Hi,
I have a 1975 XJ12C V12. I have been a motorhead for 42 years but I am new to Jag ownership. I believe that it is a good thing to improve airflow through an engine and a good set of headers generally helps if everything is set up correctly. I have been told that the stock V12 exhaust manifolds work pretty well and that headers are not worth the effort. But I am still looking around at my options. So far I have found a company in England and Australia that offer V12 headers. The Australian ones are more expensive but look to be a far better design imo. Are there any other sources to a quality made exhaust header for a V12 for my chassis that anyone can direct me to? Second question, how do I get the Australian piece to the US if I d not find any good alternatives? I have no knowledge about what the shipping options are. I checked with Fed Ex and it was a $6,300 bill. Obviously I am not doing that. DHL came in at something like $2300 but that is still pretty expensive. If I remember right shipping from England was less than $100 so I am hoping there are other options and I'd like to avoid any CA ports if possible.
TYIA
Chris

 

According to Roger Bywater, who worked on the V12 at Jaguar, there are no worthwhile gains to be had by changing the facory exhaust manifolds. If there were to be any, it would be a 5,000 plus RPM and badly affect mid range torque. The science behind this is clearly set out in his book "Moden engine technology";
I have asked two well known UK suppliers of such exhaust manfolds for dynomometer data, and they had to confess to not having any. Draw your own conclusions.

 

Thank you for your input. I read that too. Am I recalling correctly that he also said that in order for the header to be effective the primary tubes would have be 40" long and hard to package? Here is a link to the company in Australia's header. It appears to me that the primary tubes are much longer than the company in England. These look to me as if they would improve the flow. For the record I noticed that the one English company I contacted did not test their header on a dyno to compare it to the stock manifold so I agree with you about the English ones at least. Plus the picture of the English one's look an awful lot like they were simply mimicking the lines of the stock cast iron manifold and other than perhaps being slightly lighter, offer no real performance flow improvement. I should also mention that I plan on installing a programmable EFI system, slightly larger throttle bodies to my engine as well. And that my 5.3 l V12 needs valve work so I am going to swap a 6.0 V12 out of a 1994 XJS to replace my 1975 engine rather than pull the heads and have the heads redone. http://vicspiteri.com.au/products_ex...fkJd5E9RvXcUSs

 

Howsurcargo,

No help here, but a few questions.

Who's EFI are you installing?

Are you bringing the '94 XJS transmission with the 6.0 engine, or...?

Thanks!

 

Not yet sure. I am lucky enough to be in the same county as a very talented EFI shop. He was thinking Motronic or some company that begins with an "e" but it is not Electromotive. I will go with what they ultimately recommend.

I decided to pass on the auto that comes with the engine and plan on doing a Driven Man manual swap. So far they seem to have the best 5 speed conversion for my car.

 

Emerald?

 

"Emtron" is the name of the other programmable EFI system that was suggested to me to use.

 

Another book that is well worth reading is TWR and Jaguar's XJS by Allan Scott. Basically the stock exhaust is fine up to about 4500 RPM, after that it's more the mufflers that are the restriction.

Unlike domestic V8's, the V12 doesn't benefit from headers, partly because Jaguar used much larger ports than the American pushrod V8's and it's exhaust was far less restricted from the factory.

 

Hmmmm. Based on experiences I have had with other various engines over the years I am really surprised that a good quality set of headers would not improve the breathing over the stock exhaust manifold. Here is a picture of the part that I am considering. It appears to me to be of a sound design with decent length primary pipes imo.

http://vicspiteri.com.au/products_ex...fkJd5E9RvXcUSs

 

As I said, the V12 is quite different from American V8's. A standard Jaguar 5.3 does 56 hp/litre, while the best a Mustang with a 5.0 did was 43 hp/litre and the best a big block Ford did was 31 hp/litre. I'm using later net or DIN certified HP numbers, not the 1960's gross numbers that are often quite inaccurate. These are all 2 valve engines with EFI, nothing exotic in their specification and all in emissions legal trim. However, Jaguar was able to get nearly double the power output that Ford could from a 460.

I recently pulled the stock headers off a Ford 5.0 and I was amazed at how small the passages in the manifold are. The Jaguar ports are at least triple the size.

Sure, those headers look pretty. I'd want to see the before and after dyno sheets though, and more importantly the torque curves.

 

Thank you for that information. What you presented is logical and makes sense. I have asked the seller if they have done any dyno testing and see what they say.

My ultimate header on my cars would be the BMW factory M "bundle of snakes" for the S38B35 (the DOHC 4V I6) exhaust header for the M5 engine in my old 5 series. I was getting almost 1.5 hp/ci NA out of a 130k mile stock block 1988 engine.

 

A 4 valve modern engine breathes so much better than our 2 valve engines ever can. That is why those brave souls who have managed to place two AJ6 heads onto the V12 have made, with no significant changes as far as internal combustion chamber and inlet and outlet manifold work is concerned, several hundred more BHP.

Do post what the guys at Spitieri come back with.

