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All the Jaguar inlet manifolds from 75 to the very last in 97 are basically the same. Internal bore of the runners about 33mm for the whole length. the main differences are the logo on the to of the plenum replacing the ribs and the change in outside lugs to mount the late 'O' ring injector rails. So when Jaguar / TWR did the 6L engine the very same jnlet manifold went on effectively reducing flow by 13% but not a real problem for the low revs being used generally with the auto box. The manifolds I am making (remaking) were designed and made by TWR in period for better air flow and performance. They were used on the Group A XJS's and were intended to go on the 6L XJR15 which used basically an XJR-s engine. BUT the tooling had vanished from Stones Foundry. I tracked the tooling down over 15 years and acquired it pre COVID and have had a number of sets made for customers. They flow about 30% better than stock as made.. There are various online calculators online which will back up TWRs findings that the stock Throttle Body is good for upto 8500rpm on the 5.3L engine and about 7750rpm on the 6L If you have a larger v12 then the revs are lower and you may then fit TB's with the 4.2i butterfly with advantage. The even larger AJ6 TBs are simply too big just as the Lister 4 TB system is also grasping at straws. The 33mm runners are the restriction and the number and size of throttle bodies does not resolve the problem. These manifolds are 36mm at the head face opening to 41mm at the plenum so are cast up trumpets in effect. Std heads can be opened up to this (this is the diameter of the ID on the standard gasket) with the heads in situ. If you are interested drop me a line donmariamiles@gmail.com I also do the new CV14 injectors (green or grey) at £550 a set, these are the barbed with pipr type not the late 'O' ring type.
Thank You Don - I’ll send you an email.
incidentally; what makes the XJR15 engine different and allows it to be quoted at 450bhp?
The XJR15 engine is basically a stock XJR-s 6L unit with a few mods, Cams are ,415 rather than ,377. TWR only allowed about £300 for the porting on a pair of heads. These engines are dry sumped and have a baffle plate with aluminium version of the XJR9 dry sump pan. The Cosworth pistons also have a slight mod to match the little bit of work done to unshroud the sunken exhaust valve. ECU is ZYTEC which was programable with the right kit. Had a very nice set of exhaust manifolds bolted up to the heads and next to no exhaust restrictions. These engines were supposed to have the TWR Performance inlet manifolds but ended up with anodised black standard inlets as the tooling have vanished. Smaller diameter flywheel and 7.25" AP clutch (carbon) Outside oil pump was a bolt on Heidegger unit and good luck finding one of those babies
That's about it; given the Group A 5.3L engines ended up with 500BHP (with the large bore inlet manifolds) its not to unreasonable to give 450bhp to the 6L XJR15 engine with higher lift cam and better exhaust.
The XJR15 engine is basically a stock XJR-s 6L unit with a few mods, Cams are ,415 rather than ,377. TWR only allowed about £300 for the porting on a pair of heads. These engines are dry sumped and have a baffle plate with aluminium version of the XJR9 dry sump pan. The Cosworth pistons also have a slight mod to match the little bit of work done to unshroud the sunken exhaust valve. ECU is ZYTEC which was programable with the right kit. Had a very nice set of exhaust manifolds bolted up to the heads and next to no exhaust restrictions. These engines were supposed to have the TWR Performance inlet manifolds but ended up with anodised black standard inlets as the tooling have vanished. Smaller diameter flywheel and 7.25" AP clutch (carbon) Outside oil pump was a bolt on Heidegger unit and good luck finding one of those babies
That's about it; given the Group A 5.3L engines ended up with 500BHP (with the large bore inlet manifolds) its not to unreasonable to give 450bhp to the 6L XJR15 engine with higher lift cam and better exhaust.
Great project, welcome to the forum, lovely looking car. With a manual box the thing will be pretty fast too.
FWIW I say forget the tubular exhaust manifolds and Jenvey stuff. None of the exhaust manifold people I have asked have any, any at all, before and after dyno figures, and I deduce from this that their manifolds deliver bling but not go.
The Jenvey TBs will be great at high revs but will deliver poor low end and mid range performance. The OEM inlet manifolds are pretty good, and real performance gains can only be had by using the pre-HE heads and pistons, and even then up to about 4500/5000 rpm there is no advantage. Roger Bywater's book " Engine technology for the modern world" has a huge chapter on this subject which is really worth studying. The Aston Martin V12 - which is/was esentially two Ford V6s welded together - had an intake manifold almost identical to the Jaguar one you have. The difference in output is down to the valves, which is where the Jaguar unit is restricted at high revs, and the H.E. head cannot be modified much to fix that restriction. Hence TWR using the pre HE heads on their ETCC racing engine in the early to mid 1980s.
