**Post author is Sunray, can,t add which user did the replays !**

How to Choose Stacks!', 'Lots of people have been confused about this topic so I figure I would share some of the basics of intake resonance tuning here. This is a large subject in engineering filled with lots of physics variables and equations that I don't even know so treat this as a very basic guide and nothing more.And magine one intake runner going into your engine at wide open throttle (WOT) and naturally you can visualize that there is a column of air moving very quickly to the combustion chamber, pushed (not sucked!) by atmospheric pressure outside the intake.

Now imagine that we suddenly snap the intake valve closed for the compression stroke. Well what happens with that air? It does have a mass like any object so it does not want to just sit still, instead it tends to pile up against the intake valve. This piling up actually creates a sort of shock wave. This wave then bounces off the intake valve and pulses backwards through the intake toward the atmosphere.

Sorry about the hand-drawn crapness.

Now why do I care right? Well, depending on if you have a plenum or open individual runners, that pulse eventually meets another force and bounces back towards the still closed intake valve. This bouncing can occur many times in fractions of a second until the intake valve finally opens.

Now here is the REALLY interesting part, If we can time it right, we can actually have the pulse bouncing back towards the intake valve as it opens, this effect tends to â€œramâ€ a little extra air and fuel into the combustion chamber increasing the volumetric efficiency. New idea? Hell no! The car manufacturers have been engineering this into their intakes for a very long time.

How do you do it? Math! If you can't bear any math, close this window now! So the trick here is getting our intake runners to be ideal length to make the pulse timed to push air into the valve. There are other factors but they are beyond the scope of this write-up. Here is our equation: Engine rpm A· 60 seconds = engine revolutions per second. Engine revolutions per second X 360 degrees of crank revolution = Total crankshaft degrees per second# of crank degrees that intake valve is closed Ã· crankshaft degrees per second = total time the intake valve is closed at this rpm.

So as an example, we have a T series head with a cannon intake manifold and mikuni 44's with no stacks.The distance from the intake valve to atmosphere is 10.5 inches (no I didn't measure it perfectly so don't use these numbers) and I have a cam that I want to produce its real pull peak at 7000rpm. My cam has the following specs: intake opens at 39Â° btdc and closes at 81Â° abdc. Given a four stroke takes 720° to complete a full cycle this means the intake valve is open for 300° but more importantly its closed for 420 at 7000· 60 = 116.66 revs per second 116.66 X 360 = 41997.6 degrees per second 420 · 41997.6 = 0.0100 seconds the intake valve is closed at 7000rpm.

So it gets worse! The pulse we discussed travels at roughly the speed of sound which is about 1125 feet per second This means 1125 X 0.0100 = 11.25 feet the pulse travels!! Our pulse needs to travel 11.25 feet to arrive at the intake valve when it opens again. How does this happen without a massive intake pooping out from under the hood? By getting this 11.25 feet to be divided neatly by our intake runner. 11.25 feet is also 135 inches. Our intake right now is 10.5 inches.

If we divide our pulse distance by our intake runner length right now we get an even but not rounded number of 12.857 bounces. We want an even number because even numbers will see the pulse returning. 12.8 would see it getting there just a tad late. Imagine a tennis ball bouncing off a wall when you throw it. The first bounce is â€œ1â€ and the second is â€œ2â€ when you catch it (if you decide to catch it!)

In our tennis ball visual, YOU are the intake valve. We would like to tune our intake length so that 12.8 becomes 10 even or 8 even. How? Lets say we added a stack 3 inches long. Now we still have 135 inches for the pulse to travel. But now we have a total intake length of 13.5 inches. 135 Ã· 13.5 = 10 MAGIC!!!!!!

We have a perfect even number. The pulse will bounce 9 times but theoretical tenth would be it meeting the intake valve!!!! That pulse will now push just a tad more air to fill our cylinder.Now this is fine and dandy now how about if I have EFI and a plenum? Easy and actually more effective! In fact fabbing up an airbox for your sidedrafts or itb's will make this effect very nice but efi users have the upper hand here.The plenum actually acts like a spring. When the pulse comes up the runners it hits the volume of air in the plenum and that air â€œspringsâ€ the pulse back into the runner much more efficiently than ITB's or sidedrafts with stacks. Sorry guys, it's a fact.

This is why so many new cars have those crazy wild intake runner lengths. Adding stacks inside the plenum helps this effect and can make it feel like quite a boost indeed.

Have I written enough? Have I made your head hurt? I bet! But remember to take this with a grain of salt as there are other variables and only trial and error will reveal the best set-up for your machine. Experiment with inputting different engine rpm's, cam specs, and runner lengths. Enjoy! (or not)