Topic: Split - What is wrong with the stock Honda induction system?

[HondaMan]

TT:
Excellent collection of thoughts and pix! 

Here's a little more "empirical data" for you and the other readers:
Last summer (2009) I "fixed" a 750 custom and a 550 custom, both of which had pods, by directing the owners back to stock airboxes. Both bikes were suffering severe flat spots in performance in the 1500-3500 RPM range, typical of what happens when the low midrange flow is disturbed by the geometry of the pods at the carb air horns. The 750 refused to run to redline in 2nd gear. After installing the stock airboxes, both bikes lost the flat spots and readily ran to redline, neither of which was available before. Jetting in both cases had to be reduced to stock levels, as well, to prevent plug fouling, which both bikes suffered when they came to me.

Another true test goes back to 1973 when Cycle magazine tested the (then new) K&N pods without internal velocity stacks against the K&N with the internal stacks (these are real hard to find today) on a CB750K2 (HM341 pipes), and against the stock airbox with Honda paper filter. The dyno used was first run with stock airbox, then the non-stacked pods, then the stacked pods. There was less HP (I seem to remember the figure was about 2 HP) in the RPM range of 2000 to 4000 RPM with the non-stacked pods, even after the mainjet was increased from the stock 105 to 110. The bike would not redline easily in 3rd gear with those pods. The velocity-stacked pods were run at 110 mainjets and the HP curve below 5500 looked stock again, with a small increase at 7500+ RPM (said to be less than 1 HP by the writer). At the end of the pod testing, the stock airbox was refitted with the 110 jets still in place and the dyno showed a power increase of almost 1 HP from 3500 RPM to 7000 RPM as compared to the original 105 mains (which is to be expected: plugs will foul at this richer mix, though). In this test, I would like to have also seen them use the K&N filter in the stock airbox, but it was not included. My personal butt-dyno has always noted a greater willingness to run to redline on the 750s I equip with the K&N filter in the stock airbox, compared to the paper filters. But, this is just a breathing issue: the K&N passes air more easily than paper.

The "wisdom" of the era was that people could feel increased power with the pods. The truth was probably closer to this: the increase in carb noise makes the rider tend to fell like more power was being generated, and the surge in torque felt after the bike passes the flat spot they caused "confirmed" to many that it was working. (This phenomenon also seems to apply to 4-1 pipes on these bikes, but that's an argument for another day.)

Thanks for assembling such a great collection of visuals!

[kajtek]

wow, im new here but thats the kindof stuff i joined for. very helpful thanks you.

i recently purchased a cb400f which the previous owner had fitted pods to. as i understand he hadnt rejetted and it works ok, but i think it could be better (takes a lot of choke..)

initially i had planned to but some jets, fiddle around and make it run better, but im definately on the look out for an airbox after reading all that. the one thing i wanted to know is regarding the head breather 'blow by' system. i understand the environmental considerations but it seems kindof goofy in terms of engine performance. would the combustion temp not be raised by burning oil gasses?

what im getting at is could, after refitting an airbox and airbox filter in the standard housing, can i remove the breather system? obviously i would leave the breather tube in place, just vent to atmosphere. at the moment the air filter box is pretty oily and does not smell good, i prefer the idea of an oil free dry atmoshere for air filtration.

thanks for any advice

ashley

[Laminar]

The "wisdom" of the era was that people could feel increased power with the pods. The truth was probably closer to this: the increase in carb noise makes the rider tend to fell like more power was being generated, and the surge in torque felt after the bike passes the flat spot they caused "confirmed" to many that it was working. (This phenomenon also seems to apply to 4-1 pipes on these bikes, but that's an argument for another day.)

Another common term that always makes me laugh is "throttle response." If there's no quantifiable data to support improvement, just say that it improved "throttle response" and it's justified.

[MRieck]

Any questions/comments are welcome!

  The top picture shows the shape of the filter adaptors for the Mikuni 32mm TMR carbs. They address some of the issues you talk about.
   http://www.motorcyclecarbs.com/CARBURETOR_KIT_HONDA_CB750_SO_P16632C1819.cfm      BTW...they are very nice carbs.

[chickenman_26]

Lloyd,
Nice work debunking K&N's claims. With reference to this picture, what goes on the hose nipple just above the number 5? Is it the same drain hose that was fitted to the CB500 air chamber?