Depending upon funds, the Mobec system is the best plug and play kit you can buy, https://mobeck.com/produkt/jaguar-v1...e-waste-spark/
but if you have the know-how those aftermarket makes you mention will do the job and far cheaper; but as I expect you know already, getting any aftermarket ECU setup to work really well in all conditions and loads is quite a job. But some guys I have heard about have used Emerald with great success so well worth speaking to them.
Either way, good luck and keep us updated!
The only before and after dyno numbers I’ve seen tell me that a really properly designed set of manifolds will increase power by 4%.
Properly designed means based on camshaft used, anticipated power, etc
When I did mine for racing, On an XKE ( V12 roadster) The required length of tubing filled up nearly every inch of space under the bonnet.
Airflow though the engine compartment was almost stagnant. I cheated and ran them out of the Holes I cut in the rocker panels.
Racing camshafts need shorter than stock length pipes.
So you are totally correct. They are bling. Actually heavier than stock manifolds. ( the 4 castings only weigh about 4 pounds each). Then add all the required tubing
That’s why I use the stock manifolds on my vintage racing XJS V12. I also run 4 pipes out the back for more power. And so you hear all 12 cylinders at once rather than only 6 depending on what side you are on
The HP26 / HP28 are solid performers too even with a torque converter - had the HP26 in my SV8 and that was close on 500bhp - was also my XJ but that was a rather lame 300bhp - neither are a patch on the box in my Audi in response terms which is a 7 speed 'DSG' or S Tronic as Audi call it - but they're leaps and bounds better than the TH400 and derivatives track use excepted - that said I had plenty of fun with the SV8. I first encountered the DSG in a Passat R36 - I've been sold on them ever since.
I just cannot at this stage justify the hassle of making one fit into the XJS - but there are ways to get a HP26 into one.
I don't think you can buy a performance car (that isn't an EV) that has a manual these days.
The hardest part of fitting a modern auto transmission in an old car is supplying with accurate data streams so it can do its job properly. You probably all have experienced limp mode due to a simple fault in some other system in car.
We have fully a working ZF 6 speed auto in V12 XJS with stock injection system and working on comes with Mobeck EFi.
Price for ready to install kit including everything required for DIY or professional fitting from £6,000.
For further info,
Follow JagBox on Facebook, register on jagbox.uk
Test drives available.
Gurch
Sounds interesting - I actually have a low miles (42k miles) HP26 c/w torque converter sat in the garage that I keep falling over - the car is transitioning away from standard ignition the details are still being worked on, I shall follow.
Sounds interesting - I actually have a low miles (42k miles) HP26 c/w torque converter sat in the garage that I keep falling over - the car is transitioning away from standard ignition the details are still being worked on, I shall follow.
Jaguar V6 petrol has a unique bolt pattern whereas V6 diesel and V8 petrol have a common one. We have designed the adapters for the latter versions.to the 5.3 V12 HE. with GM400. Having to alter TC and adapter for BW12 on earlier cars.
Not much to update on at the moment other than to say my Getrag 290 arrived and on the face of it it seems good. Going to replace the front and rears seals just in case they have dried out as the box has not been in a car for a long time, bought it from a classic car dealer who'd had it for an unfinished project. It came without
Also, I'm just in the process of ordering the full load of stuff from Ben @ Simply Performance as follows:
CONVERSION RING 5.3 GM CONVERSION FLYWHEEL
[HK9769-TTVHD, 77-3380] TTV COVER AND 6 PAD SPRUNG PADDLE HD 240MM CLUTCH
XJS MANUAL PEDAL BOX RHD ABS - INCLUDES A £125 SURCHARGE FOR THE OLD UNIT
XJS REPRODUCTION SKI SLOPE NEW PROPSHAFT
GEARBOX MOUNT ADAPTOR
[EAC7651#] GETRAG 265 CLUTCH SLAVE CYLINDER OEM AP
[CBC6377#] XJS WITH ABS CLUTCH MASTER CYLINDER LATE OEM AP
It does feel like a bit of a shame to be removing it as there is quite a bit of Mystique attached to the whole setup.
