V12 Headers. Anyone?
 

Please don’t just reply “they won’t help.” Unless you’ve had or got a set, equal length and have some data points.

Im looking at sand bent, equal length stainless units. A bit over $3K. Not a cheap option so im looking for someone who’s actually ran some.

Headers as in intake? or exhaust?

There is a company in California that can hydraulically extrude your existing intake to max diameter (37mm?) And balance everything, leaving a glassy smooth interior for maximum flow. I will find the contact info if you like, it was alot cheaper then 3k and provides more or less the same result with stock appearance. I will be going this route soon.

Not saying exhaust headers won’t help but I think unless you spend time in the upper rev range you won’t see much difference.

I would suggest reading Allan Scott's book before you drop $3k on a set of headers. In the confines of the XJS engine bay (based on the testing by TWR) you will not see any difference in power output over changing the down pipes.

There are things that can be done to increase to power output of a stock 5.3L.

1 - Plenum is the biggest restriction NOT the runner diameter. The plenum size is way too small and the distance from the end runner openings to the plenum wall is too small. I have had my stock intake manifolds on a flow bench and they flow more than the intake port in the head.
2 - Ignition timing - in Australia all Lucas ignition cars were made to run on 91RON fuel. More advance for higher octane will increase power.
3 - Down pipes are very restrictive and in the USA contain catalytic converters. Better down pipes on a stock manifold will release power.

warrjon- what is the advance on Australia spec cars?

HE Lucas is 27° total advance. I think it could go to 25° with 95RON. The Marelli has 2 different advance curves selected with a link (for 91 and 95) but I have not found any specs on what the 95 ignition map is.

As the the OP's original question, herewith the oracle:
TT Systems Explained | aj6 engineering
As an aside, I wondered about new multibranch exhaust manifolds during my rebuild. I asked three of the most well known providers (not AJ6 as I did not want to go the TT route as my exhaust system was in great shape) two in the UK and one elsewhere what their dyno figures were compared with the OEM system. Not one of them had ever done any before and after testing. Basically, with this sort of thing for a road car you are buying glitz, not grunt!

My take, from way back, and what I worked out for me.

The V12 is in fact 2 6cyl engines, we all know that. The firing order dictates that also.

The exhaust is NOT siamesed in any way (factory that is), so 2 X 2.65ltr engines on a common crankcase.

I looked at 3ltr I4 and I6 engines, and the exhaust system specs of same. Most ran 2" front pipes and 1 3/4 tails, BUT, the sportier breeds ran 2" to 2 1/4" all the way, with "free flow" mufflers/resonators to keep them legal. I am talking FACTORY exhausts, NOT aftermarket stuff here.

My HE had SAD exhaust, Rotted boxes front and rear, and over axle pipes in lousy shape.

I went with 2 1/4 front pipes, NOT double skinned, and Lukey mufflers in front of the cradle. We tried 2 1/4 for the thru cradle pipes, NOPE, so settled to 2", and small "hotdog" style resonators on the rear.

Noise?, nope. A nice rumble barely audible at idle and cruise, but a distinct snarl when trod on.

Headers, came into the equation, and discussion with people in my work ( aftermarket parts and performance "upgrades"), and once again the size etc of the factory manifolds was deemed very good for the size of the engine, and the revs designed into it. Couple of guys yacked about power this, and torque that, etc gained by headers, and the older guys quietly went through the pro's and cons that I understood.

If the engine is going to spend a lot of time in the top 30% of the rev range, then there would be an advantage, but small, on a stock engine of any design. "Normal driving" probably more a "think there's more power" type of thing.

The main man of the outfit went on about the exhaust manifolds on most engines are a compromise of sorts, usually due to space requirements in the engine bay, and we know the V12 has plenty of room to move, but he also stated that the V12 manifolds were quite good in the flow design, and large, which was a bonus.

The under floor restrictions were the main area of concern, but we seemed to satisfy that with the pipes we used.

This was in 1994, so not Internet or Forum times as we know it now, so go it alone was the way.

I did not dyno it before and after, $$ mainly, and the exhaust was falling off anyway, BUT, the engine livened up heaps, and even more when the XJ40 throttle discs went in, and the emissions stuff left the engine bay.

I am of the belief, then and now, that some engines benefit more from headers than others, but none of my Jags have had them, and I think that what was done then in the design stages was the right thing.

I know that headers are used on the AJ6 an AJ16 engines due to constant manifold cracking, and that is a different reason than what has been asked originally.

Ok. You’ve all convinced me to put my cash into other options for my jag. I hope you’re happy. But you must admit that these are lovely work. Ceramic coated and sand bent beauties.

https://cimg8.ibsrv.net/gimg/www.jag...1211ecf91.jpeg

Someone has to put these in just because.

Maybe after a few more beneficial mods ill spring for these for Jessies 100K b-day.

I agree that they look wonderful, but none of the sellers have any figures and Bywater says they do nothing useful. But it would be great to do a dyno before and after and see! Over to you...

I've acquired a set of stainless headers for substantially less buck$ than the set above. Now I'm working out the exhaust routing. This is complicated by wanting these to go out to 60" side pipes. Easy enough, I know, but wait! I also want to merge both banks!

