# TH400 Drop in RPM?



## Roqetman (Aug 31, 2015)

My ‘65 has a TH400 transmission that was installed by a previous owner. It shifts perfectly through gears, so I’m happy with that. The issue I’m trying to resolve is setting the idle so that when it’s in Drive with the AC on it will idle at 600-700 rpm. As it is now, I have to set the idle in Park/Neutral at ~1100 rpm in order to get it to idle in Drive with AC to 600-700 range. Shifting from Park to Drive drops rpm by 300 and the AC compression drops idle another 200 rpm. I planning to install a throttle solenoid to bump up idle rpm when the AC is on. But I’m trying to determine if a 300 rpm drop is typical for a TH400? If not, what’s the cause and the fix? Any advice on what to check is greatly appreciated.


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## michaelfind (Dec 14, 2018)

Roqetman said:


> My ‘65 has a TH400 transmission that was installed by a previous owner. It shifts perfectly through gears, so I’m happy with that. The issue I’m trying to resolve is setting the idle so that when it’s in Drive with the AC on it will idle at 600-700 rpm. As it is now, I have to set the idle in Park/Neutral at ~1100 rpm in order to get it to idle in Drive with AC to 600-700 range. Shifting from Park to Drive drops rpm by 300 and the AC compression drops idle another 200 rpm. I planning to install a throttle solenoid to bump up idle rpm when the AC is on. But I’m trying to determine if a 300 rpm drop is typical for a TH400? If not, what’s the cause and the fix? Any advice on what to check is greatly appreciated.


I would say probably yes, that is to be expected if it is a stock torque converter that was installed with the TH400. Your solution sounds reasonable to me. Hopefully some experts will chime in soon.


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## Sick467 (Oct 29, 2019)

A 300 RPM drop from park to drive sounded high to me. At lunch, I checked my very mildly built 400 with the TH400 and stock stall and only dropped about 130 RPM. Warmed up it idles at 800 RPM, in gear...just under 700. I don't have AC, but the solenoid system will be you best bet if you can not find a happy medium between a higher idle w/o AC and a decent idle with. Some builds will let you get away without a solenoid idle boost, but when they don't...the slam you get going into gear is a bummer.


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## Roqetman (Aug 31, 2015)

Mine shifts very smoothly. What would cause a 300 rpm drop? I do not know what converter or if there was a shift kit installed by previous owner. I have an Olds with TH350 and it drops about 100-150 rpm when shifting from Park to Drive.


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## Sick467 (Oct 29, 2019)

Does it jolt the drivetrain when going in to drive from park? I would think it would rattle the u-joints/rear-end with a 300 rpm drop. If it does not seem too abrupt going into drive..I would check my tach for accuracy. Some of the digital timing lights have RPM readouts. Years ago, I had a torque convertor that was a higher stall in my 67/400/440TH and it had similar issues, but I won't pretend to understand how the torque convertor effects the RPM at an idle.


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## michaelfind (Dec 14, 2018)

Roqetman said:


> Mine shifts very smoothly. What would cause a 300 rpm drop? I do not know what converter or if there was a shift kit installed by previous owner. I have an Olds with TH350 and it drops about 100-150 rpm when shifting from Park to Drive.


I just read a post on timing elsewhere that discussed how you can have excessive RPM drop if your centrifugal timing is trying to come in at idle (too soon). Do you know at what RPM your centrifugal timing starts to advance the timing? If LeMans Guy were reading, he would be all over this topic for you.


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## Roqetman (Aug 31, 2015)

Michaelfind, that’s a good idea to check. My ’65 has GM HEI ignition and centrifugal advance comes in pretty low, ~2000-2200 rpm and I have vacuum advance on manifold vacuum vs ported vacuum. I have tried different spring rates on the HEI but no significant change to delay centrifugal advance to extend it to to 2800 rpm Range.


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