RZ/RD 350 & Misc. 2-Stroke Tech BBS

Technical Forums for Two-Stroke Nuts
It is currently Mon Oct 15, 2018 8:10 am

All times are UTC - 4 hours [ DST ]




Post new topic Reply to topic  [ 26 posts ]  Go to page 1, 2  Next
Author Message
PostPosted: Wed Jun 06, 2018 10:11 pm 
Offline

Joined: Sat May 28, 2016 3:15 am
Posts: 60
Location: South Florida
Ok, this will probably be a more theoretical thread...
(assuming anyone replies.. lol)
But could be helpful to some on a practical level.

So, which is better for a given combo? Larger or smaller?

What does "better" mean, for that matter?

Well, none other than the great tuner/columnist Kevin Cameron taught me years ago
(not to name drop or anything like that)
"The larger the carb bore, (within a reasonable range of course) the better the low end".
That is to say: the power curve *when on the pipe/in the powerband* is fatter down low and
drops off faster on top.
Vice versa too.
IE: Smaller bore carb more top end, more peaky.
Of course, this assumes ALL else is equal.... within the "system".

This seems counter-intuitive..
But only in terms of "four stroke think"..
Four strokes do the opposite.... bigger bore carb = more top end power.
It's all a matter of two stroke theory.

BTW... please feel free to chime in and correct my thinking on this if I get something wrong..
(but gently plz... I'm sensitive.....) :smt022 :smt033
This is, after all, meant to be a question/discussion and such..

During the overlap period... (as I'm sure everyone here knows) IE: when both ports are briefly
open to the atmosphere, resonant tuning (waves bouncing back and forth etc.) cause a nice charge of fresh mixture to be drawn into the combustion area, resulting in a denser charge when the plug lights... etc etc..
It's why expansion chambers add power... over, say, a straight pipe.

Well.... a larger tube resonates at a lower frequency..
Lower frequency = lower RPM.
Think of trombones...
A bass trombone has a larger bore than a tenor (the kind usually used), so it's range is lower.
(for a given slide position)

So, I'm wondering....
On the practical level, since I've not had opportunity to experiment, who has found this to be true with carb bore sizes, and what did you find? What is a "reasonable" range of bores.... particularly with these engines?

Ah Hah! you say.... because the other obvious aspect of the trombone occurred to you while thinking about the above?
You pull the slide out to get lower tones!
Yup...

ALL else being equal.... a longer tube (of the same bore) will resonate at a lower frequency!

This brings to MY mind.. something I've been wanting to experiment with for many years...
but never seem to find the time..

Lengthening the intake tract..
Velocity stacks for example.
The longer ones....
(it also makes the argument for reed block spacers)
Back in the day, I was often told that adding long stacks (where practical) not only help with the lower end of the power band, but also helped cure flat spots!
By keeping velocity up, at slight expense of top end HP.
(you'd probably only notice on a drag bike where absolute max HP is king)

The theory basically being...
A vibrating column of air resonates... a longer intake tract and/or larger carb bore would cause
it to do so at a lower frequency, hence lower RPM, making more power at the lower end of the curve.

Helping cure flat spots would be a very helpful side effect.
A reason for finding a way to integrate stacks all by itself, for me.
Keeping the velocity (of the vibrating column of air) up might help cure jetting out flat spot woes

It's almost certainly also why the stock airbox works well.

It's very possible to put velocity stacks inside of K&N pods, and likely others.
Perhaps one could eliminate the clunky/clumsy stock airbox, and the odd Y-Boot doohicks with greater success this way.

Thoughts on this would be MOST welcome... since this all has been banging around in my head for ages..
Practical experience would be GREAT!

TIA
Greg


Top
 Profile  
Reply with quote  
PostPosted: Thu Jun 07, 2018 11:56 am 
Offline

Joined: Tue Nov 12, 2013 2:34 am
Posts: 1141
Before addressing the intake tract question, are you claiming that smaller carbs create more power high in the rev range? Large carbs create poor low rev power as they have trouble getting the velocity necessary to atomize fuel properly. Small carbs do poorly up high as they cannot draw in enough fuel/air mixture as optimal. You are backwards at this point. Look at a big bore 2-stroke. One of the first easy power gains is to throw on some larger carbs. Carbs and pipes combo is one of the more effective complimentary replacements.

The thoughts on airboxes are definitely up for debate and you get to go through the usual airflow, velocity, pressure, and disturbance arguments.


Top
 Profile  
Reply with quote  
PostPosted: Thu Jun 07, 2018 2:10 pm 
Offline
User avatar

Joined: Sun Aug 16, 2015 5:55 pm
Posts: 796
Location: Reconquista of California
My comments are directed at the RZ350, not the application and design of let's say a math calculated correct volume, pressurized air box , air flowed design on a totally different machine and motor. So that disclaimer being said...

