What is the horsepower of a 4.2 XJ6 engine with 9:1 CR pistons

I am looking to upgrade the fi system on an XJ6 engine. In order to get the correct injectors I need to know the power output of the engine. In my case the engine has 9:1 cr pistons vs. standard 8:1, so I am not sure what the increased power should be, either at the crank or the rear wheels.

Added to the problem is the fact that I cannot find a reliable or consistent output number anywhere. It seems that there are different ways of measuring the output.

The XME fraternity have some numbers for a 9:1 engine, but I do not know if the camshafts in the XKE are different or not, which could deliver a different output.

Does anyone have any experience or idea of a realistic number?


Ray F.

As I understand it for a '72 XJ6 4.2/9.1 CR (mine), the gross SAE figure was 245bhp. It has twin 2" SU carbs. The DIN figure is somewhat less, this method was used on later cars. The XJ gurus will know more.

You‘ll have about 205 official DIN hp with a Series 3 XJ engine (larger inlet valves), L Jetronic, no cat.
The airflow meter must reduce output a bit so I would say it will make a little more.

265 SAE for a 3 HD8 9:1 engine.
245 with 2 HD8 and 8:1? That’s 185 DIN.
With emissions junk and smaller carbs it has somewhere around 170 DIN, don’t quote me on that though.

Thanks for the guidance given above. Consistent and in line with my thinking on what a 4.2 engine should produce.

I add that all emissions gubbins have been removed. Also system will run on manifold absolute pressure and not mass air flow and a standard size throttle body. Also adding distributor-less ignition.

Hopefully this will give about 260 to 270 bhp.

Unlikely without further modification but let us know!

E/XJ engines are identical, but with the years details inproved: larger water pump, coolant passages, oil cooler, larger oil pump, bigger inlet valves, better cam profiles, stronger rods, pistons all came later.

1 Like

From conversations with Ron Beattie years ago and what’s on the AJ6 Engineering web site, I’d guess that eliminating the air flow meter and matching the manifold to the head should get you to around 240 hp. Going much beyond that is quite difficult without spoiling the low speed qualities of the engine. Basically, the cylinder head was created for a 3.2 litre, not a 4.2. Of course, you can make a few more horsepower by swapping to an e electric water pump and fan. As in all things, everything is possible, but becomes expensive. For big and inexpensive gains, it may be easier to find an AJ6/16 engine.

“FI system” implies a Series III or very late Series II.

For the Series III cars these were rated a 176 SAE Net horsepower for the low compression versions and 205 DIN horsepower for the high compression versions.

Not sure about the Series II cars. About the same I reckon

Even Jaguar’s own literature contains conflicting information

SAE Net (which was phased-in circa 1972) is about the same as DIN, as I recall


The important detail is that the 176 hp S3 had a cat and the 205 hp cars didn’t.

240 just like that sounds great. I don’t understand why bosch didn’t go with a MAP sensor instead of the large and complicated spring loaded meter and why they didn’t match the ports a little better.

As you see from the other posts, Ray; numbers differ - they are sort of right, all of them, but reflect different ways of measuring, market variations due to regulations and engine management systems…

Arguably, the triple carbed was the most powerful 4,2 standard xk version produced - due to straight air flow through three throats. The power of an engine is fundamentally tied to the amount of petrol that can be burnt. Which is directly associated with how much air the engine can swallow - which relates to engine construction, timing and rpms. Supplying petrol at the right rate is relatively easy. But the 265 hp quoted will not stand up to scrutiny if compared to numbers given by later xk versions…

Added to the problem is the fact that I cannot find a reliable or consistent output number anywhere. It seems that there are different ways of measuring the output.

The air flow with the original, later, EFI ducting is impeding the xk. The amount of fuel burnt depends on the amount of air ingested - and is restricted to the ideal Lambda of 14,6 grams of air to 1 gram of fuel. More fuel added is not burnt, and will not add to power - and less fuel will reduce power…

The main problem was/is that carbs and ‘our’ EFI cannot maintain correct mixture over the whole rpm and load ranges - so to avoid flat spots the engine is set up fat.

Remember also that the xk was designed for 98 octane fuel - allowing for more appropriate timing, and more power, whatever measuring standard was used. Adding that the dist is not a precision instrument. For the US market a different advance was fitted - not aimed at maximum power, and added other emission control gadget did not improve matters…

So whatever you do; do not expect much gain with simple measures. More precision may certainly improve fuel economy - but do not expect much power increase.

