Topic: Pods Thread

[paulages]

maybe he just stepped in it? 

[mlinder]

I think, put simply, the speed with which exhaust can exit the cylinder changes the mixture of the incoming air and fuel, due to the amount of unburnt air and fuel that may remain in the cylinder after the exhaust valve closes.

[754]

When I took my mufflers off my Beemer to race it ran about the same.. very vert quiet pipes. But I bet if I richened it up a bit it would havr ran better as it was probably running lean with open pipes.. The amazing thing was I picked up 3 tenths in the 1/8.. that would be between  4 and 5 tenths in the 1/4..by pulling out the air filter.

Learn to read your plugs and jet accordingly, asking on here will only ster you near where you got to go, plug check should  determine final outcome..

[edbikerii]

Let's not forget that we're talking about a SYSTEM here.  If there are any bottlenecks in the system, the entire system will flow less, and thus require less fuel.  This includes the intake, intake boots, carb throats (and butterflies), intake manifolds, ports, valves and exhaust.

Changing any one part of the system MAY not make any difference at all, unless that part was the bottleneck.

[paulages]

as an example, i run stock carbs and airbox with the stock 4-1 (cb550F). i fabricated a freer flowing muffler, and the low end was undoubtedly impacted. i did get a higher top speed at WOT though.

[tsp37]

I can buy the theory that a restrictive exhaust increases the residual cumbustion gases in the cylinder, which can improve combustion characteristics. The residual gases contribute neither oxygen nor fuel to the mixture, but actually serve as dead mass to lower the peak temperature of combustion.  That actually lowers the available energy (and thus lowers the thermal efficiency) and reduces the NOx and SOx that form at higher temperatures - emissions. Emissions were the driver for EGR valves on cars.

The residual exhaust gases slow the combustion rate, which allows the engine to be designed with higher compression.  Drive your car with the EGR valve disconnected and notice the increase in pinging.  Notice the "check engine" light on your dash.  The hot residual gases help with fuel vaporization for more complete combustion (fewer HC & CO).  I have to dig out an old text book to verify these last two outrageous claims.

Having said that, a restrictive exhaust increases the pumping load to the engine, particularly at high load.  Residual gas will also decrease volumetric efficiency.  I can understand the logic behind tuning the intake and increasing volumetric efficiency, but not exhaust tuning.  If a more restrictive exhaust is better, why would bigger or more exhaust valves be of any benefit?

[edbikerii]

EGR is different than just a more restrictive exhaust.  EGR only comes on at part-throttle openings, because at WOT, EGR would reduce the clean air/fuel mixture in the charge, and thus reduce horsepower.

[nteek754]

Hey all, my pods  suck  yeah they SUCK GOOD AIR and make it run great have fun Craig in Maine

[TwoTired]

The residual gases contribute neither oxygen nor fuel to the mixture,

I don't believe this is true.  It could be, though, if the combustion was truly stoichiometric.  However, as I said before, our SOHC4s do not have a closed loop sensing system to adjust fuel mix based on exhaust gas analysis.  There is no ongoing compensation for air density, temperature, or humidity.  Therefore, these engines routinely run on the rich side of Stoichiometric.  Except, perhaps, on a hot day near sea level with zero humidity.  This normally guarantees there WILL be unburned hydrocarbons in the exhaust gas which are, in fact, combustible.  Retaining these gases for re-combustion with a restrictive exhaust WILL make the fuel efficiency increase, but, not necessarily the power output.  Further, if you plan to reuse these unburnt hydrocarbons, you can lean the carb mix to get closer to that stoichiometric combustion goal, while still remaining on the rich side of it.

Emission laws were the driver for lambda sensors placed in the exhaust flow, and computer controlled fuel metering, to maintain a stable mixture near stoichiometry.  You have to be very careful comparing the routine operation of ANY machine that has a "check engine" light with the SOHC4 or any engine prior to the 70's, as closed loop engine operation was simply not common on these engines.

Changing exhaust restriction on a lambda equipped engine, likely has far less effect on general engine operation due the automatic compensation that the computer provides.  It does NOT mean mixture settings are not effected.  It just means that these adjustments are hidden from you.  Applying exhaust changes to a machine where the computer sits above a seat on the machine, means the hands controlled by the computer need to adjust the mixtures of the carburetor to compensate for the exhaust changes.