I've my fair share of experience on a RZ with carbs, all on stock power valved cylinders. I must say Toomeys ideas:
(Listed under RZ 350 install guide).

https://toomey.com/documents

May just be right, as when my bikes had this kit, the performance was phenomenal considering what the bike is. My bike regularly was faster than most if not all RZs I came across. Be aware this was NOT a roadrace application, but my street bikes which had the usual mods, head skim,port work,reeds,pipes and on one Spec 2s 375cc bore.
Here's what I found out for me.
1.) Stock 26mm rejetted carbs accelerated quicker, minimum of several bike lengths.
2.) Larger carbs, did ultimately have more top end, but noticably slower on acceleration. I used 32s and 30mm , just didn't deliver the performance I wanted, where I wanted it. Like I said they weren't road race specific bikes, that's a narrow focused build all it's own.
3.) In my case, Uni filters or The 2-1 intake, always without question faster than the air box, not likley I'd be convinced otherwise, as I feel I have enough 1st hand experience.
Out if the 100s of races on RZs and Banshees, it's clear Filters are the only way to go for all out performance.Its not really even open to debate, if seeking the most power potential.
4.) . The 2-1 did make for a smoother running bike. (Probably for said reasons).
5.) The stock air box, does deliver a smoother running bike(civilized). Power realistically is not too far off running filters, but 3,4,5 bike lengths accelerating is alot, so if your looking for Maximum potential, then the frankly "tiny airbox" has to go.
6.) The 5k stumble so many seem take issue with, never really bothered me, but It's there with individual filters no doubt, I'm inclined to agree the alteration of the intake length is the main culprit, but I also feel the YPVS starts it's transition at this point as well, which accentuates the feeling for many, but as said it never bothered me on the one bike, I just powered right on through, it's only a lil hiccup..not bogging or real lack if acceleration.
After riding modified Banshees, I believe that adding larger carbs on a " big bore"cylinders engine is no doubt a benefit, not so much a stroker +4 as I've been around many stock cylinder Banshees that when ported are very very fast.

Just my 2 cents in the topic.

_________________
Banshee (Baja) race bike, Lonestar A-Arms, dual 9" L.E.Ds, FMF, Toomey,19cc domes, IMS tank, run flats.
96 GSXR SRAD, Future Yoshimura Lucky Strike rep.
85 custom Tri-Z
RZ/YZR track bike(project)
86 VFR750 RC24 Merkel replica (project)


Top
 Profile  
Reply with quote  
PostPosted: Thu Jun 07, 2018 2:54 pm 
Offline

Joined: Tue Nov 12, 2013 2:34 am
Posts: 1141
My Take on QV's response.

Questo vecchio rz wrote:

May just be right, as when my bikes had this kit, the performance was phenomenal considering what the bike is. My bike regularly was faster than most if not all RZs I came across. Be aware this was NOT a roadrace application, but my street bikes which had the usual mods, head skim,port work,reeds,pipes and on one Spec 2s 375cc bore.
Here's what I found out for me.
1.) Stock 26mm rejetted carbs accelerated quicker, minimum of several bike lengths.
2.) Larger carbs, did ultimately have more top end, but noticably slower on acceleration. I used 32s and 30mm , just didn't deliver the performance I wanted, where I wanted it. Like I said they weren't road race specific bikes, that's a narrow focused build all it's own. - For a road race application I can see the advantage of a smaller carb less tailored to max. horsepower. Since poster is looking at 'theoretical' and specifically identified high vs. low rpm performance I would say we are considering max HP at high RPM where bigger is better.
3.) In my case, Uni filters or The 2-1 intake, always without question faster than the air box, not likley I'd be convinced otherwise, as I feel I have enough 1st hand experience.
Out if the 100s of races on RZs and Banshees, it's clear Filters are the only way to go for all out performance.Its not really even open to debate, if seeking the most power potential. On side that pods are better than airbox. This is a big item of debate and focuses around the 4 items I brought up before.
4.) . The 2-1 did make for a smoother running bike. (Probably for said reasons). Personal opinion is the increased length of intake tract is similar to that of the airbox thus intake length vs. intake type is contributing to performance.
5.) The stock air box, does deliver a smoother running bike(civilized). Power realistically is not too far off running filters, but 3,4,5 bike lengths accelerating is alot, so if your looking for Maximum potential, then the frankly "tiny airbox" has to go.
6.) The 5k stumble so many seem take issue with, never really bothered me, but It's there with individual filters no doubt, I'm inclined to agree the alteration of the intake length is the main culprit, but I also feel the YPVS starts it's transition at this point as well, which accentuates the feeling for many, but as said it never bothered me on the one bike, I just powered right on through, it's only a lil hiccup..not bogging or real lack if acceleration. Have not run into this issue.
After riding modified Banshees, I believe that adding larger carbs on a " big bore"cylinders engine is no doubt a benefit, not so much a stroker +4 as I've been around many stock cylinder Banshees that when ported are very very fast. True enough. Sure the math exists on what the 'recommended' carb size for a 350cc twin is.