However, no regrets as the new ECU will be a Maxx ECU suplied by Mobeck with an all new wiring loom and components, with endless tuning flexibility.
Also quite convenient that there is a nice leather wrapped compartment in the passenger footwell for the New ECU to fit neatly into, unlike maybe some of the standard XJS cars?
Last edited by Sprintgtvgus; Jan 2, 2025 at 08:16 AM.
I think I’m also going to bit the bullet and go for tubular headers which I will get ceramic coated.
Not expecting any real gains but I think it will sound the business.
The one I’m actually less sure about is whether to get the 72mm throttle bodies over the 63mm.
I have the old throttle bodies all cleaned up and not yet re assembled, one guy told me I can get the throttle place from something like a 4.0 AJ6 car and then have the housing machined, or of course maybe I can just get them from simply Performamnce.
Not sure it’s necessary but it may help make the engine slightly more responsive. If that’s the case then I guess I also need the manifold flange matched and machined out to suit the 72mm.
A bit defensive on my part maybe - but I’m more interested in real world feedback of those whom have fitted the larger throttle bodies than what someone wrote in a book the 80’s or 90’s.
The manifold flange needs no enlargement adjustment if (for example) the AJ6 large throttle bodies are used, or their equivalent. I do not know exactly what diameter they are but they are larger than the OEM ones and in my experience with my car, together with the AJ6 super enhanced ECU, made a HUGE diffference to the car's performance and responsiveness.
So I reckon with your Mobek units enlarged throttle bodies will be worth having. Grant Francis used the TBs from the AJ6 engine and they worked fine for him.
I am not sure how relevant this Pre HE modified engine data is to an unmodified HE engine.
All I know is that the AJ6 enhanced ECU plus the larger TBs and higher flow filters I have absolutely transformed my car. It accelerated faster, particularly at high speeds, went off the line faster, and was altogether far more responsive and lively. Now all this might happen with the rest of the AJ6 stuff and standard TBs. Or, it might be as some hold, that the larger TBs open faster as far as airflow is concerned for a given throttle travel; but I doubt this as pre AJ6 it did not matter how far you opened the throttle, the liveliness was not there, nor was the plus 80 mph far better performance.
Just for info. I made 493 HP in an 5.3 on standard throttles, could not measure any conserning vacum in the menifold at 7000rpm... 5.3 V12 dyno
Best regard
Ole Mobeck
Thanks, seems like maybe the throttles are not a big deal, which inlet manifold did you use? A Don Miles one?? you also teased that you are developing your own manifold?
Thanks, seems like maybe the throttles are not a big deal, which inlet manifold did you use? A Don Miles one?? you also teased that you are developing your own manifold?
Well the car has now gone off to an independent Jaguar garage for them to start on the work. Many parts are shortly arriving from Simply Performance.
I have instructed the garage to give the car a check over and fit the engine and gearbox, fit the pedal box and other conversion parts etc etc.
Then we will pause and see where we have got to.
I may fit the Mobeck Engine Management kit myself once the engine is in.
My thoughts now turn to suspenison.
The car has brand new Bilstein B6 front shocks and all new bushings but the 4 rear shocks look to be the originals.
So trying to decide which shocks and suspension to go for.
The XJR-S shocks are supposed to be specific to the car but it doesn't look like direct replacements are available, I only have the rears which I suppose could be rebuilt....
Most simple would be some B6 rear shocks to match the fronts and keep with the original springs. Cost of Circa £580 for the rear shocks.
Then Simply Performance have the GAZ setup, although never really been that much of a fan of GAZ.
I emailed Nitron in the UK but they do not offer anything which is a shame as I have been pleased with their products befgore.
So, maybe I'll start with B6's and the standard XJR-S springs....
OR maybe consider the Aston Martin DB7 GT Shocks front and rear which would be about £1100 for a set. The Vantage GT was the final version of the DB7 with supposedly the best suspension setup.
Not had a problem with Bilstein B6's when I fitted them to a BMW, quite pleased infact, so maybe I should not try and re invent the wheel here, got to get the car back on the road and then see!?
Maybe I could try the B6 all around with the XJR-S springs first.
What other options are there for good handling without completely trashing ride quality?
Last edited by Sprintgtvgus; Feb 3, 2025 at 03:11 AM.
