SuzukiSavage.com - Splain this to me!

Dave

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Splain this to me!
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?

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Serowbot YaBB Moderator ModSquad Offline OK.... so what's the speed of dark? Posts: 28644 Tucson Az Gender:	Re: Splain this to me! Reply #1 - 05/09/12 at 10:19:57 Uhhhhh?... ... ... ... ... ... ...
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Paraquat Serious Thumper Offline SuzukiSavage.com Rocks! Posts: 2206	Re: Splain this to me! Reply #2 - 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
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justin_o_guy2 Serious Thumper Offline What happened? Posts: 55279 East Texas, 1/2 dallas/la.	Re: Splain this to me! Reply #3 - 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?
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verslagen1

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Re: Splain this to me!
Reply #4 - 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.

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greenmonster Senior Member Offline Hellooooo Nurse! Posts: 420 Vancouver BC Gender:	Re: Splain this to me! Reply #5 - 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?
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Boule’tard

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Re: Splain this to me!
Reply #6 - 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.

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Serowbot YaBB Moderator ModSquad Offline OK.... so what's the speed of dark? Posts: 28644 Tucson Az Gender:	Re: Splain this to me! Reply #7 - 05/09/12 at 14:03:28 There see?... Like Boule'tard said,... it's the Rigatoni effect... ...
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greenmonster Senior Member Offline Hellooooo Nurse! Posts: 420 Vancouver BC Gender:	Re: Splain this to me! Reply #8 - 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
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Re: Splain this to me!
Reply #9 - 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.

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thumperclone Serious Thumper Alliance Member Offline PGR rider riding with respect Posts: 6056 Grand Junction Colorado Gender:	Re: Splain this to me! Reply #10 - 05/09/12 at 14:24:25 you answered your own question "more air out" = more air in = lower fuel to air ratio
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Re: Splain this to me!
Reply #11 - 05/09/12 at 14:45:13

greenmonster wrote 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?

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.

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Dave YaBB Moderator ModSquad Offline SuzukiSavage.com Rocks! Posts: 18097 Camp Springs, Kentucky Gender:	Re: Splain this to me! Reply #12 - 05/09/12 at 15:08:58 greenmonster wrote 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 I am old enough to remember the Funicello affect! [img][/img]
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Dave

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Re: Splain this to me!
Reply #13 - 05/09/12 at 16:37:27

thumperclone wrote on 05/09/12 at 14:24:25:

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?

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thumperclone Serious Thumper Alliance Member Offline PGR rider riding with respect Posts: 6056 Grand Junction Colorado Gender:	Re: Splain this to me! Reply #14 - 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
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