strait to the point..

Bolt on B18c stock JDM 4-1 stepping it up to a Mugen 4-1 with 3 inch collector.. is this a good or bad idea....

loose power or will i yeild any gains...let me tell you also that the motor will be tuned next spring on hondata

i wanna run the mugen header 3 inch collector to 2.5 piping with a test pipe to my N1 muffler..

 

If you have enough air flowing through the motor, a 3 inch collector will only help.

 

2.5" will be fine, dont see a 3" having gains over 2.5" esp if motor is stock.

 

I think you have the potential to lose some of the exhaust scavanging/velocity with collector that big.

 

if you are trying to set up the exaust for scavenging, you will lose velocity, which means you will lose power.

 

dunno what effect running a 3" collector to 2.5" piping will have, but typically as long as there's ~2"-2.5" 'chokedown' built into the header before the collector exit, it works just fine to help the engine pull throughout the midrange. ...like a merge-cone 'megaphone' on an open-header, race-only design.

from what i gather, not much 'tuning' is done after the header collector (hence the benfit of running open-header for racing). so, it seems to me that if you bother using any shelf-type, effective mufflers (the more effective, the more restrictive, e.g. baffled designs, cats, louvered resonators), running a larger diameter exhaust will allow for less of a pressure change at the muffler(s), since the tubing itself will be able to contain more exhaust. you'll have steadier flow; it will travel more easily in the proper direction (rather than reverting upon decel when the throttle's closed, ex valves open, and cyl pressure is lower than exhaust pressure). when the pulses flow more evenly, and flow is more laminar, better scavenging is possible than when the pulses change speed and direction more often and quickly, which causes more friction and turbulence.

ask someone who's tested a HeaderTech 4-1, some which had a 3" collector and a bolt-on transition back down to 2.5". i suspect, though, that the angles and lengths of the merges/transitions are more important than diameters and lengths of relatively straight segments.

 

thats some tech stuff there