Just my 2 cents in the topic.


Top
 Profile  
Reply with quote  
PostPosted: Fri Jun 08, 2018 2:39 am 
Offline

Joined: Sat May 28, 2016 3:15 am
Posts: 60
Location: South Florida
First off, thanks to both who've replied so far.
I hope that if I seem combative it's just passionate debate, because I've come to respect both posters greatly over the years that I've been reading and learning from their posts...

Evan: the term "in the rev range" is a bit misleading.
In the powerband is more germane.
It has to do with the way 2 strokes work.
evan_calgary wrote:
Large carbs create poor low rev power as they have trouble getting the velocity necessary to atomize fuel properly. Small carbs do poorly up high as they cannot draw in enough fuel/air mixture as optimal.

This is true with 4 strokes...

I think you'd have to go absurdly huge with carb bore for the fuel to actually not atomize..
Like 45mm and above on a stock displacement RZ for example.
We all know there's a realistic/usable range of carb sizes for the stock RZ that will "work".
Something like 26mm to 30mm or thereabouts?
(and a different/larger range for bigger displacement motors)
You don't hear of people going much larger than 30mm on a stock engine..
Quote:
You are backwards at this point.

Respectfully, I think it is you who are backwards at this point.
It's what I meant by "four stroke think".
With 2 strokes.. tuning 2 strokes for power that is, moving a lot of air through the motor isn't
the key to producing power..
Just look at your expansion chamber stinger's exit size to see what I mean.
You'll notice it's pretty small diameter.... all of them are.. even ones made for max power.
It will also be a good deal smaller diameter than the headpipe section coming out of the cylinder.
It's four stroke motors that gain power with more air (gasses) flowing through the motor, together with more "squeeze" (compressing the mixture in the combustion chamber for a denser charge to then be ignited)..
Making power with a 2 stroke is a function of timing. Timing of "events"... coupled with pressure waves being used to "scavenge" the cylinder so as to draw in more fresh mixture during overlap.
It all has to do with that engineering math..

In a 4 stroke, you do that with valve timing and an X-into-1 exhaust, where another cylinder(s) help scavenge the first.
(you've noticed tuned 2 strokes don't use 2-into-1 exhausts)

To design a proper set of pipes, you need to calculate total port lengths/volumes, crankcase and combustion chamber volumes because they ALL come into play during that overlap period.
Also, the time of the overlap period vs. RPM... and add in the speed of sound.
Then, using the various formulae, you derive what size and where in the length of the pipe the cones should be.
That has to do with the timing of the negative pressure waves being reflected back at just the right moment.
Not moment as much as moments... a period... hence a power band.
(probably some other factors I've forgotten as well...)
(I've had the famous "2stroke tuners manual" since the late 70s... I'd always get through the first third or so of each chapter then give up due to that engineering math.. lol... and just get the sense of the concepts)

The (usable) RANGE of carb sizes then tailors WHERE in the powerband the curve tilts UP.
Carb bore size might contribute to the raw power output amount, but if at all, it's minimal,
and I'm not sure if it would be larger or smaller that would do that.

Again, this is to increase power..... a straight pipe will work.. it just won't make a lot of power.
Just as a huge carb will work (up to some point)... ie: you could tool around off the powerband.. just as a to small one will.
I'm guessing that a large carb.... higher end of the workable range... will spoil "drivability"..
IE: below the powerband tooling around town.... where most of street riding is done... and for
the reason Evan mentioned.

Quote:
Look at a big bore 2-stroke. One of the first easy power gains is to throw on some larger carbs. Carbs and pipes combo is one of the more effective complimentary replacements.

COMBO being the key word...
I was very careful in the OP to say "all else being equal"... meaning for a given set of pipes
and everything else.. the carb bore being the only variable.
I've believed for decades, that one should ideally get carbs/barrels/pipes as a package!
Not so easy to do these days. Not a lot of tuners anymore developing packages for the RZ 350..
The pipes and porting have to work together... along with a range of carb sizes within
a reasonable range..
To clarify:
If you deviate at all from the stock *combo* you should replace it with a combo... designed as a system by a tuner..... (preferably one who's good at engineering math LOL)

"Throwing a set of pipes on" will certainly make an improvement over what is essentially a straight pipe with catalytic converters in it..
(or in non-US countries, stock pipes that are only minimally an expansion chamber at all)
From what I've read here, most do NOT change carbs for more power with a stock motor..
Just the opposite in fact.
Spec II and others sold tailored VM30s back in the day when the bike was current..
Many have removed them, and gone back to either stock carbs, PWK 28s and others..
Again, I'm limiting it to stock (or basically stock) motors to keep it simple for now.
A few have even reported great power with the (non-US) stock size carbs and the PWK28 even with fairly modified motors (not including massive displacement increases).

