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Old 11-02-2012, 09:00 AM   #16
The Radium King
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Join Date: Apr 2010
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it’s all about moving air.

it’s all about math.

how much air is moving? for a 3.2 liter at 7200 rpm?

four stroke engine, so cylinder fills every second rotation, so moving 3.2 liters / 2 = 1.6 liters per rotation. multiply by 7,200 rotations per minute = 11,520 liters per minute. 192 liters per second. that’s a lot of air.

one could reckon that even small inefficiencies, obstacles, etc., in the intake get magnified when working with such relatively large volumes. fyi, a 1% loss on a 250 hp engine is 2.5 hp.

192 liters per second. that’s called the flow rate, where flow = volume over time. in our case this is also proportional to cross-sectional area of the piping, as air has to flow faster to move through a smaller passage. think about it; you have a pipe that you are drawing air through. flow in = flow out. now constrict the tube in the middle, creating a smaller diameter in a section of the pipe. flow in is still = flow out, but at the constriction the air will have to accelerate to pass the same volume of air in the same amount of time as in the sections of pipe before and after it.

using math, this presents as:

flow = volume / time
volume = area x distance
therefore, flow = area x distance / time
but, velocity = distance / time
so, flow = area x velocity

so, with flow being constant, a smaller area creates a higher velocity.


another thing you would have noticed if you actually did the pipe constriction thing above was that it was harder to draw the air through the constricted pipe. that is because accelerating the air is work, and work requires energy, and this energy is lost as soon as the air slows down (decelerates) on the other side of the constriction. wasted energy. exactly what we are trying to avoid. making the engine do extra work that it doesn’t have to. that is why most aftermarket intakes use larger diameter, equal diameter components.

so why would porsche do this to a performance car?

well, there are benefits to accelerating the air in the intake. if I can accelerate my air so that it is rushing into the cylinder instead of being sucked in then my engine will work less and i might actually get more power from improved cylinder fill on each rotation. in fact, if you open your engine and look at the intake runners that feed each cylinder, you’ll see that they taper slightly as they get closer to the engine. taper = reducing diameter = increasing velocity of air. porsche is working for us.

thing is, that intake valve on your cylinder isn’t always open, and when in closes you have that rushing air slamming into it and bouncing back. this forms a pressure wave that moves back up the intake and can impede airflow. what to do about that? well, if done properly, the lengths of the intake runners can be set such that the air bouncing back arrives just as one of the other valves open, using the bounce-back pressure wave to increase cylinder fill even further. that’s called a tuned intake.

note that you can tune only for a small range of rpm. the tuning is based on the length of the intake runners and the velocity of the air, but as we found above, velocity = distance over time, and changing the rpm changes the time component of the equation and throws the tuning off. all is not lost, however, as porsche has thought about this as well. if you go back to your engine, you’ll see the intake runners heading to each cylinder as discussed previously, but you’ll also see each bank of intake runners joined by two tubes. one of these tubes is called the ‘resonance tube’ and has a vacuum-operated flapper valve in it. the purpose of this resonance assembly is to open the flapper valve at higher rpm, effectively changing the intake runner length and ‘re-tuning’ your intake for higher rpm.


a few more points.

since the tuning of the intake is only working 100% for two small rpm ranges, there will still be bounce-back pressure waves occurring and impeding airflow. what you need is some way to get this wave out of the way so that your intake can be as efficient as possible. if you look at your oem intake piping between the throttle and the air box you will see a resonance chamber on it (or even a little appendix-like chamber if you have an earlier car); this is where porsche tries to stall-out the pressure wave to keep the intake as efficient as possible. my theory (ie, not science or fact) is that the great intake sound you hear when you remove this chamber and put on an aftermarket intake is actually the pressure wave interacting with the intake air; you are literally hearing your intake become less efficient.

the plenum (the chambers that join the runners together) should be designed to be at a constant, positive pressure, so that air is always available to feed the runners. the best way to get positive pressure is to have, as before, air accelerating into the plenum. similar to the intake runners, this is achieved by having your intake piping gradually reducing in diameter as it approaches the plenum, and this is where you see the porsche intake on the 3.2 liter engine reducing from 3” at the airbox to 2.66” at the throttle.

the airbox gets treated similar to the plenum, as both serve as air supplies to piping designed to accelerate air. as such, the airbox should be at a constant, positive pressure. again, to get positive pressure in the airbox we have to accelerate air into it. to do this we have a restriction (the snorkel) on the intake.

another consideration is how solids perform in a fluid. basically, accelerating fluids tend to ‘pick-up’ solids, and decelerating fluids tend to drop solids. the mississippi delta is full of mud because all the dirt that got picked-up by the faster flowing river upstream gets dropped when the river slows down to meet the ocean. similarly, decelerating the air in your airbox prior to hitting the filter allows all that dirt/bugs/cigarette butts to settle out before clogging the filter (or setting it on fire).

so, porshe is perfect, right? then why do folks show hp increases with aftermarket intakes, larger throttles, removed snorkels, etc.?

apples to apples. the boxster is a light, underpowered car (relative to the 996) designed to perform in a low speed environment (cities, twisty tracks). as such, porshe tuned the car to extract as much lower rpm torque as possible from the smaller engine. the tuning (resonance chamber, airbox/snorkel, decreasing diameter intake piping, etc.) is designed to maximise performance in the 2,000 to 4,000 rpm range AT THE EXPENSE of high rpm hp. that is, at higher rpm the resonance chamber on the intake piping that was serving to reduce the pressure wave at 3,000 rpm is now just a place where 192 liters per second have to expand and contract (wasted energy) before getting to the engine (remember that, similar to intake runner tuning, the most effective placement of the resonance chamber will vary with rpm, yet it is fixed in place). the smaller throttle body and snorkel designed to accelerate air into the plenum and airbox, respectively, in order to create positive pressure are now just obstacles to airflow as they become too small to efficiently move the volume of air we are trying to move.

if you go back and look at the dynos of aftermarket intakes, throttles, etc, first put on your ‘wishful advertising’ spectacles to try and see through all the vendor murk. next, notice that none of them show any gains, and some show losses, at lower rpms, and the benefits only start to appear at higher rpms.

and this is time I will never get back.
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