Instead of rambling for ever, selling rubbish with no figures because stuff the just doesn’t work, we spent time & money developing this engine properly from a clean sheet of paper then doing testing and long term reliability over a few years. Perhaps that makes us sound rude, well perhaps we have some right to be!

The first little question is, « why on earth hasn"t this all been done before? ».

Easy answer really.

Jaguar development engineers have been banished, because as you will quickly see, they really didn’t have a clue what they were on about, and generally messed it up.

Not suprisingly they never got any power out of the AJ6, because of the - Jaguarsport/TWR/Tomdicknharry «let"s change some anciliaries/silencers & charge stupid prices for chrome» approach.

Power is ALWAYS in the cylinder head

Merchandising is always the enemy of good engineering , unfortunately there’s plenty out there still doing it !

GRAPH1

So here’s some genuine valid flow charts of our head work from an independent superflow SF600E ;-

The vertical blue bar is original Jag max cam lift, and the bottom scale is in mm valve lift.

Flow figures are on the left.

You can’t cheat here, the figures speak for themselves

The bottom line is the original 3.6 head, the next red line is our EVO1 road head, then the blue line is from AJ16 engines, then our EVO2 road/circuit head, then finally our proper full race engine EVO3. As you can see, the AJ16 head simply doesn’t work. Now being as it was sold as an improvement, that sure makes you think!

GRAPH 2

If this has raised your curiousity read on,-

I will give you good reasons why all this works (or doesn’t work as the case may be)

Here are the curves for exhaust valve flow ;-

The more radical graphs illustrate perfectly the problems with the early AJ6 and the AJ16.

For interest, the exhaust ports are identical but the valves are different weights.

The AJ16 has much lighter one piece valves with thin stems, which look & sounds great in theory. In practice it’s rubbish.

Up to 5mm lift the early head flows better, then the AJ16 overtakes it for a bit, then it flattens out to identical at maximum valve lift

Now who would have thought it ?

Now you will probably say to me, but doesn’t the AJ16/X300 produce more power than the 4.0L XJ40 engine ?

Well in DIN terms it’s minimal.

The AJ6 4.0L manages about 230bhp on the early head, wheareas the AJ16 manages about 245-250, but more often in production.

This is explicable for 2 simple reasons.

The AJ16 inlet valves flow a little more, right close to max lift between 8-10mm, but is worse everwhere else, and truly abysmal up to about 6mm. That means the AJ16 inlet ports are TOO BIG... a Jaguar screw-up.

The AJ16 exhausts also flow really badly EXCEPT between 3-5mm of lift, in fact it’s really very poor figures but made up by one incredibly important fact.

The valves are so light, they use « toy » valve springs. Most of the power gains in fact come from reduced camshaft friction and better inlet port matching, because they were easier to align than the early head.

Have you ever seen what Jaguar did for port matching on AJ6? How about this for hideous mismatches ?

BEFORE

AFTER

MODIFICATION

It strange how so many so called « experts » persist in telling people 4 valve heads are all the same, and very little can be done to improve them.

Well, as you can see the flow on what we term the EVO 1 head is pretty much improved.

We specifically designed this to remain invisible to the eye, keep costs down, and get a gigantic increase in flow.

As you can see it’s simply masses better than any other and beats the pants off the AJ16 head right up to nearly max valve lift.

The 3.6L engine really benefits from this extra breathing and the infamous flat spots and poor torque just literally disappeared.

When you look at what we did to the exhaust breathing, there’s a giant YUMP between 5mm lift and maximum.

In fact it’s scarcely beaten by our full race EVO3 head which has BIGGER valves !

It’s nothing short of spectacular, taking the miserable 210-220bhp engine up to more like a genuine 265 and moving the maximum power up by a full 500rpm.

This speedo doesn’t lie , this was taken well before we had finished work, & it goes off the clock.

Now how do you feel about going quicker than a V12 with a 2.5L smaller engine, but standard original camshafts AND ECU ?

Tasty ?

HISTORICAL STUFF

The Jaguar design brief was very simple, and dates from the time of the Triumph Dolomite sprint 16V.

The same Spen King design team did it, and they stuck the « leyland down the plughole » symbol on the end as well.

As it was 1973, it was simply « at all costs get Jaguar"s fuel consumption under control ».

So what they did was do the opposite of good sense, and instead of making a nice compact chamber head like Cosworth, they made an odd assymetric design with excessively wide angle, and slanted over like the Dolomite.

-calling it the AJ6, they cribbed the same bore spacing as the V12, a 1mm bigger bore, and gave it the same stroke as a Triumph TR2. (which was used in a massey ferguson tractor).

What it never got right from the factory, was produce Tractor-like torque.

The torque curve is notoriously wavy, poor at slow speed....with a nasty kink & very frustrating dead spot between about

3000-3500rpm ;- In fact typical of a really lousy head/port combination, doing nothing well.

Not suprisingly it gave almost identical results to the Triumph Dolomite 16V, in fact, low compression US AJ6 models struggle 200bhp, which is only 56bhp/L and no better than the 1940s designed TR4 engine.

What we did next was to make some small improvements to the pipe lengths and clever siting of silencers, to take the maximum advantage of that terrific extra head performance, in fact a proper sports car engine that revs round to 7000.

Not bad huh and dead quiet to drive?

What I have specifically NOT done here (reserved for a later article), is detail what happens next when you change the camshaft profiles.

As you can see, on the original heads, breathing simply doesn’t improve after the original max valve lift shown by the sharp blue line.

Only the AJ16 head could benefit from a hotter inlet cam.

However with our heads, the performance is so MASSIVELY improved, the small exhaust valves are ACTUALLY breathing as well as the original large inlet ones...that’s what we call a quantum leap in performance.

What is far more of interest is, (for the next article) is what happens to our EVO1 the moment you add another 3mm valve lift.

This road head has a massive amount BETTER breathing. Adding more lift then transforms the engine, & gives it literally shed loads of extra torque. That torque is significantly higher than the V12, and starts much lower down at about 1500rpm.

So we turned an old fashioned lemon into a modern fire breathing monster like the Ford Duratec.

Anyone takers for a 330bhp engine in their XJS ? Or can you handle more ?

Webmaster