I'm going crazy, need help with trans/gear ratio
 

I'm going crazy and need help. SBC350 and TH350 transmission. 3.42 rear end, 29.6" tires, TCI 328800 rebuild kit/shift kit, TCI Breakway 240900 torque converter. 60MPH @ 3500RPM!!?? When I installed the torque converter to get it to idle in gear (cam dur. heads etc...) it fixed that problem then had these high rpms at cruise. I tore the transmission down cause TCI and Summit said that it sounds like it was slipping. I found some burnt clutches and steelies. Installed the TCI rebuild kit and now I have the extact same problem! I should only be at 2330rpm according to Summit's gear ratio calculator. I'm outta ideas...

Re: I'm going crazy, need help with trans/gear ratio
 

Hafta ask, you're SURE it's in top(3rd.) gear?

Re: I'm going crazy, need help with trans/gear ratio
 

Maybe it’s your tach!
When you’re cruising at 60 and 3500 rpm, take your foot off the loud pedal. If it still shows 3500 then your tach is wacky. If the rpm drops a bunch you know it’s converter slip.

Re: I'm going crazy, need help with trans/gear ratio
 

Your not the first to ask. I dropped it into 2nd just to be sure and the rpm jumped, put back into D and the rpm dropped back to 3500rpm. So it would appear that it is going into 3rd. Also downloaded a app to track my speed to make sure my speedo is accurate - it is.

Re: I'm going crazy, need help with trans/gear ratio
 

When I take my foot off the rpm does fall down to around 2400/2500rpm... But TCI says it's suppose to be a 2200/2400 stall so it should be fully locked up by that rpm...

Re: I'm going crazy, need help with trans/gear ratio
 

TCI website has a converter slip calculator, I plugged in my info, says my converter slip is 50%!?

Why would a new converter be slipping? what can I do to fix it?

Re: I'm going crazy, need help with trans/gear ratio
 

You are in 2nd gear!
1.52 is second gear for TH350
Adjust your linkages!:smoke:
From this page...https://spicerparts.com/calculators/...rpm-calculator

Re: I'm going crazy, need help with trans/gear ratio
 

Turf the pos tic converter and put the old one back in.
Possible the tic was labeled wrong.

Re: I'm going crazy, need help with trans/gear ratio
 

I had to pull the old one out, wouldn't even idle in gear with the stock one... So I can't really just shove that one back in.

Re: I'm going crazy, need help with trans/gear ratio
 

Holy hell I hope your right! I'm pretty sure it is but I'll have to double check everything. Those #'s are just to much bang on.

Re: I'm going crazy, need help with trans/gear ratio
 

Tweaking your timing curve will let you use the old one.

Re: I'm going crazy, need help with trans/gear ratio
 

I tried a bunch before making the purchase and couldn't get it to work.

Re: I'm going crazy, need help with trans/gear ratio
 

Did you get close to 20 initial, 12 in mechanical, 10 from vacuum all in by 3200 rpm?

Re: I'm going crazy, need help with trans/gear ratio
 

Yeah, almost exactly that.

Re: I'm going crazy, need help with trans/gear ratio
 

I calculated your rpm to be 2441 at 60 with your data

Re: I'm going crazy, need help with trans/gear ratio
 

So my shift linkage was off. I went through it and got it fixed, but didn't fix my problem. Ran a steel line for the vacuum advanced to manifold vacuum. Disconnected the kickdown cable to eliminate it. I tried to check the stall speed of the TCI converter, suppose to be 2200-2400. One way I held the brake and mashed the pedal, went to around 3500rpm. Other way was slowly gave gas till the tires started to brake loose and that was around 2400rpm. I drove in reverse and mashed the pedal, left a fantastic burnout until I stopped. Wish it would do it going fwd... when I down shift from D to 2nd it goes from 3500rpm to around 5000rpm.

See videos...(if they worked)

View My Video

View My Video

View My Video

Re: I'm going crazy, need help with trans/gear ratio
 

Usually when it flash stalls you only get another 400 rpm. It shoulda been 2800 and not 3500.
You got the wrong converter or a bad one from them.

Re: I'm going crazy, need help with trans/gear ratio
 

Make sure you are referencing actual speed with a gps, your speedo is meaningless if your truck didn’t originally come with that rear gearing, which it didn’t. (Unless you’ve corrected the speedo gearing).

Also - are you absolutely certain you have 3.42s?

Re: I'm going crazy, need help with trans/gear ratio
 

Yep. Checked the speed with a gps and its accurate. I installed the 3.42's myself. Gm 10 bolt 8.5" rear end.

Re: I'm going crazy, need help with trans/gear ratio
 

Are you sure on the rear gears , do you have enough trany fluid in it , these are basics but I have to ask

Re: I'm going crazy, need help with trans/gear ratio
 

Yes. Put the gears in myself. If anything a little to much fluid.

Re: I'm going crazy, need help with trans/gear ratio
 

Just to be clear - you're saying you confirmed that your speedometer was accurate with a GPS - or are you saying the speeds you mention in this thread are GPS speeds? If the latter, good to go. If you confirmed the speedometer was displaying accurately - then I'd ask what gear ratio did the 3.42s replace and how did you recalibrate your speedometer (i.e. for it to be accurate with a rear gearing change, the speedometer gearing would also need to be replaced to still be accurate)?

Also, can you elaborate on your shift linkage being "off"? As in, disconnected, connected incorrectly, etc.

Lastly - are you certain you have an 8 cyl tach? ( I watched one of your videos, didn't realize it was an aftermarket tach). Some are adjustable between 4,6,8 cyl - you could have it set for a 6. Just one more thing to check.

Re: I'm going crazy, need help with trans/gear ratio
 

Being in Canada, are your RPMs metric?










Sorry, just trying to lighten the mood and maybe make you smile...
Hope you get it figured out.

Re: I'm going crazy, need help with trans/gear ratio
 

Correct. I used a GPS to confirm and it is accurate. When I rebuilt the trans I counted the teeth on the drive gear for the speedo and used TCI's calculator with my gear ratio and tire size to select the "driven" gear and installed that when I did the rebuild.

When I checked the install of the B&M megashifter i couldn't get the clevis to slide in and out nicely at the trans lever at all the gear positions. So i went through and adjusted the cable and now the clevis will slide in and out of the trans lever without issue at every position as per B&M's instructions.

I do have a setable tach; 4-6-8. Its set to 8, but that doesn't mean it's working correctly.

Re: I'm going crazy, need help with trans/gear ratio
 

Haha I did laugh. I hope its something easy and inexpensive.