Hello everyone. My lovely XJ6 4.0 doing fine and I am wondering about the virtues and vices of the engine for pure curiosity. So, design intents compared to how things turned out? Everything I have read on Wikipidea is a bit too short.

 

An interesting question. As I understand it, the AJ16 was never destined for a long production run, and was a stop gap to allow Ford to launch the X300 ahead of the new V8 engines being ready. The changes to the AJ6 were pretty much the minimum necessary to ensure the X300 was competitive in terms of performance, economy, and reliability.
As you will know, a great engine, and whilst a little behind the curve in terms of performance and economy compared with modern engines, a power plant you can become attached to.
The only "vice" I can think of would be the upper timing chain tensioner, but this is by no means universal, and hardly serious.
Having owned a both an AJ16 and a V8 XJR, I much preferred the former, which is why I still have it!

 

Do tell. My prior car was an XKR coupe so I can appreciate the inline 6 vs supercharged v8 (whose nasty sounding supercharger was a distraction to the inherently amusing v8 sound).

The AJ16 is certainly smooth, and together with other components makes nice sounds.

Q1: Why doesn't it rev higher? BMW inline sixes rev higher although they are smaller displacement...

Q2: It doesnt feel as torquey at low revs the way many v6 engines feel (that are also lower displacement). Is that accurate?

Q3: anyone notice that the 4.0 with auto trans in normal (not sport mode) can barely keep up with traffic as it shifts around 2,100 rpm?

 

I think my preference for the 6 cylinder XJR was based on basically 3 factors.
- I had the 6 cylinder car from new, and it was my first experience of a supercharged car. Blew me away with the performance, and scared most of my friends witless! When the 8 cylinder car was announced, with more power and better economy, I couldn't wait to buy one. When it arrived, it was great, but wasn't such a noticeable difference in performance from the 6, and so fell slightly below expectations, whereas the 6 had surpassed them by some distance.
Secondly, with the additional cog in the gearbox, and I suspect superior shift quality, the 8 went about its business in a rather less dramatic way that the 6. Kickdown lacked the same drama in the 8 as it had in the 6, although I dare say it got down the road a bit quicker.
Finally, The sound of the 6 appeals to me, with a turbine like dimension. The 8 was relatively mute, which is undoubtedly more sophisticated, but less involving and less fun.

Turning to your 3 questions,
1) I think the straight 6 BMWs tend to have a shorter stroke than the AJ16. More revs, potentially more BHP per litre, but less torque. I have owned 320, 325, and 535 straight 6 BMWs, and loved them all (320 the sweetest engine by far), but they were all light on torque relative to power. None would be a fair comparison with a 4 litre supercharged AJ16.
2) Difficult to make an open ended comparison like this. For a given displacement , and everything else being equal, a V6 can produce a good torque outcome relative to a straight 6. Certainly some modern turbocharged V6 diesels can churn out the torque, and straight 6 is an engine configuration that seems to be heading for extinction (BMW excepted)
3) Keeping up with traffic is not a problem I have heard of for A16 XJRs or indeed normally aspirated A16s. From personal experience, I never use sport mode, but constantly delight in blowing off traffic that assumes my car is just an old Jag. On the motorway, 2100rpm would roughly equate to about 70 mph, and few pleasures compare being to being on an incline with an Audi flashing his lights behind me, and engaging kick down. Juvenile, I know, but such a simple pleasure!
Joking apart, your gearbox changing at 2100 on a light throttle doesn't strike me as unusual, and indeed would be another indication of an engine with plenty of torque. If it is doing this at wide throttle openings, you may have a fault of some sort.

 

Q1: Why doesn't it rev higher? BMW inline sixes rev higher although they are smaller displacement...

Q2: It doesnt feel as torquey at low revs the way many v6 engines feel (that are also lower displacement). Is that accurate?

Q3: anyone notice that the 4.0 with auto trans in normal (not sport mode) can barely keep up with traffic as it shifts around 2,100 rpm?[/QUOTE]

My other car is 2005 BMW 330: completely different even though it is a straight 6, much quicker and all that as expected. The Jag while it doesn't feel as torquey feels like it pulls and doesn't run out of steam, right on up to 120. I've tried both sport and normal, truth be told I can't really tell the difference, although when I push the sport button, I tend to get on the gas more. the other thing is that its a 4 speed transmission, so its not hunting for gears and having to drop 2-3 when it kicks down. Kind of like my old Plymouth Fury with a 383 and 3 speed torqueflite, the best transmission ever invented for teenagers :-)

 

Thank you Scotland and Boston!

If anyone has more technical details to share re AJ16 and predecessor please do....

Throttle body (s), cams, valves, ....

 

Our AJ16's are an undersquare design which is precisely what you want for low end torque, but does limit max revs. I think a lot of the "feel" our our engines is based on it being designed for a diesel option that never materialized. It's given us a bulletproof lower end, and the freight train type of linear torque that we experience. I notice Sport Mode makes downshifts occur with less throttle pressure, and it holds each gear longer before upshifting. I leave it in Sport Mode until the roads get icy.

