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Message started by Dave on 05/09/12 at 09:28:16

Title: Splain this to me!
Post by Dave on 05/09/12 at 09:28:16

I understand engines pretty well, and I have built and modified several in my 56+ years of existence. I usually end up tinkering with most of the engines I own at some point......and even my chainsaws have gotten muffler mods and some mild porting. I also have re-jetted engines and understand pretty well the function of the carb, the purpose of the different jets and emulsion tubes, the slide and cutaways, the needle and clips.

Since the carb is a "metering device" and regulates the amount of fuel being delivered in response to the air flow through the venturi in the carb......it seems logical to me that when you modify something in the engine that increases air flow through the carb.....the amount of fuel will also increase in proportion to the air flow increase.

Now if we ignore for the purpose of this discussion changes to compression, cams, valves, air cleaner or intake.....and only focus on changes to the muffler.........Why does a free flowing exhaust require the carb jets to be changed to increase the amount of fuel added to the air passing through the venturi? If opening up the exhaust allows more waste gasses to leave the engine and allows a little more air to enter the cylinder - why doesn't the extra air flow through the venturi pull enough fuel from the jets to compensate for the additional air flow through the engine?

Title: Re: Splain this to me!
Post by Serowbot on 05/09/12 at 10:19:57

Uhhhhh?... :-?...
... :-/... :-/... :-/...

:-?...

Title: Re: Splain this to me!
Post by Paraquat on 05/09/12 at 11:17:22

Less restriction.
Can't suck in till you push out.
More out = more in at a faster rate. Back pressure, helmholtz frequency, science.

--Steve

Title: Re: Splain this to me!
Post by justin_o_guy2 on 05/09/12 at 11:20:50

Mite need to spell that out a little more for those of us who dont know what you know. If its too much typing, can you post a link where an explanation would be?

Title: Re: Splain this to me!
Post by verslagen1 on 05/09/12 at 11:39:32

You're making a pretty big assumption... that the "metering device" is perfect and compensates for everything. Unfortunately, you're wrong. Changing the muff can affect the efficiency of the engine by allowing more unburnt gasses getting thru before the exhaust valve shuts. the reflected exhaust pulse has to shove that back in, if it aint there that's stuff's gone. and with only 8.5 compression, it's sorely missed.

Title: Re: Splain this to me!
Post by greenmonster on 05/09/12 at 12:24:13

I hate reading technical posts when I'm sick, stuff that should make sense just doesn't.

Vers, you open up your exhaust and you "allow more unburnt gasses to pass through" does that mean that we should be putting leaner jets into the carb when we put a free flowing exhaust system on?

Title: Re: Splain this to me!
Post by Boule’tard on 05/09/12 at 13:27:54

That's a very good question, which can be simplified by forgetting the stuff about the valves and the needle/needle jet/slide etc. Let's just say you have the main jet and a varying flow of air through the carb throat.

By the Bernoulli effect, the suction on the main jet increases with the square of the air velocity. The flow of gas does not increase by the squared value, it is converted to another velocity, that of the gas through the main jet. At first glance it seems it should stay in proportion.

Empirically we know it doesn't. Is that because of some complication like the position of the needle or slide? The mixture leans out as you uncork.. it does not matter if it's a CV carb or mechanically actuated slide and needle.

I am super rusty on fluid dynamics and have no time to research it. I bet Oldfeller can 'splain it. Nudge nudge.

Title: Re: Splain this to me!
Post by Serowbot on 05/09/12 at 14:03:28

There see?...
Like Boule'tard said,... it's the Rigatoni effect... :-?...

Title: Re: Splain this to me!
Post by greenmonster on 05/09/12 at 14:17:09

Oh why didn't you just say that it's a Fibonacci effect? It's all clear as mud now :-?

Title: Re: Splain this to me!
Post by Boule’tard on 05/09/12 at 14:21:44

A partial answer occurred to me. The suction on the main jet is due to both the Bernoulli effect AND vacuum generated by the restriction of the stock air filter as the engine is pulling air out of the carb throat. Going with a less restrictive air filter removes some of that vacuum, so at a given flow rate, there's less gas going into the stream.

But that only explains the lean-out when you uncork the intake side.. not the additional leaning from a less restrictive muffler.

Title: Re: Splain this to me!
Post by thumperclone on 05/09/12 at 14:24:25

you answered your own question
"more air out" = more air in = lower fuel to air ratio

Title: Re: Splain this to me!
Post by verslagen1 on 05/09/12 at 14:45:13

7D687F7F74777574696E7F681A0 wrote:

No, think 2 stroke reverse cone exhaust.
with a 2 stroke, the exhaust open 1st and most of the exhaust runs down the pipe, then the intake opens sucking it a fresh charge.
And quite a bit of the fresh charge gets into the exhaust, yet both intake and exhaust are still open.
This pressure wave finally hits the reverse cone and reflects back to the piston. This pushes the fresh charge in the exhaust pipe back to the piston. This creates a super charge effect in the compression chamber.

A similar effect happens with the 4 stroke, just not a severe.

