Intake Resonance Pulse Tuning: How to Choose Stacks!

[sunray]

Had a lot of questions about this subject over at toyota3tc forums lately so I wrote this and figured I'd post it here too. Let the brain hurting begin!

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.

Imagine 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 “ram” 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 ? 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 ? 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 39? btdc and closes at 81? 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?

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 to the intake valve. 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 “1” and the second is “2” 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 ? 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!

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 “springs” 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. Note in the picture below that the plenum has a stack inside it. That stack's length would be tuned for an ideal pulse.



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)

[Jackrabbitslim]

Holy shiznit. This is perfect... I'm making a couple intake right now and this will help with all my project. Great post. Thanks a lot man. I love to hear about some of the other variables. Maybe one day we could collaborate on something for building headers. Thanks again.

Bry

PS...see you really are my hero... hahaha

[AndrewE]

I learned something

[BenR]

cool.

but my question is do you want to boost peak power or tune for a more even power band?

[liam75]

AAAHHHHH MY BRAIN!


Good info to know, i had heard that intake runner length made a difference in power out put. It made sense, but i didnt know why.
Thanks Chris, you old farts may have more use then just funny drunk ramblings afterall.

[paulcorolla]

The only downside to this knowledge is the fact that it is hard to get different length air horns. And it gets frustrating knowing it could be better.
There is also maths for figuring out the ideal location of the throtle plate and injector location relative to the intake valve.
Some of this theory can be seen on the Hasselgren FA 4ag, if you have seen pics of it with the injectors mounted outside the air horns.
Good stuff.
Thanks Chris, now I have to do a bunch of math!

[sunray]

cool.

but my question is do you want to boost peak power or tune for a more even power band?

Well you actually can only have peak power. The only way to try and have the best of both worlds is to use variable intake runners. Toyotas first attempt at this was TVIS. Modern engines use two sets of runners that are switched by a throttling valve. And some wild ford designs on the focus have runners that are telescopic.

One thing I forgot to mention (because I decided to join Brett's experiment) was that ITB's and sidedrafts benefit from this pulsing effect much better by creating an airbox for them that is roughly 60% of the volume of your displacement.

[MPTness]

i read this from subaru tech. forum

imo tvis is giving too big of an unknown variable when calculating this type of stuff...

[bcae92]

Good stuff Chris!

I have another request though: Do you have anything on exhaust theory with regard to scavenging, etc. that you could post up?

[sunray]

Good stuff Chris!

I have another request though: Do you have anything on exhaust theory with regard to scavenging, etc. that you could post up?

Well that is a neat subject that has some very recent new theories people are talking about. I'll see what I can come up with. The conventional ideas about exhaust tuning are changing but so far everyone is agreeing about the basics of scavenging depending on head design.

[Jackrabbitslim]

i read this from subaru tech. forum

imo tvis is giving too big of an unknown variable when calculating this type of stuff...

If you're making you're own intake, stacks.. whatever... why would you keep t-vis?

[sunray]

The only downside to this knowledge is the fact that it is hard to get different length air horns. And it gets frustrating knowing it could be better.
There is also maths for figuring out the ideal location of the throtle plate and injector location relative to the intake valve.
Some of this theory can be seen on the Hasselgren FA 4ag, if you have seen pics of it with the injectors mounted outside the air horns.
Good stuff.
Thanks Chris, now I have to do a bunch of math!

Paul, rather than changing horns everytime, just fab up some spacers for the base of the horns. As long as they meet smoothly, it will achieve the desired effect.

Now those injectors outside the horns is usually used in staged injection. The idea is to try an give the air and fuel just a little bit more physical time to atomize together and increase tubulent at higher rpm. At say 10,000rpm there is very little time for a port injected fuel charge to mix.

[sunray]

If you're making you're own intake, stacks.. whatever... why would you keep t-vis?

Remember guys that TVIS was a very early and crude attempt at finding a low end velocity combined with high end volume. It unfortunately was not as effective as today's designs due to it dispensing with port tuning. Also, variable intake runners work best today thanks to variable cam timing and lift to match rpm conditions.

This doesn't mean we should not use some of this knowledge to develop our older engines further. Sort of borrow some of the engineers hard work in r&d.

[bcae92]

Well that is a neat subject that has some very recent new theories people are talking about. I'll see what I can come up with. The conventional ideas about exhaust tuning are changing but so far everyone is agreeing about the basics of scavenging depending on head design.

Thanks, that would be awesome since I haven't heard any of these new theories yet.

[paulcorolla]

"And some wild ford designs on the focus have runners that are telescopic."

I had an SVT focus for a couple years or more, it had variable cam timing on the intake cam and a dual runner intake manifold. You could hear the manifold clunk back to high rpm short runner mode when you turned the car off. It was sweet.
Mine was quite modded but 180~hp @7000 rpm and 150ft/lb from 3500 is pretty impressive for a na non honda. haha, its the touque that made it fun. but it still revved to the top so easilly.

sweet pics



I have a modified SVT focus header on my mango!

[davew]

If you really want your brain to smoke!! Go to the library check out Scientific design of Exhaust and Intakes systems book by Smith and Morrison.

They used 16 variables, including valve opening, exhaust timing, interference actions and air temp etc. for the tuned intake resonance filling. Also it is not a direct simple harmonic, the reflective waves will actually delays crank rotation as you increase RPM and there is a loss of strength with each reflection.

For those you want the simpler method try David Vizard's Carburators and Intake Manifolds, How to build horsepower Vol.2 this book has both ram tube and plenum info.

Dave W