I've never seen the actual torque curve of this engine but it does feel unusual in that I get a bigger kick-in-the-back just AFTER it upshifts when I've floored the throttle. It's a completely different experience than an engine biased for higher rpm horsepower.

 

Torque curve Jaguar X300 4.0

 

A picture is worth a thousand words! Now thats about as Canadian prairie-flat a torque curve as I've seen. A downshift at redline would increase the available torque. Makes perfect sense with what I feel! I bet my XJR's dyno would look similar, just both at higher numbers.

Many thanks for posting this Keesh.

 

Hi Weisberg,
It's a shame you aren't in the UK so couldn't attend my seminar earlier this year on the Jaguar AJ6 and AJ16 engines. All your questions would have been answered. Below are some extracts from my presentation, though they might be a bit difficult to follow without the explanation that went with them.








As has been stated elsewhere in this thread, the reason for the low peak power speed of the 4.0L engines is their unusually long stroke of 102mm. If the torque curve is plotted against the mean piston speed, rather than engine speed, which take into account the long stroke, then the torque curve will look quite competitive for an engine with no variable valve timing or pressure charging. The naturally aspirated AJ16 engine achieves a peak torque of 400Nm from 4.0L, so 100Nm/litre. This is still a competitive value today, especially for a non VVT engine. The torque per litre is often equated to the equivalent mean pressure on the piston during the power stroke. This is called Brake Mean Effective Pressure (BMEP). This value for the AJ16 engine is very similar to the higher power, but smaller capacity BMW M5 i3.8L inline 6 engine. It is just that because the BMW engine has a shorter stroke it can achieve this BMW at a higher engine speed, and so make more power.


I hope this helps.

 

It is a shame I am not in the UK to attend your lectures. I can follow the equations, and I am quite satisfied with your answer. Thank you.

The engine and ZF 4 speed transmission make a good combination. My only curiosity is the effect of having a 4.01:1 rear axle ratio vs the standard 3.58:1 for my spirited city driving. I know. sell it and buy an XJR, but it was Dad's and I cannot part with it.

 

What an educational thread!

It almost hurts to hear it was somewhat of a "placeholder" for the V8 Jaguars. I definitely prefer the interior and exterior of the X300 to the X308. Our AJ16s are as bulletproof as they come, and broken in they get extremely good mileage comparatively--I'm getting around 22-24 in the city, and more than 30 highway according to the trip odometer!

I think whatever Ford "did" that they usually "don't" to these fine engines was an excellent move. Great amount of power and incredible balance all around...and they sound fantastic, IMO. Any of us owners can testify to the smoothness and quiet grumble we hear in the cabin, that's for sure. Not to mention how cool they look (I've had quite a few compliments), and how easy they are to wrench on! I'm considering reducing the compression ratio to that of an XJR, adding a turbo and seeing what kind of power I can get, though I'm not overly experienced with that kind of stuff. Still, looking at a Nissan RB25DE, a 2.5L straight six with similar torque-to-horsepower, and how loaded those things can become, it's more than too tempting...

 

I have met someone in Australia who has built an uprated supercharged AJ16 engine which it is claimed produces 580bhp after correcting for transmission losses from wheel horsepower measured on a chassis dyno. The size and running speed of the supercharged used, together with the boost pressure produced are consistent with this claim.

 

Would be very interesting to understand the details, challenges and what was done to achieve those results.

 

There are too many detail changes to put them all in this thread, so I have emailed them to you direct. The conversion is based on an Eaton RVT1900 running at 2.5:1 drive ratio. A 90mm throttle body was fitted together with an adaptor to the SC inlet. Everything else was custom made. The exhaust system used my design of flanges and primary and secondary geometries. The ECU was remapped to use the signal from a higher flow rate airflow meter and larger capacity injectors were fitted.

 

The AJ6/AJ16 is a essentially a "truck motor" that was designed for heavy (4000 lbs) car equipped with automatic transmission. The engine was optimized for low RPM, this is especially true of the 4.0 variant.

I suspect that the combustion chamber of AJ6 and AJ16 was designed in the early 1980s, before CFD (Computational Fluid Dynamics) software gained widespread use. It's not uncommon to gain 80-100 horsepower on a 5.0 Mustang engine of the same vintage by slapping a modern set of cylinder heads on it. I suspect that AJ6/AJ16 could use an improved head. Sadly, there is probably no market for it, so it's unlikely to happen. The 4.0 is a long-stroke motor, so revving it high isn't going do happen. The factory never optimized these engines for high RPM use, which is a logical choice for XJ40, XJ300 and XJS that they were installed in.

Good things about AJ6/AJ16 family:
- bomb-proof bottom end (almost diesel-like)
- Inline 6, when made with quality materials and balanced well is a very strong configuration, arguably more so than a v8
- DOHC and fairly high compression ratio

Bad things about AJ6/AJ16:
- Low power for displacement
- 4.0 is a long-stroke engine, so it won't rev into high RPMs which is how the high horsepower figures are made
- weights more than an American 6 liter V8 engine

The way to make horsepower on a normally aspirated engine is:
- Displacement (AJ6/AJ16 got plenty of that)
- High compression (AJ6/AJ16 got that - 10:1)
- High RPM (not going to happen on any AJ6/AJ16 over 3.6L displacement due to long stroke)
- High flowing cylinder head (AJ6/AJ16 leaves much to be desired in that respect)

You could probably build a high-revving version of 3.6 with a better flowing head. In fact, I toyed with this idea. This endeavor would require a significant investment of time by people who know more about cylinder head design than I do. There's no money to be made here, so unless this project becomes "labor of love" for someone qualified it won't happen.

