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Exhaust overview
XJ Saloon S1,2,3
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TT exhaust explained

The Jaguar V12
V12 Performance
Stretching the V12
E-type EFI conversion
D Jetronic
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Article: V12 Engine
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Article: XK Engine
Article: 4.2 EFI

The Jaguar V8
V8 Performance

AJ6 Superchargers
Supercharging article

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Updated August 2016
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1. How is a TT exhaust system different to a big bore one.

A big bore exhaust system merely relieves any restrictions to flow that might exist. As the main restriction on most Jaguars takes place in the 1st silencer boxes, replacing these with our Silencer Substitute Pipes achieves a major part of the gain available from a Large Bore System. Our TT (Twin Tube) system consists of tuned length resonant pipes which play an active part in the scavenging process and help to draw exhaust gases out of the engine. A TT system can therefore increase performance much more than the big bore system and can do so even in the mid-range speeds. Really effective performance systems thrive on high gas velocities - which means pipe sizes do not actually need to be very big. It is instructive to look at the modest pipe sizes found on cars made by such as Ferrari - they are hardly forced to skimp on pipe size yet undoubtedly provide the necessary performance.

2. Can you supply multi-branch exhaust manifolds for my Jaguar V12?

Well we could - but the trouble is that there isn't enough space to install pipes of the right dimensions to work properly. If the dimensions are wrong then the system can actually have a negative effect. There is virtually no overlap of exhaust pulses into the cast three-into-one manifolds of the V12 (and 6 cylinder engines) so replacing them with separate pipes doesn't reap much improvement for the cost and trouble involved. As the TT system keeps each group of three separate for a considerable distance it provides the most viable solution.

3. Can you supply a TT system with much bigger tail pipes?

Yes, but large tailpipes tend to generate an unpleasant exhaust note and can be excessively noisy. We therefore do not recommend tailpipes larger than 2.5 inches diameter for road cars although we can provide larger pipes which are sleeved down inside the rear silencer. The tailpipe ends can be square cut, angle cut, rolled over or twin pipes can be provided at extra cost.

4. Can the TT system be supplied with catalysts?

We are investigating the possibility of adding low loss catalysts in the position normally occupied by the first silencer boxes to satisfy the requirements of some markets where a visual catalyst check is mandatory. It is not practical to incorporate catalysts further forward in a twin pipe system but a lambda feedback sensor (heated type) can still be used which means most cars will still pass a yearly or biennial emission test without the catalyst being present.. Contrary to some beliefs UK cars do not have to have a catalyst to pass an MOT test.

5. Can you supply an exhaust system with straight pipes running under the rear axle assembly to avoid the restriction of the bends around the drive-shafts?

Actually it is well known that bends in an exhaust system have no significant effect on performance. The bends around the axle typical of a Jaguar are hardly restrictive at all and there is nothing to be gained by having a straight pipe run.

6. Why don't you have a cross-over pipe between banks on your performance exhaust systems for the V12?

It is true that a crossover can be of benefit to conventional V8 engines (with two plane crank) but only because they have an uneven firing sequence along each cylinder bank, which creates the rather delightful V8 exhaust note. A consequence of having two cylinders firing consecutively on each bank is that the flow rate is briefly doubled, then later in the cycle the long gap between firing impulses halves it, compared to the average flow rate. The temporary high flow rate means the V8 can benefit from using larger exhaust pipe sizes than other engines and a cross-over pipe can help to even out the impulses. It is not obvious but the Jaguar V8 has such a balance pipe within the main silencer box.

A V12, on the other hand, has equally spaced firing impulses along each bank creating an even sequence of pulses. There is no flow variation to require a balance pipe and the system does not have to contend with brief periods of peak flow so pipe sizes can be smaller. The only reason the V12 E Type had a balance pipe was simply to tune the exhaust note.

7. Isn't a quad throttle system better than large single throttles on the Jaguar V12?

No, because the quad throttle layout goes against some of the accepted principles of good manifold design with regard to flow bias so is actually worse for air distribution than the standard arrangement. Quad throttles don't perform as well as properly sorted single ( to each manifold) throttle systems and they add unnecessary complication which is one reason why the layout was never used on either the TWR Group A XJ-Ss or the XJR15 racers.

8. Can I just buy large throttle disks from you and bore out my own throttles?

No. There is much more to it than that. We do sell the throttles as complete assemblies but other work is necessary to get the best from them.

9. Can you sell me a cold air intake kit for my V12?

It is not something we offer although we do add supplementary intakes below the filters as a means of increasing breathing area without introducing excessive noise. At high speeds the air reaching the intakes behind the radiator is much cooler than at low speeds so the performance gain of cold air intakes is less than might be imagined. It is easy to forget that warmer air is better for fuel economy so the standard 5.3 arrangement is a better compromise than it might seem.

