why tune after gears?

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bryantaylor

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can anyone explain to me WHY you have to get your car tuned after a gear swap like everyone says you have too? i can't see why its needed when you are not changing anything going in or coming out of the engine, but i wanted some more input.
 
You don't need a "tune" for the engine. You need a chip or a PCM reflash to compensate for the new shift points.

So it is a tune for your transmission.
 
ALSO, the MAIN reason is.....wait for it......the ECU is governed based on the RPM's of the drive shaft. The stock top speed is 126ish, with 4.10 gears I was topped out at 96MPH!!!! ;(

You need a chip/tune (depending on the year) to get rid of the speed governer.
 
We drove my 98 base home from Detroit area without a tune, and it was a short time til I got my new chip from Lonnie (BOC). I went from 3.07 to 3.73. It was actually quite interesting and fun driving it without the chip, but just the improvements in shift points etc with a chip specifically made for those gears made the car much more enjoyable to drive.
 
RPMs change when you change gears, so maybe it's just that it's nicer with specifically designed RPM shifts? I don't know, it's not like it wasn't driveable, and everyone said I didn't need a chip for new gears, it was just better when I had a new chip specifically for my new set up. Prior to that I had a chip set up for the 3.07's and I also have a shift kit on my car. Maybe that made a difference? I'm talking out of my ass now so I'll shut up. ;)
 
Several inputs determine the shift points on the 4R70W.

Transmission Electronic Control System

The following is a list of the actuators and sensors used in controlling the transmission.

Powertrain Control Module (PCM) (12A650)


INPUTS

Engine Coolant Temperature (ECT) Sensor (12A648)

Intake Air Temperature (IAT) Sensor (12697)

Transmission Oil Temperature (TOT) Sensor (7H141)

Manual Lever Position (MLP) Sensor (7A247)

Brake ON/OFF (BOO) Switch (13480)

Transmission Control Switch (TCS)

Electronic Ignition (EI) System (12A297)

Air Conditioning Clutch (ACC) (2884)

Mass Air Flow (MAF) Sensor (12B579)

Throttle Position Sensor

Transmission Speed Sensor (TSS) (7M101)

Vehicle Speed Sensor (VSS) (9E731)
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The TSS and VSS will send the 'wrong' signals to the PCM when you change the rear gears ratio.

The transmission speed sensor (TSS) is a magnetic pickup, located at the output shaft ring gear that sends a signal to the PCM to indicate transmission output shaft speed.


Transmission Function:

Torque converter clutch control, shift scheduling, used in determining EPC pressure.


Symptoms:

No converter engagements, harsh shifts, abnormal shift schedules.

DTC: 639
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The vehicle speed sensor (VSS) is a magnetic pickup that sends a signal to the PCM. This VSS signal tells the PCM vehicle speed.


Transmission Function:

Modifies upshift schedules.

Symptoms: Shift engagement/disengagement (hunting) on grades.

DTC: 452
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The Mark VIII PCM was 'tuned' for 3.08 (base) and 3.27 (LSC) gear ratios. Yes, you can drive the car after gears without a chip or PCM reflash but the PCM will think the car is going faster than it actually is, not to mention the calculated 'load' will be different. Of course the 'speed limiter' will be off as well.

Depending upon how agressive you drive you may notice minimal difference or a significant difference.
 
so the only real sensor that causes the problem is the TSS? because the VSS would be working right with the corect gear installed in it.

also what is wrong without the tune? does it shift early or something like that?
 
bryantaylor said:
so the only real sensor that causes the problem is the TSS? because the VSS would be working right with the corect gear installed in it.

also what is wrong without the tune? does it shift early or something like that?
Click to expand...

The VSS will be close but not 'correct'. But you can fix that with a speedcal. The TSS is the transmission channel that controls proper torque converter clutch control, shift scheduling and EPC pressure.

The calculated engine load from the other sensors will be off since the TSS will be mis-matched proportionately by the speed difference determined by the change in gear ratios. Hence, during 'normal' acceleration, the car will shift early and the TCC schedule will be affected as well along with the EPC pressure which will be high. WOT will also have short shifts not allowing the optimum performance since the car will be shifting short of the pre-determined power band.

It's not so much 'wrong' as it is not 'correct' for many reasons. I doubt any serious implications would result but you would be nowhere near peak performance.
 
bryantaylor said:
how does our ECU know when to shift the car? i always thought it was TPS and RPM that tells it to shift?
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JP beat me to it. :)

bryantaylor said:
so really the only reason for a tune is to get rid of the speed limiter?
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For this to make sense to you, let me tell you this. I HIGHLY recommend getting a chip on a STOCK Mark 8. That is how much improvement these cars get from a simple tune and shift schedule overhaul. Get an SCT 4 bank (or a tuner if you're good) and you'll be VERY pleased. http://www.fastpartsnetwork.com/content/view/19/

Be2lose said:
What? Why would you think that?
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+1!

bryantaylor said:
so the only real sensor that causes the problem is the TSS? because the VSS would be working right with the corect gear installed in it.

You are correct, but like JP explained, the VSS does not completely control the shifting of the transmission and most importantly the torque converter lock-up.

also what is wrong without the tune? does it shift early or something like that?
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Honestly, it shifts like crap. It will go into "lock-up" at about 20mph and bog down unless you put your foot into the throttle. Mine would go into lock-up while I was coasting through a parking lot looking for a spot to park at MAYBE 15 mph and it would NOT come out of lock-up unless I romped the gas.

If you're accelerating from a light at normal throttle, you'll hit about 40 mph and it will start bogging because it will be in 4th (OD) and drop into lock-up so again, its slam the gas to get it to downshift into 3rd or tap the brake pedal to get it out of lock-up.

If you've paid attention to the stock Mark shifting and rpms, you'll know exactly what I'm talking about, expecially if you've got exhaust mods and can hear the engine bogging.
 
thanks for the info guys, its good to know. i do plan on getting a moates setup and burning my own chips. i am assumeing there is someplace in the EEC tuning software you actually put in your gear ratio?
 
bryantaylor said:
i do plan on getting a moates setup and burning my own chips. i am assumeing there is someplace in the EEC tuning software you actually put in your gear ratio?
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These are questions you need answers from moates. ;)

I believe it is so on the OBD2 flash tuners, but I'm not sure on the EEC-IV chip burners.
 
bryantaylor said:
i am assumeing there is someplace in the EEC tuning software you actually put in your gear ratio?
Click to expand...

in the sct software there isn't a "blank" or spot that you just type in your gear ratio.
which is a good thing.

IF the software DOES have a gear ratio blank or a pull down menu, RUN VERY FAST away from that software.

there is more to adjusting for gears than typing in 373's.

your working on adjusting for the differences between "vehicle speed" vs "engine speed".

the rear gears aren't the "last variable" in the chain that affect engine speed vs vehicle speed.

what about tire height?
 
yes, i am talking about my 95. all this auto trans stuff is new to me, i have always had 5spd cars. need to just do a t-45 swap and call it a day, lol
 
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