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Now I was reading on stealth326.com, that volumetric effeciency goes down with higher RPM. Now i'm guessing that its because the valves can't open and close fast enough to maintain 100% VE. So are systems like Variable Valve timing, and DOHC to increase the VE %?
Basiclly what I'm asking is, is it correct that vvt liberates power from the engine rather than adding to it, like a turbo or N02?
Please don't ask me for details, I know nothing! :oops:
Volumetric efficiency is simply a measure of how much air you can move in and out of the engine per cycle. The stealth guys making a blanket statement like that are just wrong. VE is not on any kind of strict, universal positive or negative correlation to RPM.Quote:
Originally Posted by Jet Black
Most street engines do not ever operate at 100% VE. To begin understanding the relationship between VE and RPM, you need to realize how much head design, valve design, cam design, and cam timing (among other, more complicated things) affect airflow. As RPMs increase, volumetric efficiency increases, to a point. Logical enough: As you begin to move air through your engine faster and faster, it gains momentum. As speed increases, momentum increases (because the mass of the air is obviously not changing). This increase in momentum "packs" the cylinder full of air more and more as the speed increases. You hit a point, however, where the head simply cannot flow enough air to feed the engine at speed, and less and less air packs into the cylinders. The speed is still there, but the required mass isn't. Get what I'm saying?
Naturally aspirated cars that are well-tuned can often go above 100% VE at higher RPM, due to the momentum of the air. Every engine, of course, will run out of air at some RPM; the head is a restriction no matter what. The only engine that would be able to run at 100% VE would be an engine with no head (nothing inhibiting the flow of air), and that's just not possible. This flow of air is what determines your powerband, in case you were wondering. Of course, you can play with cam timing, valve shape and actuation, combustion chamber design and intake and exhaust tract modifications, which will change the flow of air through your motor, changing your peak VE, changing your powerband and maximum power.
On to vvt systems. A normal valvetrain can only operate in one mode: the specs you get from the factory. It doesn't matter if you're at 1000rpm or 9k rpm, the valves still lift the same amount. This, of course, is inhibitory to airflow at high rpm. So you ask, why not just put in a 9k rpm cam? Simple enough: a high rpm cam will leave the valves hanging open longer and larger, and generally with more valve overlap (the time when both intake and exhaust valves are open). At 1k RPM, the air isn't moving fast enough to effectively fill the combustion chamber and stay in the combustion chamber. It's very hard to describe without pictures, but that loping, I'm-going-to-die-at-any-second idle sound is caused by air going in the intake port and straight out the still-open exhaust port. Not much combustion going on in there. No combustion = no power. When you have a low-lift, short duration cam, the air fills the cylinder evenly and stays in there, maintaining your idle speed. VVT systems try to combine both of these advantages: the daily driving and low end power of a small cam with the screaming top end of a big cam. It's still only a small step in the right direction though. Engine dynamics changes per every single RPM, and the ideal valvetrain would adjust lift and duration for every RPM. Two stage vvt systems are better than one stage, but still very restrictive. An ideal system would probably consist of solenoids, activated by a computer, that are attached to the valves that could regulate via the electric pulse how far to open the valves. It's probably in the works somewhere.
So, in the end, vvt systems do "liberate" power from the engine by allowing it to flow differently at different RPM. Valvetrain dynamics play a critical role in determining the characteristics of your engine, as you can hopefully begin to appreciate now. It gets infinitely more complex from here. I could spend days writing, but I won't. https://www.thegalantcenter.org/styl...lt/biggrin.gif
Hope I helped a little, post up any other questions you have.
just run a leaf blower to your intake manifold so you save money on filling bottles of...air...
VVT systems are getting more advanced as time goes. I believe porsche and ferrari offer 3-stage systems at the moment, but I could be wrong. I would predict that in the next 10 years we will be seeing a solenoid-activated valvetrain. Think of how much easier that will be on everybody! No timing to worry about when rebuilding your motor....https://www.thegalantcenter.org/styl...lt/biggrin.gif