Exhast pipe changes and fuel management

Hydroboy35th said:

In short, yes.., you will need some sort of fuel management!

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Let's see...air and fuel enter the engine through the intake and the mixture of air and fuel is determined by everything in the intake...the airbox assembly, filter media, etc. After the mix enters the engine and is burned it is expelled through the exhaust. All said, yes altering the exhaust significantly affects how the engine performs (for example try totally plugging up the exhaust) but altering the exhaust has little, if ANY effect on whether or not the mix is lean or rich. Whatever the mix already is (lean or rich) can be magnified by altering the exhaust (because a less restrictive exhaust lets the engine "pump" more efficiently and, for example, existing lean conditions are more noticeable) but the actual mix itself stays pretty much the same because everything "mix wise" happens before the exhaust. Most of the time, exhaust changes don't require jetting (carbs) or re-mapping (FI) but when one starts altering the air intake, jetting or re-mapping is almost always required. That's not to say that if you change exhausts AND still fatten her up a little that you won't have much better performance (and usually less mpg). You just won't be near as "green".

No offense to anybody, but there is some flawed logic and theory being disseminated here.

Lets simplify and assume that the intake can flow 75 air units, the engine can flow 100 air units, and the exhaust can flow 75 air units. The engine in this configuration will only ever see 75 air units, because that's the max that either the front end, or rear end of the air path can flow. So, if I add an exhaust that can flow 100 air units, the engine will still only see 75 air units, because the intake is still the choke point. If I leave the stock exhaust, and add an intake that flows 100 air units, the engine still only sees 75 air units, because the the exhaust is the choke point. Now if I add an exhaust and an intake that both flow 100 air units, the engine now sees 100 air units. In this example we assume absoltues. In reality, this model is imperfect because changes at either end affect airflow velocity, which also affect airflow volume. The problem with our engine is that a fuel injector only flows a fixed amount of fuel. So if you make a change to the front, or rear end of the air flow path that affect the total volume of air moving through the engine, YOU NEED TO CHANGE THE AMOUNT OF FUEL FLOWING THROUGH IT TOO!

Now before you get all excited and refer back to S&S's findings, I'm not saying emphatically that changing the exhaust and keeping the stock exhaust, or vice versa, will burn up your engine, I'm just saying that IF YOU MAKE CHANGES TO YOUR ENGINE YOU SHOULD KNOW THE EFFECTS! Otherwise, you do so at the risk of your engine.

If you still think I'm full of $%^@, do some research on carburetion theory.

Good luck with your bike!

cycho said:

So if you make a change to the front, or rear end of the air flow path that affect the total volume of air moving through the engine, YOU NEED TO CHANGE THE AMOUNT OF FUEL FLOWING THROUGH IT TOO! ...do some research on carburetion theory.

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I don't see any mention of the Manifold Absolute Pressure (MAP) sensor here. As I understand EFI theory increased air flow results in a change in the MAP and a proportionately greater ON time for the fuel injector; i.e it does "change the amount of fuel".

I would be researching EFI theory although carburetion theory is very helpful in understanding Electronic Fuel Injection since EFI is designed to mimic every good aspect of the technology that it replaces.

Unfortunately no amount of theory can replace the accurate measurement that Honda used to create the EFI map with the stock components. The best solution for a modified exhaust and/or intake is to take dynamic measurements (at all engine loads and speeds) of the oxygen content in the exhaust. You can mount your own air/fuel ratio gauge and install an oxygen sensor in your exhaust or pay for a dyno run. The gauge allows you to first see if you need to remap and you can recheck if changes are made over time. If a fuel controller is needed an adjustable one would allow you to make the corrections you specifically require.

Worst case scenario without analysis would eventually be burnt (leaking) exhaust valves and loss of power.

cycho said:

No offense to anybody, but there is some flawed logic and theory being disseminated here.

Lets simplify and assume that the intake can flow 75 air units, the engine can flow 100 air units, and the exhaust can flow 75 air units. The engine in this configuration will only ever see 75 air units, because that's the max that either the front end, or rear end of the air path can flow. So, if I add an exhaust that can flow 100 air units, the engine will still only see 75 air units, because the intake is still the choke point. If I leave the stock exhaust, and add an intake that flows 100 air units, the engine still only sees 75 air units, because the the exhaust is the choke point. Now if I add an exhaust and an intake that both flow 100 air units, the engine now sees 100 air units. In this example we assume absoltues. In reality, this model is imperfect because changes at either end affect airflow velocity, which also affect airflow volume. The problem with our engine is that a fuel injector only flows a fixed amount of fuel. So if you make a change to the front, or rear end of the air flow path that affect the total volume of air moving through the engine, YOU NEED TO CHANGE THE AMOUNT OF FUEL FLOWING THROUGH IT TOO!

