Roger Bywater is selling a product, of course he will tell you his is best.
Personally, I wouldn't take as gospel anything someone who was responsible for the serious compromise that is the Jaguar V12 cylinder head and intake design....a lot of what he has written flies against modern performance engine design, but that is to be expected of someone who got out of the game over 30 years ago. The fact he and his team struggled to get 300hp (and it's more likely 270hp if you look at the performance of the vehicle) out of the V12 in production trim, then had to have an outsider redesign the combustion chamber (to replace the hopelessly inefficient and prone to carbon build up one they designed) says it all.
Think about that.
Even a short tube, free flowing header will be far, far more efficient than the stock V12 castings. Tuned length is great, but if whatever you bolt on creates less backpressure than what is there, you WILL see performance gains.
If Aston martin managed to squeeze tuned length headers into the engine bay of the same platform, it can be done.

 

coup[e pix of Bob Tullius car,XJR5 slider inlets, and his XJS 1977, won many races in TRans-Am. WOW 8000 revs in 1977! helps to hang around the Daytona track .

you will have to rotate specs pic, things have sure changed ,but shows 44 was on the right path, but politics get into the way of progress!

 

Ron,
Thanks for the specs on the early Group44 XJS.
Something seems off though.
Unless he's smoking those fat Goodyears through 1st and 2nd, a car with 475HP, and a 3.77:1 final drive should go 0-60 faster than 5 seconds, and have a lower ET through the quarter mile.

 

Typhoon
FYI, Mr Bywater did not design the V12 head or block. It was designed mainly under the direction of Harry Mundy and Walter Hassan, I believe. Further, I think you are also mistaken about your unpleasant description of his abilities, which his success as a valued consultant to all sorts of respected businesses contradicts.


All engineering is a compromise between time, cost, budget, materials, and perhaps most of all, the available technology and the state of the knowledge and understanding of the time. My own opinion is that Jaguar managed to produce a reliable and excellent V12 mass production road-car engine that no other firm before had managed to do. Perhaps more importantly, that was the objective. It powered in carburetted form a car acknowledged to be at the time the best saloon car in the world, regardless of price, the Series I XJ V12.


As to your point that using outside advice to improve a design is evidence of failure, I challenge you to find me a firm, in any field whatsoever, let alone automobile manufacture, that does not use specialist advice in some aspect of its activities. I think you owe the man an apology.
Greg

 

I don't know who you think you are, but you have neither the right, nor the authority to suggest I owe anyone an apology for stating facts.
Your arrogance knows no bounds, does it?
If you would like to talk about unpleasant, how about the way you passive aggressively attack anyone who refuses to share your views on a subject?
Frankly, I'm tired of seeing you always trying to have the last word on every subject and proclaiming to have every answer to every problem.

 

Jeeeez! Take a step back...Greg is in France...have some compassion, have you seen the news toady? You've been spouting diatribe over on the XJ sedan board too. It's just a vehicle.

You have a classic case of JEOD or Jaguar Engine Obsessive Disorder... The Jag v12 is an engineering marvel but there have been many production V12's before it and many after it. Cases in point... Ford introduced a v12 in 1932 and there was a Ford V12 in production until 1948 and the Packard "Twin Six" was in production from 1916 to 1921. GMC put V12's in trucks from 1960 to 1965.

 

i agree Zilla, a little optimistic numbers, most likly 400-425hp,(1977) and revs like 7500rpm, ive heard of 5.3s rev 7500 from factory BUT never seen one, my experience is HP flattens out around 6800-7000rpm!

and road race tyres of that era, did not hook very well, and slow shifting, so many variables!!

but never the less great for 1977, but road tests say the Corvettes were still faster on acceleration, but couldnt carry the speed thru the corners or brake as well,maybe XJS better car balance or something!

found this old pic of Tullius maybe a little BS, like any racer or fisherman,
LOL.

 

Hey Zilla, after looking closer to the specs, 44 XJS had a super high ratio 1st gear(a true road race 1st gear), he did 80mph in 1st gear, so it would be a slow 0-60, slipping clutch and chugging to get rolling!

he had a 1st ratio of 2.14, where today we have many with a 3.14 1st ratio, makes for a much faster launch!

 

Headers do work on a V12, If you take off a cast iron manifold and have a look at it you will quickly come to the conclusion that just about anything must be better. Racing V12 XJS in the 80's we were not allowed to fit headers, we did try making very tight tubular manifolds and cast spray them to look like cast iron manifolds but couldn't get them to fit under the shields, We used a famous guy in the UK called Mike the Pipe. We gave up as I always remember Mike saying he just couldn't torture piping to do the turns necessary to fit in the space we required. We contented ourselves with weeks of work hogging out the insides and tig ing up the holes if we went through. Years later I found out that TWR had succeeded but they had made larger exhaust covers to allow the tubular's to fit out of sight.
IMHO Roger is wrong here, very wrong actually. I would like to hear from anyone who has been delighted or even happy with the ECU updates........ Enough said.
The Lister pattern of headers was by contrast universally well received and has been copied by many, it may not be the ultimate exhaust but it doesn't take too much to improve on standard. Its neat, packages well and does give gains. I dont sell them , but its a thought !, lol

 

ECU satisfied updates, I was having stalling problems between shifts on my 5sp manual conversion. I sent AJ6 the ECU because of reading about the "override control" in the ECU. Whatever they did worked for solving my situation. I did not get any "throw back in the seat" performance from this mod nor was expecting it.
Lawrence

 

I'm more than happy to offer my car as the test mule for these new headers you'll be offering

 

Nearly added this bit in my post,
There is a difference between a manual ECU and the usual automatic ECU. I know AJ6 did/do the mod for the Auto ECU and do know it works to stop the problems you mention, Used to be about £60 but that was many years ago.

 

I was wrong with the dimensions of the pistons

 

That's an interesting photo. It appears that the XJRS piston is actually shorter (from the wrist pin up to the piston top) than the 5.3, indicating a possible lower compression ration in the XJRS.

Can you post a photo of just the 5.3 and the XJRS pistons connected with a wrist pin, and your scale laying across the top?

 

Yes of course, tomorrow. And add pre HE

 

You need to remember that the XJR-s piston is for a 78 crank so would be 4mm shorter in height

 

You're so right. I had forgotten about the increased stroke of the XJRS. I wonder what the CR would be if you put those in a 5.3...thinking along the lines of the recent post regarding using pre-H.E. pistons in an H.E. for purposes of forced induction.

 

I recon about 7.5:1

 

XJRS's piston looks like forget

 

Thanks Nikolay,
Those are VERY informative photos.
Yes, the XJRS piston does look to be forged.
If it actually IS forged, and if it can get the compression ratio of a stock H.E. motor down to around 7, or 7.5, it would seem to be a perfect choice for large amounts of forced induction on a 5.3. Although I'm not sure how difficult it may be to find a set.