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Discussion Starter · #1 ·
I have a tune and typically I drive with Cruise on ergo tune is off. Tonight I was driving down this long hill by my house and as usual I had th car in 5th using engine braking to maintain speed. With the tune off, the car starts @40 MPH and will accelerate to @55 if I don't watch it. Tonight for some reason I had the Cruise off (Tune is on) and the same hill in 5th gear the car held 40 MPH. Had me thinking, am I in 3rd and not 5th. I checked and it was 5th. Felt like the car was actually being held back. Surprised me. never notice before. Ideas?
 

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Compression itself cannot be altered from a tune. Someone else might have an idea on what happened though
 
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Discussion Starter · #4 · (Edited)
12 Eco 6mt?
Yes and I probably was in 3rd gear or the downpour of rain was the resistance. It just felt like the shifter could have been in 5th. I was hoping that I actually had engine braking but probably the rain. I will try it again this morning.
 

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Compression itself cannot be altered from a tune. Someone else might have an idea on what happened though
Actually it can. A tune will increase boost, therefore the higher pressure will squeeze more air/fuel into the cylinders increasing compression.
However I don't think this is the case for this specific situation. Sounds more like the tune calibration which should be fine. Timing events will have this effect as overlap can bleed or hold cylinder pressures.
 

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i think the higher boost only changes your dynamic compression, not static compression(or vice versa) which is determined by stroke, piston design, heads, ect. which would only slow you down at higher rpms like any car. i think it was there is a good chance you were in 3rd or it was the heavy rain, it will slow you down quite a bit. i noticed it while driving any of the cars ive ever owned, ive actually found it to be quite a task to maintain speed in a strong head wind while driving my cruze.
 
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Discussion Starter · #7 ·
It didn't must a been in 3rd or the ponding puddles with the downpours we had last night. Probably the puddles bc never felt that much engine braking b4 in any gear with this car.
 

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The tune could alter your valve timing (don't know what your specific tune is like), which would change your dynamic compression, which would increase your engine braking. The valve timing is specifically altered by the stock tune to keep the dynamic compression low at cruise/coast, which improves engine efficiency/fuel economy.
 

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I agree, sounds like the tune is futzing with valve timing. I suspect in the "Eco" mode of the tune (or on the stock tune), the Cruze leaves the valves open longer when coasting for the MPG benefit. Kinda annoying when you're going down a steep hill, but the benefits do pay off in normal driving in DFCO mode.
 
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