So no, leaning out the jets won't stop gas getting into the exhaust. But it will reduce the power produced.

This is why open exhausts don't work as well.

Title: Re: Splain this to me!
Post by Dave on 05/09/12 at 15:08:58

253027272C2F2D2C31362730420 wrote:

Oh why didn't you just say that it's a Fibonacci effect? It's all clear as mud now :-?

I am old enough to remember the Funicello affect!

[img][/img]

Title: Re: Splain this to me!
Post by Dave on 05/09/12 at 16:37:27

4559445C415443525D5E5F54310 wrote:

you answered your own question
"more air out" = more air in = lower fuel to air ratio

You are not considering the fact that more air moving through the venturi will cause a greater vacuum and pull more fuel with the additional air. When you are running along in your bike and holding the throttle steady.....and crest a hill and the bike speeds up from 50 mph to 55 mph....the additional air that the 5 mph increase caused is matched by the additonal fuel the carb provides.....and you don't need to do a jet change. However......if you were to increase the flow of air through the engine by 10% by changing to a less restrictive exhaust.....it is time for a jet change. Why?

Title: Re: Splain this to me!
Post by thumperclone on 05/09/12 at 17:53:02

because of the greater suction/venturi effect the jets cant deliver enough fuel cause now they are too small

Title: Re: Splain this to me!
Post by engineer on 05/09/12 at 18:31:12

Dave has a very good question and when people first sat down and designed carburetors they probably hoped the things would work as Dave has suggested and to some extent they do. Dave is suggesting that a linear relationship exists between fuel flow and air flow, if one doubles, the other doubles etc. But as Boule'tard has stated it doesn't quite work that way.

Air constantly changes in density as it flows which is why modern cars have detectors to measure air mass flow so the computer can determine how much fuel to inject. Air is a compressible fluid and fuel being a liquid is not compressible, they behave differently. If you double the flow through the venturi you will draw in more fuel through the jet but not necessarily at the same increase in rate. The two do not follow a straight line if you were to graph air flow vs. fuel flow. Also in combustion it is the mass of the fuel and mass of the air that count whereas in a carburetor it is velocity and pressure that do the metering and so the carburetor only indirectly meters out the correct mass of fuel for a given mass of air.

Having said all that, Dave is essentially correct and even simple carburetors do a reasonable job, look at the carb on an old "hit and miss" engine. At one time whatever made an engine run was considered good enough. Then people started asking the carb to be efficient under all conditions and it has become more complex trying to do it. That's why the good old carb is almost obsolete and is being replaced by fuel injection.

Title: Re: Splain this to me!
Post by Serowbot on 05/09/12 at 18:37:06

... and I actually got that... !... :)...

Now I can sleep in peace tonight,... thanks... ;)...

Title: Re: Splain this to me!
Post by Oldfeller on 05/09/12 at 22:18:39

3538223B3223362533570 wrote:

That's a very good question, which can be simplified by forgetting the stuff about the valves and the needle/needle jet/slide etc.

I am super rusty on fluid dynamics and have no time to research it. I bet Oldfeller can 'splain it. Nudge nudge.

::)

OK, here's my take on the whole mod it to make more power thing.

We tell them to rejet.

We tell them to put on free flowing muffler.

We tell them to put on a free flowing air filter.

You guys are getting tied up in your knickers over the order of things like A causes the need for B or C. False assumption there, you can do them in any order but you will likely wind up doing all of them as they complement each other's improvement effects.

Jetting generally gets tossed to the end of the bus since you fine tune the jetting AFTER you finish muffler and filter mods (or you get to do it two or 3 times as some of us have done)

I wish carbs were that good as to be totally self adjusting -- they aren't. Sorry.

Title: Re: Splain this to me!
Post by bluesrockdoc on 05/10/12 at 05:30:13

GOOD thread, thanks to all.

Title: Re: Splain this to me!
Post by Routy on 05/10/12 at 06:02:08

Good thread indeed,....I think Engineer said it best'

A carb is designed to work exactly like Dave said, but like was said, a carb is not perfect.....like EFI is much closer to being.

This is the main reason I always advise not to rejet, untill there is an obvious reason to do so, and then to do it one step at a time.
W/ some here, there is a bad mind set that x mod = x jet. Its just not that cut and dried. Many times no rejetting is required after a muffler mod,.....and that just mite be because the carb really did do what its designed to do.

Title: Re: Splain this to me!
Post by 360k+ on 05/20/12 at 15:41:58

At the risk of kicking this old hoss again, after reading this thread
yesterday something occurred to me... I have a friend who used to build high perf jet boats. One day I was asking him about the motors he used (usually big Olds 455s, Chevy 454s, etc.). I remember he told me that increasing the HP beyond a certain limit was fruitless because you could only force so much water thru the pump. He explained that was why the big top fuel drag boats are always prop driven. That made me wonder about Dave's premise here and the difference between gaseous (air) and fluid (fuel) dynamics. What I mean is, the ratio of air and fuel wouldn't be constant because air can be compacted many times under force whereas fuel can't, and their two volumetric rates would be different. I dunno, just thinkin...