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[mikalias]

I recently changed my exhaust system on my 2014 cvo road king from the stock system to the Harley Davidson version of the super trapp.  These are not slip ons but the complete system.  I have been told by a lot of people that they system will cause the mixture to run lean because of the increased flow and that it will damage my valves over time.  I fact one guy at a HD independent repair shop told me not to ride the bike but trailer it down to a shop with a dyno and have a turner installed.  My problem is I don't understand how better flow in an exhaust system can affect the intake system as when the intake valve closes and then the exhaust valve opens the fuel metering is totaly isolated from the exhaust stroke and should not be effected by the exhaust system. I can understand how this can happen in a two cycle engine but not a 4 cycle.  Am I wrong about this?

Mik

[grc]

Actually exhaust flow can and will affect the fuel mixture, but not to the extent those people you talked to tried to make you believe.  In the old days before closed loop engine management systems, any change that could increase air flow through the engine would adversely affect the air fuel ratio.  These days you have a closed loop engine management system with oxygen sensors in the exhaust, and the engine management system uses the sensor data to automatically adjust the fuel setting to maintain the prescribed ratio.  A simple exhaust change is unlikely to cause a big enough change in air flow to exceed the capability of the system to self adjust.  So the recommendation to avoid riding and just trailer the bike to a dyno to have it tuned is way out in left field, and I'm guessing the recommendation came from some old geezer who still hasn't heard about modern engine management systems.  And just in case you think I'm picking on old geezers, I am one.

Now, to further confuse the issue I will say that I still recommend having the bike tuned by a fully qualified professional tuner.  The tune that comes stock was designed first and foremost to meet strict emission standards, and not necessarily to make the engine run as smoothly and reliably as it can and should.  A good tune is well worth the cost even on a totally stock Harley.  The trick is in finding the fully qualified and ethical pro you need to do it right.  Don't assume that any place with a dyno actually has fully qualified people on staff.  We have some very good pro's on this site; ask them for recommendations for a good tuner in your area.

Jerry

[mikalias]

Actually exhaust flow can and will affect the fuel mixture, but not to the extent those people you talked to tried to make you believe.  In the old days before closed loop engine management systems, any change that could increase air flow through the engine would adversely affect the air fuel ratio.  These days you have a closed loop engine management system with oxygen sensors in the exhaust, and the engine management system uses the sensor data to automatically adjust the fuel setting to maintain the prescribed ratio.  A simple exhaust change is unlikely to cause a big enough change in air flow to exceed the capability of the system to self adjust.  So the recommendation to avoid riding and just trailer the bike to a dyno to have it tuned is way out in left field, and I'm guessing the recommendation came from some old geezer who still hasn't heard about modern engine management systems.  And just in case you think I'm picking on old geezers, I am one.

Now, to further confuse the issue I will say that I still recommend having the bike tuned by a fully qualified professional tuner.  The tune that comes stock was designed first and foremost to meet strict emission standards, and not necessarily to make the engine run as smoothly and reliably as it can and should.  A good tune is well worth the cost even on a totally stock Harley.  The trick is in finding the fully qualified and ethical pro you need to do it right.  Don't assume that any place with a dyno actually has fully qualified people on staff.  We have some very good pro's on this site; ask them for recommendations for a good tuner in your area.

Jerry

Well, I'm still confused by the fact that when the intake valve is open the exhaust valve is closed and when the exhaust valve is open the intake valve is closed so how could the low back pressure or high back back pressure of an exhaust system have any effect on the fuel mixture.  Unless it is as you say the engine management system that maybe see different oxygen amounts but they have been saying this about exhaust system causing lean mixtures way before engine management come in to being.

Mik

[HDGearHead]

Well, I'm still confused by the fact that when the intake valve is open the exhaust valve is closed and when the exhaust valve is open the intake valve is closed so how could the low back pressure or high back back pressure of an exhaust system have any effect on the fuel mixture.  Unless it is as you say the engine management system that maybe see different oxygen amounts but they have been saying this about exhaust system causing lean mixtures way before engine management come in to being.

Mik

The opening of the intake and closing of the exhaust valves overlap to assist in scavenging of the exhaust gases.  The amount of overlap in degrees is determined by the cam.

Check out the following two youtube videos:

Camshafts (Part 3) - Camshaft duration is explained
Camshafts (Part 4) - Valve overlap.

[timo482]

i really understand the confusion...

there is a LOT of information - and misinformation flowing around all the time.

before changing anything on a engine - full thought and understanding really needs to happen.

first - the bike came with 02 sensors that adjust the fuel input to match what is going through the engine. & can often adjust enough to make it o.k.

second - current valve timing does have overlap - where both valves are open at the same time for a tiny bit of time.

third - changing exhaust or air cleaners or both DOES change the fuel mixture a bit..

the whole important question revolves around "how much" and unfortunately the answers are all murky and grey and unknowable till you have the bike on a dyno with a COMPETENT tuner at the controls figuring out what is going on. that is why changing the exhaust voids your engine warranty and they the EPA folks get all wound.

its your bike - do whatever you want.

just remember - if you break it there are lots of folks who will make sure its your fault - and it just plain sucks

to

[mikalias]

The opening of the intake and closing of the exhaust valves overlap to assist in scavenging of the exhaust gases.  The amount of overlap in degrees is determined by the cam.

Check out the following two youtube videos:

Camshafts (Part 3) - Camshaft duration is explained
Camshafts (Part 4) - Valve overlap.

Well thanks HDGearHead, that clears that mystery up for me but now I have to worry if my bike is in serous need of a tuner.  I'm thinking of buying a Vance & Hines Fuelpak 3 wonder if that will solve the mixture problem?

Mik

[mikalias]

i really understand the confusion...

there is a LOT of information - and misinformation flowing around all the time.

before changing anything on a engine - full thought and understanding really needs to happen.

first - the bike came with 02 sensors that adjust the fuel input to match what is going through the engine. & can often adjust enough to make it o.k.

second - current valve timing does have overlap - where both valves are open at the same time for a tiny bit of time.

third - changing exhaust or air cleaners or both DOES change the fuel mixture a bit..

the whole important question revolves around "how much" and unfortunately the answers are all murky and grey and unknowable till you have the bike on a dyno with a COMPETENT tuner at the controls figuring out what is going on. that is why changing the exhaust voids your engine warranty and they the EPA folks get all wound.

its your bike - do whatever you want.

just remember - if you break it there are lots of folks who will make sure its your fault - and it just plain sucks

to

I bought the exhaust system from the Harley dealer so does that still void my waranty?

here is the link to the system I bought

http://www.harley-davidson.com/store/screamin-eagle-high-flow-exhaust-system-with-nightstick-mufflers

Mik