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#92939 - 11/29/17 01:36 PM TH350 rpm drop
ACH57 Offline
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Registered: 05/21/07
Posts: 58
Loc: Clarkdale, AZ
What should the rpm drop be on a TH350 going from neutral into drive? Setup is a fresh 250 engine and fresh Trans from a local builder with a Summit converter-12” and 1700-2100 stall.

I’m running conventional ATF and have not done the first service yet.

When I put it in gear the rpm drops about by at least 500 and it lurches forward like maybe the converter is not releasing all the way. The engine bogs down too so I have to set the idle at 1200-1300 or it wants to stall. Unfortunately, the builder has closed his shop but I may be able to track him down. Any suggestions are appreciated.

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#92940 - 11/29/17 03:53 PM Re: TH350 rpm drop [Re: ACH57]
panic Offline
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The Summit converter was probably described as "1700-2100 stall" based on the characteristics in the most common application: Chevy 350 small block, which has far more torque at that speed than your 250. Stall speed will always be proportionate to the square root of torque.
This means that without anything wrong the stall speed will be significantly less than advertised, and your idle speed drop into gear will be greater.
The converter doesn't release, the only internal change occurs when the car rolls out.
500 does sound high, but I can't see a specific remedy.
Perhaps someone here has an idea?

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#92941 - 11/29/17 04:48 PM Re: TH350 rpm drop [Re: ACH57]
ACH57 Offline
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Registered: 05/21/07
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Loc: Clarkdale, AZ
After some more checking I think I may have an air leak and/or timing issues.

Would you recommend a higher stall speed converter? Or Smaller diameter like 10"?

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#92942 - 11/29/17 05:10 PM Re: TH350 rpm drop [Re: ACH57]
panic Offline
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First place I would look: how much initial spark advance are you running, and what carburetor?
Do you have a vacuum-controlled valve body?
Smaller diameter is one way to increase stall speed, but there are many other factors including turbine fin clearance, angle, and shape. A 10" converter will also act as a light-weight flywheel and give better throttle response but may be more stall than you like. You should be above your stall speed at your cruising MPH or gas mileage suffers.
What stall to use is partially dependent on your traction (8" is pointless with small tires), how hot the cam is (to idle cleanly), do you have power brakes, and when does serious torque begin.

Tell us more.

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#92943 - 11/29/17 06:09 PM Re: TH350 rpm drop [Re: ACH57]
ACH57 Offline
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Registered: 05/21/07
Posts: 58
Loc: Clarkdale, AZ
Car is a ‘64 Chevy II, 3.08 posi rear end with 26” tires

Engine: 250 bottom end with cast pistons, balanced and zero decked with about 200 miles since rebuild, Tom’s Thrifty cam (212 degrees duration @ .050 lift, .512 lift at valve, 110 lobe separation), timing has been set at 10* initial on a large cap HEI distributor with external coil and I've tried full manifold vacuum and ported vacuum

Head: 70’s small plug head with smog ports plugged, intake lumps, 1.84/1.6 valves, Offenhauser intake, 3stud exhaust manifold with 2.5” exhaust and glass pack muffler

Carb: 0-80457sa (4160 style) Holley in 3 barrel configuration (one primary barrel plugged), with quick change secondary spring housing and secondary jet metering block added for adjustability

Trans: typical TH350 with Moroso adjustable vacuum modulator and the aforementioned Summit 1700-2100 stall 12” converter
Manual disc brakes

Cruising speed is about 2200 rpm at 55 mph


Edited by ACH57 (11/29/17 06:14 PM)

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#92946 - 11/30/17 02:46 PM Re: TH350 rpm drop [Re: ACH57]
ACH57 Offline
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Registered: 05/21/07
Posts: 58
Loc: Clarkdale, AZ
Any suggestions on a better TC are appreciated.

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#92947 - 11/30/17 04:55 PM Re: TH350 rpm drop [Re: ACH57]
Mean buzzen half dozen A.K.A. Hank Offline
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I would run an 11” with 2400- 2600 stall
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#92956 - 12/05/17 12:05 PM Re: TH350 rpm drop [Re: ACH57]
70Nova Offline
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I had a customer bring a big cam 350 and auto trans 70's chevy to me, complaining about stalling. He had jacked the idle speed to 1300rpms to try to remedy this, to no avail.

I adjusted the idle to about 700 and the problem went away.... at higher idle rpms the converter is not locked or engaged but the fluid is "flashing" the internal parts when it should not be doing anything, and t makes the converter try to grab. "Flashing" may be the wrong term here, not sure. Splashing may be more accurate.

Anyway, I know it sounds backwards, but LOWER your idle to where it belongs. See what happens. Lower idle will not splash fluid so much in the converter and cause resistance.

Jan

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#92957 - 12/06/17 07:13 AM Re: TH350 rpm drop [Re: ACH57]
tlowe #1716 Offline
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AHC57,
Have you gotten any further with this? That idle sounds really high. I would expect 800 at the most.
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#92958 - 12/06/17 08:34 AM Re: TH350 rpm drop [Re: ACH57]
strokersix Offline
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Maybe the 1300 idle speed has the blades open enough to be off the idle circuit. Then when shifted into drive it can't pull from the idle ciecuit and stalls. I'm not a carb expert, just a thought.

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#92959 - 12/06/17 01:57 PM Re: TH350 rpm drop [Re: ACH57]
panic Offline
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Loc: Hempstead, Long Island, NY
I agree, this is possibly the problem. If the throttle disc is open past the idle and transition holes, they stop working as designed. The disc should be between the holes at idle.
2 cures:
1. Increase your initial spark advance by 5-10° (and deduct this amount from your mechanical advance curve to achieve the same total), which will raise your idle speed and allow the speed screw adjustment to be relaxed and lower the throttle disc. It may also increase throttle response.
2. Drill a hole in the throttle disc close to the idle/transition holes. This allows more air to enter at a lower throttle disc angle, which will properly mask those openings. Typically, the hole is 3/64" to 5/64".

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#92962 - 12/07/17 04:06 PM Re: TH350 rpm drop [Re: ACH57]
ACH57 Offline
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Registered: 05/21/07
Posts: 58
Loc: Clarkdale, AZ
Thanks for the tips. I currently have the carb off reworking the vacuum lines and checking everything for leaks. I'll get it back together with everything cleaned up and double check what it is doing.

When I would drop the idle below 1000 rpm the only way I could keep it running in gear was to blip the throttle and the accel pump would provide fuel. The problem was not quite as pronounced when the trans oil was warmed up.

I'm leaning towards it being a converter that is too large and/or too low of a stall speed for my engine's torque.

My engine guy suggested going old school and trying a Vega converter which is basically just a smaller diameter and will raise the stall right?

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#92963 - 12/07/17 05:26 PM Re: TH350 rpm drop [Re: ACH57]
Mean buzzen half dozen A.K.A. Hank Offline
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Yep sounds like the torque converter is not matched to your engine. A Vega converter is a 10” and should stall about 2600- 2800 Rpm
MBHD
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#92971 - 12/10/17 08:49 PM Re: TH350 rpm drop [Re: ACH57]
Mean buzzen half dozen A.K.A. Hank Offline
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The Vega converter is not strong. I broke one when I was not making that much power. Maybe 200-259 Hp range, but I was running a full manual shift valve body.
That shifted very harsh.
MBHD
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