IR lengths and plenum sizing for carbs

[naukkis79]

This is a pressure trace, fourth harmonic
explain to me what's bad about it

Your diagram shows first, second and third harmonics. Tuning for harmonics is only for overlap period and for low rpm setups have about zero effect on anything. What matters for tuning is max cylinder vacuum to intake close point to achieve high pressure to intake closing point. It's more than just frequency as target is to achieve max VE not max pressure.

With IR-setups it's very easy to have different length velocity stacks to change tuning point, shorter stack means more power top end power, taller stack increases mid end power. Mild camshaft setups don't give a shit for harmonics, top end power is intake flow limited and shortening manifold will increase top end power. But tuning only for top end power is just stupid.....

And good IR intake length is usually as long as you can fit in. Intake from head to carb should be as straight as possible for even fuel distribution, velocity stacks could be bend to 90 or even 180 degree if needed.

[modok]

count the peaks of the green line 1,2,3,4
FOURTH HARMONIC

If this is a difference of terminology i would welcome learning alternative definitions, so i can understand them

[user-23911]

count the peaks of the green line 1,2,3,4
FOURTH HARMONIC

Well, no it's not.

Every pipe has a resonant frequency.
A harmonic (by definition) is a multiple of that resonant frequency.


Or it can be a length which is a fraction of it.





It's no different from the uneducated masses talking about ignition coils charging when they don't.
It's because they never did the relevent training and qualifications that go with it.


So anyway.........now that's defined as gigo.

Does anyone have a picture of any of the Mclaren Can Am cars.......you know........the ones with the really long velocity stacks sticking out the top.
I wonder how they would fit into pipe max?

[user-23911]

count the peaks of the green line 1,2,3,4
FOURTH HARMONIC

Count the number of degrees between the peaks then see how many there would be over the whole 720 degree cycle.

About 6 or 7 of them.

[digger]

count the peaks of the green line 1,2,3,4
FOURTH HARMONIC

Well, no it's not.

Every pipe has a resonant frequency.
A harmonic (by definition) is a multiple of that resonant frequency.


Or it can be a length which is a fraction of it.





It's no different from the uneducated masses talking about ignition coils charging when they don't.
It's because they never did the relevent training and qualifications that go with it.


So anyway.........now that's defined as gigo.

Does anyone have a picture of any of the Mclaren Can Am cars.......you know........the ones with the really long velocity stacks sticking out the top.
I wonder how they would fit into pipe max?

Look at the BMW stuff it's not very long even on the lower rpm stuff . Back in the 80s the runners were a bit longer but still less than 20" and ive tested shorter runnners on those with no loss in torque anywhere and big power power gains likely due up being able keep csa small when you have a nice straight shot at the valve that the shorter length inherently allows .

Pipemax is pretty good but you don't always want to tune the intake to peak hp rpm for all applications (as per pipemax suggest) as you can get some decent area under the curve gains with an inch or two longer with minimal peak hp losses. Going much longer boosts mid range but hurts topend so tradeoffs become substantial

[modok]

count the peaks of the green line 1,2,3,4
FOURTH HARMONIC

Count the number of degrees between the peaks then see how many there would be over the whole 720 degree cycle.

About 6 or 7 of them.

actually 5.1, but i don't see how that matters.
We are not interested in how the pipe would work OFF the engine, but how it works when connected to an engine, specifically, the one it's connected to.

I may in the future also add in parenthesis to (peaks per cycle) when speaking about induction lengths. You can call it whatever you want, i don't care, as long as we are understanding eachother.

[user-23911]

When IVO happens, the intake pressure is at a peak when ideally it would be a trough.
So it's the wrong length for that RPM but will be a better length for a different RPM.

But that's at 7750 RPM.......you tune length for peak torque.
The japs do anyway.

Studying that chart a bit more, each cycle seems to be about 150 deg. 5 peaks and 5 troughs over 720 deg.
that makes it the 10th harmonic, to calculate the length..........6.86 metres/22 (following from my earlier post)
Works out at about 12 inches.
But if you made the pipe longer, the peaks and troughs would be further apart and IVO could be made to happen in the previous trough?

