Header primary size - is 40mm too small?

**confuzion** · 23-09-2010 02:09 PM

Hey all,

B18CR in an DC2R

I hunted for a while looking for some 2nd hand JDM headers and wasn't too keen on the 4-1's that were around, so I bought some 4-2-1 M&M Honda headers from http://www.nengun.com/m-and-m-honda/exhaust-manifold and are on their way. They are a reputable JDM brand and very popular over there, plus were cheap.

During a period of boredom I did some more research and noticed that all the other good brand headers use something like 45mm pipe diameter primaries, whereas the ones I bought have 40mm primaries.

In your opinions will I be losing alot of power compared to a header using 45mm primaries?

I'm keen to do a before and after dyno, but if the common census is that they are crap I might just sell them now..

Thanks all

**dougie_504** · 23-09-2010 11:29 PM

No worries. The important things to consider for power are the design for range-related gains (4-2-1/mid range or 4-1/high range) and the size of the single pipe outlet that connects to your cat.

45mm is probably more beneficial for an engine head that has enlarged exhaust ports, but you'll be sweet with 40mm.

Stock header is very restrictive anyway, and you'll save a few KGs in weight alone when you change the header. So I reckon upgrading this part is always a good idea

**lycan** · 24-09-2010 01:22 AM

If see the shape that is good. I think 45mm too small at least 2" for 1800cc engine. So that will have better flow. However I am interesting with the result dyno. Will you put in the forum? I want to know the improvement.

**NSPYRE** · 24-09-2010 01:35 AM

OP: thats a pretty dam good price, let us kno how it turns out. got the TODA header myself (45-50-60). be interesting to see if the smaller primary and larger collector make a difference

Originally Posted by lycan

If see the shape that is good. I think 45mm too small at least 2" for 1800cc engine. So that will have better flow. However I am interesting with the result dyno. Will you put in the forum? I want to know the improvement.

45mm primary too small.... wat kind of 1800cc engine are u thinking of building??? o.O

**confuzion** · 24-09-2010 04:33 PM

Thanks for the replies all.. At this stage (even though I'd love to) I'm not interested in porting/cams etc so should be OK.. I will post up the dyno before and after once done..
2 inch = 51mm, havn't seen any headers with those size primaries, maybe custom, and you'd want to have some big mods/hp?!