Th350 vacuum and shifting - Chevelle Tech
Transmission & Driveline Transmissions and Differentials

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post #1 of 8 (permalink) Old Aug 24th, 19, 5:42 PM Thread Starter
Ryan
 
Join Date: Mar 2014
Location: Tacoma, WA
Posts: 25
Th350 vacuum and shifting

Hey gents,

Currently trying to raise my part throttle shift points on my th350. It’s shifts to second very early and into third almost immediately after. I’m aware of the modulator adjustment and the changing of weights and springs. I’ve played around with both with no success. With that being said, here’s something I’m going to try. Let me know if my thinking on this is way off. I’m going to try to reduce the vacuum signal that my trans sees at the vacuum modulator from the intake by installing a reducer in the vacuum line. I bought one of these vacuum connectors from the local parts store. I’m going to snip the tip off of the side nearest the intake and leave the necked down side nearest to the modulator. In my mind this will work. But I can also see it not working. In the likely event that I’m way off base, does anyone know a way to reduce a vacuum signal.

Because it will get asked,..
383 stroker
200 cc alum heads
11:1 static compression
237 duration in, 243 duration exh hyd roller cam
108 lobe center
Dual plane Eddy RPM intake
Holley HP 750
14 initial, 36 total
3600 (tight) converter
3.55 gears
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1970 Malibu
383, 10.3:1, etec 200 heads, isky 280 mega, holley 770,
th350, 2600 stall,
12 bolt, 3.55 eaton posi
no time slip yet

just got out of paint
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post #2 of 8 (permalink) Old Aug 24th, 19, 5:45 PM Thread Starter
Ryan
 
Join Date: Mar 2014
Location: Tacoma, WA
Posts: 25
Caveat:
Yes my kick down cable is adjusted to where it is fully extended when my carb is fully opened.

1970 Malibu
383, 10.3:1, etec 200 heads, isky 280 mega, holley 770,
th350, 2600 stall,
12 bolt, 3.55 eaton posi
no time slip yet

just got out of paint
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post #3 of 8 (permalink) Old Aug 24th, 19, 6:48 PM
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Tom
 
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Location: Firestone, CO
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Re: Th350 vacuum and shifting

Changing vacuum and adjusting the modulator will only move the shift points a little and makes the shifts harder and less pleasant. The proper way to change shift points is with the governor. If you have changed weights and springs and not seen a difference it may be that your governor or the bore in the trans case is worn out. Measure the lands on the governor, they should measure .800 or damn close and ensure the bore in the case isn't torn up. The governors are available from Sonnax new or drop by a trans shop, they usually have dozens of them laying around.

70 Camino...Roller

69 Malibu Stock 307, Reverse Manual TH350, 4,500 Stall Edge Converter, 4.56 Spool 35 Spline 12 Bolt

76 C20 VortecPro 496 628HP 655 lb/ft TH400 Edge Converter, 4.10 14 bolt
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post #4 of 8 (permalink) Old Aug 24th, 19, 7:31 PM Thread Starter
Ryan
 
Join Date: Mar 2014
Location: Tacoma, WA
Posts: 25
Quote:
Originally Posted by Tommy the Cat View Post
Changing vacuum and adjusting the modulator will only move the shift points a little and makes the shifts harder and less pleasant. The proper way to change shift points is with the governor. If you have changed weights and springs and not seen a difference it may be that your governor or the bore in the trans case is worn out. Measure the lands on the governor, they should measure .800 or damn close and ensure the bore in the case isn't torn up. The governors are available from Sonnax new or drop by a trans shop, they usually have dozens of them laying around.
Thanks for the response. I think I’ll try a new weight and spring tonight and see if it changes. I bought a digital scale today so I can weigh the weights. While I’m in there I’ll mic the lands. So I guess I was off on the whole vacuum thing. I was about out of ideas.

1970 Malibu
383, 10.3:1, etec 200 heads, isky 280 mega, holley 770,
th350, 2600 stall,
12 bolt, 3.55 eaton posi
no time slip yet

just got out of paint
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post #5 of 8 (permalink) Old Aug 24th, 19, 9:07 PM
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Tom
 
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Location: Firestone, CO
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Re: Th350 vacuum and shifting

I assume you're replacing the inner weights and springs and that will set your WOT shift points and does have a small effect on part throttle but the biggest improvement you can make for light throttle, particularly the 1-2 shift, is a skeleton outer weight. Corvettes, F bodies and Syclone/Typhoon, possibly others, with 700r4s used them and they are interchangeable with 350 and 400s. I couldn't find any recently when I built the 400 in my truck so I cut my stock ones, it's not too difficult. The outer weights aren't available in the aftermarket. If you use a skeleton outer weight you'll get a much higher 1-2 light throttle shift point and you can back your vacuum modulator way out and have a higher RPM shift that is butter smooth when driven nice. It's pretty magical when you get it all balanced.

70 Camino...Roller

69 Malibu Stock 307, Reverse Manual TH350, 4,500 Stall Edge Converter, 4.56 Spool 35 Spline 12 Bolt

76 C20 VortecPro 496 628HP 655 lb/ft TH400 Edge Converter, 4.10 14 bolt
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post #6 of 8 (permalink) Old Aug 24th, 19, 9:59 PM
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Tom
 
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Re: Th350 vacuum and shifting

Here's what the skeleton weight looks like-

*This isn't my picture. It came from nastyz/28 through a Google search.
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70 Camino...Roller

69 Malibu Stock 307, Reverse Manual TH350, 4,500 Stall Edge Converter, 4.56 Spool 35 Spline 12 Bolt

76 C20 VortecPro 496 628HP 655 lb/ft TH400 Edge Converter, 4.10 14 bolt

Last edited by Tommy the Cat; Aug 24th, 19 at 10:15 PM.
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post #7 of 8 (permalink) Old Aug 24th, 19, 10:52 PM Thread Starter
Ryan
 
Join Date: Mar 2014
Location: Tacoma, WA
Posts: 25
Quote:
Originally Posted by Tommy the Cat View Post
Here's what the skeleton weight looks like-

*This isn't my picture. It came from nastyz/28 through a Google search.
Oh wow. I’ve been researching and working on this for months and have yet to hear of that. Thanks

1970 Malibu
383, 10.3:1, etec 200 heads, isky 280 mega, holley 770,
th350, 2600 stall,
12 bolt, 3.55 eaton posi
no time slip yet

just got out of paint
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ppg arrival blue metallic
black SS stripes w/ blue pearl
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post #8 of 8 (permalink) Old Aug 24th, 19, 11:13 PM
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Tom
 
Join Date: Oct 2006
Location: Firestone, CO
Posts: 1,059
Re: Th350 vacuum and shifting

You have to dig pretty deep to find that stuff. A lot of hobby enthusiasts view auto transmissions as black magic and prefer to let pros handle them. People who build trans like it that way and don't go throwing their "secrets" up on the internet for the world to see.

70 Camino...Roller

69 Malibu Stock 307, Reverse Manual TH350, 4,500 Stall Edge Converter, 4.56 Spool 35 Spline 12 Bolt

76 C20 VortecPro 496 628HP 655 lb/ft TH400 Edge Converter, 4.10 14 bolt
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