Mods and jetting

[Rcannon]

Is it possible to draw a conclusion on performance of an engine by changes a person made to jetting after the mod?

If a person changes a pipe and has to increase the main jet size, is it reasonable to assume the pipe helped?

Thanks! I hope someone might understand my thoughts?????

 

[Moto Squid]

I suppose it all depends on how it is affecting the engine. When I put Vforce reeds in the rich jetting became normal, but now with some more additions I'm jetting at stock settings and richer at times.

 

[Rich Rohrich]

Originally posted by Rcannon
If a person changes a pipe and has to increase the main jet size, is it reasonable to assume the pipe helped?

Unfortunately no.

If the scavenging of the engine is compromised at some point because of a change in pipe tuning it might force you to jet richer to make sure the remaining available oxygen can find a partner for combustion . Because there is old exhaust waste left in the combustion chamber there will be less oxygen available to react with that fuel so the end result would be less power. That same pipe might tune correctly at a different rpm and improve the scavenging at that point allowing the jetting to be leaned in one of the other circuits. That's just one simple example in a long list of interactions that take place when you make a pipe change or some other internal mod.

It's pretty common on two-strokes for good porting to force you to lean the jetting because the engine is becoming more efficient.

Hope this helps.

 

[Rcannon]

Rich, thank you. It does very much!

 

[Moto Squid]

ok then...so on my 02cr125 after putting on a thinner base gasket, 2 head gaskets, opening up the exhaust port and putting on a modified head and I have to richen the jetting does that mean that the mods aren't that effecient??? It's now got much better mid-top power but falls off the pipe a lot quicker. Is this because there's a magic rpm now where the scavenging becomes poor really quick? Engine mods are a can of worms I tell ya' :confused:

 

[NO HAND]

Rich, that's very interresting! I was wondering about this.

 

[Rich Rohrich]

Originally posted by Moto Squid
ok then...so on my 02cr125 after putting on a thinner base gasket, 2 head gaskets, opening up the exhaust port and putting on a modified head and I have to richen the jetting does that mean that the mods aren't that effecient???

Maybe maybe not. Why you need the fuel is another question entirely. Do you need more fuel because you've improved the breathing or do you need it because the chamber has an excess of waste exhaust gas trapped and the oxygen you did trap is having a a tough time finding fuel to react with? All richer jetting means for sure is you need more fuel to get a combustible mixture.

The salient point is, it's worthwhile to look beyond what seems to be the obvious because you just might find a deeper understanding of engines and performance. :thumb:

 

[Studboy]

:worship:  Thank You! I've pondered this question for a long time, but now I know for sure!!! :worship:

 

[Moto Squid]

I know my mind is too small to be wondering on its own....BUT....now you've got me thinking Rich, how can I determine if I'm trapping more exhaust and is that always a bad thing? This is the type of stuff that makes my brain turn spin, hehe maybe that's why school is so boring :p