Jump to content

If I install just the fmf power core 4. Do I have to rejet?

Recommended Posts

Hello everyone, I have a 2004 crf250r and would like to get an fmf power core 4. I would like to know if I would have to rejet? I'm looking to get the powerbomb header in a few months and know I have to rejet them but not sure about with just the slip on. Thank you!!

Share this post


Link to post
Share on other sites

Probably not, depending of course, how well jetted your bike is now. Most slip ons do not add any power, they just make the bike louder (less muffling effect) Stock mufflers flow just as well. When you replace the header, you may have to up the main by one size. as it will flow better.

The hot setup is the power bomb and the stock muffler.

  • Like 1

Share this post


Link to post
Share on other sites

I don't think I would want to buy one you DIDN'T need to rejet.   If optimum jetting for both the existing pipe and the planned purchase are the same,  chances are they act alike.   FYI I own a 2004.  I would save my money for a cylinder and case modification if it hasn't already been done.  

Share this post


Link to post
Share on other sites

I just purchased the powerxore 4 for now because my the person before me had beat the stock muffler up and the packing flys out no matter what I do. I would rather buy fmf rather that stock. But thank you to both of you!!

Share this post


Link to post
Share on other sites
9 hours ago, KTM_rider_EK_17 said:

No, slip on mufflers are developed to avoid having to rejet. You will have to mess with the jetting after a header though.

Uh, no.

If you go from stock intake, AND EXHAUST, to opened up intake and free flowing exhaust, you will be pulling more air to the motor, but not more vacuum across the jets, so yes, you would need to rejet to compensate.

The real question is what state of tune and jetting is your bike in now, to know what jetting changes are needed, if any.

Share this post


Link to post
Share on other sites
2 hours ago, THE KRAN said:

you will be pulling more air to the motor, but not more vacuum across the jets

How?

If the engine is not pulling more vacuum than before, then how is more air going into the engine now?

Share this post


Link to post
Share on other sites
8 hours ago, dirtjumpordie said:

How?

If the engine is not pulling more vacuum than before, then how is more air going into the engine now?

Laminar flow of the air, not vacuum from the motor, is increased.

Share this post


Link to post
Share on other sites
Laminar flow of the air, not vacuum from the motor, is increased.


The flow of air, as in an intake, has nothing to do with the exhaust. The slip on muffler will increase the flow of exhaust fumes. A full exhaust, as in a header and a muffler, will increase the flow of exhaust fumes and effect the performance of the engine but has nothing to do with how the air gets into the engine.
  • Like 1

Share this post


Link to post
Share on other sites
2 hours ago, KTM_rider_EK_17 said:

 


The flow of air, as in an intake, has nothing to do with the exhaust. The slip on muffler will increase the flow of exhaust fumes. A full exhaust, as in a header and a muffler, will increase the flow of exhaust fumes and effect the performance of the engine but has nothing to do with how the air gets into the engine.

 

Your right it doesn't. Thats why it wasn't mentioned.  HOWEVER adding an exhaust will do very little if the intake is restricted, as you volume of air/fuel mixture will be small.

You have to address them both at the same time.  There must be a volumetric synergy of the intake and output of gasses.

 

The header is a tuned pipe. All it does is resonate at a frequency that the engine designers prefers. Aftermarket headers resonate higher or lower, changing the power band.

The muffler is an extension of that tuned pipe. That is why some are short and small diameter, and some are long with large diameter, or any where inbetween.

 

 

 

 

Share this post


Link to post
Share on other sites
Your right it doesn't. Thats why it wasn't mentioned.  HOWEVER adding an exhaust will do very little if the intake is restricted, as you volume of air/fuel mixture will be small.
You have to address them both at the same time.  There must be a volumetric synergy of the intake and output of gasses.
 
The header is a tuned pipe. All it does is resonate at a frequency that the engine designers prefers. Aftermarket headers resonate higher or lower, changing the power band.
The muffler is an extension of that tuned pipe. That is why some are short and small diameter, and some are long with large diameter, or any where inbetween.
 
 
 
 


Smart man. I can stand behind that. I guess I read your original post wrong. Sorry dude.
  • Like 1

Share this post


Link to post
Share on other sites
4 hours ago, THE KRAN said:

Your right it doesn't. Thats why it wasn't mentioned.  HOWEVER adding an exhaust will do very little if the intake is restricted, as you volume of air/fuel mixture will be small.

You have to address them both at the same time.  There must be a volumetric synergy of the intake and output of gasses.

 

The header is a tuned pipe. All it does is resonate at a frequency that the engine designers prefers. Aftermarket headers resonate higher or lower, changing the power band.

The muffler is an extension of that tuned pipe. That is why some are short and small diameter, and some are long with large diameter, or any where inbetween.

 

 

 

 

If the new exhaust is more efficient at facilitating evacuation of exhaust gasses during the exhaust event, then there will be lower pressure in the cylinder upon completion of that exhaust event.

If the new exhaust allows for a lower pressure condition at the end of the exhaust event, that also means the lower pressure condition in the cylinder will be present at the beginning of the intake event.  

Since the cylinder pressure is lower at the start of the intake event, there will be more vacuum during the intake event.  Thats one of the ways in which exhaust systems effect the volumetric efficiency of the engine.  

It basically gets more of the exhaust gasses out of the cylinder.  Which makes room for a larger amount of fresh intake charge, and also increases the pressure differential between the intake tract and the cylinder.

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Reply with:


×