 

I believe a reasonable summation of their position is that they do not install their "extractors" on stock engines. It is combined with intake work and a programable EFI system. That the V12 responds very well to increases in airflow. They never said anything about changing the cams, just opening up the intake as well and accurately controlling fuel and spark. They said the results are very pleasing.

I am planning on installing larger throttle bodies and using a programable EFI system with the stock intake manifold. Is one intake manifold superior to the other? I notice the 5.3 intake manifold has a ribbed plenum and slightly more robust webbing. The 6.0 manifold has Jaguar script on it and less webbing. Is the internal diameter of the runners the same for both of the intakes? If there is no difference I was thinking about using the earlier manifold to minimize any visual clues of there being a 6.0 V12 instead of the expected 5.3. I read somewhere of someone who modified the intake manifolds but died and no-one has stepped up to continue that process. Are there drawings of what needs to be done somewhere?

 

I wouldn't bother with larger throttles. TWR was asked by Jaguar in the late 80's to evaluate the 5.3 with an eye to producing a "Jaguar Sport" version using the parts and knowledge that TWR had gained racing the V12 in the Group A European Touring Car championship series. They did a series of dyno tests adding and swapping parts and then doing a dyno run to measure the effect

They found that the 5.3 intake manifold runners actually should be 3-4mm smaller diameter, as that produced more torque. They put the racing larger bore intake manifold on a standard 5.3 and recorded a reduction in power until 5,500 rpm, when it came back to the unmodified power level. There was an increase in power in the 6500-7500 RPM band, but that's not where a street car will ever be driven. Indeed, a stock engine will have trouble staying together at 7500 rpm.

They increased the displacement and used the standard manifolds and noted that they worked much better on the larger engine. A 15% increase in displacement produced a 27% increase in power. Larger throttles were only effective on the larger engine, in conjunction with other modifications and only at 5,500 RPM and greater. They concluded that the stock throttles can easily support a true 500hp. Even at 6000 rpm the gain from the larger throttles was only 2 hp. Spend your money elsewhere, as a large throttle gives a slight benefit in a very narrow (and high) RPM range.

 

I wanted to thank everyone who responded. It was good information. I decided to stick with the stock exhaust and stock throttle bodies and focus on getting the engine running correctly in my car.

 

Yes get the engine running property first. Let it run in the dark and look for all the HT sparks and sort those 1st, Make sure your fuel lines to the injectors are in good condition and without cracks!
Remember these engines are 30 - 50 years old now. Changing HT leads , cap/ rotor and the little injector pipes is quite cheap and good insurance. If you have vacuum advance/ retard on the dissy make sure it actually works, the bellows inside could have failed many years back.
Big throttles don't work, well not on anything anyone on here is likely to build anyway.
If you want to know what works look at what was done to the TWR XJS, don't believe anything anyone else says. TWR used very special cast exhaust manifolds totally unlike the std items You can see a copy on the Mobeck site, You cannot afford these but it does indicate there is an advantage to be gained here. TWR certainly didn't use the standard inlet manifolds either, they cast special larger ones but kept the standard TB size, if you get into doing the maths on all this you will see why and why the larger or even quad TB's as used by Lister are a waste of time.
The whole system was designed for low revving automatics so the inlets manifolds are relatively small in ID and the exhausts didn't really need to do much.
Now if you go to the 6L engine its even worse and the inlets and exhausts are the same as the 5.3, they are not sized up, and if you want more power you really need to open up or change both.
Good luck.
Don

 

Thank you for the information. At this point I am planning on doing a swain heat coating on my exhaust manifolds, maybe extrude hone port them, a slightly larger diameter exhaust, just one set of mufflers at the rear, I have no cat, and using all of my 5.3 ancillaries on my 6.0 to keep it stealth. I am also planning on using a Driven Man 5 speed conversion which comes with an aluminum flywheel and the high torque starter. I also ordered an Ishihara crank scraper. I used one in my BMW. Honestly I could not tell you if it made a difference or not since I did not do a before and after dyno test. But the concept seems to make sense and it was not that much money. I will post updates as they happen. Lately I have been focusing on my 1969 Ford C600 truck with the NAPCO 4WD conversion.
Chris H

 

I'm a big fan of extrude hone. The intake & heads on the BBC in my '77 jag were extrude honed in Irwin, PA. I also had them do my intake on the engine for my MK2.

 

One thing that wasn't mentioned in the previous discussion is the difference in flow between the Pre-HE and HE heads. The HE heads (that your 6.0 has) swirl the incoming air/fuel mixture to get a more complete burn. The part throttle fuel economy increase was remarkable, but it is a lot harder to make big power with the HE heads than with the flat heads.

Also, The Driven Man has a reputation. I have no connection with them, but poke around a bit before ordering...

 

I am not focused on big power with my Jag XJ12C. It is more of a luxury cruiser with all of the weight and chassis flex it has. I have other cars that are more suited for big power.

But please enlighten me about Driven Man. What do i need to be mindful of?