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...not 100 HP. There aren't any bolt on mods that can free up that kind of power. I have those headers, a 2.5" free slowing stainless exhaust system, and tons of other mods. At best the exhaust will give you 20-35HP, but could allow other mods to be more effective. The best reason to go with a system like this is the weight savings. The next best reason is the heat soak after shut down is greatly reduced, those old cast iron manifolds sit there for hours baking all of your under hood components.
It is a huge job to install this system, but having said that, I love mine. But you won't be running around with 400HP, without major internal changes to the motor (heads w/ larger valves, larger bore pistons, etc.).
Dave
I will get some of the mods that can be done. I think you are way underestimating headers + larger injectors....with other mods and a few thousand you can build the 5.3 up to even higher.....
Regarding post that 'headers don't add much'...is not true. On a V-12 XJS 5.3...if you put tube headers, free flow on back, larger injectors and something like a K&N intake you will get a HUGE increase in power. Something like 100HP on a car I recently saw. Will drop your 0-60 from mid/high 7's to low 6's.
thats rly contradictory. If such simple mods could increase the power by 100hp it would definitely drop the 0-60 lower than low 6s
When I first posted this thread, I was in the market for an XJS with the intensions of adding some mods for performance. The first thing I considered was a set of headers. For most applications, it is the first performance add on because it is typically the most bang for the buck. Because of the complexity of the installation, I know on a V12 XJS this isn’t so. Since my first post on this matter, I acquired a V12 XJS and I am going to fab my own set of headers. Below is a picture of a set that we built for an Aston Martin. Granted there are many, many differences, but the complexity is about the same. As far as horsepower increases, there is a significant difference between one header design and another. Aston Martin V12 Headers | Flickr - Photo Sharing!
I'm not estimating or underestimating anything, I have the stainless headers, I have the higher flow injectors, 2.5" exhaust system, Cold air induction + K&N's, larger throttle plates, a chip in my ECU from AJ6 engineering, etc, etc, etc. All of that together doesn't even add up to 100HP. It certainly feels like it does, but my 6 liter started at 305HP, and is now at about 350HP. All of this work, and dyno was over 10 years ago though, so I'm probably starting to lose some of those HP's by now...
There is no more to be squeezed out of her without head work with larger valves, cams, increased capacity with pistons and sleeves, etc. 400HP is the max you can expect from the 6.0L engine with HE heads, and that is with all of the above done. If you want more HP's than that, you have to use modified pre-HE heads, which involves major work, new, custom pistons, etc. The 5.3 stock is about 260HP depending where you are in the world btw. You would be very lucky to get 300HP with headers exhaust and injectors (which you don't need because the stock injectors are capable of keeping up with only those mods, and the ECU's are pretty good about not going lean...) in an otherwise stock 5.3.
Mine were made in England, I bought them from Bob Tilly I think the name of his business is V12-Sales???, they are beautiful, and they were $1,400 US 10 years ago...and a huge job to install. I will try to find the pictures I took while I was test fitting them. They are wrapped now, and hard to see under the air boxes, etc.
Dave, beautiful car. Jamey, I am not sure if I would make these for anything except for my own application. There may not be enough of a market to develop such a creature. I was also entertaining the idea of fabricated intakes. It seems very popular now.
It can be done without the bender. Buy mandrel bends and fab from there. If you purge the welds with argon during the welding process, the finished tube will have the same integrity as a tube that was bent from a single length. Some of the criticle issues in header fabrication is tube size selection, tube length, collector secondary sizing, welding techique and a few other items. The bends are available from quite a few sources.
Whilst I realise this is a very old thread about headers/ exhaust modifications the nature of the thread seemed to be about potential power gains.
I suspect 30bhp (10%) gain is not far off for a better exhaust, I have always believed that for 6.0l the restriction on breathing is more at the inlet end.
I can't think how an inlet manifold could be designed to be worse than standard!
In no particular order the flaws with the standard would seem to be the internal diameter of runners is too small, the length of runners too short,
Plenum volume increased, finally inlet runners equal length.
Not inexpensive but large power increase even with HE heads
I have a 6l XJR-S and I'm about to fit Hayward & Scott tubular manifolds. Yes, they are expensive, but there are other benefits to fitting these. I've had mine ceramic coated, so I'm hoping that the benefits include, reduced under bonnet temperatures, weight savings, reduced heat soak and along with a package of other changes better performance and sound.
Whilst changing the headers I am deleting the CATS and the middle silencers, so I'm expecting some sort of improvement in sound and performance although as others suggest this may not be a dramatic improvement. Either way, if and when I get round to doing some work on the inlet side I know I have an exhaust that will aid throughput. Couple of pics of the Cermachrome covered manifolds..
07-18-2011, 04:45 AM