Otherwise... what do we mean by "big bore" and which "pipes"?
It DOES matter!
Big bore motors probably never used the original pipes... so which aftermarket pipes did it have before?
Bigger displacement.... particularly MUCH bigger, will of course need bigger carbs to feed it..
Bigger than stock anyway.
Even then, pipes/carbs along with barrel port scheme contribute to a RESONANT SYSTEM!
Albeit a very different one than stock.....

A 2 stroke with expansion chambers, definately IS a resonant system..
Manipulating that is what tuning is all about.
That's just a fact.
Larger volume resonant systems, as well as longer ones, will resonate at a lower frequency...
That too is a fact.
Quote:
The thoughts on airboxes are definitely up for debate and you get to go through the usual airflow, velocity, pressure, and disturbance arguments.

What this seems to demonstrate, is that a longer intake tract is helpful... in SOME way..
It also shows that LESS airflow is actually beneficial.... (up to a/some point) because the stock airbox is obviously more restrictive of airflow than the other options..

Questo:
Thanks for the practical, experience based info!

Toomey wrote:
You must remember when you remove the
stock box, you also remove some of the intake tract length. This machine needs, and
indeed is designed to have a certain intake tract length to take advantage of the sonic
activity in the system. When people use the individual filters on the back of each carb,
the resulting shortened intake tract length upsets the sonic wave activity in the intake
tract and there is a power loss down low. Even more important, there is a marked
increase in the "spit-back" of fuel due to this upsetting of sonic wave activity that leads
to a thorough soaking of the filters, further reducing airflow and compounding the
problem. This length is restored with our 2:1 unit so sonic wave activity is restored and
the filter is up and out of the way of any spit-back. End of problem.

I remember reading all that by Toomey back in the day.. and it guided my thinking... to this day really.

During that same conversation, Keven Cameron kept repeating "IT'S REED LIMITED!"..
He even got a bit agitated because I wasn't getting it and kept pressing him..
I was asking him to build a motor for me for roadracing at the time.... in the 80s.
He eventually said.. if you want to spend that much $ for horsepower, maybe you should consider just buying a new TZ250.......
Otherwise... just understand that a few modest mods make sense, just leave it at that and race it!
He DID suggest (enthusiastically) to get an aftermarket CDI system..
I felt it silly since the bike came with one lol.... wasn't long before that they came with points LMAO
Only to find out many years later that IT was the reason the bike would rev beyond the powerband easily with short track gearing..
NOT the carb size..
He also strongly recommended welding (and truing) the crank, and while at it..
He was willing to do some minimal porting.... like cleanup and transfer work etc, not timing changes... throw in a set of Wisecos.. Reeds...
It never happened though.. I guess partly due to what he'd convinced me of, plus money was tight with the rest of a racing budget.....
So I soldiered on with the toomey Y-boot and jet kit..
(upped the pilot and got a few other mains, tho it never needed more than 1 or 2 sizes either direction)
Always did want to fix the top end problem though.... and for years thought a bigger carb would do it.... but what it needed was a Zeel... or static-plate... He'd recommended a Motoplat FWIW..

To my mind, the takeaway from the Toomey comments, with respect to this discussion... has to do with intake tract length...
Pretty much what I was saying..

So I still wonder...... what about the longest available velocity stacks inside of a larger/longer pod type filter???
Been wondering about this for all these decades..
Dam shame I just don't have the time/means to try it out...
Not for a long while yet anyway.
I'll post if I ever get around to it.

Incidentally, I'm currently working on a set of PWK28s for when I get the bike running again.
Sure... a tiny bit larger seems just about right.. But I'm long since persuaded any bigger (with basically stock engine) is a mistake.
I never liked the VM30s either.
Other than the theorizing, I'm trying to figure out what size carbs for a future "all out" big CC RZ build.... and how to go about determining what size... beyond so-and-so says X works out great..
I'd like to KNOW why I should choose bore size X vs. bore size Y for my highly modified build..
That and the lingering question about adding intake tract length..

I never minded the little stumble much either.... but it's sort of a "tuner's pride" thing to eliminate it..
And yes... I've always believed it has to do with shortening the intake tract length..
(compared to the stock airbox... all alternatives do that)

The stock airbox is just not for me... lets leave it at that.

Quote:
The 2-1 did make for a smoother running bike. (Probably for said reasons).

This is what I've heard since the early years consistently... tho a smidge less power than pods... if even enough to really matter. Why I raced with the Y-boot till moving on..

More tomorrow... suddenly it's gotten really late!


Top
 Profile  
Reply with quote  
PostPosted: Fri Jun 08, 2018 1:26 pm 
Offline
User avatar

Joined: Fri Nov 01, 2013 8:34 pm
Posts: 1322
Location: back in the hills
There's more to the airbox than intake length. Resonance...........Hemholtz.............Volume...........that annoying 5K RPM stumble.......which occurs in big 4 strokes as well as our little smokers. And the ever present government sound regs.