Cheers,

[tsp37]

Prior to concerns with emissions, the carburetors were set to run a little rich for performance.  A little rich is good, a little lean is terrible.  That's why we like to increase the jet sizes on EPA friendly engines.

If the fresh charge is fuel rich, the exhaust will be fuel rich.  If the fresh charge is lean, the exhaust will have oxygen.  Mixing the exhaust with the fresh charge will only aggravate the inlet condition.  However, the effect is very slight.  But you cannot lean the intake to take advantage of rich exhaust.

My reference to the check engine light was an attempt at comic relief. 'Tis mad, 'tis true.  Assume for the sake of simplicity that all engines are open-loop.

EGR systems do not restrict the exhaust flow, although some require backpressure for proper operation.  My Chevy P/U is one such beast.

Which takes me make to the original issue.  Exhaust restrictions have no bearing on the A/F mixture on a four stroke engine.

[TwoTired]

Prior to concerns with emissions, the carburetors were set to run a little rich for performance.  A little rich is good, a little lean is terrible. 

The carbs runs rich because Air density, temperature, and humidity are variable.  And, you must not let the engine go lean when these factors stack up to create a lean condition...ever. The SOHC4 does not have any means to adapt to different oxygen content ratios in the inlet air. Therefore, for most common operations, the engines run on the rich side of stoichiometry.  Not so much because of desire, but because of necessity.

If the fresh charge is fuel rich, the exhaust will be fuel rich.  If the fresh charge is lean, the exhaust will have oxygen.  Mixing the exhaust with the fresh charge will only aggravate the inlet condition.  However, the effect is very slight.  But you cannot lean the intake to take advantage of rich exhaust.

Says who?  Honda did exactly that with the 77-78 model CB550K's, for example. The 4 into 4 outlet diameters were reduced in size which elevated chamber pressures and reduced cylinder scavenging.  The engine, cam, head, valves, intake runners, carb venturi diameter, and air filter all remained the same.  These bikes get the best economy of all the CB550K models,  have the smallest jet sizes, and also produce slightly less power than the previous K models with the stock 4 into 4.  They also had the lowest emission numbers.  Seems a perfect existence proof.  What analysis data am I missing?

EGR systems do not restrict the exhaust flow, although some require back pressure for proper operation.  My Chevy P/U is one such beast.

I venture to guess your Chevy P/U does not rev as high as the SOHC4 and the air velocities through the cylinder are not comparable.  Lots of chevy P/Us have an Ox sensor and a closed loop fuel metering system.  Further, the SOHC4 does not have an EGR system or an Ox sensor closed loop fuel metering system, last I checked. 
In stock form, the SOHC4 exhaust system does have back pressure.  If you simply change the exhaust with one that has similar back pressure/ flow characteristics, the jetting required remains the same. If you change the exhaust for a different back pressure/flow characteristics, the jetting will need to be adjusted in order to have the same rich cushion that ensures the engine will never find air conditions that cause it to go lean.

Exhaust restrictions have no bearing on the A/F mixture on a four stroke engine.

Sorry, but I still see no justification, data, or reasonable rationale for that statement, and must reject on that basis.

[paulages]

TT has provided a pretty great theoretical analysis. now, i'll reiterate mine: i've witnessed dramatic differences in performance with nothing other than a change in exhaust at the muffler. i'm not saying that it can't be tuned out, but the change is dramatic and obvious and to say that it's horse#$%* is just foolish.

[Ibsen]

There is only one accurate way to tell who's right or wrong. Take a Dyno test with a stock exhaust, air filter and jetting, and then with any mods you might have done to the exhaust and air filter but with the same stock jetting. The Dyno test will tell you exactly what's going on and what you have gained, or lost, and in which rpm area:

[nteek754]

There is only one accurate way to tell who's right or wrong. Take a Dyno test with a stock exhaust, air filter and jetting, and then with any mods you might have done to the exhaust and air filter but with the same stock jetting. The Dyno test will tell you exactly what's going on and what you have gained, or lost, and in which rpm area:




 Well this may be true for getting exact numbers BUT I dont need a dyno to tell me I have more power or less power  with my currant setup pods (proper jetting) , KERKER. And as far as that goes with just putting on the  4 into 1 kerker and stock airbox and no jetting and no baffle just cone piece wow! night and day hands down, and better gas milage,  hmm go figure  have fun Craig in Maine

[Ibsen]