That part number shows up in the CB550 air cleaner fiche. If so, how is it supposed to be routed. Thanks.

It is a reduced diameter tube and it routes down to behind engine and in front of the swing arm.

Then it is the same tube as on the CB500. Thanks. BTW, that reduced diameter tube has a foam disc in it where it expands to fit on the nipple. This is just a moisture drain, I assume? Hose #6 isn't available anymore, but I've fitted one that has a cap in the end that gets removed occasionally
for draining the breather residue.
Stu

[754]

Some of us are running custom frames, and or different carbs. So the stock induction is not going to help us.

 Honda is an excellent Company with an excellent engineering staff, but they have built a few things that were less than stellar.

 Are the engineers right most of the time, yes. Do they sometimes have do-overs.. yes..

 There are many different styles of riding on here, what works for some, may not work for others.

 Some of us like our bikes stock, some like changing them..

 I have to ask you this TT, what engine mods have you done to your bike , by this I mean induction, carbs, engine, & exhaust?

[BobbyR]

I run a stock airbox and I work for a Japanese Company and I work in the Engineering Dept with Japanese Engineers. Here is what I learned. Japanese engineers are meticulous in testing. They will NOT over complicate, they tend to think in a very linear fashion, and are totally results oriented. The Honda engineers would not have designed the airbox and all of the associated hardware assemble and to mount if it was not absolutely necessary to do so.

They are quite cost conscious. If they would they would have gotten the same result slapping some Pods on the carbs, they would have. Pods are cheap to produce and much quicker and easier to install than the stock airbox. Time and motion is money in a production environment.

The airflow around a bike is very dynamic just going in a straight line. When you add tilting the bike for turning you change the airfilow around the bike, they needed to design something that would supply a constant air supply to the carbs during those maneuvers and also crosswinds, rain, snow etc.

Since you trust their engineering enough to take your 30-40 year old motor up to the redline, why would you doubt their air delivery system?

  

[754]

Bobby, A few things, pods were not available when the 750 came out.
 The engineers are constrained by several  boundaries , when working on the design.

 Excess intake noise is out.
 Bike being affected by washing it is out... things like that.

 They are shooting for a result that fits most of the people most of the time, under nearly all conditions. A reliable system.

[Laminar]

I have to ask you this TT, what engine mods have you done to your bike , by this I mean induction, carbs, engine, & exhaust?

Nothing, of course. The bikes were and are the absolute pinnacle of perfection.

[MJL]

I enjoyed that.  It made a lot of sense to me.

I have no need for pods but I do have a question.  Would a single air filter that is large enough to cover all four throats, with built in venturies, one or four of those cones and the filter part further back towards the frame be a much better alternative.  I doubt there is anything like that made, just thinking.  Like I said, I enjoyed your posts.

You described a factory 750 airbox.



:edit: I ran pods on my 650 for awhile (less than a summer) and ditched them. I didn't notice much if any performance increase, more intake noise, and got a lot of filter oil on my pants from the foam pods.

[Really?]

I enjoyed that.  It made a lot of sense to me.

I have no need for pods but I do have a question.  Would a single air filter that is large enough to cover all four throats, with built in venturies, one or four of those cones and the filter part further back towards the frame be a much better alternative.  I doubt there is anything like that made, just thinking.  Like I said, I enjoyed your posts.

You described a factory 750 airbox.

Woohoo, I got something right!   

I guess what I was really going for in that is something that gets the same results but would also suit the people that prefer the pod type route.  If it looked good and fed the same or improved demands of a flow that is not disturbed, someone might be able to create (or build a kit) something like this, they have something to sell (if possible).  But then, I was also curious for myself.

[mlinder]

I don't know why people call this stuff "theory". It's not theory. It's well tested and documented physics factualities.

[Laminar]

I don't know why people call this stuff "theory". It's not theory. It's well tested and documented physics factualities.

