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Old 01-24-2007, 05:31 PM   #1
page02wrx
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Default Understanding Volumetric efficiency

Got a quick question on VE....

How does increasing VE add a denser charge into your motor? I know it gets the air out faster but really... if the cams and valves are set in time with your rpm's then they are still open and closed the same amount of time...

I've tried searching but the only things I have found is exactly what I have said.... how increasing how fast the air gets out increases your efficiency.... I just don't understand that if your valves are on a time limit how more or less air can enter/leave faster.... Does it do something like let the piston come up more freely on the exhaust stroke making the piston on the power stroke easier to push down?
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Old 01-24-2007, 08:41 PM   #2
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err what the hell i didnt mean to post this here?
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Old 01-24-2007, 09:07 PM   #3
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Old 01-24-2007, 09:14 PM   #4
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I still haven't read anything on why it adds power, i'm just reading how the charge is moved quicker.... one site says it lets you rev higher which allows for more power which is WRONG. Higher rpm's does not equal higher power.... maybe if I had a big turbo?
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Old 01-24-2007, 11:24 PM   #5
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VE is a measure of how efficient the engine is at moving air. Higher VE means you're moving more air through the engine for a given condition. More airflow is the POTENTIAL to make more power. Since the amount of power an engine CAN make is almost a direct function of airflow, you want the engine to eat air as efficiently as possible.

As to the whole "potential" versus "actual" part, there are several things which will keep you from making the engine's full potential power at a given airflow including:
-- Friction losses (roughly a funciton of RPM squared)
-- Pumping losses (the energy it takes to push the exhaust gasses out as well as the energy it takes to draw the intake charge in)
-- Spark effect (knock limit preventing you from running the ideal MBT spark)
-- Fueling effects (thermal limits causing you to run fueling richer than the optimum 12.7:1)

Since increasing VE usually doesn't make any of the above effects significantly worse, it's a good thing for power. However, something like increasing boost will increase backpressure and therefore exhaust pumping losses, potentially heat the charge and hurt your spark efficiency, and cause more energy to be released to the exhaust, which hurts your fueling efficiency. Bottom line, higher VE = more airflow with few detrimental compromises. More airflow = more power potential. Therfore, higher VE = good.
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Old 01-25-2007, 01:28 AM   #6
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What I think the OP is missing is that air has inertia. It takes time to get moving. A high duration cam will open valves earlier to get the air moving quicker. This makes for a higher VE at high revs. At low revs, this overlap means that some of the air goes in and right out the exhaust. This is bad for economy, and not done in a factory car.
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Old 01-25-2007, 03:58 PM   #7
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Quote:
Originally Posted by bofh View Post
What I think the OP is missing is that air has inertia. It takes time to get moving. A high duration cam will open valves earlier to get the air moving quicker. This makes for a higher VE at high revs. At low revs, this overlap means that some of the air goes in and right out the exhaust. This is bad for economy, and not done in a factory car.
Thanks a lot it makes sense now

and TMessick thanks a lot for your post.

I have a thread in 2.0L powertrain, sorry for posting here it was a mistake... You can go there if you want to further talk about it.

Again sorry, if a mod sees this thread please delete it as I have one in 2.0l powertrain.
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