Reed valves & hard back filters- resonance

[RdsOnly]

I have sean many 4 strokes guys use hard back K&N's- work great. Not so much on  a reed valve 2 stroke- why?
Push back [resonance] Reeds are designed to push back the wave form of the crack case [to some extent].
Think of it as (+) and (-) waves. Piston goes down = + pressure to the carb. out see and feel spit back out of carb.
Piston goes up (-) wave- pressure sucks fuel from carb into crankcase.
Up& down it goes until it hits a (RPM) frequency that all is good harmonized or equal. Flows to it's optimum in & out.

Spit back: happens even with reed valves at RPM's so there must be a wave that does that, Right.
If that wave hits a hard surface and reflect back into the intake and disrupts the flow, we call that a stumble.

Simply changing from hard back to soft- foam will remove that stumble - jetting maybe required. Due to flow increase.

Please comment on this subject if your results very.


JT
(rdsonly) dead

[elliottles1]

Hi,
my experience in my younger years when k+ns were very popular was that you could not jet big enough to stop it running weak, i seized an engine before i learnt. all went away upon removing them. now i use soft filter pods and no problem. so fully agree with your post.
cheers
les.
happy holidays!

[m in sc]

yup. actually did my final engineering paper on this a LONG time ago in college.

not nearly as bad on a piston port, and totally fine on one with correctly made spacer between the cab and filter. I ran k&n's on my S2 and put down 59 rwhp with them with alinear '4 stroke' looking curve.

the very long cone style i see on banshees are probably ok, but you cant package them in a motorcycle frame typically. these are typically 6,7, 8" long. 

[Pravin]

yup. actually did my final engineering paper on this a LONG time ago in college.

not nearly as bad on a piston port, and totally fine on one with correctly made spacer between the cab and filter. I ran k&n's on my S2 and put down 59 rwhp with them with alinear '4 stroke' looking curve.

the very long cone style i see on banshees are probably ok, but you cant package them in a motorcycle frame typically. these are typically 6,7, 8" long.

Hi SC ,

This is interesting , do you have any example of the spacer you mentioned above ? how it can be made : )

[teazer]

We tend to think of a single wave in each cycle but in reality there are many reflected waves per cycle at all revs, but their impact may change at different engine speeds.

As the piston rises from BDC, incoming air flows in through the reeds but as the flow starts inwards, it sends a negative wave backwards which hits that flat filter end and as with all wave energy, when reflected of a closed end it returns with the sign changed from negative to positive, so that positive wave now travels through the gas back to the reeds picking up fuel on the way.

That cycle repeats somewhere around 4 to 6 times per intake event and the magnitude of the pulse (pressure wave) decreases each time it is reflected, so what the carb "sees" is an oscillating pressure wave through the incoming gas flow.  I studied fluid dynamics a decade or two earlier than M in SC and we didn't have complex computer models back then - or even simple computers either if it comes to that.  And I am pretty sure that I lost that knowledge years ago.

Thinking about the issue, one of the pressure pulses outwards will start as the piston starts to fall and push gas back out of the carbs and that pressure wave will get out through the reeds faster than they have a chance to snap shut.

On a TD3 or TZ, or R5 street bike without filters you can easily see a standing wave reaching back almost to the rear fender and some of that mix always coats the back of the motor.  On a reed motor there is less of a mist to see for sure but the problem was identified by greater minds than mine years ago with hard backed filters which allow the reflection.

I don't think it's worse on a reed motor than a piston port.  I suspect that no one was interested in testing on piston port street motors after reeds were introduced.

[m in sc]

all the triple guys tested it, so its been done. the 'hot ticket' for a lot of guys was a 45 or 90 degree elbow at the bellmouth, that disrupted the wave enough not to cause the reversion.

on the piston port motors, i tested using radiator hose, and started way longer than this and kept cutting it back until i just started to feel a difference. its hokey, but it worked:



i then made some out of straight tube and a small insert as a 'crush ring' and this was the final:



from what i have experienced, the reeds are actually a bit worse, but a narrower window of reversion, as they physically push air back closer to the carburetor. however, all motors will act different depending how they are tuned so there's no real 'magic bullet' minimum as far as length etc. But this is exactly why the 'y' boot works so well in most applications. its long enough to cover and exceed most reversion issues.  also the straighter the runner at the bellmouth, typically, the less turbulent the flow into the bellmouth, so this helps as well. 

On the tzr, its literally 2 straight sections of tube into a big airbox plenum with a very large panel filter above it, stock. 

as a spacer example on terrible cone k&n's:

this is when I ran that S2 on a dyno, they usually lag at 4-5 k. no porting, programmable ignition, stock heads, s3 (400 carbs) and denco chambers.  note on this one, the 2 lower numbers are where the rear tire slipped on the drum of this ancient dyno.  the main number isn't the key, the shape of the curve is. the top where it goes squiggly is where the rev limiter was set. 

[busa1300]

If you have worked on RZ/RD/RZV500R, or RG500's you know the airbox portions that fit over the carbs themselves have a hard backing only about 1 1/2-2 inches from carb. Although on the Yamaha there is a 90 degree bend in the intakes before the carb which could make some kind of difference.
I will admit getting the Yamaha tuned in just right at every RPM/throttle position is a challenge after mods have been made. Because the carbs are always sideways with no room with bodywork.

[m in sc]

but they also curve out away from it some. that deflects the wave, as do the 90  and 45 degree elbow setups people use on h1 & h2s.