Definitions 1, 3, and 5 would all apply, although there's a significant amount of 7 in this thread.

the·o·ry

1. a coherent group of general propositions used as principles of explanation for a class of phenomena: Einstein's theory of relativity.
2. a proposed explanation whose status is still conjectural, in contrast to well-established propositions that are regarded as reporting matters of actual fact.
3. Mathematics. a body of principles, theorems, or the like, belonging to one subject: number theory.
4. the branch of a science or art that deals with its principles or methods, as distinguished from its practice: music theory.
5. a particular conception or view of something to be done or of the method of doing it; a system of rules or principles.
6. contemplation or speculation.
7. guess or conjecture.

[Markcb750]

Ditto on the factualities, them makses gooder physices

[mlinder]

I don't know why people call this stuff "theory". It's not theory. It's well tested and documented physics factualities.

Definitions 1, 3, and 5 would all apply, although there's a significant amount of 7 in this thread.

the·o·ry

1. a coherent group of general propositions used as principles of explanation for a class of phenomena: Einstein's theory of relativity.
2. a proposed explanation whose status is still conjectural, in contrast to well-established propositions that are regarded as reporting matters of actual fact.
3. Mathematics. a body of principles, theorems, or the like, belonging to one subject: number theory.
4. the branch of a science or art that deals with its principles or methods, as distinguished from its practice: music theory.
5. a particular conception or view of something to be done or of the method of doing it; a system of rules or principles.
6. contemplation or speculation.
7. guess or conjecture.

But #2 is completely lacking. This stuff is not "still conjectural, in contrast to well-established propositions", which is probably the most important bullet point in this list concerning wether something is "theory" or not.
#4 is not applicable in this case
#6 is not happening here.

Therefore, not theory, but proven formulae.

[mlinder]

Ditto on the factualities, them makses gooder physices

lurn gooder fisix wif Mraks new book, "lurn gooder fixis wif Mrak!"

[turboguzzi]

TT, compliments on the work.

The subject has surfaced a few time sin the hipo forum and hipo - velocity stacks related posts.

Go around a Moto gp or superbike or F1 paddock and you will not miss it: all of them have airboxes. its since the 90's that the beneficial effects of having a still air plenum are recognized. The beneficial effects of good stacks from well before. One of the highest priorities i have for my racer is to build an airbox around the current open running Keihn CR's.

what did change formt the time the cb's were designed is that today airboxes are "tuned" and they have about a X10 volume to do the work right i.e. a liter bike's got a 10 liter airbox, the Cb's are far from that ratio. also, the section shown through the system is obviously at the center, the two outer carbs dont have such a nice air flow.

even though, a so-so airbox is way better than none, not to mention the bad effects of side winds on pods, rain etc.  

Now TT, all you have to do is compile a similar post about oversize tires and how they slow you down, another topic were people have trouble accepting the truth...

TG

[Laminar]

Definitions 1, 3, and 5 would all apply, although there's a significant amount of 7 in this thread.

the·o·ry

1. a coherent group of general propositions used as principles of explanation for a class of phenomena: Einstein's theory of relativity.
2. a proposed explanation whose status is still conjectural, in contrast to well-established propositions that are regarded as reporting matters of actual fact.
3. Mathematics. a body of principles, theorems, or the like, belonging to one subject: number theory.
4. the branch of a science or art that deals with its principles or methods, as distinguished from its practice: music theory.
5. a particular conception or view of something to be done or of the method of doing it; a system of rules or principles.
6. contemplation or speculation.
7. guess or conjecture.

But #2 is completely lacking. This stuff is not "still conjectural, in contrast to well-established propositions", which is probably the most important bullet point in this list concerning wether something is "theory" or not.
#4 is not applicable in this case
#6 is not happening here.

Therefore, not theory, but proven formulae.

While basic fluid dynamics is not #2, it's still entirely appropriate to call fluid dynamics "theory," based on definition #3.

Also, the applications of the aforementioned fluid dynamics are most definitely #6 and #7.

In fact, it is a near certainty to cause turbulence right at the entrance to the carb and reach well into the throat.

No POD or filter placed at the carb inlet can offer that isolation or buffering from turbulent air flow.

I estimate the 750 path is a bit shorter , around 4-5 inches.

Statements like these ARE conjecture - they're not conclusions reached as a result of direct testing, but they are estimations and predictions made based on assumptions.

Not to denigrate any of the info that TT has presented here, but just to be clear that what we're reading is estimations of real-world performance based on a number of assumptions.


