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

[TwoTired]

Thank you for confirming what I said, what, 10 posts ago?  No matter how much vacuum you apply, once you have reached the maximum flow for a given orifice, NO MORE FUEL WILL FLOW.  Does everybody agree on that?

No.  And you simply CANNOT prove it!  Even if a committee votes in your favor.  Committees don't create physical principles.

The smaller the orifice (the straw) is, the less liquid will flow.  Everybody agree on that?

Only when the pressure differential across the orifice is the same.

The smaller the orifice is, the lower the vacuum (meaning less pressure differential) required to reach the maximum flow through the orifice.  Everyone agree on that?

No.  Unless you have also added turbulence to the flow, which is very very unlikely in the SOHC4 carb.

At WOT, the main jet has reached its maximum flow.  Everyone agree on that?

Main jet still flows at a rate determined by the pressure differential.  If you change the pressure differential at WOT then the amount of fuel flowing though it changes as well. As in switching from the stock filter membrane to a filter membrane with less pressure drop across it.

[kghost]

The smaller the orifice is, the lower the vacuum (meaning less pressure differential) required to reach the maximum flow through the orifice.  Everyone agree on that?

At WOT, the main jet has reached its maximum flow.  Everyone agree on that?

1. No.....the pressure on one side of the orifice vs the other will dictate that.

2. No not necessarily...only that the jet is limiting flow based on pressure ratios.


Edit: damn Lloyd you beat me.

[sangyo soichiro]

Are you guys taking into account the viscosity of the fluid?

[kghost]

Are you guys taking into account the viscosity of the fluid?

No its pretty much constant for car gas.

lets assume for discussion.....temperature, specific gravity etc are constant.

[crazypj]

[quote

[/quote]

 Daammnn, lot of replies since yesterday,  

 Fed up with seeing that diagram, it's not correct. (but I guess it's OK to explain basic operation?)
 
Pilot jet has less effect than shown and main jet has more effect at lower throttle openings (just drill out a main jet and see how well thing runs off idle, well before  main jet 'should; be having an effect)

 With Kei-Hin 3 jet carbs, it's even further away from what happens (due to being CV, bypass drilling's, etc)

 Guess bunch of people will have to take TT's word for what happens, even if it goes against 'common sense' and preconceived idea's

PJ

[Retro Rocket]

even if it goes against 'common sense'

I think if you understand all of the parameters then it is common sense.
I don't think it could have been explained any better...

Mick

[sangyo soichiro]

The bernoulli equation I saw quoted assumed viscosity was negligible (i.e., it assumes everything is frictionless).

Do the frictional forces change with fluid velocity?

[crazypj]

Yeah, just keep airflow below 760ft/sec and your OK 

PJ

[Laminar]

Fed up with seeing that diagram, it's not correct. (but I guess it's OK to explain basic operation?)

Basic operation is what I was going for. Someone suggested that mains only matter at WOT.

[sangyo soichiro]

Boy, wouldn't this bike look cool with PODS?!   

[kghost]

You will no doubt know that the main Jet acts as a restriction only at Wide Open Throttle.

Why is that?

No...guess again Laminar.

Basic operation is what I was going for. Someone suggested that mains only matter at WOT.

See....not what was said. Restrict being the word you were looking for

The bernoulli equation I saw quoted assumed viscosity was negligible (i.e., it assumes everything is frictionless).

Do the frictional forces change with fluid velocity?

Yes.... overall.....depends on the diameter of what your flowing thru and the surface as it relates to boundary layer.

As TT noted laminar vs turbulent flow matters too

[sangyo soichiro]

The bernoulli equation I saw quoted assumed viscosity was negligible (i.e., it assumes everything is frictionless).

Do the frictional forces change with fluid velocity?

Yes.... overall.....depends on the diameter of what your flowing thru and the surface as it relates to boundary layer.

As TT noted laminar vs turbulent flow matters too

I bring this up in relation to the discussion about whether the main jet has a flow limit.  Not about the pods vs. box discussion.  Just wanted to make that clear. 

[Laminar]

You will no doubt know that the main Jet acts as a restriction only at Wide Open Throttle.

Why is that?

No...guess again Laminar.

Basic operation is what I was going for. Someone suggested that mains only matter at WOT.