I cannot really see why engine power should influence choise of injectors…?

xj6 85 Sov Europe (UK/NZ)

1 Like

Thank you all for the thoughtful and insightful comments. They certainly have helped to clarify and guide me.

I am not looking to increase power but to match the injectors to power needs. Proper flow rate of the injectors is important. Oversized and at low revs the engine becomes difficult to tune to run smoothly as the duty cycle of the injectors cannot decrease sufficiently. Undersize and the injectors have to run at 100% duty cycle (recommendations are to aim for 85% to 90% max duty cycle) and there is a risk of running too lean with resultant damage. So knowing the correct number avoids having to spend money on different size injectors in order to get it right.

From what I have read above something over 200bhp and less than 265bhp is an honest number. I think I will pitch for 240bhp.

The restrictions that some have highlighted above are valid and will remain after my modifications are done. Chiefly, I think, is the size of the throttle body which looks far to small for the needs of 4.2 litres. I remember a Harry’s Garage video where he improved the injection system on his XJ12C. A major contributor to an increase in power that he obtained was to increase the size of the throttle bodies. I suspect the same benefits would accrue to an XJ6 engine.

Once my engine is on the dynamometer I will know the right number. But, alas, that knowledge is still some considerable time in the future as my project progresses slowly!

Unless you are going to race the car, it seems pointless to attempt to increase horsepower available and go to that much trouble to do so. The cars have adequate power, and most owners do not stress them. An engine swap would be the way to go if you want a lot of power, and then you are putting an extra strain on the rest of the car, not to mention killing the value of the car in the process.

1 Like

What is this going in? Did I miss that somewhere?

205 is DIN, 265 is SAE net, don’t confuse them! Both result in about the same power as installed in a car.

I‘d like my XK to have more power.

1 Like

Mike you did not miss anything, but your instinct is spot on. I am replacing a 100bhp lump in an Austin Healey 100/6 with the XK engine. I am putting it in front of a 5 speed transmission.

So, while not necessarily wanting to increase the output of the XK engine, I am certainly beefing up the Healey.

1 Like

It’s going to be epic in an Austin-Healey without any power increase. Too much power and torque and you’ll want wider wheels and … the whole process can escalate to the point that a pretty little English sports car becomes a 427 Cobra. It brings me back to my previous suggestion of an AJ6/16 as possibly cheaper, easier and lighter (or less heavy).

Peter your suggestion was pursued by me early on. AJ6 engines are two-a-penny and cheap-as-chips in the US and on the face of it makes a good choice instead of the usual V8 conversions that one sees in Healeys.

Several factors settled me on the XK engine. Foremost was the idea that the Healey really did deserve the XK engine. But we know where that went within BMC. secondly, the exhaust side of the XK matches the Healey engine. The AJ6 is on the opposite side, hence complications with getting the exhaust out the back. The XK exhaust plugs right in to the Healey system. Finally but as importantly, the engine mounts of the 4.2 land right on top of the Healey mounts. Similarly the length of the engine and transmission put the rear mount squarely on the Healey gearbox mount. This makes installation easy with no modification to the Healey frame or body.

The sound of a nicely running XK engine is a joy to hear and would suit the Healey better than the raw sound of a V8. So that is also a decider. I owned a Mk1 manual with OD for many years and the joy of driving that car and the sound and feel of that engine was maybe the biggest deciding factor in my choice.


That makes a lot of sense especially for the exhaust which is already a tight fit under the car. There’s some precedent for your mod. I have a vague memory that the Healey company built an A-H 4000. I think they used the Rolls-Royce 4 litre engine from the big Princess R. They may have improved the body-chassis structure, maybe welding them together. Anyway, from what I can recall, it all worked very well. Of course, BMC/BMH/BL or whatever they were at that stage weren’t interested.

I ran all of my Jaguars (E-Type, XJ6 (2), XJ-S, XJ12) on a dynamometer to get real numbers at the rear wheels. Here is a link to information on some of my cars that I posted on Jag-Lovers years ago.


Indeed, Ray - so you better describe the EFI system you plan to install…

The original set-up for the Jaguar EFI was; constant fuel rail pressure - the ECU opened the injectors for a prescribed time, typically in the millisecond range, to inject the appropriate amount of fuel. The injector size was not really related to horsepower, but more related to engine volume…so…?

And Paul’s experience with dynos is illustrative…

xj6 85 Sov Europe (UK/NZ)