As an aside, I was curious about this statement:

Ilust. 013
Honda inlet coupler.  Notice the ramped inlet?  Whatever velocity at the wide bell diameter, the speed will be increased as it transitions to the smaller diameter.  This velocity change helps convert turbulent flow back into laminar flow.

It's been a couple of years since I've done any CFD but an increase in velocity means an increase in Reynolds Number, which means turbulent flow is more likely.

[mlinder]

Why do those definitions of "theory" seem somewhat contradictory?

[Laminar]

Why do those definitions of "theory" seem somewhat contradictory?

Why does any word have more than one definition?

[chickenman_26]

I have to ask you this TT, what engine mods have you done to your bike , by this I mean induction, carbs, engine, & exhaust?

Nothing, of course. The bikes were and are the absolute pinnacle of perfection.

I think it's unfair of both of you guys to imply that TT doesn't know what goes on for real if he hasn't made these mods on his own bikes. I don't see him criticizing anyone or trying to convince anybody to change to stock induction here, only to give everyone info that might be useful if problems did crop up after an induction change. At least a person would be able to look for flow issues caused by mismatched surfaces, regardless of what carbs or filters might be installed. And BobbyR's observations are right on the mark. There are a lot of folks who feel they can do better with $20 aftermarket "solutions" than all of the engineers with years of education and field experience. I think they're fooling themselves, but everyone's entitled to do that. That's what's made companies like K&N household words.

Stu

[Laminar]

I have to ask you this TT, what engine mods have you done to your bike , by this I mean induction, carbs, engine, & exhaust?

Nothing, of course. The bikes were and are the absolute pinnacle of perfection.

I think it's unfair of both of you guys to imply that TT doesn't know what goes on for real if he hasn't made these mods on his own bikes. I don't see him criticizing anyone or trying to convince anybody to change to stock induction here, only to give everyone info that might be useful if problems did crop up after an induction change. At least a person would be able to look for flow issues caused by mismatched surfaces, regardless of what carbs or filters might be installed. And BobbyR's observations are right on the mark. There are a lot of folks who feel they can do better with $20 aftermarket "solutions" than all of the engineers with years of education and field experience. I think they're fooling themselves, but everyone's entitled to do that. That's what's made companies like K&N household words.

Stu

My post was in reference to previous posts TT has made outside of this thread.

[BobbyR]

I have to ask you this TT, what engine mods have you done to your bike , by this I mean induction, carbs, engine, & exhaust?

Nothing, of course. The bikes were and are the absolute pinnacle of perfection.

I think it's unfair of both of you guys to imply that TT doesn't know what goes on for real if he hasn't made these mods on his own bikes. I don't see him criticizing anyone or trying to convince anybody to change to stock induction here, only to give everyone info that might be useful if problems did crop up after an induction change. At least a person would be able to look for flow issues caused by mismatched surfaces, regardless of what carbs or filters might be installed. And BobbyR's observations are right on the mark. There are a lot of folks who feel they can do better with $20 aftermarket "solutions" than all of the engineers with years of education and field experience. I think they're fooling themselves, but everyone's entitled to do that. That's what's made companies like K&N household words.

Stu

+1 i would imagine you understand very well the effect of air flow over mismatched surfaces. 

[crazypj]

 TT is completely correct, there isn't anything better for a stock bike.
 K&N, et.al. shorten the intake so much it's just about impossible to rev engine high enough to benefit from any form of 'pulse' tuning.
Pods are just too close to the carb/valve head.
 several years ago I used various formula's to calculate intake length on various motors, pods usually needed an extension tube between 9"~16" to work as well as stock airbox (in one case, it would put pods about level with rear tyre )
With highly modified engines and radical cams operating at very high rpm, you can make them work properly with a few 'modifications' to filters, (cutting up stock intake trumpets and fitting inside pods is possible but not easy) otherwise, it's a 'style' thing, (and very difficult to jet carbs so they work even close to as well as stock)
 Even race bikes I've worked on have intakes too short (riders don't like look of extended intake, particularly if someone with short stubs beats them)
Everyone is doing same things so it all evens out though

PJ

[mlinder]

Why do those definitions of "theory" seem somewhat contradictory?

Why does any word have more than one definition?

Oh, these are multiple definitions? I thought these were a lit of prerequisites for "theory".

In that case, I reject all but #2 for this particular situation.