IR lengths and plenum sizing for carbs

[user-23911]

Intake valve opens at 3rd harmonic,

You're still confused.

Intake valve opens at 3rd peak after TDC.
What happens at TDC.....nothing.

The wave STARTS at IVO....ends at the next IVO.
Everything in a 4 stroke follows a 720 degree cycle.
There's regular peaks every 150 or so crank degrees, you can't see all of them due to what else is going on.

It's NOT the 3rd harmonic even if it's convenient for you to think that it IS.

[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

That pressure spike at IVC isn't harmonic, it's result from accelerating intake flow to near mach speed with about 0,5bar vacuum at 400 crank degrees and relieving resulting high speed air inertia at IVO. That inertia ramming is main tune point for intake manifold, not harmonics. And it's tuned with flow versus time(intake length), as flow grows with higher initial vacuum it is possible to tune for broad rpm range, more flow at less time in higher rpm range.

the pressure spike at TDC is the fundamental. Fundamental wave is considered the first harmonic. The first reflection of that wave is called the second harmonic. It is this way in most branches of science. I see our difference is just terminology! I didn't make it up, it was like that when I got here.


Yes I also agree inertia ram OR I would say, how the length suits the fundamental event, does become the MORE important factor, but because the cycle to cycle harmonics will always be present you know it will be the third or fourth, just a question of which one.

[desmo]

The best part of the P. Smith scientific design book is the graphs. He has the details of the setup and the pressure trace it produces, and that's ALL true. The author does struggle with building a model to explain what he was seeing.

used to be GT power, Lotus, and....anther i can't recall, but now I think there are a lot more. I'm not up to date, but, I figure it's been out there long enough I bet they all reverse engineered eachother by now.

The author struggles because he knew nothing about the subject, he only knew how to write and sell a book. If you bought the book looking for “how to build inlet and exhaust systems”, you would be disappoint, but if you were trying to find the answer to long held questions on why, it is the “Bible”.

The only point he got right, was to publicise the work done by John Morrison, his work at the time was the key to understanding, why altering the length of the inlet and exhaust pipes worked to change the torque of the engine, we always knew it did, but didn’t know why.

Morrison’s work added sound waves into the case, not only was there air pressure waves acting in the engines pipes, there were also sound pressure waves acting independently through the systems, that enhanced the pressure differential in the cylinder, to give the high Volumetric Efficiency that we get now.

[user-23911]

the pressure spike at TDC is the fundamental.

No it's not.
The fundamental is the frequency at which the pipe will resonate at.

[modok]

So you don't know what to call it either?

[digger]

Inevitable

[pcnsd]

Inevitable

[modok]

I think I meant BCD actually. Might have been drinking a little at that time.

[naukkis79]

You're still confused.

Intake valve opens at 3rd peak after TDC.
What happens at TDC.....nothing.

The wave STARTS at IVO....ends at the next IVO.
Everything in a 4 stroke follows a 720 degree cycle.
There's regular peaks every 150 or so crank degrees, you can't see all of them due to what else is going on.

It's NOT the 3rd harmonic even if it's convenient for you to think that it IS.

Harmonics in intake tube is coming from intake valve closing pressure spike. I see 3rd harmonic, which is about 3x 150 degree apart from IVC to IVO.

So what you see is pressure spike from IVC which made vacuum at about 680 degrees which reflects back 1st harmonic at about 40 degree. Every subsequent harmonic is lost some momentum but 3rd when ivc opens is still over atmospheric pressure. When tuned to harmonics you tune your wanted harmonic to max power rpm as that tune hurts power rpm range above that. If you tune to max torque rpm you probably have engine which have both torque and power peaks at near same rpm.

[ptuomov]

You're still confused.

Intake valve opens at 3rd peak after TDC.
What happens at TDC.....nothing.

The wave STARTS at IVO....ends at the next IVO.
Everything in a 4 stroke follows a 720 degree cycle.
There's regular peaks every 150 or so crank degrees, you can't see all of them due to what else is going on.