There are much cheaper ways of making an X300 run like a scalded cat. The frame rails on XJ300, late production XJ40 (1992.5 and up) and XJS are wide enough to fit an American V8. An LS1 with a mild head and cam package will be upwards of 400 Hp. Make that over 500 with forced induction. If the budget is tight you can find a late production Ford 5.0 from a junk yard for a couple of hundred bucks and slap a pair of aluminum heads on that puppy for another $500. That will be good for at least 320 Hp. Bonus points - you have plenty of manual transmission choices. The downsides are typical of an engine swap.

 

To your point, the fella in Australia mentioned in post 13 above has spent a few years and a huge amount of money to get 580HP out of the AJ16. So it is possible, but you essentially have a completely custom engine and control systems.

That all being said, most things follow a curve of diminishing returns. So it would be really interesting if we could get 80% (or even 50%) of that performance for 20% of the cost. But in reading what he did, I'm not so sure it is possible.

I've got some thoughts about getting the 10% lower SC pulley and increasing intercooling capacity (bigger radiator or maybe killer chiller) but our options are certainly limited since there is so little aftermarket.

...now if there were a bolt-on kit for an M112 or a "twin screw" SC, that would be great. Although I like to tinker and fabricate to a point... I'll be the first to say I really prefer something in kit form that I know has been tested and works. No such kit however exists for the X300.




.

 

Juha,
An interesting post, and much to agree with, but I am surprised you do not mention torque at all. In the real world, it is arguably even more important than BHP to the driving experience, and certainly is as far as the X300 is concerned.
Your list of ways to pep up bhp doesn't mention forced induction, which is of course the route Jaguar pursued, and interestingly so now has almost the entire industry.
As for fitting a Ford 5.0 V8 and putting aluminium heads on it to make 320bhp, why would I go to all that trouble to make marginally less power than my standard XJR already has (326 bhp) in a far less refined manner?

 

The 4.0L AJ6/AJ16 engine is indeed a long stroke (90mm bore x 102mm stroke) engine so is better suited to producing high torque at lower speed than making high specific power (eg 100bhp/litre) at higher speeds. The 4.0L AJ16 engine produces 400Nm (ie 100Nm/litre) which is still to this day a respectable output for a naturally aspirated engine, especially one without variable valve timing or direct injection and without the benefit of any computer simulation techniques to help with optimisation. With its 102mm log stroke, it would be difficult to tune the engine to produce peak power at a high engine speed, by modern standards. However, it would be quite practical to tune this engine to produce peak power at over 6000rpm, as this would only equate to 20.2m/s mean piston speed. This is quite a modest piston speed to make peak power. The 3.8 litre 6 cylinder BMW M5 engine of the same period produced its peak power at 21.8m/s with its 94.6mm stroke engine running at 6,900rpm. The standard rev limiter on the AJ16 engine is set at 5,950rpm, but could be raised by reprogamming the ECU. I would disagree that the port design of the AJ16 is in some way inherently unsuitable for producing high power outputs. The valve/ piston area ratio is reasonable for a 4 valve pent roof combustion chamber design. The mean port velocities would be reasonable (108m/s) at a mean piston speed of 20m/s. The port to valve angle axis is about 40 degrees and the intersection of port to valve axis allows for a large inner corner radius. With careful modification it is quite possible to achieve a port flow coefficient of over 0.68 at high valve lifts. This is quite respectable. If the cam profiles, intake and exhaust geometries were optimised to produce peak power at 6000rpm, it should be possible to produce about 300bhp from a Jaguar AJ16 engine. However, nobody has ever commissioned me to produce such an engine for them.

 

It should be possible to hook up either VEMS or Megasquirt to AJ6/AJ16 which opens up interesting tuning possibilities. It is also possible to replace PROM chips in the ECU with a daughterboard making the original GEMS system programmable. I would venture to guess that about 15-20Hp can be 'found' by just reprogramming ignition advance and fuel maps, due to conservative nature of Jaguar's original programming.

A decent supercharger shop should be able to make a hybrid unit out of a ported M90 case and M112 parts. The stock Jaguar EFI uses MAF, so it should be able to compensate for extra air getting into the engine up to a point. Eventually you start bumping against the limitations of the injectors. You can either crank out fuel pressure, which is a hack or install bigger injectors and reprogram the fuel maps.

With a 6K RPM redline, IMHO a normally aspirated AJ16 should be worth at somewhere in the neighborhood of 300 Hp if the head flows well. May be it will need some porting, I have not yet messed with it to have an informed opinion. Rigging an aftermarket ECM or making the stock ECM programmable IMHO is the starting point. The latter is only practical if someone already knows that code (i.e. has disassembled it and knows where the ignition advance and the fueling tables are).