[JamRSW6]

Are the factory style Turbo 400 vacuum modulators adjustable? Looks like there is a screw on the end in the picture attached. Any thoughts about turning the screw clockwise or counter-clockwise and what effects it would have? Thanks, Jimmy

[skullbucket]

If there is a screw in it then get one of those small pocket screw drivers and turn it clockwise about a half a turn and it will shift later and harder so you know your in third.

[Jim Moshier]

Yes they are.

[Schurkey]

Yes they are...after you break the spot-weld they often use to hold the calibrated adjustment.

[Cliff R]

Adjustable modulators will provide the tuner with a small range of adjustment and it will for the most part only effect light part throttle operation. With heavy and full throttle the vacuum falls off enough that the modulator no longer becomes a player, very similar to a distributor vacuum advance.

Most will provide an adjustment range of about 3-5 mph from one end to the other. Make very small adjustments and drive the vehicle after each one till you get it dialed in where you want it......Cliff

[Formulajones]

I usually have to make 1 full turn at a time to see any real change.

Elevation has a huge affect on them as well. Up here at 5,000 ft I had it adjusted for nice part throttle shifts. But when I drive it down to sea level the darn thing would get into 3rd gear before I hit 20 mph.

So I basically had to adjust mine for somewhat longer part throttle up shifts at 5,000 ft. so the car would shift more normal when down at sea level. And you'll notice the change as well. Car shifts harder and holds shifts longer up on the mountain, but down in Phoenix the shifts soften a bit and are at normal shift points.

[Schurkey]

I thought that was the "big deal" behind the huge factory modulator--they were barometric compensated.
Thought they had the aneroid bellows inside them so they weren't affected by altitude or air pressure, which is why they're large.

Am I wrong?

[Cliff R]

Air pressure isn't the big player it's the additional throttle angle applies at high altitude for "normal" driving. At lower altitude the engine makes more power everyplace, so less throttle angle to drive around, and higher vacuum at the same time. Since the modulator is a load sensing device, it shifts later/firmer with more throttle angle and lower engine vacuum produced by that deal.

So if one were to adjust the modulator for desired shifts at 5000', for example, and you took the car down to sea level, it will shift earlier and just a tad "softer" in the "normal" driving range......Cliff

[Half-Inch Stud]

Originally Posted by Schurkey

Yes they are...after you break the spot-weld they often use to hold the calibrated adjustment.

DITTO.

If you tink your TH400 isn't sensitive enough to the Vac Modulator, then go ahead and swap the Modulator valve with the other type valve: either identified with/without 1 ring or technically identified by the ramp cut (shallow/less shallow) into a landing.

One of those Vac Mod Valves allows the Trans to be more sensitive to Vac Modulator push aganst the Line Pressure vs Gov pressure balance. The other valve less sensitive. Something like that; why believe me when you can look it up. HIS

[Formulajones]

Engines do make more power at sea level, but they also make more vacuum.

I see a solid 3-4 inches change in vacuum from 5,000 ft vs ~1000 ft or less. Which in turn does affect how it's pulling on the modulator and change part throttle shift points at all light throttle angles.

At 5,000 ft I can pull away from a stop. Extremely light throttle, and basically won't even shift to 3rd unless I either hit about 30 mph or I completely let off the gas. I can keep the car at ~25 mph with light throttle and it will stay in second gear. But I need it adjusted that way up here for the setup to even work normal down at sea level. Actually I like the fact it holes 2nd like that with around town driving and no rear gear to speak of (3.07), it's at least a little more snappy when I need to accelerate without pushing so hard to force a down shift.

Shurkey I understand what you're asking and I can't explain why it's that way. 3 400 turbo cars here that all do the same thing. One has a smaller aftermarket adjustable modulator in it. Acts the same. "shrug"