IR lengths and plenum sizing for carbs

[digger]

One direction or another the flow starts during overlap as caused by pressure differentials in inlet exhaust and cylinde, r not the piston motion at all

You close the valve such that you trap the most mass and this inevitably means some reversion due to the mechanical limitations of valve train. You don't close it when pressure are equalised.

[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.

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.

Yes, I agree.
sorry I didn't read it correctly the first time. I'm glad you GET that. Would you expect changing the IC point to make any real difference in this condition? NOPE
Doesn't really make a difference, just so happens the valve is closing also. Such strong wording I thought it had some greater agenda.

[modok]

I did want to mention.....why I initially just said "fourth harmonic" is NOT because the cycle to cycle harmonics are MOST important, in fact, they take a back seat to how the tuned length works with filling the cylinder during the stroke itself.
How the length suits the primary event IS more important for sure, BUT we know the cycle-to cycle harmonics WILL be there, so you know what you want is going to be either 3rd or fourth (cycles per cycle).
That's more or less the basis of pipemax for instance. Just random example, you know it's going to be 12 or 16, not 14.5, and that can really help shorten your learning curve. A lot less to fiddle with.

So....why do some engines use the third and some the fourth? Well, some combination of duration and size of the intake compared to what you are filling.
Relatively SMALL intake tract and one valve per cylinder, many cases the flow will be peaked for a period in the middle of the induction stroke, and during this period....it's delaying wave action (in elecrical terms, it's clipped?). No pressure will build till the flow slows, and if the flow is just peaked out for 20-40 degrees, it's not slowing. It's not going to start climbing pressure until the flow slows down so you need a shorter length to have a faster reaction.

The port injected, four valve, you might have BIG pipe, which won't get clipped like that at all so they are a lot more likely to use the third. In fact, from looking under the hoods most do.

[modok]

Harmonics are USUALLY, Cycles per cycle
http://www.hersheyenergy.com/harmonics.html

IF pipemax, like some of you here, is using an alternative definition, that would explain A LOT

[digger]

Harmonics are USUALLY, Cycles per cycle
http://www.hersheyenergy.com/harmonics.html

IF pipemax, like some of you here, is using an alternative definition, that would explain A LOT

here is Larry's explanation of the numbering from the pipemax forum for exhausts, for the intake not sure my guess would be same nomenclature as the primary

Primary Pipe Harmonics :
v1.0 to v3.98____new version 4.00 with real Numbers
1st Harmonic = real 1st Harmonic "one-end Closed Tubes"
2nd Harmonic = real 3rd Harmonic "one-end Closed Tubes"
3rd Harmonic = real 5th Harmonic "one-end Closed Tubes"
4th Harmonic = real 7th Harmonic "one-end Closed Tubes"

Collector Harmonics :
the Collector Tube Lengths are properly numbered.... they act like "Open-end or both ends open tubes" ,
and use both ODD and Even numbered Harmonics, that is 1,2,3,4,5,6,7,8 so on
so there is no change to this in new PipeMax v4.00

[user-23911]

Primary Pipe Harmonics :
v1.0 to v3.98____new version 4.00 with real Numbers
1st Harmonic = real 1st Harmonic "one-end Closed Tubes"
2nd Harmonic = real 3rd Harmonic "one-end Closed Tubes"
3rd Harmonic = real 5th Harmonic "one-end Closed Tubes"
4th Harmonic = real 7th Harmonic "one-end Closed Tubes"

So it's like I said 7 pages back........odd ones only.
Got there in the end.

[modok]

well, I don't know about all that but I think you just add one, from the looks of the results.
He's probably calculating time from valve close to valve open so not "counting" the primary wave as being a harmonic? Something like that.

[modok]

Joe I've already figured out what your system is so I can translate it. It's acoustic method.

You know electricity too tho. I know you do.

[user-23911]

Confusion caused yet again by using incorrect definitions.


The lengths tie in perfectly with my measured factory EFI manifolds
Don't even need computers or simulations.

[modok]

They indeed do, but if there are THREE ways of naming harmonics, which one do we use?

[digger]

it does matter what you call it. it only matters how it performs

[modok]

yeah. why would we want defined terms and standardization? it's not like we are french. Thank god.
Cheers to that!

[naukkis79]

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.

That just point of view, but cylinder filling over atmospheric pressure is coming from flow's high speed inertia not piston moving. If you close valve before cylinder fills up and piston decelerates you will still get near 2 bar pressure spike at back of intake valve. Goal is to trap that inertia made pressure into cylinder which also softens that final pressure spike but as revs rise and VE drops intake always will close too early and leftover pressure spike grows.

[naukkis79]

The lengths tie in perfectly with my measured factory EFI manifolds
Don't even need computers or simulations.

Most factory EFI-manifolds aren't tuned to high harmonics, they are helmholz-tuned with small plenum, target is to increase plenum pressure temporarily with outgoing flow from runner, with then increases pressure differential in some other cylinder runner within same plenum. You get much more energy from first harmonic than subsequent so with plenum tuning to other cylinder there's much more potential to gain even in low revs.

[ptuomov]

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.

That just point of view, but cylinder filling over atmospheric pressure is coming from flow's high speed inertia not piston moving. If you close valve before cylinder fills up and piston decelerates you will still get near 2 bar pressure spike at back of intake valve. Goal is to trap that inertia made pressure into cylinder which also softens that final pressure spike but as revs rise and VE drops intake always will close too early and leftover pressure spike grows.

If your position has moved 90 degrees from “totally wrong” to “just point of view”, that’s progress. However, another 90 degrees are still needed.

You do have a high pressure in the intake port and cylinder when the intake valve closes anywhere within the power band but it’s simply incorrect to think that intake valve closing causes that high pressure.