Th350 to TH2004r RPM Drop Questions

MrSony

Geezer
Nov 15, 2014
6,109
113
Des Moines, Iowa
For those that have done it, what was your drop? Right now with 235/60/15s (26.1"), 3.42, and my th350 im at 3k at 70. Fun for about 10 minutes, and that was 2 years ago.

Calculator says 2k rpm with .67 OD. Thinking about running a 3k lockup converter as well.

What say you guys?

also whats the lockup drop? is it a percentage of something? A set amount?
 

pagrunt

Geezer
Sep 14, 2014
6,542
113
Elderton, Pa
Should be able to find a gear ratio calculator to figure it out. I thought I had a good one bookmarked but I can't find it on my list.
 

MrSony

Geezer
Thread starter
Nov 15, 2014
6,109
113
Des Moines, Iowa
Should be able to find a gear ratio calculator to figure it out. I thought I had a good one bookmarked but I can't find it on my list.
i did. i was more or less wondering how accurate they are and real world experience driving a 3 vs 4 speed.
 

64nailhead

Goat Herder
Supporting Member
Dec 1, 2014
2,898
113
Upstate NY
The rpm drop on my sons for lockup is between 250 and 300 rpms. We ran it without locking it up for a long time.

How are you going to control lockup?
 
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64nailhead

Goat Herder
Supporting Member
Dec 1, 2014
2,898
113
Upstate NY
And should’ve added, that locked up it runs 2500 at 75mph. Unlocked about the same rpm at 70. With 26” tires the lockup kinda sucked at 65 and below. But you’ll find that HP and TQ at lockup speed affects the feel a bunch. The lower the HP and TQ the smoother it will feel. His motor makes about a 1hp per cube NA and it’s a 421.
 

pontiac guy

Master Mechanic
Oct 28, 2016
492
93
Royse City, TX
The calculation is accurate always. If good data goes in good data comes out. All the variables are fixed. The only place you get ambiguity is from converter slip. That is dependent upon the quality of the converter. At a steady state it should get to about 90% ratio of applied vs output speed or better. Otherwise you just make heat and you burn up your fluid. At a guess if its 90% efficient and you are seeing 3000 RPMs the lock up should get you 300. If you aren't using the lockup. You need to go into the trans. I think there's a check ball you ditch. I haven't done an automatic in a long time but I remember that the 700 oils 4th thru the converter and service life is decreased if you don't ditch the check ball and use a specific non lock up converter. I think the 200 is ok just being unplugged. Verify that info first tho or just go lock up. 200-300 RPMs isn't a lot but it's significant if you get my meaning. They didn't go to all that work designing the system for fun. It made a difference in a normal driver. Perhaps for your purposes it's not a big enough difference to retain.
 
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Oct 14, 2008
7,081
113
Melville,Saskatchewan
You will be right around 2K at 70, actually it will be 2100 rpm.
 

MrSony

Geezer
Thread starter
Nov 15, 2014
6,109
113
Des Moines, Iowa
The rpm drop on my sons for lockup is between 250 and 300 rpms. We ran it without locking it up for a long time.

How are you going to control lockup?
aftermarket kit. deactivated with the brakes.
 

fauxfour2

Greasemonkey
Apr 21, 2020
101
43
I have the b & m lock up converter box. Set it to come on at 65. With 3.73 gears i am at 2k at 70. I agree i see a 200-300 rpm drop with the lock up. My car originally had the claymore aka th200.
 
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