IR lengths and plenum sizing for carbs

[hoffman900]

Here is the smoothed data with the 1st harmonic (34" Intake Length) and 2nd harmonic (20" Intake Length) added.


(right click - "open in new tab")

Pretty much performed how you would expect it to.

[modok]

Very good illustration of all things discussed.
Port energy is real?

According to the way it’s calculated, added volume (length) should increase port energy. As we see, it does not correlate.

I posted a whole slew of posts from Clint, Neels, and Jon S which state what you’re seeing in the above.

I’ll try other lengths later. I’ve done some work out at 19”-20” long.

The Yoshimura stacks are gimmicky. They’ll tune to whatever the short stack height is.

well, the 'port energy" calc could be expanded and corrected to take into account that the farther up the tract you are the less it counts....but I'd rather not, in fact, lets never speak of it again besides a basic concept that ram is real. When you averaged the curves it was clear to see that the longer it was the more torque it makes until it runs out of tune, quite predictable. I think we are all in agreement there. The question at this point IMO, is when and why and how of how low a harmonic you can go.

[hoffman900]

Very good illustration of all things discussed.
Port energy is real?

According to the way it’s calculated, added volume (length) should increase port energy. As we see, it does not correlate.

I posted a whole slew of posts from Clint, Neels, and Jon S which state what you’re seeing in the above.

I’ll try other lengths later. I’ve done some work out at 19”-20” long.

The Yoshimura stacks are gimmicky. They’ll tune to whatever the short stack height is.

well, the 'port energy" calc could be expanded and corrected to take into account that the farther up the tract you are the less it counts....but I'd rather not, in fact, lets never speak of it again besides a basic concept that ram is real. When you averaged the curves it was clear to see that the longer it was the more torque it makes until it runs out of tune, quite predictable. I think we are all in agreement there. The question at this point IMO, is when and why and how of how low a harmonic you can go.

Dependent on lots of things. Use? Time spent in a given rpm range? Acceleration time? Etc.

Outside of racing with a datalogger, most won't be able to answer those questions, other than the first one.

[user-23911]

The simulation seems to fit quite well.

From my own measured examples (17 inch and 26 inch)........the longer runner is used at low RPM, shorter at higher, switch over at about 4K.......you'd expect the 26 inch to peak at 2500 and you'd also expect the 17 inch to peak at 5500?
I wonder how that fits?

But in real life they DO work ........because they're factory designed.

Here is the smoothed data with the 1st harmonic (34" Intake Length) and 2nd harmonic (20" Intake Length) added.

By the 1st harmonic, you really mean the 3rd?

[digger]

MadBill,

Here you go. I took the trendlines out as it's just too much.


(note: right click and hit "open image in new tab" to see it expanded).

It's interesting to see where they diverge and converge. I struggle the most with filling in that 3500-5500rpm range. I can gain there, but it kills the top end and vice-versa. I can fix it with cam timing, but none are available for this engine with what I need, and being a sliding rocker type, the lobe will have to be a proprietary to this particular engine.

Such is the life with a single cylinder

can you plot Inlet pressure and cylinder pressure at both 4750rpm and 7750rpm (11" and 13.75") maybe on separate graphs

then plot the Mass flow vs crank angle for the same rpm and runners

and efficiency trace the Ceff for the same rpm and runners?

[Erland Cox]