Now before you get all excited and refer back to S&S's findings, I'm not saying emphatically that changing the exhaust and keeping the stock exhaust, or vice versa, will burn up your engine, I'm just saying that IF YOU MAKE CHANGES TO YOUR ENGINE YOU SHOULD KNOW THE EFFECTS! Otherwise, you do so at the risk of your engine.

If you still think I'm full of $%^@, do some research on carburetion theory.

Good luck with your bike!

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Quick!!! You better spread this info to all of the major OEMs specifically HD that offer "performance" slip-ons and exhaust systems and use as a sales tool "no rejetting or re-mapping required". Just teasin'! Now specifically to your point...we need both fuel and air - the "mix". The amount of air that gets mixed with the fuel is way more dependant on it's path (and the restrictions therein) to the cylinder head than the restrictions on its way out. And it goes without saying you should NEVER make an exhaust change without closely following up to see if all is still good regarding the mix. Likewise, you should NEVER just automatically fatten things up without, again, checking to see that all is good with the mix. I didn't respond (at least I didn't mean to) in absolutes because there are so many variables out there in types and styles of systems, variations in manufacturer specs, and so on. What I will claim as MY absolute is that in MY experience, simply installing better performing & better sounding mufflers or exhaust systems on motorcyles and doing NOTHING to the OEM intake has never caused a lean condition. Never.

garc said:

I don't see any mention of the Manifold Absolute Pressure (MAP) sensor here. As I understand EFI theory increased air flow results in a change in the MAP and a proportionately greater ON time for the fuel injector; i.e it does "change the amount of fuel".

I would be researching EFI theory although carburetion theory is very helpful in understanding Electronic Fuel Injection since EFI is designed to mimic every good aspect of the technology that it replaces.

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Never heard that one before. I'd think you'd need a forced induction system to increase the preassure in the manifold to the point that the ecu would adjust it. In any case, the amount of adjustment the ecu can make for this sensor (which as you probably know is intended to compensate for changes in altitude) is not enough to compensate for intake/engine/exhaust changes, otherwise DynoJet et al would be out of business.

I realize that this bike is fuel injected, but like you said, the principals are the same, and there is a lot more out there on carburetion.

OJGP said:

Quick!!! You better spread this info to all of the major OEMs specifically HD that offer "performance" slip-ons and exhaust systems and use as a sales tool "no rejetting or re-mapping required". Just teasin'! Now specifically to your point...we need both fuel and air - the "mix". The amount of air that gets mixed with the fuel is way more dependant on it's path (and the restrictions therein) to the cylinder head than the restrictions on its way out. And it goes without saying you should NEVER make an exhaust change without closely following up to see if all is still good regarding the mix. Likewise, you should NEVER just automatically fatten things up without, again, checking to see that all is good with the mix. I didn't respond (at least I didn't mean to) in absolutes because there are so many variables out there in types and styles of systems, variations in manufacturer specs, and so on. What I will claim as MY absolute is that in MY experience, simply installing better performing & better sounding mufflers or exhaust systems on motorcyles and doing NOTHING to the OEM intake has never caused a lean condition. Never.

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Teasing aside, HD and all the other manufaturers offer an IMPLIED WARRANTY when they make these statements, because if the product doesn't do what they say, they are liable. When manufacturers make these claims, you can be fairly certain that they've done extensive testing to ensure their products aren't going to cause problems. Back to my original point, and no disrespect to S&S, but are they offering a warranty to everyone who mods their bike like S&S did? Don't think so. So in this case, and I think we agree here, if you make changes to your bike, you should know and understand their effects.

I still disagree with you on your air path theories. Modifying the engine, and or the exhaust, can change the amount of air flowing through the engine IF they are the restrictive part of the path. More air = leaner mix. Period.

I do agree that in most cases, changing the exhaust and leaving the stock intake will probably not dramatically affect anything mix wise. Stock airboxes are fairly restrictive. However, I still believe that, in the absense of a manufacturers implied warranty, you should know what you are doing to your bike.

tony_fury said:

With regards to the Fuel Management system, I'm stuck with the California emissions fixed fuel management system (ARB) non adjustable. Now CobraUSA recommends the FI2000R for their pipes (ARB) for California models. With that said, I have reservations with another fixed setting. They want $200.00 bucks for the Fuel Management module that cannot be adjusted. I have to see what the effects are by adding the reaper exhaust tips.
I need an address out of state to get the adjustable one...
Thanks for the feedback

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Dude, I think someone at CARB just scared a vendor or somehting here. As far as I know, It's not illegal to sell non CARB compliant devices in our state, it's just illegal to use them on anything besides off road, closed course competition vehicles. As soon as the 49 state models are more widely available, I'm sure you'll be able to get one from one of the bigger vendors. That's my plan anyway. For my Fury race bike, of course