I know that the 997 GT3 header will fit on our 996 Gt3's, curious if the 991 version will fit. Any install issues like clearance or port fitment? I would be building a custom exhaust from the header flange to exit.

Thanks guys for your input.

 

Doubt it... The M96/79 and M97/76(77) are effectively the same cylinder blocks and the MA175 is just a different thing altogether.

Besides fit, you also have to consider scavenging efficiency when you move to wider-bore headers.

 

I understand exhaust flow issues associated with bore diameters in primary and cats. That being said I see that 997 and 991 headers are being marketed as an interchangeable part from a number of sources.

Knowing the 997 fits the 996 I was trying to wrap my head around what the actual differences were with a 991 model as it relates to the geometric properties of these headers.

 

 

 

Different spacing on the flange for the bolt holes between 997 and 991. I ran into a fitment issue once back when I was at Fabspeed when we sent a set of their 991 GT3 Race Headers to a fellow overseas with a 991 Cup. Turns out early year 991 Cups were still using the 997 generation engine and couldn't get it to work...oops!

 

 

Well known fact that the 991.1 Cups (up to MY2016) ran the Mezger engine. Different story with the street cars though--all 991 GT variants are all on the 9A1 architecture.

Off the top of my head the bore spacing doesn't even line up if you put two sets of headers side by side.

 

Do the opposite, get the 991 Center Muffler and built custom headers to the OEM exhaust.

 

Correct, the earlier gen 991.1's use the Metzger engine and utilize the 997 part number exhaust system. I can confirm the 991 GT cars/later model 991.1 and 991.2 Cup's (9A1 engine/headers) are different spacing wise and do no fit anything on a Metzger engine.

 

 

It fits, and agree that in comparison the 996 headers look like toilet rolls taped together

Your best compatibility may be the 997.1 GT3/RS/Cup headers coz they're made to flow with a 3.6.

FWIW, 997 cup headers are mated with [200] cell cats. Could be 100 I dunno. More free-flowing and louder but defo has lifing issues if you're buying them used. Had to replace mine when the engine was rebuilt at ~150 hours... also I think the 996 cats reside in the muffler (I think)... your conversion will be more extensive than just a header swap.