XJS ( X27 ) 1975 - 1996 3.6 4.0 5.3 6.0

V12 Headers. Anyone?

 
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Old 04-18-2019, 01:26 PM
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Default 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.
 
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Old 04-18-2019, 04:55 PM
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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.
 
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Old 04-18-2019, 05:21 PM
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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.
 
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Old 04-18-2019, 06:21 PM
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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.
 
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Old 04-18-2019, 09:47 PM
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warrjon- what is the advance on Australia spec cars?
 
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Old 04-19-2019, 05:45 PM
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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.
 
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Old 04-20-2019, 02:20 AM
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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!
 

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Old 04-20-2019, 06:35 AM
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Old 04-20-2019, 07:15 AM
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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.
 

Last edited by Grant Francis; 04-20-2019 at 07:21 AM.
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Old 04-23-2019, 11:48 AM
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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.



Someone has to put these in just because.

Maybe after a few more beneficial mods ill spring for these for Jessies 100K b-day.
 
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Old 04-23-2019, 12:31 PM
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Originally Posted by JigJag View Post
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.

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...
 
 
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