And the big one............VELOCITY.

_________________
There is only one difference between a madman and me. The madman thinks he is sane. I know I am mad.
Salvador Dali


Top
 Profile  
Reply with quote  
PostPosted: Fri Jun 08, 2018 7:33 pm 
Offline
-----
-----

Joined: Fri Oct 03, 2003 11:11 pm
Posts: 3458
Location: Southern Ontario, Canada
Funny, I put a 2-1 on and ran it for a few years. I took great pains to dial in the jetting and get it running as well as it could (with stock carbs). I went back to a lightly modified stock airbox and found the bike far more pleasant to ride. I later went to PWK28's with the airbox and it was better still. Out of interest maybe I'll try the 2-1 with the PWK's, but for me the biggest advantage of the 2-1 was that it was way easier to remove if you had to mess with anything. I recall someone adapting a Banshee style airbox to an RZ with one big filter, IIRC it was MBSteve on a salt flats bike.


Top
 Profile  
Reply with quote  
PostPosted: Fri Jun 08, 2018 9:19 pm 
Offline
User avatar

Joined: Tue Jan 20, 2015 8:18 pm
Posts: 118
Location: The Land of Francorchamps
a good (one that can flow more air than the engine) airbox is always better than seperate filters or an "add-on" duct with a filter attached to it. The bigger the volume of "still air" that can be drawn from, the better.
and even gases have inertia. the longer/bigger a stationairy column of gas is, the longer it takes to get up to speed. once it is up to speed you could say it has a ramming effect at the end, but that only makes up partly for what you lost at the beginning.

and Helmholtz ? someone once made a very nice explanation of how it works in 2-strokes :

Quote:
"A Helmholtz resonator consists of a volume connected to a duct". That is what Wikipedia tries to tell you.
But that is a Helmholtz resonator in its simplest form; one that you won't find anywhere in an engine.
What you do find in an engine is an intake tract with a variable cross-sectional area. This tract is from time to time connected to a variable volume (the crankcase) through a very variable window (the intake port / reed valve / rotary inlet).
The crankcase is connected to a number of transfer ducts with variable cross-sectional areas, which are from time to time connected to a very variable volume (the cylinder) through a number of very variable windows (the transfer ports).
The cylinder is from time to time, through a very variable window (the exhaust port), connected to an exhaust pipe with a very variable cross-sectional area who at the same time doubles as a volume. This pipe volume is constantly connected to a big volume (the outside world) through a tailpipe with a constant cross-sectional area and constant entry and exit windows (thank God, finally someting that's not variable).
We call this a compound Helmholtz resonator .
The various papers also tell us how to calculate the resonator's frequency:
" frequency = speed of sound / (2*pi)* Sqr ( cross-sectional area of the neck / ( volume of the resonator * effective neck length ) ) ".
O yes, the speed of sound... It is dependent on temperature, which is not really constant in the intake tract and the crankcase, rather variable in the transfer ducts and very variable in the cylinder and the exhaust pipe.
Now the above frequency formula is not exact; it is an approximation that is usable as long as the volume of 'the' tract is very small compared to the resonator's volume.
So when engines are concerned, that formula goes very far out the window.


I'd say : make it as short as possible.

_________________
if it runs, you can race it !


Top
 Profile  
Reply with quote  
PostPosted: Sat Jun 09, 2018 4:07 am 
Offline

Joined: Tue Nov 12, 2013 2:34 am
Posts: 1141
Evan: the term "in the rev range" is a bit misleading.
In the powerband is more germane.
It has to do with the way 2 strokes work.
evan_calgary wrote:
Large carbs create poor low rev power as they have trouble getting the velocity necessary to atomize fuel properly. Small carbs do poorly up high as they cannot draw in enough fuel/air mixture as optimal.

This is true with 4 strokes...

This simply is not true in practice that smaller carbs generate more power 'all else equal'. Throw some 22mm carbs on your RZ and let us know how the dyno curve looks. There are optimal ratios for all of these things such as your crankcase volume, reed to intake size, pipe dimensions etc. If you throw some 32mm on a stock bike with stock everything else, yes, you are unlikely to create more power but you are equally unlikely to do better with some 20's. The power pulses flow through the carb then return back through thus still interacting despite returning. The intake and exhaust side of a 2-stroke also have different 'jobs' to do so the dimensions of a stinger are not a good example of why the intake side should be equally restricted. It is not '4-stroke' think. The problem with going to extremes exists in the same realm regardless of 2 or 4 when you get wild. You also don't have to jump to a 45mm to run into this problem.