I don't disagree with you QTRHOSS7. Experienced riders and those who have experience with tuning carbs have a feeling for what's going on. And I'm sure you and many others have got what it takes to get it right.
But since there seems to be a lot of theory being discussed it might be better to be able to refer to a test diagram. On the diagram I posted the Dyno test showed that the main jets needed to be enlarged a couple of sizes to get the full effect of an air filter modification which increased the air flow. The rpm area where the larger jets was needed the most was in the +/-5000rpm area, just in the transition area between the needle jets and the main jets. And I got the impression this is what the discussion was all about .
The benefit with the Dyno test is that it measures the air/fuel mixture and the CO2 and it can tell you in a pretty accurate way if the problem are related to the idle mixture, the mid range mixture controlled by the jet needles and the needle jets, or the WOT mixture that is controlled by the main jets.

Btw, here's some jetting rules of thumbs:

http://www.motorcyclecarbs.com/Jetting_Rules_of_Thumb_W43C551.cfm

[tsp37]

Let's try again, starting with the my first assertion:

The exhaust system has no effect on the jetting on an SOHC4.  The exhaust side is decoupled from the intake by virtue of the nature of the four-stroke engine.  Furthermore, the mass of the residual exhaust gases is small in comparison to the incoming charge, and the mass of residual oxygen or HC is smaller yet.  The residual exhaust gas has no significant effect on the incoming fuel-to-air ratio.

Lean intake/rich exhaust
In regards to adjusting the intake mixture because of residual fuel or oxygen, think of the intake/cylinder/exhaust system in terms of mass (or atomic) conservation.  The hydrogen and carbon atoms are fuel, and the oxyen atoms are, well, oxidizers.  The same mixture of atoms entering the cylinder must be leaving cylinder must be in the cylinder in a steady state.  You cannot have excess oxygen in the intake and excess fuel in the exhaust.

Lean/Rich Performance
From the reference: "To obtain the maximum power from an engine, one should run near [an equivalence ratio] = 1.6 with a compression ratio and inlet pressure such that imep = klimep."  The equivalence ratio is the F/A ratio divided by the stoichemetric ratio.  An equivalence ratio of unity is stoichemetric, an equivalence ratio greater than unity is rich.  IMEP is the indicated mean effective pressure, and KLIMEP is knock limited indicated mean effective pressure.  In the simplest terms, a moderately rich mixture delivers more power.  That does not mean that a rich mixture provides better efficiency.

Ref: Colin R. Ferguson, Internal Combustion Engines, Applied Thermosciences, 1986.

Honda 550
I don't know why Honda reduced the outlet diameter in the 550 head, but it may have something to do with reducing the Mach no. at the exhaust valve.  Increasing the back pressure to the valve would do that.  If the exhaust gases in the cylinder are cooler than in previous models (because of a lean charge?), the sonic velocity is lowered and the Mach number increases at the exhaust valve.  It may also be to retain residual exhaust gases, providing the benefits that an EGR system provides.  There is much speculation here on my part; should you choose to challenge me, I would find it easier to insult your mother than to back it with research.

[paulages]

should you choose to challenge me, I would find it easier to insult your mother than to back it with research.

 

you've obviously got plenty of theoretical analysis. i'd simply like you to analyze the dramatic differences i and others have felt with nothing other than a change in the exhaust. this is a real world thing, not a textbook reference. i'm very open minded, and certainly not disagreeing with your analysis, i just think it's funny that you are arguing theory against something that can be felt at the throttle and the rear.

[mlinder]

Paul, I think he's not denying a possible performance change, but he's questioning the idea that a less restrictive exhaust requires jetting changes....
I totally see the login in his argument.
However, I also had to go up from 120's to 125's when I drilled holes in my baffles..... went from fine to lean soon as I drilled them out, with the 120's.
His argument makes perfect sense, but the fact that there was a measurable difference in the AF ratio going from stock hm341's to slightly drilled out hm341's tells a different story.