[Geoff2]

Ben,
Just pick a convenient tract length....& build it! A particular tract length will only give an improvement over a narrow rpm range. Chrysler found this out with the long ram 413 engines of 1960. The engines used two 4bbl AFB carbs, on 30" long intake runners/manifolds [ two ], with the carbs sitting above the exh manifold on each bank. This length produced a 10% torque boost at around 2800 rpm. Chrysler then made a shorter version of the ram manifolds that gave the boost at higher rpm. They then went to the famous cross ram, as used on the Max Wedge 413-426 engines, & these gave a boost at very high rpm.

[user-23911]

When IVO happens, the intake pressure is at a peak when ideally it would be a trough.
So it's the wrong length for that RPM but will be a better length for a different RPM.

Woops,you want a peak not a trough so it' s tuned for that RPM but the tuned RPM isn't right unless it's a 9K rev limiter.

[pcnsd]

Reading through this thread makes it clear to me, there is little to no consensus on intake resonance tuning for IR applications. This is likely because an IC motor is a dynamic environment that does not precisely fit resonance theory. When the valve is closed and the throttle open, it should resonant like an organ pipe open at one end, on every odd harmonic. (1,3,5,7...) When the valve is open and the throttle open, it should act as a Helmholtz resonator on every harmonic (1,2,3,4,5...). To complicate things further, the Helmholtz volume is ever changing and there are variable frequency standing pressure waves being generated with both negative (peak piston demand) and positive (valve close event when RPM and cam events are not aligned) pressure and the pressure wave events do not align precisely with the resonant tuning over anything but a few fixed rpm ranges. It is a tough environment to divine.

I am sure most have seen this from "The Scientific Design of Exhaust and Intake Systems" by Philip Smith and John Morrison.
It would seem to indicate that in IR layouts, longer (within practical limits) is better. In it I see only the importance of the intake ramming cycle and the practical application of contained mass and velocity. Yes, there are clear peaks likely tied to resonance, but no indication that a particular length is better than another other than that of "Longer"

Effects_of_Runner_Lengths_BW-1024x6.jpg

[ptuomov]

Plus if you can make the runner longer while keeping it straight, you can make it larger diameter and add top end without compromising the bottom end.

Some Helmholtz resonator manifolds on a V6 that have two three-runner plenums produce naturally two torque peaks. That’s great because then one can tune the header to the rpm in between those peaks and have a very broad rpm range over which the intake-exhaust pressure differential is about right. Where the intake is out of tune, the header pulls a good vacuum during the overlap. Car factories loved that sort of arrangement in the 1980’s and 1990’s before variable valve timing. When they could use a header, they added an active resonance valve between the plenums to fill up the valley in the torque curve.

[digger]

http://www.emeraldm3d.com/articles/emr- ... th-intake/

[BenE64]

http://www.emeraldm3d.com/articles/emr- ... th-intake/

Thank you for that link! Found that very helpful and interesting. Also the statement that under 80% throttle the length doesnt make much difference, brings a bit of reality back, it is a street car.

I have currently got it to around 13-14in in length to end of the trumpet. I am looking at building 45-90 degree bends on top so i can extend that if possible. It is getting close to the OEM length runner now. I have run out of room to go heaps more

[modok]

I do appreciate that there is interest in the subject, sorry if I can't explain it proper but it's just so dead simple I don't know what it is that I need to explain. When a throttle is open then it does not matter where it is located in the system, because it's open. The venturi is not movable, so where it is does matter, and if you don't have one then you don't need to worry about it.

[user-23911]

Some Helmholtz resonator manifolds on a V6 that have two three-runner plenums produce naturally two torque peaks.

Another case of wrong terminology.

A helmholtz resonator (again by definition) isn't a straight pipe, it's a spherical chamber with a narrow neck.

The formula to work out it's resonant frequency needs input for volume (actually radius) as well as neck diameter.
Not length of the pipe.

https://en.wikipedia.org/wiki/Helmholtz_resonance