Top
 Profile  
Reply with quote  
PostPosted: Sat Jun 09, 2018 7:08 pm 
Offline

Joined: Mon Sep 27, 2010 7:21 pm
Posts: 488
Location: Wellington New Zealand
JanBros wrote:
a good (one that can flow more air than the engine) airbox is always better than seperate filters or an "add-on" duct with a filter attached to it. The bigger the volume of "still air" that can be drawn from, the better.
and even gases have inertia. the longer/bigger a stationairy column of gas is, the longer it takes to get up to speed. once it is up to speed you could say it has a ramming effect at the end, but that only makes up partly for what you lost at the beginning.

and Helmholtz ? someone once made a very nice explanation of how it works in 2-strokes :

Quote:
"A Helmholtz resonator consists of a volume connected to a duct". That is what Wikipedia tries to tell you.
But that is a Helmholtz resonator in its simplest form; one that you won't find anywhere in an engine.
What you do find in an engine is an intake tract with a variable cross-sectional area. This tract is from time to time connected to a variable volume (the crankcase) through a very variable window (the intake port / reed valve / rotary inlet).
The crankcase is connected to a number of transfer ducts with variable cross-sectional areas, which are from time to time connected to a very variable volume (the cylinder) through a number of very variable windows (the transfer ports).
The cylinder is from time to time, through a very variable window (the exhaust port), connected to an exhaust pipe with a very variable cross-sectional area who at the same time doubles as a volume. This pipe volume is constantly connected to a big volume (the outside world) through a tailpipe with a constant cross-sectional area and constant entry and exit windows (thank God, finally someting that's not variable).
We call this a compound Helmholtz resonator .
The various papers also tell us how to calculate the resonator's frequency:
" frequency = speed of sound / (2*pi)* Sqr ( cross-sectional area of the neck / ( volume of the resonator * effective neck length ) ) ".
O yes, the speed of sound... It is dependent on temperature, which is not really constant in the intake tract and the crankcase, rather variable in the transfer ducts and very variable in the cylinder and the exhaust pipe.
Now the above frequency formula is not exact; it is an approximation that is usable as long as the volume of 'the' tract is very small compared to the resonator's volume.
So when engines are concerned, that formula goes very far out the window.


I'd say : make it as short as possible.

Frits has a great turn of phrase.

_________________
496 Cheetah. TSS PVs, PWK35s, Ignitech, RGV(ish) chassis


Top
 Profile  
Reply with quote  
PostPosted: Sat Jun 09, 2018 8:26 pm 
Offline

Joined: Sun Sep 30, 2012 6:28 pm
Posts: 55
Location: Cedar Rapids Iowa
I see in your original post you mentioned reed spacers and I want to give my recent experience with them. Running a 84 rz350 with pipes and stock carbs with a Dynajet kit in them. Y boot with big K&N.
I tried a set of 6mm reed spacers. Rode the bike about 15 miles with them. What they did was pull the power range lower in the RPM's. Instead of producing power all the way to 10,500 plus it made the power sign off about 9500. No over rev, felt really dead on top. Some of that could have been my gearing 15-40 so it Rev'd really quick thru the range. Maybe if I had stock gearing it would have worked better. I ended up taking them back off. I guess they did what they were supposed to do but I didn't like them. I don't have the 5,000 rpm stumble on my bike so I can't say it it would have helped with that. Just my 2 cents.

_________________
84 RZ350 Allspeed GP replica pipes, Stock carbs 380 mains, 30 pilots, Y air boot/ k&n filter, TDR Carbon Reeds, Zeel, stainless lines, TDR R1 caliper adapters, R6 blue dots.


Top
 Profile  
Reply with quote  
PostPosted: Sat Jun 09, 2018 8:39 pm 
Offline
User avatar

Joined: Fri Nov 01, 2013 8:34 pm
Posts: 1322
Location: back in the hills
Quote:
..........it is an approximation that is usable as long as the volume of 'the' tract is very small compared to the resonator's volume.


To accomplish that we use the airbox volume and intake area/length. Not the entire system.

In our case the resonant frequency is used to stuff the cylinders in the period just before the pipe's efficiency range when the unaided incoming charge would be disrupted by the "double dipping" pressure wave. The infamous 5K stumble. It's why it appears when the airbox is removed. The carb bore size assists (see above) the pressure wave. It's why the stumble becomes easier to tune away with other carbs sans airbox. It works on big twin four strokes too which need it depsarately.

Properly sized boost bottles will do the same thing as an airbox. Reference Harry Barlow for that one.

_________________
There is only one difference between a madman and me. The madman thinks he is sane. I know I am mad.
Salvador Dali


Top
 Profile  
Reply with quote  
PostPosted: Sat Jun 09, 2018 10:29 pm 
Offline

Joined: Sat May 28, 2016 3:15 am
Posts: 60
Location: South Florida
Regarding airboxes..

