Turbo theory question

[Mr Peepers]

Has anyone else noticed that boost comes on quicker in lower gears than higher(5spd)? Example: maintaining speed at 4k rpm in 2nd and mash it, it reaches max boost sooner(watching the gauge, not seat of the pants) than 3rd at 4k rpm, same with 3-4 and 4-5. I know an engine needs load to create boost, but why would more load in higher gears spool slower than lower? Maybe not vac to 0 psi, but 0-xx psi. Assuming the rpms are past boost threshold why is this? Maybe I'm just crazy
Also, I've never come across a good(or understandable) reason why stutterbox creates boost in neutral. Is it because the throttle plate is open and it loads off the inertia of the rotating assembly? Or is the retarded timing more a factor, sending energy from the combustion out the exhaust valve?

-Curious

 

[Defiant]

Higher heated gas flow into the turbine, and less backpressure on the compressor. Which is not to dispute the speculation regarding your relative sanity, if present.

 

[daren_p]

When you say boost comes on quicker in lower gears how exactly do you mean? Are you saying the time it takes in seconds is shorter? (this I would agree too) or the rpms at which you reach the same boost level are lower in a lower gear (this I wouldn't agree too).

 

[twinturbo3k]

You are noticing the time lag in boost, not RPMs. If you are in 5th at 3500, WOT it will spool in lets say 1.5 seconds. If your at 5000 RPMs in 5th WOT it will spool in .75 seconds. When you are in 2nd gear and punch it at 3500- instead of 1.5 seconds till full boost your RPMs will go up as your boost goes up in less time than it would take to spool at 3500, so by the time you hit full boost youll be at 4.5-5k which reduces your spool time because your RPMs go higher much quicker than in 5th gear, higher RPMs = more gases through your engine = more gases through turbine = more energy creating turbine acceleration = creating boost. Sorry if that was confusing its early and im attempting to try to get my ideas into text.

 

[Mr Peepers]

Defiant- Should I assume that there is a certain point when the load actually increases lag(at the same rpm)?

daren_p- By lag I mean the time before full boost is reached when the rpms are already past the boost threshold, and I agree that the threshold is lower in higher gears, and I'm assuming it's because the time spent in the lower rpms.

twinturbo3k- I was thinking the exact same thing, but I was tempted to say if you go WOT in 3rd at 4k rpm, and WOT in 5th at 4.5k rpm, 3rd will still reach xx psi with all other factors being the same. I would still have to wonder if there is a point in rpm when lag starts to increase again, depending on the turbo, cams, exhaust, etc. I cannot verify this myself since jacks don't roll as well as tires

Thanks for the replies

 

[PieEyedPiper]

Some of the replies were difficult for my semester-is-over-smashed brain so im not sure if this was addressed, but, I believe I have it.

The reason you percieve boost to build faster during lower gears is because of the gears themselves.
Turbo or not, the car will rev through the rpm range much faster in a lower gear, correct?
So, during say a 3 second burst of WOT, how much exhaust is being emitted from the engine during the 2nd gear pull vs the 5th gear pull? The answer is much more from the 2nd gear pull.
Why? Because the engine has revolved that many hundreds of times more in that 3 second timeframe during the 2nd gear pull vs. the 5th gear pull.

While it is true that heat and engine load have a dramatic impact on spool characteristics of a given turbo system, it is not enough to negate the larger amount of exhaust emitted from the 2nd gear pull, thus spooling the turbo faster.

 

[dsm-onster]

Some of the replies were difficult for my semester-is-over-smashed brain so im not sure if this was addressed, but, I believe I have it.

The reason you percieve boost to build faster during lower gears is because of the gears themselves.
Turbo or not, the car will rev through the rpm range much faster in a lower gear, correct?
So, during say a 3 second burst of WOT, how much exhaust is being emitted from the engine during the 2nd gear pull vs the 5th gear pull? The answer is much more from the 2nd gear pull.
Why? Because the engine has revolved that many hundreds of times more in that 3 second timeframe during the 2nd gear pull vs. the 5th gear pull.

While it is true that heat and engine load have a dramatic impact on spool characteristics of a given turbo system, it is not enough to negate the larger amount of exhaust emitted from the 2nd gear pull, thus spooling the turbo faster.

Great explanation. Thus no need for me to reply, though I was eyeing this thread for a while. Perception has so much to do w/ what we feel in the hindquarters and on any other gauge but the rpm gauge. The Tach tells all.

 

[Mr Peepers]

PieEyedPiper-
So if I understand what you are saying correctly, I should be able to restate it like this: WOT 2nd gear at 4K rpm, by the time it reaches full boost it will be at 5k for simplicity sake. Now, 4th gear(any higher gear than 2nd will work) 4k rpm WOT it reaches full boost at 4.5k rpm(guessing). What you're saying is, there is more exhaust gas going through the turbine with the extra 500 rpm before the maximum pressure is reached, resulting in less lag. Assuming lag is measured in seconds, no load is on the car before going WOT, and 4k rpm is past the boost threshold. Feel free to completely disagree, I'm just here to satisfy my curiosity.

Something else I was just pondering, I'm assuming lag decreases as rpm goes up when past the boost threshold, but would it have the least lag around the top of the HP curve and start to increase in lag after that?-Assuming it's within the turbo's capabilities. Or does it depend more on the turbo's efficiency map/range?

 

[dsm-onster]

Something else I was just pondering, I'm assuming lag decreases as rpm goes up when past the boost threshold, but would it have the least lag around the top of the HP curve and start to increase in lag after that?-Assuming it's within the turbo's capabilities. Or does it depend more on the turbo's efficiency map/range?

I'm a bit too tired to follow the first paragraph, but I like this one above. I agree. And many personal logs concur.

 

[absolute_DSM]

Some of the replies were difficult for my semester-is-over-smashed brain so im not sure if this was addressed, but, I believe I have it.

The reason you percieve boost to build faster during lower gears is because of the gears themselves.
Turbo or not, the car will rev through the rpm range much faster in a lower gear, correct?
So, during say a 3 second burst of WOT, how much exhaust is being emitted from the engine during the 2nd gear pull vs the 5th gear pull? The answer is much more from the 2nd gear pull.
Why? Because the engine has revolved that many hundreds of times more in that 3 second timeframe during the 2nd gear pull vs. the 5th gear pull.

While it is true that heat and engine load have a dramatic impact on spool characteristics of a given turbo system, it is not enough to negate the larger amount of exhaust emitted from the 2nd gear pull, thus spooling the turbo faster.

Thats the way I saw it when I first read this post before anyone replied. I just didnt think that anyone else would think the same.