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Go Back   Chevy TrailBlazer, TrailBlazer SS and GMC Envoy Forum > 2002 - 2009 TrailBlazer/Envoy Tech > 02-09 Vortec 4.2L I-6 Engine and Drivetrain > Engine Tunes/Mods

Engine Tunes/Mods Performance Engine Tuning/Engine Modifications

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  #1  
Old 03-16-2006, 08:35 PM
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johnner1999 johnner1999 is offline
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4.2l variable timing?

I'm sure this has been asked but I was getting odd results using search....


At what RPM does our 4.2L engine switch from the "little" cam profile to the "big" profile? This engine seems very linear (almost electric) feeling and I can't feel a surge in HP over grunt of torque like I have with other variable timing systems. And I awesome our engines are only variable on the intake side correct?

Maybe it doesn't even have variable timing
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  #2  
Old 03-16-2006, 08:48 PM
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The cam phasing is on the exhaust cam on the 4.2L's. Not sure what conditions cause the cam to 'adjust', but I know whatcha mean about the 'electric' feeling. My 2005 4.2 w/ the 3.42 rear is no neck snapper off the line, but she seems to pull forever! This would be one fast truck were it not speed/rpm governed.

G/luck
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Old 03-16-2006, 08:50 PM
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I think you have a misunderstanding on the concept of Variable Valve Timing.

Your camshaft lift and duration are always the same its a physical part of your engine.

The Variable timing changes the point at which the spark plug fires. The PCM optimizes the timing automatically .. its not going to give you any more power, it just makes your engine run efficiently. Depending on the timing of the spark, and the position of the valve overlap, etc .. it can produce more vacuum on the intake side.

- Dan
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Old 03-16-2006, 09:05 PM
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Quote:
Originally Posted by Ddubb
I think you have a misunderstanding on the concept of Variable Valve Timing.

Your camshaft lift and duration are always the same its a physical part of your engine...
Whaaaat?? The 4.2L has variable valve timing on the exhaust cam. There is a 'cam phaser' positioner motor that changes the degree of rotation on the exhaust cam.

Joel
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Old 03-16-2006, 09:09 PM
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Quote:
Originally Posted by Ddubb
I think you have a misunderstanding on the concept of Variable Valve Timing.

Your camshaft lift and duration are always the same its a physical part of your engine.

The Variable timing changes the point at which the spark plug fires. The PCM optimizes the timing automatically .. its not going to give you any more power, it just makes your engine run efficiently. Depending on the timing of the spark, and the position of the valve overlap, etc .. it can produce more vacuum on the intake side.

- Dan
Ignition timing is variable in many cars with electronic controls (more commonly known as spark retard).

Variable timing in the I6 is as stated above - cam degree phasing from an actuator.

Variable lift is something different entirely - Honda's VTEC system uses a separate high-rev cam lobe profile, while BMW's system uses a secondary cam actuator on the cam followers.
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Old 03-16-2006, 10:35 PM
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All I know is that in my Acura the engine really kicks you in the pants around 5-6k rpm when vtec kicks in. Almost has a turbocharged feel because of the lag (not nearly as drastic of course).
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  #7  
Old 03-17-2006, 01:56 AM
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I can feel the "vtec" type kick around 4000 rpms on ours.
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Old 03-17-2006, 09:06 AM
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Are you sure its only on the exhaust ?? The 3.9 with Variable Valve timing is on both the Intake And Exhaust.

They adjust equally. So while the Valve timing is Infinately adjustable, thats only true to a certain extent. It directly corresponds with Ignition Timing. You still only have a small window of Variable timing to work with, or the engine would not have sufficient compresion.

The Camshaft itself physically never changes its effect on the valves opening and closing .. if this were a Ferrari it would be a different scenario, but this is no Ferrari, and not a VTEC either.

So that being said, technically, it may make your vehicle more powerful by its design, but that being Normal operation of the engine, it increases efficiency more than anything else by optimizing both the ignition and valve timing .. within a few degrees, not anything really extreme like using a different camshaft or roller rocker ratio to give more lift and duration which would dramatically increase power.


- Dan
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  #9  
Old 03-17-2006, 09:23 AM
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Variable Exhaust valve timing...

I definitely feel mine "kick in" over 3500 RPM or so.

From the GM technical description of the I6...

"For the first time in a GM truck engine, variable valve timing (VVT) will be featured. The VVT on the Vortec I6 includes an exhaust cam phaser. Cam phasing changes exhaust-valve timing within a range of 25 degrees as engine rpm and operating conditions change. The result is linear delivery of torque, with near-peak levels over a broad rpm range, and high specific output (maximum horsepower per liter of displacement) without sacrificing overall engine response and driveability."
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  #10  
Old 03-17-2006, 11:02 AM
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It should begin kicking in around 3000-3500 rpms, which is also about the point the TQM really allows the throttle to start openning to match your right foot. If you're on the gas from a stop, you should notice a nice kick in the pants after the tach crosses 3000 rpm.

If you want the truck to feel more gutsy down low, there are a couple of tunes available which remove most of the TQM.....Wester's Garage has a great tune for the I6 and there will be a couple of more on the market shortly.
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