(I personally don't like them... take up space, make things harder to work on, ugly too)

Ironically, Kevin Cameron comes up again... honestly just a coincidence.. I hadn't spoken with him again nor even read his stuff for a couple of decades, till I stumbled on to this one thing a few years back.
An article he wrote advocating airboxes..
I think the thing was that this seemed odd for us older fellows... I wonder if anyone here is old enough to remember the 70s and before...
Where NO airbox was anything but a heavy/restrictive nuissance, and everyone knew to dump them.
Cameron explained that the new crop of bikes... both street and race.. benefit greatly from a *PROPERLY DESIGNED* airbox.
I have no doubt that a properly designed airbox is "better"... Probably in every way.
But no individual has the means to properly design one..
Engineering staff, many many dyno hours.... an established engine design to make it for, etc..
I have no doubt that an airbox meeting those criteria is "better" than no airbox at all.
Incidentally, he explained that "better" means a specific volume and resonant frequency, along with whether
it's to be positively pressurized or not.

It's just not possible for me to believe that the original Yamaha airbox is "best" for all the possible variants of RZ motors in use.
Certainly not ones hundreds of CCs larger in displacement....
Maybe the stock airbox... or any airbox design, might have enough flexibility in its parameters to accomodate all possible pipes/carbs/port schemes around the stock displacement.... but I can't imagine that plus huge displacement changes.......

Add to this that ANY mods to the stock airbox change it's design in unpredictable ways..
Just removing the "snorkle" changes it a LOT..
All you then have is "pants seat" changes.... maybe even improvements.. maybe not.

This many years after the RZ being discontinued, the original airbox just isn't practical for many..
I don't think it would be the best choice for my future "all out" build.... much greater CCs.. etc.
The whole point here, was to see if it's possible to find a way to determine what carb bore a good choice for a given displacement and WHY.
Where in the powerband would larger/smaller bore (within that sensible range, say from 28mm to 36mm) does smaller vs. larger bore make what difference...
Also, would longer intake tract change that, and in what way.
To finally establish a way to determine what to use in lieu of an airbox.
Beyond so and so said such and such.... "this worked for me" and the like.
Also, some don't have the orig airbox... there aren't many available, and cost a LOT..
Then, cut it all up on a guess.... either your precious original or an expensive epay one..

If anyone IS designing a new airbox for RZ350 based engines please let me know!
Though I would hope they've had the ability to do all that complex math, AND the dyno hours on ALL the possible combinations of displacement/portings/carbs/pipes!

I'm not sure that airbox vs. not is partly just a solution in search of a problem... since they run OK without them.

But at all costs I don't want this to become an "airbox vs. non-airbox" thread!

Hey T.Rex! Thanks for chiming in.. Roger is it?

I'm not terribly concerned about sound regs.... as I'm not looking to design and sell a line of motorcycles..
I rather like a little intake noise for my personal ride..

Resonance, I already addressed above..
Suffice that a longer pipe resonates lower (column of air).. so does one of larger diameter.
Just a fact.

Helmholtz.. I'm sure you both have a point on that..... but I'm not convinced that the original airbox fits all possible engine configurations.... and I can't design one from scratch..
Not so hard to just make the intact tract longer...
The how and why of it would be nice to know... particularly with regard to carb bore size.

Velocity: Yes... very critical... well velocity stacks are thought to increase velocity.... again, all else being equal..
Like if you went to a larger carb bore to achieve a particular effect.... a longer stack might restore the needed velocity.

Hi RuZty.. thanks for contributing!

RE: the 2 into 1 y-boot thingie..
I dunno.. Toomey Spec2 and others swear by it... I ran with it for years, even raced with it, and it wasn't in any way a problem.
Just maybe not ideal.. can't say for sure.
It seemed to be universally agreed back then, and still now I think.... that it "runs better" with that than individual pods.
Also seems like all have always agreed that the stock airbox makes the bike "more plesant" or more "civilized" to ride around on.
Maybe even lose the 5k stumble issue..

Personally, and this is just me.... the LAST thing I'd want, is a civilized RD or RZ!!!
ACK! Maybe I'm just a hooligan?? :smt107 :smt080

If I wanted civilized, I'd just restore a Honda-Dream.. :P

Does seem to make the argument for longer intake tract length though.
Whatever else...

Why not try a set of the longest stacks you can get.... many on epay.... together with a reedblock spacer..
See what results you get..
If jetting changes dramatically, you'd know you're on to something!

I must confess that I've had this belief for a while, and haven't been able to test it..... that adding length to the intake
tract in the RZ350 would be of benefit.
Might even eliminate the anomaly that causes the 5k stumble thing
At least that's been my suspicion.