It's NOT the 3rd harmonic even if it's convenient for you to think that it IS.

Harmonics in intake tube is coming from intake valve closing pressure spike. I see 3rd harmonic, which is about 3x 150 degree apart from IVC to IVO.

Intake valve closing causes no pressure spike. Near tuned rpms, it’s a non-event as far as wave generation goes. It’s the decelerating and then accelerating piston near the BDC that causes the high pressure wave in the intake tract.

[naukkis79]

Intake valve closing causes no pressure spike. Near tuned rpms, it’s a non-event as far as wave generation goes. It’s the decelerating and then accelerating piston near the BDC that causes the high pressure wave in the intake tract.

You are totally wrong. It's incoming air inertia which increases pressure at BDC. Too early intake close will make that spike to grow(which increases tuned harmonic at IVO, one point more to tune harmonics to max power rpm range). What you state is opposite to that but it's plain wrong.

If that pressure increase was made from piston moving we could have max about 75% VE with normally aspirated engines.

[ptuomov]

Intake valve closing causes no pressure spike. Near tuned rpms, it’s a non-event as far as wave generation goes. It’s the decelerating and then accelerating piston near the BDC that causes the high pressure wave in the intake tract.

You are totally wrong. It's incoming air inertia which increases pressure at BDC. Too early intake close will make that spike to grow(which increases tuned harmonic at IVO, one point more to tune harmonics to max power rpm range). What you state is opposite to that but it's plain wrong.

If that pressure increase was made from piston moving we could have max about 75% VE with normally aspirated engines.

If you are of the school that thinks the intake runner waves are caused by the air column slamming into the closing intake valve, you’re not alone but you’re still wrong. It’s been printed in the Hot Rod magazine, too, but it’s still completely wrong.

The truth is that the high pressure wave in the intake tract is caused by the piston decelerating relative to the air flow into the cylinder and that starts happening before BDC of the intake stroke.

[modok]

Intake valve closing causes no pressure spike. Near tuned rpms, it’s a non-event as far as wave generation goes. It’s the decelerating and then accelerating piston near the BDC that causes the high pressure wave in the intake tract.

Ah, i didn't get that the first time, but yes, you could say the intake valve does not cause it.
i would say in any case, you have flow, you stop flow, there will be a pressure spike. pressure and flow, current and voltage, That's just how AC works, right joe??

[ptuomov]

Intake valve closing causes no pressure spike. Near tuned rpms, it’s a non-event as far as wave generation goes. It’s the decelerating and then accelerating piston near the BDC that causes the high pressure wave in the intake tract.

Oh....if you are saying there is no pressure spike then you are probably wrong.
We could debate what causes it, sure, but dude, it's there.

There’s a big pressure spike in the cylinder (and shortly after that in the intake port) that is caused by the piston decelerating relative to the air flow around the BDC. That’s the main event.

At the tuned rpm, you’re closing the intake valve when the pressure on both sides is about the same and there isn’t much flow. If this weren’t the case, then you’d close it either earlier or later. Closing the valve when there is little pressure differential and little flow means that valve closing isn’t going to generate any sort of meaningful pressure pulse. The pulse is already there in the cylinder and intake runner by that point.

[user-23911]

At the tuned rpm, you’re closing the intake valve when the pressure on both sides is about the same and there isn’t much flow.

Opposing flow.

There's flow in the intake port trying to get into the cylinder.
The piston is already on it's way up and trying to push the air back out of the cylinder, opposite to the port flow.


The intake valve closes to trap the air in the cylinder........the flow in the intake port bounces off the closed valve.


Conservation of energy.
Waves.
The original energy to get the air moving in the first place comes from the piston movement.

But you can't just stop the movement of gas without the pressure changing.
As above......Modok......V = L di/dt (eg. ignition coil)
Or in other words the induced pressure is proportional to the change in flow.