Why can you run leaner with a performance pipe?

[Colorado KDX]

Can anyone tell me why you can run leaner with an aftermarket
pipe and silencer? compared to stock pipe and silencer. Is
it increased air flow and lowwer eng. temp? or better fuel burn?
or something else? Is this also true for reeds?
Thanks, Colorado KDX

 

[canyncarvr]

Leaner than what?

Most likely leaner than stock setting...just 'cuz stock is so rich.

Is that what you mean? Leaner than stock?

Different pipes jet differently. I run quite a bit richer jet-set when I use a KG-30 (rev) than with a KG-35 (torque).

 

[Colorado KDX]

canyncarvr-so are you saying that if you jet the bike correct
(not stock rich)- you will not have to change the jetting for a
KG-35? what about using FMF Torque Reeds? will this require
rejetting? If so, do you KNOW WHY?

 

[canyncarvr]

Carbs operate due to differences between atmosphere and internal air pressures. Changes made to an engine that change that pressure differential will require jet changes to be made to keep the air/fuel ratio correct.

It is outside atmospheric pressure that causes gas to be pushed through the carb jets into the incoming airstream.

Take the airbox mod for example. Removal of or drilling holes in the cover REDUCES the pressure differential..the engine is trying to pull air against less resistance (less obstruction). All other things being equal, the airbox modification will cause the engine to run lean(er) because there is less pressure differential so less gas is pushed thru the carb.

Put a small straw in your mouth..suck as much air as you can, compare that to sucking on a piece of 1" pvc. Your cheeks don't collapse at all with the larger opening compared to the smaller.

Putting on a pipe effects the engine in a couple of ways. It may flow more air from the standpoint of size and volume ability. But, in a two stroke the pressure waves created in the expanding/collapsing cones have more of an effect on engine operation than the 'amount' of air you can get through the pipe based on it's size.

Rule of thumb is adding boyesen reeds or a radvalve will call for a leaner pilot and main. That would mean that modification is changing the airflow in such a way that the pressure differential INCREASES, so you need to make the gas 'harder' to get.

AHA!!..That's two opposing statements! Increased differential with a stock airbox is bad..but increased differential with addition of reeds is good?

Well...that isn't happening on the same side of the carb, 'eh?

Have you been through CDave's carb theory? Check out:

http://justkdx.dirtrider.net

Click on 'tech tips' and 'carb tuning'.

Basically EVERYTHING/ANYTHING will require rejetting. Temperature, elevation, riding style, induction modification (type of filter, how it's oiled), exhaust modification (type of pipe, S/A or just a silencer with NO arrestor).

Who said this...someone on this forum...'Jet a rider's bike, he can ride for a day. TEACH a rider how to jet.........' well, you get the idea.

 

[Fred T]

I thought that was feed a man a fish he eats for a day but teach him to fish and eats for life. My bike likes perch, salmon causes too much spooge.

 

[Colorado KDX]

canycarvr-thanks for the great clear stated info-I have one more question- A old two-stroke boat racer told me that I can get a better
fuel burn by boring out the center of my main jet. Making sure
that you do not change the jet size just increase the area after the
inlet. He says that this causes the fuel to atomiz better and this was
a trick that a lot of racers used to do. Is there any truth to this helping performance? Thanks Colorado KDX

 

[canyncarvr]

That's assuming a whole bunch of stuff.

In a word, yes. That's part of what dynajet does, and jetting technology in general for years has included different 'flow rates' and efficiencies based on differently shaped entry orifices.

In a word, no. How are you going to determine to what angle/size/depth you are going to modify the orifice for 'better atomization'? By hooking up your benchtop AtomizerMan?

Take your pick!;)

While there are many shadetree mechanic smoke-n-mirrors 'secrets' that DO wonderful (if mysterious) things to an engine's performance, I'd bet there are many MORE of those same types of 'secrets' that are just plain silly....and some of them are detrimental!

You'll be far ahead of 90% of riders if you know how to jet your bike using jets that are over-the-counter available.

Fred: Yeah. Salmon is too fat.....too rich? ;)

 

[Rich Rohrich]

Originally posted by Colorado KDX
A old two-stroke boat racer told me that I can get a better
fuel burn by boring out the center of my main jet. Making sure
that you do not change the jet size just increase the area after the
inlet. He says that this causes the fuel to atomiz better

There is ZERO truth to that old wives tale (it's been around for AGES). The point where the fuel is sheared by the moving air stream is no where near the mainjet. The inlet and outlet shape of a well made main jet is fairly precise. Boogering it up with a drill bit will only serve to disrupt flow and in many cases limit the bulk flow potential of the jet. In other words it leans the main, which accounts for the claims people often associate with this "hot trick" :silly: Guys who find an "advantage" with these tricks tend to find the same reults by jetting leaner. Plastic mainjets had a similar "voodoo" following in the 70s and 80s. Poor consistency between jets was the root cause of the claims associated with them as well.

The most common reason you can jet leaner after a pipe or porting change has more to do with changes in the scavenging efficiency of the engine than anything else. Air Fuel ratio is nothing but a function of what is happening in the combustion chamber. How much fuel needs to enter the chamber to ensure all the available oxygen has a fuel molecule partner to team up with. When an engine scavenges poorly at a certain load or rpm there is a larger portion of residual exhaust gas trapped in the chamber. As the percentage of trapped waste exhaust increases it makes it more difficult for the the oxygen molecules to find a fuel partner, so the standard fix is to add MORE fuel by jetting richer thereby increasing the chances that the oxygen will have a partner and there will be a reasonably good chance of reacting the fuel. As you limit the amount of residual exhaust that is trapped this "extra" fuel is no longer required and the jetting can be leaned in the same range. Combustion temps and horsepower usually go up while EGT goes down as a result of faster more complete combustion. The tricky part with two-strokes is a pipe may scavenge better at one rpm/load range causing you to lean the jetting while another load /rpm range may have the opposite effect because the pipe is longer tuned for that range and doesn't scavenge as well. As the pipe tuning becomes more rpm specific and power output increases with the usual powerband narrowing this dual effect on the fuel curve tends to become more pronounced.

Reeds tend to have more of a carb/pressure differential specific effect on the carburation then pipes or porting.

 

[Colorado KDX]

Thanks for all the Great Info!
Colorado KDX

 

[3KDXXR2]

I thought the saying goes like this: Give a man a fish and he eats for a day, Teach a man to fish and he wil sit in a boat all day and drink beer.

 

[canyncarvr]

Here come 'd flak!

You can (could) order 'different' jets for the normally aspirated 2.2 mitsu motor (in things like daytonas). The inside diameters were the same, the ENTRY to that diameter would vary in size and shape and finish.

But, these 2.2 screamers being able to eat mustang 5.0s and camaros for lunch, I'm sure they weren't perFORMance oriented ;)

cheers.....and, yes...I'll have another beer, too!