3.31 to 3.73 swap throttle cutting out!

[Smoon]

Hey guys,

Long time reader, first time poster here! The forums have been great in helping me make some decisions regarding the mods I wanted for my S550 GT mustang

I just got my 3.73s done today, without a flash to the ECU for the speedometer. However, now on 3rd gear and up, the throttle cuts out around 1500 to 2k rpm, similar to hitting a governor. Except that if I hold on the throttle, it will still creep up the RPMs ever so slowly. This has made the car essentially undriavable as it takes a very long time to rev up to the 50+ mph speeds. I am still able to rev higher and fine with lower gears, particularly first and second. Havent punched the car as I'm still waiting for the gears to break in.

I have heard there shouldnt be any issues running with a misreading speedo with the gear swap. Any of you guys here ever hear of a similar issue or will I have to flash the car? Doesn't seem like its the gear specifically, as it seems software related and not actually mechanical. There are no CELs or other indications on the dash. Any help or input from you guys wouldbe appreciated!

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[Johnboy7701]

Do you have any tunes in your car at all? Same thing happened when I got a tune. Had to datalog the issue and about 3 months later they finally fixed it.

 

[Smoon]

No sir, no tunes at all. The computer hasn't been touched yet.

I can't really think of what could possibly cause this issue... I can't imagine that the computer actually reconciles a certain wheel to engine speed/tranny gear?

I'm wondering if anybody with this generation of Mustang has gone from 3.31 to 3.73 without a ECU adjustment? The shop I had install the gears insists that its the computer, and using the tuner will fix the problem. They're thinking that the level of change/difference is causing the computer to go sad panda status.

 

[2015 Silver GT]

Why a gear swap but no tune? The gear swap was really pointless.

 

[Smoon]

Why a gear swap but no tune? The gear swap was really pointless.

I'm not sure I follow? I wasn't aware that a tune was absolutely required for operation with a gear swap, other than for speedo calibration. I had forgotten to mention that I am on the 6-speed manual transmission, so I assumed I could get by without the tuner for now.

 

[2015 Silver GT]

My point is a tune would have been money well spent. Switching to those gears was pointless since there is little if any performance gain and now you have a car that's undriveable.

 

[Smoon]

I went with the new gear ratio to get more of the "seat of my pants" feel from the increase in applied torque. I have owned a different Mustang that had a tune on it, which felt great, but I also had an automatic which was adjusted along with the tune. From what I have seen others say, a tune on a manual transmission isn't quite as worthwhile. Also, I was admittedly a little paranoid about having the ECU touched this early. An old friend of mine worked at a dealership service department, and he had mentioned that when warranty issues came in for any engine problems that a tune was the first thing they looked for as a scapegoat.

Cost for cost, a tune/tuner vs gears/labor were similar in price for me. And I felt the gear ratio would give me more of what I wanted in particular.

Regardless, there is no point in discussing alternative paths now, since the gears are what I have decided to upgrade and already have had done. I noticed in your profile that you self tuned your car; in your experience, do you think a misreading speedo can cause the throttle to cut out?

I'm really trying to figure out if the diff shop screwed something up, or whether this really does require an ECU adjustment to work properly in today's Mustangs.

 

[2015 Silver GT]

You shouldn't need a tune for a gear swap except to adjust the speedometer. Which a tune can't actually do on your car. But that shouldn't be a reason your car isn't running right. I think it's unrelated.

 

[gsxr1300]

You need the flash tool from Whipple to tell the PCM the new gear ratio.

 

[Road Dog]

Just Curious

Smoon, did you disconnect the negative battery cable before doing the gear swap? Just asking because the car relearns a lot of things after a power reset. I'm wondering if the PCM would adjust after it gets power back and you start to drive it?

 

[Smoon]

I actually went ahead and bought a CAI/tune combo from American Muscle. The tuner should let me calibrate the speedometer.

I'm just very confused as anyone I've talked to has never had throttle issues after a gear swap... and I'll be pretty sad if I find the calibration doesn't fix the issue. I can't find a correlation as to why the throttle would cut out at this point. On the other hand, I haven't talked to anyone that has completed a gear swap on the S550 Mustang; I'm wondering whether something may have changed in the programming since then that might account for a funky wheel speed.

I just thought about possibly resetting via pulling the negative cable too I'll let you guys know how that goes.

I do also have the Ford Racing catback - although this was added on a couple of weeks ago and I wouldn't think that this could cause any issues.

Thanks for all the input!

 

[gsxr1300]

Tuner is not going to let you adjust for the new gears , you need the tool from Whipple.

 

[gsxr1300]

http://www.mustang6g.com/forums/showthread.php?t=36247&highlight=swap+gears

 

[15RRGT]

Good info Jason thanks for posting that :cheers:

 

[Higgs Boson]

You shouldn't need a tune for a gear swap except to adjust the speedometer. Which a tune can't actually do on your car. But that shouldn't be a reason your car isn't running right. I think it's unrelated.

HPTuners can do this, the whipple tool is not the only option. of course, if your tuner doesn't use HPT or you don't have it, then I guess you need the whipple tool.

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