Here's what I am currently planning to do. 2.5" Header downpipes into 90's turned inward towards the centerline. Into a pair of Siamesed U pipes for the merge, out and through flexible connectors to allow engine motion, out to the 2.5"-3" side pipe adapters and finally into to 3" SPs.

I do not believe that I will have any issue with ground clearance. This will still be above existing lowest projections on the suspension. I am concerned about heat transfer into the sump, so I'm planning a ceramic on extruded alluminum heat shield between the exhaust and sump, as well as ceramic coating the whole U merge assembly out to the flexibles.

Anything else I should be concerned about?

rough idea...

https://cimg9.ibsrv.net/gimg/www.jag...b66d02ed7c.jpg

Jigjag,, do it and let know the results,, my experence with other engines is CERAMIC when pipe gets HOT almost glowing, when cool the shine has gone to dull rough finish, but still an insulating temps!
BUT my V12 exhaust manifolds have been 25yrs, and Ceramic is fine, because the Pre HE chamber is a cool running temperature, the HE is a much higher temps! (also more fuel effeicent)! more complete burn!
also drops seats out more often! DUH!
https://cimg2.ibsrv.net/gimg/www.jag...dec2ec06db.jpg

pic of side pipes , i like it , can hear ALL 12 cylinders from the drivers seat!
https://cimg2.ibsrv.net/gimg/www.jag...655fd8df64.jpg

This is not a good design. Exhaust doesnt like to change direction and really doesnt like U-tuns The ferrari V12 owners learned an X pipe does offer up some modest mid range gains but Ferrari heads and exhaust systems are in another league. I suspect an X pipe on a Jag would do nothing but change the tone. Just get some high flow CATS and chambered center mufflers, that will suffice with stock cams. For headers to scavenge effectively, you need a camshaft with some over lap... or when both valves a open for a period called the 5th cycle where the out going pulse sucks the cylinder clean and induces a mild draw for a moment through the intake valve. The stock V12 cam and pocketed exhaust valve has no chance at accomplishing this.

Also on a V12 the exhust and intake event overlap from side to side so with this design there could actually be some reversion. On a V12 you should use an X pipe far down down stream from the heads....an H pipe or a pressure wave termination box both can both be closer to the engine.

If you want more performance from your V12 ditch the heavy TH400 and fit a 4L60e, you;ll loose 100 LBS and have a lower 1st gear and gain 4th gear with a locking coverter. This is the 1 upgrade that "Could" pay for itself over time in fuel savings. The performance from this upgrade would dwarf what was to be gained from an exhaust upgrade even if it was designed and tuned by a F1 team on a dyno. Better yet, install a 6 speed but it's much more involved.

Be careful with your pipe sizes. Just to give an example, the super charged 6 cylinder XJR with 322hp runs a single 2.25" pipe until it splits to go to the underfloor cats. A guy on the X300 forum did all sorts of work with big throttles and big exhaust and it destroyed his low end torque. Sure, it made more power at 5500 rpm, but it was slower on an acceleration run than a standard car because of the lack of torque. He ended up chasing his tail making more modifications to bring his cars's performance back to the level of a standard car.

Thanks for the feedback! Just to be clear as to my intentions, I’m not looking to see a performance gain from either the headers or the pipes. I want a subtle grumble at idle, a deep growl when pulling, and a roar when enraged.

The headers are an engine appearance enhancement, access improvement, weight reduction, and will alter the sound of the exhaust. Side pipes check all the same boxes. Looks, access, weight and sound.

As I said, I’m not looking for power, but I don’t want to do harm either. As far as creating reversion, I’d love to know the math or better yet to know a website that could help me calculate the proper distances for each merge to best suit the specific needs of this engine. But I’m not British and my last names not Leyland. Around here there’s a proud tradition of just doing stuff and seeing if it goes better. In this case, I’m willing to take some relatively inexpensive chances and see if it still goes as well.

The routing would be best if it didn’t have to take any turns at all. But it must. Hell, look at the stock system, all the cats and mufflers and resonators and small tubes bent like chewed straws through the irs. And it goes. The only other way I see to merge both banks is at the rear before the axle, and run the side pipes rear to front. That is moronic, so no side pipes, just mufflers and dump it before the wheel. Blah.

This gets me a huge change in looks, access, weight and some semi-random change in sound. <rolls dice>

Any suggestions for a better merge?

As for reversion, I’ve modeled and animated the intake overlap on each bank when I was trying to understand the manifold better, but not the exhaust. This final 2:1 merge is quite far from the exhaust valve at this point, but I’ve not done any math.

I suggest you read this from Roger Bywater:
TT Systems Explained | aj6 engineering
and this:
XJS Systems ? V12 & 6 Cylinder | aj6 engineering
With a standard engine, on a road car, these systems are the best you can get.

As for the twist and curves (and smaller pipes) thru the IRS.....

In many circles the prevailing 'wisdom' is that what happens that far aft is not of much consequence, as the exhaust has already 'slowed down' so much. Especially on a mild engine such as the V12. Sounds reasonable although I'm sure there's a lot more science involved and many variables

As for the merge. I have an X-pipe just aft of the trans.

https://cimg0.ibsrv.net/gimg/www.jag...154bec9e04.jpg


Cheers
DD

Rule of thumb for calculating head loss in pipes is that the bend where the radius is at least 3X the diameter of the pipe doesn't have any effect on flow. I.e. a bend of 6" radius on 2" pipe isn't a significant restriction.