Pod filter?

[magoconnor]

My bad...
I believe the advertised duration is 0.006.

[ratdog]

No, that can’t be right. Unless I’m reading it wrong. I think it says the duration in degrees. The intake valve stays open for around 240 degrees, a racing cam would be closer to 300 degrees. How can the duration be a fraction of a degree?

[dexameth]

Maybe I'm using the wrong terminology... I'm talking about the length of tube between the carb and filter, that calculator is describing the length from the valve opening back. With this carb I've only messed around with the tube between the filter and carb and only once had a longer intake on but it was a totally different carb so I can't cross compare those results.

[magoconnor]

I am not quite sure what I put in my coffee...
Here is calculator where the advertised cam duration is needed.
www.wallaceracing.com/runnertorquecalc.php

[magoconnor]

Here is a thread concerning the subject. 49ccscoot.proboards.com/thread/23058/intake-runner-length-experience-needed?page=2

[ratdog]

Maybe this is too much info, but I’ll try to keep it simple

Frankly, this might be all wrong because I’m trying to do this from memory

What we are trying to do by changing intake length is time the pressure wave created by opening the valve to arrive back at the valve just before it closes. Sort of an extra push just before it seals. The number we used to use was 30 degrees. You have to actuly have some opening to push that little extra thru

Of course the time required will very with the RPM. At a lower RPM, there is more time for the valve to be open

So on these little scoots, we know that RPM we want is about 7500, because that about where the CVT makes the engine run. Of course those of you that have modified your CVTs can insert the RPM you are running at

Now what is happening is the opening of the valve creates a pressure wave that runs up the intake track until it hits a big open space or the air cleaner. That pressure wave collapses and creates a new pressure wave coming back down the track until it collapses at the open valve, and another pressure wave starts back up.

So we know the RPMs. That’s about 125 RPS. Or about .008 sec for one rev, right? So if it takes .008 sec to do 360 degrees, that would be about about .00002222 sec for one degree

Now you’ll have to trust me on this one. We used 1125 ft/sec as the speed of that pressure wave at sea level at the temps we would find in the intake track. That’s about 13500 inches per sec

Now comes the important part that I dont know. How long is the intake valve open on these little guys. My guess is about 240 degrees. But we want the pressure wave to arrive about 30 degrees before closing so we are looking at how far does that pressure wave travel in 210 degrees? Well if I did it right, 210 degrees should be about .004666 sec

So in .004666 sec, at 13500 in per sec, the pressure wave will have traveled about 63 inches. Or an intake length of 31.50 inches! Well that’s way to long, so how about we try the second trip out and back. That’s about 15.75 inches. Remember that is from the valve seat to the end.

Now these are guesses, I don’t have the parts infront of me ( I’m sure someone could actually measure this stuff. T I’m going to call the distance through the carb at 3” and from the outside of the head to the valve seat at about 2.75”. And I would guess the intake runner to be about 5”. So if those numbers are close, we have about 10 3/4 inches to the face of the carb which would leave us an intact runnerof about 5 inches

Anyone still with me?

[magoconnor]

Sweet info, thou the math seems to escape me, my intake runner is around 5 inches.

[ratdog]

I did it on the fly instead of making nice formulas, but basicly we are figuring 7500 rpms. At that speed it will take how much time to move 210 degrees
Now that we T, how far dose our pressure wave move in that time? Of course it has to move from the valve to the end and back so we have to devide D by two

We can’t use an intake that long, so we let that pressue wave create a second pressue wave and time that to arrive just before the valve closes. So in effect we need D/4.

The cam duration is an educated guess. With real cams, we get specs to allow us the get the exact duration. The speed of the pressue wave comes from testing long ago. Just FYI, it is different for figuring the exaust, because the exaust is much hotter.

Now a “racing” cam would have more duration. So the pressure wave would travel a longer distance

But if your using a racing cam, you should be using a higher RPM, which means less time for that pressure wave to travel.. so higher rpms would make this measurement shorter

[dexameth]

Good info, ratdog! Really, this has opened my tuning up a little more and now I want to obtain a degree wheel to really put this all to use.

But let me clarify, we are talking about the intake from the head to the carb length right? Not from the filter to the carb.

[ratdog]

Actually we are talking from the VALVE to the air cleaner.  My guess is that it’s about 2 1/2 inches to get to the outside of the port and about 3” thru the carb.  I’m guessing the intake tube is about 5 inches (I’m sure this varies from bike to bike).  So about 10 1/2” to the face of the carb

[ratdog]

Btw, it also works for the “correct” length of the exaust. The speed of the pressure wave is different because of the difference, and we would be looking at the timing of the exaust cam, but still it’s just a matter of timing

[AtariGuy]

I'm intrigued to look into this further. Got any additional available research material? And how much does that pressure wave speed vary based on elevation above sea level? I will try to get a degree wheel on this glixal a9 cam in the next week.

[ratdog]

Don’t rember all, but I do remember that at 750 ft, it was 1107ft/sec.

To get the “real” numbers, you also need a dial indicator set up the dial indicator to measure valve movement. The number I always used was the valve moving .004” to get the “start” point. This was because a cam may be .001” above the cam circle for 300 degrees but it only gets measurable valve movement for 240 degrees. While you are at it, also set up on the exaust side so we have a real number to figure the exaust pipe length.

Great idea!