Speaking of 4 strokes...
One example of the added intract length thing I happen to know about, concerns the first gen GSXR750.
Another bike I had at the time..
The frist 2 seasons (86 & 87) that Yoshimura raced them, they used the stock rubber
(opened up to match carb bore) intake "manifolds".
They sold their own proprietary magnesium bodied version bored to 34mm of the stock non-US carbs.
The TM-33 was sold by the rest of the aftermarket, Particularly Sudco/Mikuni-USA.
The TM-33 was a bored to 33mm and recalibrated stock carb.
The stock carb differed in size in various countries... from around 29mm to 31mm or something like that.
When the RS series of Mikunis came out, they went to those.... 36 then 38mm... still with stock manifolds.
The next season... a major change.... (the bikes were suddenly rocket ships.. esp. out on the Daytona banking)
Different carb type.. BUT with a longer metal pipe-like adapter between carb
(it now screwed in to carb body, and were available in different lengths) and stock intake manifold.
They also went straight to 40mm!!!!!
Image
Image
Image
Image
(first 2 pix = early setup, second 2 pix = later setup)

EDIT: Ack.. not sure why the pix aren't showing up... but if you right click on each and open in new tab, they come up..

Sorry I couldn't find better pix (surprisingly since it's the Schwantz superbike)
BUT you can easily see that the latter version, the carbs are much farther away from the head!

Again, not at all to name drop.. honest... but Nabe @ yosh.. who I'd been working with me on tuning the bike for
racing (I'd spent enough.... he ported the head to superbike spec...) swore to me THIS was the KEY!
That I REALLY should get these new carbs!!!
But 3 new carb sets in 3 years at $$$$ was just to much so I passed.. and just tried to make my RS carbs work.
Turns out he was right, and I missed out..
At Daytona later that year or maybe it was the next, he explained to me that that was the "breakthrough".
They eventually understood that the stock intake length was to short... a design flaw......
Worked well enough on the street with stock airbox and pipe... but as soon as you changed from that..
anything...
a similar "flat spot" around the same RPM (maybe it was 6k.. who remembers) emerged.
Any other changes just shifted it up further in the RPM band.. Cams/timing, ignition, carb bore size... pipe changes..

They never went back from those carbs which accepted this screw-in pipe-like affair..
It wasn't very long after that that the bikes came out with fuel injection.. so the next step for them
was to transition to that for racing as well.

I realize that is a 4 stroke... and that changes things.. But it taught me that the original intake tract length
may not be ideal.. and THAT has made me wonder all this time.

It also raises the question of where to lengthen... upstream or downstream of the carb?
That is very likely to differ between 2 and 4 stroke.. and even any 2 of either type..

So would adding length be better? Permit a larger carb? Would that add power at top or bottom of the powerband?

_________________
Greg


Last edited by 2bang on Sat Jun 09, 2018 11:43 pm, edited 3 times in total.

Top
 Profile  
Reply with quote  
PostPosted: Sat Jun 09, 2018 10:36 pm 
Offline

Joined: Sat May 28, 2016 3:15 am
Posts: 60
Location: South Florida
T.RexRacing wrote:
Quote:
Properly sized boost bottles will do the same thing as an airbox. Reference Harry Barlow for that one.

So what is "properly sized"? :smt003

Must admit, I had been wondering... why a crossover pipe if you need a 2 into 1 yboot or an airbox for that matter..
Also been wondering if a boost bottle would make up for some of what an ideal airbox would add...

FWIW


Top
 Profile  
Reply with quote  
PostPosted: Sat Jun 09, 2018 10:51 pm 
Offline

Joined: Sat May 28, 2016 3:15 am
Posts: 60
Location: South Florida
Srhiskx wrote:
I see in your original post you mentioned reed spacers and I want to give my recent experience with them. Running a 84 rz350 with pipes and stock carbs with a Dynajet kit in them. Y boot with big K&N.
I tried a set of 6mm reed spacers. Rode the bike about 15 miles with them. What they did was pull the power range lower in the RPM's. Instead of producing power all the way to 10,500 plus it made the power sign off about 9500. No over rev, felt really dead on top. Some of that could have been my gearing 15-40 so it Rev'd really quick thru the range. Maybe if I had stock gearing it would have worked better. I ended up taking them back off. I guess they did what they were supposed to do but I didn't like them. I don't have the 5,000 rpm stumble on my bike so I can't say it it would have helped with that. Just my 2 cents.

Thanks for chiming in!

Did you have the spacers ahead of or behind the reed cages??

I used 15-40 for short track gearing (Loudon).. it easially revved beyond 9500 tho the power died dead at 9500..
This is with same setup... toomey pipes/jet kit/ yboot with big K&N..
NO spacers...
SO: I wonder... if you had stock ignition how can it be that it would rev past there and still be pulling (without the spacers)????

_________________
Greg


Top
 Profile  
Reply with quote  
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 26 posts ]  Go to page 1, 2  Next

All times are UTC - 4 hours [ DST ]


Who is online

Users browsing this forum: No registered users and 6 guests


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum

Search for:
Jump to:  
Powered by phpBB® Forum Software © phpBB Group