When I built my last set of real headers the primary pipe length was 27” for a 7,000rpm peak.
If I took it up to 8500 I could shorten it to 53” But at that RPM I was forced to use really high grade steel for the valve springs to last. ( formula 1 stuff)
There isn’t enough room in the XJS engine compartment ( that’s 27 feet of tubing).
Robert Knodt did it and if you look at his set no air can flow through the engine compartment.
I got away with it in a series 111 XKE because that is open except for the 1” sq tubes that form the frame and even there I had to cut away some of the body to get it to fit.
I can try to make a set of 180 degree headers and it’s possible that I don’t have to block off all the air flow out of the engine compartment.
While I know the firing order of the V12 I just can’t mentally picture which pistons are 180 apart on the same side.
I’ve got a few spare engines but no space to take one apart. Anybody help me out here?

 

Correct me if I'm wrong, but I believe 180° headers are meant to equilibrate the uneven exhaust pulses of crossplane V8s. No other engine configuration would profit from such a corrective.

 

The stock cast Iron exhaust manifolds are very good. First they weigh only about 4 pounds each so a little over 16 pounds, which is lighter than all that tubing will be. Then if you do make a proper set of equal length headers you gain at most a 4% increase in horsepower.

I hate to admit it but the reason I’m considering making a set of 180 degree headers for the vintage race car is because of sound. Enzo Ferrari used 180 headers on his cars. As a result they have a tone, a sound, that clearly identifies it as a V12. Yeh, the earlier 3 liter engines because of the small diameter of the tubes have a scream that won’t be present in a bigger 5.3 liter V12 but it will have the V12 sound.
Then since it’s a race car with no muffler needed I’ll run it straight back under the center section of the differential and on back to either 6 or 3 brass megaphones.

 

Sorry I didn’t answer you. Any engine benefits from 180 headers. Ferrari always uses that design. Hence the sound.
Ford GT40 in the 1960’s used that as well but they put it over the transaxle.
Typically a V8 takes exhaust from one side and bundles it together so the sound order is 180/ 90 / 180 / 270.
Because we have 60 degree firing each of the 4 manifolds can put out 180 degree pulses 60 degrees apart but they are blended together just above the catalytic converter Where they get jumbled up. It’s also why the engine sounds like it’s reving so fast when you listen to the exhaust pulses at the rear. You are hearing all 12 cylinders. When you hear a v8 you are hearing 6 cylinders and a stumble.

 

The V12 has basically sequential firing order 60 degrees apart. So it’s easy for each exhaust pulse to be timed not to interfere with the adjacent cylinder. The stock cast iron manifolds do that already. ( and they only weigh about 4 pounds each)
unlike the V8 which may have adjacent cylinders 270 degrees apart, 90 degrees apart, or 180 degrees apart.
The only reason to do a 180 header on a V12. Is we have 3 pulses of 4 strokes 3x4=12
To get the full 12 cylinder sound we need to time the exhaust pulses in order to separate them.
if you listen to the stock 4 pipe exhaust system it sounds like the engine is running very fast at idle.
But a look at the tach will show the proper 650 rpm.
6 tailpipes the idle sounds like the proper idle speed but clearly very busy. We could then use smaller diameter pipes and get a higher note.
Due to Jaguars larger size than the Ferrari 3 liter engine It won’t be the high scream or tearing silk, but it would give a more interesting note.

 

This might help you.