[paulages]

Paul, I think he's not denying a possible performance change, but he's questioning the idea that a less restrictive exhaust requires jetting changes....
I totally see the login in his argument.
However, I also had to go up from 120's to 125's when I drilled holes in my baffles..... went from fine to lean soon as I drilled them out, with the 120's.
His argument makes perfect sense, but the fact that there was a measurable difference in the AF ratio going from stock hm341's to slightly drilled out hm341's tells a different story.

you exemplified my point, mark. a performance change would have to be rectified with jetting unless the exhaust was just completely incompatible with the rest of the system. i guess i assumed that part was implied in my statement.

maybe if you think in terms of extremes: if the headers were closed off to the size of a dime or less on a 4-1, it's hard for nayone not to see how that wouldn't effect exhaust flow. the exhaust has to go somewhere, right? as TT noted, the 550F is only different than the early K models in the exhaust department, yet runs a 2 size smaller main. so if any existing back pressure was tuned in stock (say for emissions, as TT noted), this could be restricting the free flow of gases at high RPM's. i don't necessarily think this point is being debated, but rather what effect the remaining pressure on the exhaust side has on the incoming mixture. this is where the theory hits the road. i can't argue the theory, only that opening the exhaust up has clearly caused many people to run leaner. if that's being disputed, i'd just be curious to hear an explanation.

[MRieck]

Let's try again, starting with the my first assertion:

The exhaust system has no effect on the jetting on an SOHC4.  The exhaust side is decoupled from the intake by virtue of the nature of the four-stroke engine.  Furthermore, the mass of the residual exhaust gases is small in comparison to the incoming charge, and the mass of residual oxygen or HC is smaller yet.  The residual exhaust gas has no significant effect on the incoming fuel-to-air ratio.

What about reversion? Especially at high RPM's. Add a cam with more duration and it is even more pronounced. Lobe seperation also comes into play with single cam engines as it directly effects overlap.

[edbikerii]

I think I'm starting to understand tsp's question.  It is more complex than it seems.  I think the answer can be explained this way:

Air is compressible, but liquid fuel is not.  The greater vacuum generated by freer exhaust can pull more air through the carburetor bores, but not more liquid fuel through the fixed-size orifices in the jets.

In other words, more air getting pulled through the engine (higher vacuum) at WOT due to free-flowing exhaust means that more fuel must be mixed with that air, but the jets can only flow so much incompressible liquid fuel per second no matter what the vacuum is, therefore larger jets are required.

What do you think TSP?  Am I understanding your question properly?  Does this sound like a plausible answer to you?

[edbikerii]

Just in case it is not clear why the vacuum is higher when the exhaust is free-flowing, let me try to visualize the behavior with a more restrictive exhaust:

When there is backpressure, the pressure in the cylinder remains high throughout the exhaust stroke, and through the exhaust valve closing.  When the intake valve opens on the intake stroke, there is less vacuum available to pull in fresh air/fuel mix because there is still a higher pressure of burnt exhaust gases remaining in the cylinder.

[tsp37]

When there is backpressure, the pressure in the cylinder remains high throughout the exhaust stroke, and through the exhaust valve closing.  When the intake valve opens on the intake stroke, there is less vacuum available to pull in fresh air/fuel mix because there is still a higher pressure of burnt exhaust gases remaining in the cylinder.

Qualitatively, this is true.  However, I don't believe there is really enough residual to effect the inlet vacuum significantly.

On a car, WOT actually decreases the vacuum.  You have the highest vacuum at idle with the throttle plate nearly closed.  An open throttle allows the highest pressure at the intake valve.  The slider in a motorcycle carb changes the flow in a way I don't understand.

What about reversion? Especially at high RPM's. Add a cam with more duration and it is even more pronounced. Lobe separation also comes into play with single cam engines as it directly effects overlap.

I assume by inversion that you mean the fresh charge or exhaust going the wrong way for a short time because of valve overlap.  Now you are getting into inlet tuning which is a little mysterious to me.  The "wrong way" gases are still very small compared to the "right way" gases.  Inlet tuning and overlap tend to narrow the power band of an engine - you get lots of torque within a narrow RPM range.  The fact that our CB's are very happy at high RPM - the higher the better - can probably be explained by the inlet geometry and valve timing.  The ideal RPM is just a little faster than the real engine can manage.  I understand the for the 90's Nighthawk, Honda sacrificed the high end performance to broaden the power band.

I don't simply ignore as fools those who claim that rejetting was necessary after the installation of a free flowing exhaust, but I would only trust a dyno test as Ibsen suggested.  Had I been absolutely certain of myself, I would have never posed the question.