10. Why do I need to have the ECU modified when I improve the engine's breathing?

Quite simply, more air requires more fuel. On the V12, having MAP load sensing, minor breathing improvements can exploit the slight over-richness that the standard ECU provides to limit temperatures at full power but significant improvements will always require some sort of change to the fueling Engines with MAF load sensing will often compensate automatically as long as the increased flow does not extend beyond the area mapped by the ECU.

11. Do I need to have the ECU modified when I fit a manual transmission?

The problem is that over-run cut off can produce unpleasant oscillations as the car rolls to a standstill. It is therefore good practice to delete this function from the ECU - as Jaguar had to hurriedly arrange after introducing the manual 3.6 XJ-S.

12. Do I need to upgrade the Lucas Constant Energy ignition system on my HE V12?

No. The Lucas CE system is extremely powerful (more so than most so called "high power" after-market systems) as befits a very high compression 12 cylinder engine. It is quite adequate for racing V12s running at 7000 r.p.m. and beyond.

13. How does a Super Enhanced ECU cheer up a Jaguar V12 engine?

Mostly by sharpening up response to throttle movement although there are subtle changes to the main mapping to liven the engine in the mid-range.

14. Can you just send me the upgraded chip for me to put in the ECU myself?

Only if the ECU is a 26CU or 36CU type (from 92 MY face-lifted cars). Earlier cars have the chip soldered in and require circuit changes as well so the ECU must be sent to us for upgrading.

15. How does a Plus Torque kit improve mid-range performance?

The modifications to the individual tracts of the inlet manifold improve breathing by exploiting pulse effects according to long established principles.

16. If I fit a Plus Torque kit or a TT exhaust system will my car still pass an emission test?

Yes, as long as the engine is in sound condition and the feedback sensors are still working. The only difficulty (for the TT system) is in places where a visual inspection for catalysts is mandatory.

17. Can you test and / or repair my ECU?

Yes, depending on the car. It is something we do routinely on bench top test rigs which reproduce in-car situations.

18. If I just fit a smaller pulley to the supercharger of my XK-R won't I get an easy power increase?

Not really. It will provide increased torque but at high speeds the raised temperature of the air leaving the supercharger largely negates the potential for a significant power increase. (see our feature on supercharging for a full explanation).

19. Can you supply a richer chip for my 4 litre AJ6 engined XJ-S or XJ40?

We could - but there isn't any point. It is necessary to get more airflow into the engine so the first thing is to reduce the restriction of the airflow meter (MAF) and if this is correctly calibrated the ECU will still fuel the engine correctly so will not need to be modified. The only way a chip change can provide increased performance from the 4 litre is by advancing the ignition timing, which places the engine at risk of piston failure. Actually one can achieve the same effect by just moving the crankshaft trigger sensor forward a few degrees - but it is still a risky thing to do.

20. What is the difference between road and race version of the Plus Torque and TT kits?

Essentially the tuned pipe lengths are shortened to improve breathing higher up the speed range to benefit power rather than torque.

21. Can you supply high lift camshafts to cheer up my road going XJ-S?

We could but we found that it is not a very productive thing to do on most Jaguar engines so we only recommend a change of camshaft for racing and then only when the induction and exhaust systems have been uprated. The problem is that a longer duration cam lifts the breathing efficiency of the valve gear to higher engine speeds but the standard induction system is unable to keep up. The end result therefore is a loss of low and mid-range torque without any worthwhile top end gain. In fact many years ago we made some short duration, reduced lift, cams for the V12 that boosted low end torque and knocked about 1 second of the 0-60 time. We discontinued them because we found that it was almost impossible to sell something that gave less top end power - even if it did make the car faster in most situations!

22. Can you supply ported and polished cylinder heads?

We could, but the design of Jaguar ports are among the best anywhere and we found long ago that money is much better spent in other areas so it is not something we normally provide. Port modification, other than cleaning up any casting imperfections, would normally only be worthwhile on a full race engine. Polishing ports is actually a complete waste of time - it just looks nice on the bench. The shape of the ports is the most important issue. The surface finish needs to be smooth in most areas but controlled roughness on the inside of bends can be beneficial by helping the air to follow the inside of the curve - rather like the way dimples in a golf ball pull the air in behind to reduce drag and extend the flight.

23. Can you supply big valve heads for my HE V12?

The design of the HE cylinder head with the exhaust valves deeply pocketed in a compact combustion chamber makes it difficult to find any advantage from increasing the valve size. The larger valve head just gets closer to the combustion chamber wall thereby choking the flow-path. In the circumstances there is not much point in fitting larger inlet valves.

Larger inlet and exhaust valves can be useful on the earlier flat head engine but only when the engine is already heavily modified in other ways.

