I'm in the planning stages of my first manifold project and was hoping for some advice from those in the know.

Here's the details:
'06 Audi A4 2.0T
T4 twin scroll cast collector (304SS)
½" 304SS head flange
304SS sch 10 pipe

My issue is that I want to run 1.25" runners instead of 1.5" to aid spool. 1.5" is a bit much for this motor and the planned power levels. I have read a few posts on stepped runner manifolds and was wondering if this is something I should consider. The collector (which I already have) is designed to weld to 1.5" pipe. If I have to, I'll just use 1.5" pipe throughout, but I would like to start with 1.25" at the head flange and then step up to 1.5" at or just before the collector. Is this more headache than it's worth? Where should the transition be, or does it not matter? Any advice is greatly appreciated.

 

on a twin scroll, i have not see any benefit....

i have played around with this a lot on the engine dyno; triY's, stepped headers, etc.

pm me if youd like some info

 

I guess what I'm really after is that I want 1.25" runners and the only cast T4 TS collector is for 1.5". Should I just use 1.5" for everything or will I get the spool benefits of a smaller runner by doing a stepped design?

 

I don't know that stepped headers make much of a difference on a turbo car . . . but I have no real world dyno data in front of me so it could.

My primary deciding factor would be if the port size matches up better to the I.D. of the 1.25 pipe or 1.5 pipe . . .

 

Alex from BP Autosports did a comparison going from a 1.25" runner to a stepped runner 1.5" manifold and gained spool and power. Same dyno, same day, same boost, same timing.

 

I'm planning on using this head flange:
Exhaust Manifold Flange, 2.0T FSI, 2.0 TFSI, 2.0 TSI - 034Motorsport | Performance Parts for Audi, Porsche, and Volkswagen

They claim the ports are matched to the oem gasket and can be fir to 1.25" or 1.5" weld els.

itspools - that's what I wanted to hear. Do you happen to have a link to a thread with the info?

 

Yep, made a tangible difference on a 2JZ application over a straight 1.25" manifold. I saw a 32whp and 27lb/ft gain with slightly faster spool and different auditory qualities.

Here's the post with the info:

https://honda-tech.com/welding-fabri.../#post49890729

 

The man himself speaks^^^

 

 

if the diameter of the steps are chosen correctly, it will make a world of difference

i have seen improvements throughout the whole graph

I did not see much benefit on a twin scroll, but on a regular t4 (and t6) flange, there was...as well as a huge improvement on the tial vband housing.

but with the tial vband housing, we did another little twist to that manifold too.

 

I'm building a stepped 2JZ T4 twin scroll twin gate unit with a new collector design currently, I'll be interested to see how it works.

 

Alex's is the only dyno ive seen on stepped vs none. However i think that in the case of the JZ it needs to be VS an all 1.5'' runner manifold. Which is a pretty standard size for them. Not negating his findings, but on high hp cars 1.5 could be beneficial power wise, and slightly distorting the gains of the test.

 

I'd love to do a straight 1.5" manifold, but it's hard to fit 1.5" pipe on a GTE head flange. If I get a few dollars together to build a straight 1.5" manifold, I'll do it for ***** and giggles.

 

That on a high HP jz vs stepped 1.25 would definitely be interesting. I never seen any results posted from collin as in the power department. Stuff has been lookin rad lately btw.

 

Once I get mine back together, I'll probably give it a shot. I wouldn't expect any details from him about any of that stuff. Thanks bro, I'm having an absolute blast with it!