4th gear stumble
 

I'm posting this here with the idea someone may know what's happening. I went to the track today in terrible heat and humidity...it was awful and odd for this time of year where i live. Anyway best of the day was 12.7 at 110mph. 1.8 sixty. My question is in the vid if you listen to the car go thru the gears once it goes into 4th it stumbles....anybody have any idea what it could be...I'm thinking tune related but I'm not sure thank you.

https://youtu.be/3oEmLH751S4

I take it this doesn't usually happen? If it was the converter locking or something you'd get that every time - so I'm going to assume that's not the case.

It sounds like you might have tickled the knock sensor pretty good and it sucked out some timing for a bit. I didn't hear a stumble (actual mis-firing) but more of just laying over and then recovering, which is what you'll get if the PCM grabs a bunch of timing out from under you (it recovers pretty fast - stock tune will roll that timing back in much slower). The fact that you mentioned it was very hot and humid may support this this theory, as heat will lower the threshold for knock anyways.

If it was a one time thing I wouldn't get too concerned, but if that starts being repeatable, you're gonna want to get on some sort of data acquisition platform and see what's going on.

Good lick against the Mustang!

Thanks alot I appreciate the help. This is the first time down the track with the long tubes and my new tuners full trans and dyno together. It did it on every pass. I thought i could feel something but I wasn't sure because with the helmet on and the exhaust it was only when I watched the vid that I heard it. Believe me....I lost that race by 1000th of a second .we both thought I won till we seen the slip. I did beat a Shelby Gt 350 r by a car. So do you think that lay over which is a better description is from the heat and humidity. I'm wondering why it would o only do it at that point..i could feel the car stop pulling for an instant. I think that cost me a mph or 2 and maybe a tenth.

If it does it every time I definitely would want to get some scans to see what's going on (a $20 bluetooth OBDII adapter and free Torque app on your phone would get you there if you have no other options). Depending on what it is, I would bring it back to your tuner for further review. I would send them a link to your video and ask for their input.

I do think lay over is more appropriate - I don't hear it misfiring at all, it sounds like it loses some power, and then comes back in. No hint of that in the first three gears, just 4th where the load is the highest. It almost sounds like the converter locking - I don't know if that's a thing with 6L80's or not, but it's about the right spot in the pull to do that. I would make sure it's not that before going any further.

I brought up the question if it was the converter locking upto my tuner ....he checked my file and said its set to lock in 4th. Because it does it evey run...the exact same time that may be it . I go see him wed morn. And your right..there is no misfire or anything..just an rpm lose.

That's what it is then - just the converter locking. I didn't know that was a 6l80 thing - but I've not paid much attention to the automatic stuff. What converter is in the car - I assume it's something designed for WOT lockup.

Its a Circle D 3200. Seems to work well but i guess Wed will be the day to fined out. :happy0180:

Let us know what you find - but I'll bet that's by design. It's become a fairly common practice to lock the converter on the back half of the track once to make up some of the efficiency lost with a torque converter. It's usually worth a couple MPH.

Ok....i appreciate your advice.:D

Yep it was the converter locking up. So the tune has been changed ...went with the tuner for a wot pull and its just fine. He reduced some more tq management and it seems to have a little more grunt down low.

Makes sense - good to know. If you don't know it's doing that I can see how you'd think it was something like a stumble. Probably not something you notice on the street all that often, because how many wide open 3-4 shifts do you get to make on the street?

Your right....i didn't notice on the street. I usually get to the track way earlier ..like May. But its all good. Now we are talking about how much torque can be safely removed for alot better hole shots. He only has 15% removed but I've read 50% plus and even completely removed...that's another story lol

I would pull it all and let your right foot control how far into the danger zone you wanna go. Especially with a good converter I bet it would pick up a lot on the 60'.

I'm going Saturday finally. Two broken diffs, an axle, a slave cylinder, the whole cam job, I've been down nearly 11 weeks this season. It if doesn't go 11s I'm lighting in on fire!

Lol......have faith man.....have faith.