RPM Drop When Torque Converter Locks

[randy.powell]

Hi all - I have searched for hours on the board here trying to figure out how much of a RPM drop I should expect when my 700R4 with a locking converter locks up.

Background - 350 SB, 700R4, 4:10 rear.

The mech that put the trans in says it will only be around 200 RPM. I think it should be more. I'm not seeing/feeling it...

Bottom line - I don't think the TQ is locking...

Thoughts?

TIA,
Randy

[bonnieclyde100]

mine seems to be 200-300 rpm maybe 400

[Bigdav160]

Roughly 10% depending on load.

So if you're running 2500 rpm in fourth under light cruise the rpm drop should be 250 rpm with the converter locked

[jjzepplin]

You should feel it. It is slight and correct for about 2-300 rpms. Mine is loud and rumbly so I can hear in the exhaust. I also have a switch so I can turn it off for testing and carrying heavy loads. You really shouldn't use fourth gear for towing or loads anyway.
I just got all the parts for my tach dash to confirm this just haven't had time to put it together and install it. What system are you using for the lockup?

[randy.powell]

From what I can tell he only hooked it up for vacuum control. With the old inaccurate tachs we have (well me anyway) it's hard to see a small drop like that...

So if you guys are only seeing a small drop in RPMs then apparently he isn't blowing smoke...

My best guess is I'm running around 2000/2100 RPM at 60 with the 4:10 and 29" tall tires. Does that sound close to what y'all are seeing?

Again - thanks to all!
Randy

[geezer#99]

Punch in your numbers and check it out.
http://www.advanced-ev.com/Calculators/TireSize/

[67ChevyRedneck]

Originally Posted by randy.powell

From what I can tell he only hooked it up for vacuum control. With the old inaccurate tachs we have (well me anyway) it's hard to see a small drop like that...

So if you guys are only seeing a small drop in RPMs then apparently he isn't blowing smoke...

My best guess is I'm running around 2000/2100 RPM at 60 with the 4:10 and 29" tall tires. Does that sound close to what y'all are seeing?

Again - thanks to all!
Randy

700R's aren't vacuum controlled. They have a throttle valve (TV) cable that connects to the carb. They have an electronic lock up. It's possible he didn't hook that up? I used this:

http://www.bowtieoverdrives.com/cata...php?ITEMID=332

A simple lock up test is to get up to cruising speed where you think it's locked up, and blip the throttle just a little bit, if the rpms move a good bit it's not locked up, if the tach barely moves, it's locked. A couple hundred rpm drop is about right.

I have a 3.73 rear and I think mine only turns 2300 @ 70 mph so those numbers sound about right.

[bonnieclyde100]

Originally Posted by 67ChevyRedneck

700R's aren't vacuum controlled. They have a throttle valve (TV) cable that connects to the carb. They have an electronic lock up. It's possible he didn't hook that up? I used this:

http://www.bowtieoverdrives.com/cata...php?ITEMID=332

A simple lock up test is to get up to cruising speed where you think it's locked up, and blip the throttle just a little bit, if the rpms move a good bit it's not locked up, if the tach barely moves, it's locked. A couple hundred rpm drop is about right.

I have a 3.73 rear and I think mine only turns 2300 @ 70 mph so those numbers sound about right.

in stock form thats true but you can get a kit that will control the electric solinoid with a vacuum switch

[BLT gmc]

Best way I have found to tell if the converter is locked is to lightly tap the brakes at cruising speed. If it is wired properly and working properly, when you tap the brakes the converter will unlock, the engine will speed up slightly. As soon as you return to cruising the converter will lock and the engine RPMs will drop again. Bruce

[67ChevyRedneck]

Did a little googling... didn't know about the vacuum switch kit. I'd still recommend using one in conjunction with a brake kit. Nothing like slamming on the brakes and having the motor stall with power brakes...