressure in high gear. Might be some pressure loss?

Does this car have RPM limiters of any nature.. if so try to borrow some others... or get new ones
(not sure if NSS cars use them)

I'm thinking the transmission guy is missing something awry in one of the clutches or the pump. Deep pan or shallow?

If the VB allows a line pressure gauge tap, a gauge plumbed into the cockpit might show a pressure drop at top end. If the center tap on the case is blocked, a gauge at the front servo will work.

I'm sure it has a MSD chip for max RPM.
I will find out.

He is going to install another transmission and see what happens.
So we'll see what happens.

I'll update when he gets results
Thanks everyone....

What do you mean he saw slippage on 3rd gear clutch?
What did he see exactly.
Any issues with change in the stall speed or 60’?

If you have a spare valve body I would try that first.

take a look at the lines to the trans cooler - are they a soft braided or a rubber line - you might be sucking one closed - also look at the cooler itself junk inside can act like a valve restricting flow

Also could be the valve body acting up - or the shifter is not aligned perfectly

Both cooler lines are under pressure at all times when not in park.

I don't disagree - but pressure and flow are two different things in a hydraulic system

Could be either clutch pack(s) or converter?
If the clutch pack is slipping, you should see the increased wear of the clutches and heat in the steels.
Some info in rear clutch pistons and bellview springs:
https://www.youtube.com/watch?v=V9Zl4gay5Jo&t=39s

wrong seals in clutch pack ?

isn't it around 30 lbs. or less pressure(maybe 15 to 20 Lbs. ) to the tranny cooler

My earlier reply was directed at the statement that the cooler lines, if rubber, could be sucked closed.
They can’t because they always see pressure.

You are correct, the lines normally see 15-30 psi but varies.

It would be interesting to see how much flow makes it to the trans for guys that mount their cooler at the back of the car.

while maybe the word sucked is not the best choice - lines fail - if you have worked around hydraulic systems long enough you will see a soft line failure - like a brake hose that fails and blocks flow but does not burst - what happens is the hose breaks down and the oil gets between the layers and literally can collapse the line closed - like a valve

"It would be interesting to see how much flow makes it to the trans for guys that mount their cooler at the back of the car. "



could you explain, please ? i don't quite understand, as i'm kind of a dunce at times.
thanks !

Hydraulic system “friction”. Think of electric circuits, the measurement for resistance.

i get it now.
thank you for the explanation !

I'm thinking steel or aluminum will have less internal friction, resistance, to fluid flow than neoprene or rubber. lines.
I'm not sure about PTFE, Teflon lined hoses internal friction
One way to find out is by doing a flow test in gear

A novel idea, a cooler flow test.

my old noggin is beginning to understand now !
thank you guys for helping me out !
the electrical circuit analogy made it click. also, the steel/aluminum tubing vs hose.
with the cooler flow test idea, i'm thinking comparing stacked plate to tube and fin ? if so, would/should the internals of the cooler be how much larger [size wise] than the in/out lines to the transmission ?

Actually you want to compare “ delta”. The difference between the output flow at the front cooler line fitting versus the return line entering at the trans.

Everything in between has a direct effect on the return volume. Pressure is a useless measure for this.

The less that comes back could have a negative impact on geartrain durability and a direct impact on stall.

is there an inline "flow meter" type of gauge one could use to check this ?
i still enjoy learning things i don't know, even though i'm old.

Most of the transmissions I tested (727,46/47/48) produced about a quart of flow at idle in less than 10 seconds, in neutral, at approx. 65-75 degrees.

That’s at the front cooler line fitting, one by the bell housing.

Works fine with the spare transmission in the car.
So on to the next problem.

So then the problem was inside the transmission which makes sense given the symptoms. The trans guy who looked at the trans and said it was okay must have missed something.

Yep.
Could be a casting issue.
The case is probably going to be thrown out.

I have seen the rear clutch pack slip and have the plates look good......but they were not. Usually only in high, or going into high. Were these ever really checked out?

Why would the rear clutch pack slip only in high gear? If it was slipping why wouldnt the frictions and steels show it? The rear clutch never begins its engagement with the input shaft spinning. So it is subjected to minimal slippage. If for some reason the trans was loosing pressure in 3rd only it should show in the front clutch. As far as the case itself. The passageway to the forward clutch feed isnt much over a 1" long. Pretty easy to see if its cracked. Its my feeling if there is no clutch damage that possibly the converter fill is being restricted or bleeding fluid internally. That would raise rpm and not show signs of slipage.
Doug