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03 Trailblazer 4.2 I6
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Discussion Starter #1
I have a 03 TB 4.2, and was curious if it would harm anything to manually shift the automatic transmission to maybe let it rev to somewhere 6,500. The auto shifts at 6,100 at most, and I would like just a little more out of it. Another question (not that I plan to), has anyone reved the 4.2 to around 7,000 without modding it? I'm guessing that's not good for it, but I'm just curious lol
 

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The factory RPM shift points are pretty much the right place to be unless you go into the engine and have it critically balanced along with all the rotating parts.

You've got airflow, columnar velocities, valve angles, cam profiles, exhaust restrictions (some are programmed IN and need to be there), and ultimately the length and diameter of your exhaust system. It all counts.

Then you'd need to have a reprogrammed ECM to allow sufficient fuel and ignition timing to assist getting to a higher RPM anyway.

Any 'improvements' you'd try to get would be compensated for by the ECM anyway. I digress......

Manually shifting the transmission won't really hurt anything (maybe) - not like you might think anyway. The only caveat to that statement is that you will be engaging the braking-part of the gear that you are upshifting to. Those extra parts, pistons and servos require substantially more fluid and it CAN harm the total shift pressure and flow quality to the normally applied clutch/band and create a faster wear problem to the friction material.

There's another consideration --- the Line-Bias Pressure will be max'd out to anticipate the need for higher-than-normal pressures and oil flow.

This puts heavier-than-normal wear on the front pump and associated seals and gaskets, not to mention the electrical servos and switches.

Think of it this way ---> well - it's complicated - but I'll try here... and I'm talking about MANUALLY UPSHIFTING HERE.
  • There are some calibrations that change this apply/invoked/release flow chart.
  • It matters that you know the RPO code for your transmission and the 'as-built' considerations that one could check off on the order form.
IN MANUAL LOW/1ST GEAR INVOKES:
1/2 shift solenoid is active, 2/4 band applied, overrun clutch is applied, forward clutch, the forward sprague is holding, low roller is holding, Low/Reverse clutch is applied
  • NOTE - many calibrations will NOT allow 1st gear start-up from standing and instead starts in 2nd gear to help low traction conditions.
SHIFTING TO MANUAL 2ND GEAR INVOKES:
2/3 shift solenoid is active, 2/4 band is applied, overrun clutch is applied, forward clutch is applied, forward sprague is holding
  • NOTE: some calibrations will not allow 1st gear to start in this position, and instead will start in 2nd, skipping 1st all ogether.
SHIFTING TO MANUAL 3RD GEAR INVOKES:
NO shift solenoids are activated, overrun clutch is applied, forward clutch is applied, forward clutch is applied, forward sprague is holding, 3/4 clutch is applied.

Manually shifting into 4th, is like any other time you would start off it OD - or 4th gear anyway.

But - by manually UPshifting your transmission you've been invoking more devices that really are considered 'braking' devices - these are to RETARD the vehicle while going down hill. This takes the load off the brakes and can help control the vehicle speed.

This is what the lower gear ranges are typically used for and they aren't really for UP-shifting,.

I hope this is clear and helps you some - and I believe that I got this sequence correct.
 

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03 Trailblazer 4.2 I6
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Discussion Starter #3
Ok well thanks for all that info! I do use the manual gears however for engine braking in the snow, so I've found a use for them!
 

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Ok well thanks for all that info! I do use the manual gears however for engine braking in the snow, so I've found a use for them!
Then you're one of the good ones. Just be careful if you dont have Posi or GovLok, 'cause downshifting will make one front and one rear wheel stop.... and on snow it's never a good idea to be sliding 50% of your tires.

Most people own an auto and then manually upshift because no steenking GM engineer is gonna tell THEM!

Those are the same people who don't use their parking brake either.

BTW.... I do not know, if on TBs, that ABS works in 4wd.
I know that on my other GMs, ABS is OFF in 4wd.
So far there hasn't been enough snow to actually test this where I live.

That is NOT an invitation for southerners to move to Montana.
Montana is full.
Dont even think about it.
 

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03 Trailblazer 4.2 I6
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Discussion Starter #5
Yeah I never go down through gears fast and always pay close attention to avoid that. Just using the foot brake is pretty sketchy in my truck in the snow, so slowly downshifting is a lot safer
 
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