I have a 96 Z28 that just turned 70k miles. I just had a new converter installed and I picked it up today. The converter is an FTI 3600 9.5" lockup. I am getting what feels like a vibration feeling under part throttle. Mainly under heavy acceleration in lower rpm when the converter is slipping the most. It does not do it throughout the RPM band, just when the converter should be slipping the most. I can feel the vibration in my foot/gas pedal. Normal driving is fine, highway speeds under lockup is fine. The car currently has stock heads and cam and 3.23 gears (upgrades coming this winter).

I called FTI and they think it is in my tuning which needs adjusted to compensate for the converter. I do have a mail order tune on the car now that I have been happy with. I had no issues prior. This is my first stalled 4l60e so throwing this out to see what the consensus is among those who have stalled autos. Is this common and I just need a retune?

 

anytime you change stall speed you have to update the tune to accomodate for it. I'm not the one that you spoke with but I can try to further help. does the vibration frequency change with engine speed or vehicle speed? does this vibration surge or jerk the drivetrain (similar to manual car clutch chatter) or is more of a NVH type thing like a driveshaft or tire out of balance?

 

Thanks for following up! It's hard to explain but this feeling is limited to the rpm range where the converter would slip the most. I can take off slow and not feel it, or when locked up it's fine. I feel it the most in my foot on the pedal when accelerating hard. The best way to explain would be it feels like traction control engaging. The frequency doesn't change, it's constant within the rpm range it occurs in. I'm sure you're correct and it's the tune but if you think I should look at other items let me know. I know it's not the converter, I was just throwing this out to see if others had any similar experiences when adding any higher stall converter.

Even without being able to really get on the car hard, I can tell this converter is going to transform my car. I can't wait to be able to really test it. This is the main reason I went with your converter because of the awesome support, so thanks again!

 

Just to follow up. For the LT1 OBD ll platform, what tables should be adjusted in the tune to compensate for a higher stall? I got a response back from the guy that tuned it and he said there really wasn't anything to adjust for the trans to compensate for a converter. There isn't much support anymore for tuning LT1 cars so outside of taking the car back to the shop to look for a mechanical issue, I'm not sure what in the tune should be adjusted.

 

It sounds to me like the engine is torqueing over under load and maybe the exhaust is hitting the floor or trans mount is not in the same place and torque arm is hitting. The transmission must be removed to replace the torque converter and most likely something was not put back in the same place. Does it vibrate with trans in park or neutral, (no load on engine) when reviving the engine to the same RPM that you would feel the vibration at when accelerating?

 

Only happens under load. That's a great thought about the exhaust or something moving under load and possibly hitting the floor. That makes sense when enough torque is applied it is shifting something and hitting. I have long tube headers and the y pipe is very close to the floor on the driver side. Saturday I am going to take it back to the shop and put it up on their lift so we can examine under it and see if that is the case. Thanks for the suggestion.

 

Following this theory hold the brake and slowly rev the motor and see if you can recreate the vibration. If you do that would further support this diagnoses.

 

Great suggestion. I'll test this tomorrow afternoon and let you know.

 

I tested this theory. Up an incline it was worse and very noticeable trying to accelerate from a stop. I then tried in reverse and no noise. Turns out it was the driver side header collector/ y-pipe hitting the floor. I have newly installed poly engine mounts and the driver side long tube collector was always close but I never felt it with the stock converter. I guess with the added stall, the engine torques upward on the driver side just enough now causing the header to hit the bracing right in that area, thus causing a very bad vibration that could be felt. Good news it was no major issue. I will likely remove that header to clearance the collector area correctly but I was able to do enough with it on the car to where it won't hit most of the time.

To FTI, this converter is amazing and now that I could test it better knowing the trans is fine, this really transformed the auto.

 

Thanks for the up-date. Glad you got it figured out.