Why are headers 2.5" and the cat backs are 2.37"(60mm)?

[Bon87]

Most JDM manifolds are 2.5" correct?
But why are exhausts like spoon, mugen and tanabe 2.37" (60mm) piping instead. Why didnt they match it with the header collector at 2.5"
I've just noticed that the TODA header is 60mm

I've currently got a Mugen 4-2-1, I believe its 2.5" and the rest is oem exhaust. But I have a Mugen TL muffler to go on(no b-pipe)
I was going to get a custom 2.5" straight through header back made up. BUT the Mugen TL muffler is is like 57mm to 60mm piping... so kinda restrictive right? if you get me? but its the same if I had the proper Mugen TL b-pipe, 2.5" header into a 2.37 cat back...... I hope this makes sense......

 

[blinx9900]

why do hot dogs come in a package of 10 but the hot dog buns come in a package of 8? Some things in this life just don't make sense my friend.

-why does ph make an F sound?
-why is it called taking a crap? your not taking anything, your leaving one!
-why do we park in a DRIVEway? but we can drive in a PARKway?

the list is endless lol...

 

[Bon87]

Not the type of reply I was looking for but oh well

 

[blinx9900]

Not the type of reply I was looking for but oh well

Sorry, i always wondered the same thing and could never find an answer, I'm guessing its just a mistake caused by confusion due to the difference of metric and standard measuring systems. Kind of lost in translation just like Spoon Sports Ventuli gasket, its supposed to be venturi but it got lost in translation...

 

[RagingAngel]

I think part of the reason has to do with the fact that in Japan, Hondas are not normally tuned to the levels that we find in the states.

There probably isn't a single Honda that has 900whp in Japan, while in the states there are at least half a dozen that I know of, and probably more than I guesstimate.

The point I'm trying to make is that manufacturers will design systems for the public and the public just might not know any better but my other guess is that the power increase from going 2.37in to 2.5 isn't much so they likely decided it wasn't necessary.

Anybody serious enough will fabricate their own exhaust and this is evident in Japan as there are many suppliers who offer custom tubing/mufflers.

The Toda is a true 2.5inch at the collector, the 60mm refers to the diameter of the "cone" right before, otherwise a 2.5in exhaust donut wouldn't fit.

 

[rvm]

I think part of the reason has to do with the fact that in Japan, Hondas are not normally tuned to the levels that we find in the states.

There probably isn't a single Honda that has 900whp in Japan, while in the states there are at least half a dozen that I know of, and probably more than I guesstimate.

The point I'm trying to make is that manufacturers will design systems for the public and the public just might not know any better but my other guess is that the power increase from going 2.37in to 2.5 isn't much so they likely decided it wasn't necessary.

Anybody serious enough will fabricate their own exhaust and this is evident in Japan as there are many suppliers who offer custom tubing/mufflers.

The Toda is a true 2.5inch at the collector, the 60mm refers to the diameter of the "cone" right before, otherwise a 2.5in exhaust donut wouldn't fit.

I totally agree

 

[jugbugz]

Are the ones measured in Japan 2.36" ID or OD?

My exhuast system is 2.5" OD and 2.36" ID. My collector is the same.