See....not what was said. Restrict being the word you were looking for

What? The main jet affects fuel flow across a broad part of the RPM range by restricting it.

You need to quantify what you're saying with math instead of throwing out random statements.

[HondaMan]

I'll bet those of you who have read the Carburetors chapter in my book are having a good time watching this discussion, too. 

[kghost]

What part of Restriction are you having difficulty accepting?

Let me see if I can be clear.

A restriction = something limiting flow.

The main jet restricts flow at Wide Open Throttle.

Doesn't mean that it does not flow fuel anywhere else in the RPM range or throttle position. Its a limiting factor when the slide or butterfly valve is fully open.

At other than WOT it works in conjunction with the needle valve, the idle valve, and the pressure differential to meter fuel.

Once again...limiting/ restricting at WOT .....nowhere else.

In the midrange for example....needle taper and pressure differential are controlling the quantity of fuel flowing.

The needle acts as a variable jet.

Bear in mind that theres a bit of overlap between the circuits.

Like Idle to midrange.....otherwise you'll get flat spots and bogging in transition.

BTW...These are not random statements....its actually how the physics and science behind carbs works.

I'll bet those of you who have read the Carburetors chapter in my book are having a good time watching this discussion, too. 

No....its like trying to explain to children theres no Santa.

[TwoTired]

I'll bet those of you who have read the Carburetors chapter in my book are having a good time watching this discussion, too. 

Hmmm...  Looking suspiciously like a shameless plug there.   

[LBM 550F]

     I just have a simple question, My very stock 1975 550F runs very well but is missing its complete air box cover (snorkle piece) I'am still using oem honda air filters but will switch to a Uni this summer to cut costs. Is the lack of a cover on the air box hurting performance ?? or just a noise supression device?
                    Thanks..L

[Cormac]

So then, the stock carb configuration is tuned around the fact that the stock filter media limits air flow, correct? I know, the air box also smooths air flow, bonus... How can it possibly be a good thing to limit or restrict air flow? The math I have always seen indicates that more air (oxygen particularly) in addition to the correct ratio of fuel produces more power. In addition, the ability for an engine to more readily fill the chambers with that air/fuel mix increases volumetric efficiency, which is good from both an economy and power stand point. So it seems to me, that the stock carb tuning was not the optimal configuration, but rather a patch job to deal with a crappy filter media that was the only thing available at the time... Ignoring the GI "ladies stockings" trick of WW2. Also seems to me that we should be overjoyed at the ability to retune the carbs to work with a more free flowing filter that creates less of a pressure differential at the media. Let the thing BREATHE. And we should be doing so in a manner that creates a suitably clean, smooth flow... And should allow for a hardtail or Konged frame, and not look like a big black box.

[MRieck]

     I just have a simple question, My very stock 1975 550F runs very well but is missing its complete air box cover (snorkle piece) I'am still using oem honda air filters but will switch to a Uni this summer to cut costs. Is the lack of a cover on the air box hurting performance ?? or just a noise supression device?
                    Thanks..L

It's part of it.

[MRieck]

Those old air boxes had several functions...one was keeping noise down even in 1975. Noise regulations started. Blowby catch was another.  People get wrapped up in other stuff...I understand but I'm not a part of it.

[kghost]

So then, the stock carb configuration is tuned around the fact that the stock filter media limits air flow, correct?

The stock paper element probably limits to some degree but remember that the honking big filter supplier one cylinder at a time. Aftermarket filters in the stock shape are available.

How can it possibly be a good thing to limit or restrict air flow? The math I have always seen indicates that more air (oxygen particularly) in addition to the correct ratio of fuel produces more power. In addition, the ability for an engine to more readily fill the chambers with that air/fuel mix increases volumetric efficiency, which is good from both an economy and power stand point. ?

The main thrust of TT's post is that the stock airbox provides volume and laminar flow. The area and the shape of the velocity stacks in the stock airbox are critical to smoth flow. Both contribute to Volumetric efficiency. Keep the air flowing and avoiding turbulence being the main objective.

So it seems to me, that the stock carb tuning was not the optimal configuration, but rather a patch job to deal with a crappy filter media that was the only thing available at the time... Ignoring the GI "ladies stockings" trick of WW2. .