I have heard tales of guys who put a free flowing LOUD exhaust on their two-stroke bikes and swore that they had more power.  In fact, they were blowing more of of the fresh charge straight through the cylinder and a dyno test would have shown that they were very wrong.  There was also the smart guy who said he "weaned" his two-stroke engine off of two-stroke oil by adding a little less oil to each successive tank of gasoline . . .

[edbikerii]

TSP, this is a great read:  http://www.amazon.com/Scientific-Exhaust-Systems-Engineering-Performance/dp/0837603099

When there is backpressure, the pressure in the cylinder remains high throughout the exhaust stroke, and through the exhaust valve closing.  When the intake valve opens on the intake stroke, there is less vacuum available to pull in fresh air/fuel mix because there is still a higher pressure of burnt exhaust gases remaining in the cylinder.

Qualitatively, this is true.  However, I don't believe there is really enough residual to effect the inlet vacuum significantly.

On a car, WOT actually decreases the vacuum.  You have the highest vacuum at idle with the throttle plate nearly closed.  An open throttle allows the highest pressure at the intake valve.  The slider in a motorcycle carb changes the flow in a way I don't understand.

What about reversion? Especially at high RPM's. Add a cam with more duration and it is even more pronounced. Lobe separation also comes into play with single cam engines as it directly effects overlap.

I assume by inversion that you mean the fresh charge or exhaust going the wrong way for a short time because of valve overlap.  Now you are getting into inlet tuning which is a little mysterious to me.  The "wrong way" gases are still very small compared to the "right way" gases.  Inlet tuning and overlap tend to narrow the power band of an engine - you get lots of torque within a narrow RPM range.  The fact that our CB's are very happy at high RPM - the higher the better - can probably be explained by the inlet geometry and valve timing.  The ideal RPM is just a little faster than the real engine can manage.  I understand the for the 90's Nighthawk, Honda sacrificed the high end performance to broaden the power band.

I don't simply ignore as fools those who claim that rejetting was necessary after the installation of a free flowing exhaust, but I would only trust a dyno test as Ibsen suggested.  Had I been absolutely certain of myself, I would have never posed the question.

I have heard tales of guys who put a free flowing LOUD exhaust on their two-stroke bikes and swore that they had more power.  In fact, they were blowing more of of the fresh charge straight through the cylinder and a dyno test would have shown that they were very wrong.  There was also the smart guy who said he "weaned" his two-stroke engine off of two-stroke oil by adding a little less oil to each successive tank of gasoline . . .

[MRieck]

What about reversion? Especially at high RPM's. Add a cam with more duration and it is even more pronounced. Lobe separation also comes into play with single cam engines as it directly effects overlap.

I assume by inversion that you mean the fresh charge or exhaust going the wrong way for a short time because of valve overlap.  Now you are getting into inlet tuning which is a little mysterious to me.  The "wrong way" gases are still very small compared to the "right way" gases.  Inlet tuning and overlap tend to narrow the power band of an engine - you get lots of torque within a narrow RPM range.  The fact that our CB's are very happy at high RPM - the higher the better - can probably be explained by the inlet geometry and valve timing.  The ideal RPM is just a little faster than the real engine can manage.  I understand the for the 90's Nighthawk, Honda sacrificed the high end performance to broaden the power band.

I don't simply ignore as fools those who claim that rejetting was necessary after the installation of a free flowing exhaust, but I would only trust a dyno test as Ibsen suggested.  Had I been absolutely certain of myself, I would have never posed the question.

I have heard tales of guys who put a free flowing LOUD exhaust on their two-stroke bikes and swore that they had more power.  In fact, they were blowing more of of the fresh charge straight through the cylinder and a dyno test would have shown that they were very wrong.  There was also the smart guy who said he "weaned" his two-stroke engine off of two-stroke oil by adding a little less oil to each successive tank of gasoline . . .

When you get an engine spinning at 9,000, 10,000, 12,000 to 20,000RPM charge is going in both directions with any type of cam. It seems to me you are looking at internal combustion as a static event which is incorrect. It is a multi dynamic process with fluid events. You also don't take into account free flowing systems with small, tapered head pipes etc which effect effect flow but that is fair as these are old bikes. I think you also underestimate bulk volume change and it's profound effect on both the the intake and exhaust at low lifts BEFORE the actual pushing or pulling of charge. It is interesting how fuel has to be pulled from engines secondary to installation of a good system on a a FI bike. How come a good pipe on a busa will make 8 HP with no other changes except a fuel map? On a ZX12....up to12HP?