So the tranny guy put this TH400 back together, almost everything was replaced.
When he tore it down, the autopsy showed that among other things that were a disaster, the 3rd gear clutches were toast.

So he has it in the car, he hooks it up to a pressure guage and runs it on the lift and watches the line pressure. Everything was fine except in 3rd gear at cruise, line pressure being about 90psi, stayed at 90psi when the throttle was increased suddenly.
So the pressure wasn't coming up to meet the torque put out by the engine. I'm probly explaining this wrong but the vacuum modulator wasn't changing things like it should and that was because, (I think) this cam doesn't make enough vacuum.
Bottom line, the pressure isn't coming up to meet the torque of the engine.
So even when the tranny was new, it was only running 90 psi at any time in 3rd gear no matter if I jumped on the throttle or not.

So he's working on that problem.
He explained to me what he was going to do but I can't remember what he said now, I'm going to write it down next time. Something to do with a spring? In the shift kit?

He said he won't be satisfied until he sees the trans pressures where they should be when they should be. Once we get it good with the tires in the air, he runs the gauge on a temporary line into the car and drives it to make sure trans pressure is where is should be under load.

He told me most likely, no transmission shops check for this.

Low vacuum would increase line pressure not decrease it, so it is not your cam.

The modulator valve in the case under the vacuum modulator could be sticking and I would check that first because it is easy to do.

Did you try unhooking the vacuum to the modulator? That should show a higher pressure, around 160 psi with a stock pressure regulator spring.

You won't get much load to cause a drop in vacuum on a lift so it is better to use a vacuum pump hooked to the modulator to watch line rise.

Your builder could also have a leak in the 3rd gear circuit that may or may not have existed before the rebuild.

3rd gear clutches are the weakest clutch in a TH400 or Th350 but this is easily resolved if the builder knows how to dual feed this circuit.

What does it mean to dual feed the circuit?

To use both "sides" of the direct apply piston, 3rd and reverse for more apply area.
Ask your builder about it. If he doesn't know, find a different builder.

You'll never get it to live at 450+HP without dual feeding the clutches and it will live much longer at less power and have a firmer 2-3 shift if you do it.

The TransGo Reprogramming kit does it, but its circuitry gets into trouble at about 500HP.
Transbrakes and reverse pattern valve bodies also do it.

He does it first on a lift and if that all checks out, he hooks up a temporary pressure gauge and monitors pressure as he drives it.

I didn't describe it right to you.

I don't know what I'm talking about.

But what I think my memory reliably retained is:
The pressure at cruise throttle was 90 psi but it didn't come up to the reguired 160+ psi when the throttle was increased. He couldn't get 3rd gear pressure to ever come up past 90. Which he said is bad and would fry my 3rd gear clutches. I think he said 1st and 2nd were fine, I'm not sure.


Here is a different subject.
Is the part I'm thinking of called a clutch drum? A 1st and 2nd clutch drum? He recommended I go buy a billet steele clutch drum. It cost me about 95 bucks.
Does that sound familiar to you?

Good choice in buying a steel drum. You've adverted one problem.

I'd buy your trans guy lunch, or supper later.

The steel forward hub is what you are talking about. It isn't an issue until over 750 HP and I haven't had one break even at those levels.
It is a good idea on a transbraked unit but not necessary on an street/strip Th400 until you start making big power.

Your builder seems to be looking in the right places from what it sounds like but I hope he knows how to dual feed the directs at your power level and I hope he did so on your trans.

I would recommend you ask him if he dual fed the direct clutches and blocked the reverse passage so the pressure doesn't leak off in 3rd gear.

Pressures drop in 3rd on a stock Th400 because of the modulator function.

Check the modulator valve for modifications...

The steel forward hub is what you are talking about. It isn't an issue until over 750 HP and I haven't had one break even at those levels.
It is a good idea on a transbraked unit but not necessary on an street/strip Th400 until you start making big power.

Your builder seems to be looking in the right places from what it sounds like but I hope he knows how to dual feed the directs at your power level and I hope he did so on your trans.

I would recommend you ask him if he dual fed the direct clutches and blocked the reverse passage so the pressure doesn't leak off in 3rd gear.

Pressures drop in 3rd on a stock Th400 because of the modulator function.

Check the modulator valve for modifications...

He dual fed it.
I asked him yesterday.


We drove around with a line pressure guage hooked up and watched the pressure as the tranny did its job.

Everything worked very well for street use. The Hughes 3500 converter was much more solid than I expected. (I've been used to this crappy unidentified 9" converter that was slippin and slidin before and never was making contact with the crank)

When before, I could never feel the gear changes and it felt like just one long geared trans, now I definitely feel the shifts and they kick you good.

Here is what was weird;
When I got on it in 3rd gear, the pressure would initially spike up from a 90psi cruise pressure to about 125 and then go right back down to 90. So the tranny guy, was starting to suspect the valve body wasn't good. So we came back and just to be sure, he hooked up a vacuum gauge and we did it again. (At idle with vacuum line disconnected, the tranny made 150 psi)

I realize the theory is, when you jump on the throttle, the vacuum should decrease and trans pressure should increase to meet the applied torque of the motor and not let clutches slip. (I think that was how it was told to me)
I make about 9 inches of vacuum at idle.
So we got back out there and this time, while I drove he watched the guages. I jumped on it in 3rd and the vacuum went away like expected but then, the vacuum started building back. I was on it, accelerating and the vacuum, disappeared INITIALLY but then built right back up, despite my foot being on the gas.

So when the vacuum came back, of course, the trans pressure decreased to 90 again because it thinks we're cruising.

So now I have to figure out why my engine loses vacuum initially but then builds it right back up, even though my foot is almost to the floor.
So the tranny guy, doesn't suspect the valve body any longer. He told me the engine is not really working that hard which is why the vacuum is building back. Its a 3160 pound car with a BBC.

So we're going to explore the avenue of hooking up a micro switch, that goes to a solenoid that would be in line to the vacuum line that goes to the modulator. This solenoid would cut all vacuum to the modulator should I jump on the throttle and need high trans pressure.

You could just run a mechanical modulator like I use on the turbo setups I do.

You could just run a mechanical modulator like I use on the turbo setups I do.
Can you tell me more about these?

It replaces the vacuum operated modulator with a mechanical modulator that operates with a cable to the throttle linkage. Line pressure increases proportional to throttle opening.

So we road tested the new trans.

With a vacuum gauge and a pressure gauge. I'm still going to need a vacuum solenoid or a mechanical modulator or something like that when I go to the track or dyno.

But I did try a couple things.
I drove on a 95 degree day to watch trans temp (and engine temp) and it ran MUCH cooler than it did before. I didn't have a trans temp gauge on it before this rebuild but the motor ran hotter.
Now, the motor and the trans, on a 95 degree day ran at 180 for engine and 160 for trans.

On the freeway, from a straight start in 3rd gear at about 2500 rpm, I nailed the throttle to see where it would flash to.
I was disappointed to see 4800 or so. I was hoping for 4k. Its a 3500 Hughes stall.

One thing it did NOT do before the rebuild:
It caused the front shocks to snap to full extension and hazed the tires. I wasn't expecting the tires to break loose a tiny bit or I wouldn't have done any test like this.
When I mean snap, I mean snap, Its definitely didn't do that before.