shift points

niceguypmp3

Well-Known Member
one thing i've noticed that's different between our buick lacrosse and my TB is the shift points. our lacrosse with light throttle will upshift and lock the converter with the rpm being as low as 1100 rpm and it will still pull and accelerate with that low of an rpm with the good old 3800 v-6 engine but the TB doesn't seem to like having the rpm lower than about 1400 or will unlock the converter. Surely these powerful inline 6 engines can handle pulling with rpm's as low as 1100 or 1200 with light throttle and not hurt them?!? that would probably help the fuel mileage too if light throttle with the rpm's that low while putting around town. the lacrosse also allows you to give more throttle to pull a hill before a downshift than the tb too which allows lower rpm to pull most hills fairly easily which also equates to better mileage. Can a tune do these changes and can the 4.2 handle it while a 3800 can with ease?
 

TequilaWarrior

Well-Known Member
You're comparing apples and watermelons....

Your Buick LaCrosse (you didn't provide a year) could weigh 1000 lbs less than the trailblazer. The thrust-to-weight ratio alone would be 25% better all other things being equal (and they're definitely not equal).
Your Buick LaCrosse is front wheel drive (unless a 2010+ equipped with AWD) while the TB is rear wheel drive or 4wd (you didn't specify). Comparing FWD to RWD, there is no real comparison at all.
The TB is essentially A TRUCK (body on frame) with a back seat and no bed. Your LaCrosse is a unibody sedan - modern spaceframe, lightweight design.
FWD transmissions don't shift like RWD transmissions. The V6 torque curve and the I6 torque curve are radically different. Gear Ratios between the transmissions/transaxles are dissimilar. Aerodynamic cross-sections are polar opposites. etc... etc... etc...

In short, you can't compare the two...
Were we comparing a TB to a TB xl... holding all major factors the same (engine, 4wd, year) we could make some valid comparisons for fuel mileage, hp, etc...
 
OP
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niceguypmp3

Well-Known Member
ok, a better comparison then is the 2003 TB and a 1987 Chevy caprice classic with the weak 305 v-8 and the 4 speed auto trans. it does the same as my 06 lacrosse. it will upshift and will hold the higher gear or lockup with the rpm's down to around 1,000 rpm and it will let you drive around like that and if you are light on the throttle, it can accelerate from that low rpm without unlocking the converter or downshifting. and of course a big old full sized caprice is heavy and rear wheel drive.
 

TequilaWarrior

Well-Known Member
Sorry, still not a great comparison.
While it is a body on frame vehicle, it still has a curb weight approximately equal to your LaCrosse, about 3400 lbs - THAT's why it behaves very similarly to the LaCrosse. The 305 will have a horspower/torque profile similar to the modern 3800, too. VERY generally speaking, "V" engine horsepower and torque curves are approximately the same shape regardless of displacement and cylinder count. The 4200 I6 has a unique torque curve - being an inline 6, and a fairly large one at that - and the PCM is programmed from the factory to maximize the delivered torque - in order to maximize that torque the PCM will prompt transmission shifts. The PCM in the TB controls the engine and the transmission and uses each to complement the other.

Also, keep in mind that this I6 is "drive by wire"... your throttle pedal is not directly connected to the throttle body. The pedal has a sensor the PCM reads and the PCM adjusts the throttle valve based on several factors - including engine load, current RPM, pedal position, engine temperature, air temperature, etc., etc., etc. That 1987 caprice has a throttle cable from the pedal to the carburetor. It has a computer, but it's inputs are very limited and only control the needle position for air/fuel mixture. The transmission in the caprice has 2 "inputs": vacuum from the intake and a cable from the throttlebody ("kickdown" cable). The kickdown cable increases the likelihood of a downshift but does not necessarily command it. Vacuum is used to time and improve shifts.

That being said... a tune of the PCM in your TrailBlazer can provide a perceivable improvement in horsepower and anecdotally has provided some improvement in fuel economy (although it's kind of an "or" not an and.... enjoying the power burns the gas, so to speak).
 

MRRSM

Lifetime VIP Supporter
Just my :twocents: ... But in the end... It is ALWAYS About "The Math":

Even though this Article gives you the lazy out of "Skipping the Bravo Sierra and just *Clicking* on the Green Link* that takes you directly to "The Transmission Shift Points Calculator"... The Answers to your Questions are also explained on Page One:

 
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