24. Do you have a supercharger kit for my HE V12?

It is not a practical thing to offer. The ideal engine for supercharging would have a compression ratio of about 9:1 with valves and porting that are able to handle considerably more flow than standard. The HE V12 has a compression ratio of between 11:1 to 12.5:1, depending on year and market, and also has deeply pocketed exhaust valves that are not well suited to higher flow rates. Supercharging beyond about 6 p.s.i. (anything less is hardly worth the bother) requires some form of inter-cooling which adds to the practicality of fitting everything into the available space.

Because the compression ratio has to be lowered the HE V12 must be totally rebuilt before it would be suitable for supercharging. It surely makes more sense to rebuild it to 7 or 8 litres and forget about supercharging. Either way this is not a cheap exercise.

25. Can you supply the parts to build an 8 litre V12?

No, but people like Rob Beere can do so. We can provide appropriate induction and exhaust systems and an ECU programmed to suit.

26. How much should I advance the ignition timing to improve performance?

You can only advance the timing safely if the octane rating of the fuel is increased. Otherwise you will place the engine at serious risk of suffering piston damage as a result of high speed detonation - which is not always audible before striking. To put some perspective on this Jaguar officially recommend that the 12.5:1 compression European HE V12 (1981-88) should have the timing retarded by 5 degrees in order to safely use 95 octane (RON) unleaded fuel instead of the 97 octane originally specified. We think this is probably a bit over-cautious and that 3 degrees of retard should be adequate, conforming to the general guide that reduction of 1 octane number in the fuel requires approximately 1.5 degrees of retard.

27. What is the difference between detonation, knock and pre-ignition and how might it effect my engine.

This a very deep subject. A concise explanation will be found in our "Introductory Technicalities" section. Note that harmful knock does not necessarily give audible warning before causing damage and is not the same as low speed "pinking".

28. If the HE stands for "High Efficiency" surely isn't the HE V12 the best starting point for a high power V12 engine?

No. The "High Efficiency" label refers to part throttle fuel efficiency where the HE was a major advance over the pre-HE flat head engine. The pocketed exhaust valves are a major limitation on the power potential of the HE engine.

29. What is a "Flat-head" V12?

The original V12 had flat cylinder heads and the combustion chambers were formed by deep bowls in the piston crowns - an arrangement often referred to as a "Heron head" after the originator, the highly regarded aircraft engine specialist Sam Heron. The unobstructed valves make this engine a much better proposition than the HE as a basis for a high performance power unit.

30. Do I really have to throw away the head bolts and fit new ones if I have to replace the cylinder head on an AJ6 engine?

No. The bolts can be re-used without any risk of failure. It is a fallacy that they have to be renewed and no such recommendation will be found in the ROM.

31. Can I use unleaded fuel in my Jaguar?

Yes. The only stipulation being that it must be of the correct octane rating for the engine. Valve seats are high quality inserts so are not susceptible to excessive wear in the absence of leaded fuel.

32. Do you have a price list in $s?

It is not practical to have separate price lists for different currencies because the exchange rates are not consistent. If the figure is multiplied by the /$ exchange rate of the day then the result will be the $ price for that day. The exchange rate at the time of any transaction is the one which will apply.

33. Can you send me power curves for the performance of your products?

Individual engines can differ so much, for example V12s could easily vary by 20 b.h.p. from the showroom and the differences get wider still with time as the engine ages, added to the vagaries of dynamometers and rolling roads, that it is it is quite impossible to say that bolting X conversion onto a Y engine will give Z power.

There is also the dilemma that anyone quoting honest power figures would not look so good compared with someone else who makes exaggerated claims. We think it is better to simply indicate that a particular conversion can be expected to increase torque or power by a certain amount or percentage.

34. What effect does a Plus Torque conversion have on fuel economy?

It rather depends on how it is used. Best economy is achieved from spark ignition engines by running near to full throttle (not at high vacuum according to some popular beliefs) and at moderate engine speed. Hence use of the highest possible gear will generally deliver best economy. Improving responsiveness and mid-range torque makes this more easily achievable in normal driving so can aid highway cruise economy, however use of full available performance will always use a lot of fuel.

35. What LPG conversion would you recommend for my XJ-S V12?

Whilst the low fuel cost is undoubtedly attractive, there is an unavoidable quandary about expecting people to spend money to save money, so most LPG conversions are built and installed down to a tight price. Quality, therefore, generally seems to be suspect and one has to wonder about who would want to buy a converted car 5 years down the line. Considering the considerable loss of boot space and the little known prohibition from undersea tunnels (LPG converted cars are not permitted through the Channel Tunnel, amongst others) we think that anyone unduly worried about the running cost of a Jaguar should really just buy a smaller car. One might just about make an argument for an LPG conversion on a car that covers a high annual mileage but we are constantly amazed at the number of people who think about it for a car that only covers a few thousand miles a year.

AJ6 Engineering, 60, Henshall Road, Bollington, Macclesfield, Cheshire SK10 5DN, England
Email, Tel/Fax:- 0044 (0)1625 573556