No it was engineered to keep the engine clean and develope power for street use. Paper filters such as the stock unit continue to be used the world over. Its really not some bodged together contraption

Also seems to me that we should be overjoyed at the ability to retune the carbs to work with a more free flowing filter that creates less of a pressure differential at the media. Let the thing BREATHE. And we should be doing so in a manner that creates a suitably clean, smooth flow... And should allow for a hardtail or Konged frame, and not look like a big black box.

TT's point is that while the filter media in aftermarket pods is less restrictive....it creates turbulent flow. There is no volume in the pods to allow the airflow to smooth out. The little stacks in the stock airbox aid in smoothing flow as well. The other point was steps in the aftermarket pods create turbulent flow as well. More air is good if its smooth. If not it can actually inhibit flow and volume.

Reading his post and understanding the principles will help you understand why theres a Big Black plastic box.

[Cormac]

I have read TT's very fine posts, and do understand why there is a big plastic box. My point is, that while the stock filter assembly is very well thought out, and very well engineered, it is not the ultimate solution.
Paper filters are still used the world over primarily due to cost considerations. There are a multitude of other filter designs that perform far better (including less pressure differential across the filter media) in both filtration and flow, excluding oiled cotton filters. Even though I use K&N clones on most everything, except my Jeeps, they got real K&N's... due to shape constraints, otherwise I would have gone cheap-o.
Ultimately, what I was saying is that there IS a better way to do it. Pods, unfortunately are not it. Though they can be made to work, and look far better. I DO understand the limitations of the pod design, I DO understand turbulence and flow. I have an extensive library dedicated to all things gearhead oriented. Including several books on carburetors, one for every type I own (excluding these...) and a very good general one called Carburetors, and another dedicated to intake design. Do not talk down to those you do not know, unless they state something that is factually untrue. Speaking of which, whether or not there is a honking big filter, if there is a pressure differential (which TT has time and again stated is necessary for proper tuning) created across the filter media then the filter is not flowing as much as the engine can ingest. If the flow rate of the filter was sufficient, then there would be no vaccum created. Granted, all filters create a impedance to flow. The trick is to find the one that creates the least impedance. And yes an aftermarket filter in the stock assembly does fit the bill. However, that still leaves the issues of size, location, and style to be addressed.

Cheers.

[HondaMan]

I'll bet those of you who have read the Carburetors chapter in my book are having a good time watching this discussion, too. 

Hmmm...  Looking suspiciously like a shameless plug there.   

Actually, I'm trying to support your part of the topic. Especially the part about the velocity stacks and laminar flow discussions. I drew pictures of it all in the book, hence my comment: owners can look it up while they read it here.

One of the misperceptions (that you're helping to clear up, here) that is so common is the part where "the fuel is sucked up into the vacuum in the throat...". Well, it can look that way, but that's not how they work, except during choke mode where the air can only enter the throat through the emulsifier tube on the mainjet, so it drags a lot of fuel in with it by accident. This choke mixture is never metered, but just one step away from the "ticklers" of the ancient Amal -type carbs.

It's exactly the laminar flow, meshed with the height that the needle jet is sitting above the edges of the throat, that keep it from letting fuel lift into the throat until about 1/8 throttle or so. At that point, the laminar layer thins out enough that the moving, lower pressure, air touches the top of the jet and the fuel then has enough lift from the bowl to make it up into the throat.

(You'd be surprised at how many e-mails I get each week that ask "Is the book ready yet?"). 

[Simpson]

So what I gather is Pods create turbulence/eddies and TT's threads can also create turbulence/eddies.
I will not be replacing my stock air box with a TT.
 

[MRieck]

So what I gather is Pods create turbulence/eddies and TT's threads can also create turbulence/eddies.
I will not be replacing my stock air box with a TT.
 

That's funny. K&N's...not "pods"...I hate that term...do have use. I have presented factory "stacks" etc to change length. Aftermarket  carbs also incorporate adapters from the intake side which add length. I haven't seen much when it comes to removing the box except there are some conditions where the stock box will not "work". Personally I don't care but experience has lead me to different solutions. That is primarily driven by the fact I don't do anything stock. I find stock pretty boring BUT THAT IS ME. A mailbox works great stock....that's about it for me. Not wanting to bust B's but..well...I guess I am.