It is extremely difficult to properly calculate intake and exhaust lengths.
It must be an empirical calculation based on tests.
Because the wave is NOT travelling with the local speed of sound during the valve open phase.
On the intake side it travels with the SOS minus the intake velocity that differs over the port length and with crankshaft degrees.
On the way back towards the valve the wave travels with the SOS plus the port velocity.
To further complicate things there is further speeding up of the wave from the pressure but I have to look deeper in Blairs book for that.
Time lost going slow on the way out is not regained going fast on the way in because the time spent going slow is much longer than the time spent going fast.
When the intake valve is closed and there is no flow velocity the wave travels with the SOS.
Valve open tuning is the most important and the wave created here by first the exhaust and then the piston is what creates all the following waves.
They are echos of this first wave that actually it is not one wave but a depression over several degrees that makes the air rush in.
The trick is to catch the last reflection of that depression returning as a pressure wave at valve closing.
The good thing for us is that the wave changes with rpm and can be useful for a larger rpm span than a simple harmonic.
It is good to have pressure at the intake valve as the valve opens but going through different harmonics this will only happen at certan rpm:s.
Best is to try to get intake and exhaust phased so that intake is in tune as the intake valve opens when exhaust is out of tune.
And having exhaust suction at intake valve open when there is no positive intake pressure.
Especially with IR or you will get a exhaust pulse up the intake that will make the carb draw fuel on the way in, the way out and on the way in again.
This will make the engine run very rich and if it is really bad it will wet the plugs and cause misfiting long after you are out of the bad tune.
You can see it on the öamda with the engine first running rich and then lean.
Lean is wet plug misfiring in this case.

Erland

[Geoff2]

If I was the poor OP trying to make sense of of all this, I think I would pick a number that is convenient, make the inlet tract length....& go from there...
At least the headache would improve...

[BenE64]

If I was the poor OP trying to make sense of of all this, I think I would pick a number that is convenient, make the inlet tract length....& go from there...
At least the headache would improve...

20171208_205453_resized.jpg

Half done 2kg of wire and die grinded 1.75kg back out again to reshape the oem base. Averaged 14in from trumpet to intake valve. I could make the middle carb trumpet an inch taller and the rear carb an inch taller again. the front carb wont have enough head room to go more. Next one I'll do different again. Learnt alot though.

[user-23911]

Go make yourself 6 trombones, bolt up to an engine dyno and report back?

[Erland Cox]

That looks nice!

Erland

[cjperformance]

Go make yourself 6 trombones, bolt up to an engine dyno and report back?

[BenE64]

Go make yourself 6 trombones, bolt up to an engine dyno and report back?

Will you settle for a gtech and the same stretch of road for back to back testing?

Nearest engine dyno is 6 hours away, not sure I can afford the inflated mining industry pricing structure once there.

[naukkis79]

According to the way it’s calculated, added volume (length) should increase port energy. As we see, it does not correlate.

It will. But for engine power port energy means nothing, it's trapped air volume which means. But as we have more port energy we could use it to keep intake flowing longer, more intake cam duration can be used without losing torque in effective powerband.

[naukkis79]

Here is the smoothed data with the 1st harmonic (34" Intake Length) and 2nd harmonic (20" Intake Length) added.


(right click - "open in new tab")

Pretty much performed how you would expect it to.

Only first and second harmonic tuned lengths have any meaningful use, every other intake length is too short. For that rpm range only usable intake track length is second harmonic 20" to have help from tuned intake length. If we want to be in powerband from 3500 rpm we could use about 10% more intake cam timing with second harmonic tuned intake than with shorter intakes and gain power everywhere from 3500-8000 rpm range.

[hoffman900]

Here is the smoothed data with the 1st harmonic (34" Intake Length) and 2nd harmonic (20" Intake Length) added.


(right click - "open in new tab")

Pretty much performed how you would expect it to.

Only first and second harmonic tuned lengths have any meaningful use, every other intake length is too short. For that rpm range only usable intake track length is second harmonic 20" to have help from tuned intake length. If we want to be in powerband from 3500 rpm we could use about 10% more intake cam timing with second harmonic tuned intake than with shorter intakes and gain power everywhere from 3500-8000 rpm range.

Define "10% more cam timing". If I have seat-to-seat duration of 315*, are you saying it needs to be 346*

Where does the ICL have to be? Does that change?

Sorry man, I'm not really buying it. Also, you have to look at where the rpm range is. That's what transmissions are for