Hello All!

I own a 1985 Series III XJ6 with a 4.2 litre. Looking for ways to increase torque and HP.

Suggestions?

 

You can increase compression by shaving a few thou off the head. But that will only be a minimal increase. Someone might supply "Racing" cams on the internet somewhere. Since the 4.2L XJ motor has such a long stroke its hard to increase torque without doing radical things.

Installing a "Hot wire" Air flow sensor should help a little bit as well.. But you might need to go with an aftermarket ECU to interface with it.

Exhaust extractors, Larger exhaust might increast torque a little as well... but there isn't much you can do without going turbo/supercharged (and there is plenty of room in the engine bay to do that), you might need to drop your compression though to allow it to benefit fully from having forced intake

 

Nitrous Oxide.

 

engine swap and fabricate the bay to accommodate the new motor is best option.
other than that, no reliable option.

 

The modern NOS systems are very reliable and you can match and tune the additional fuel to make the car run leaner or richer when squeezing as you wish. This is a very simple way to add as much HP as your car can handle.

I went with the NX system (not installed yet) because they appear to have better flowing valves, better reliability, etc.
With NOS, whatever the jet size says it will give you is RWHP that it WILL give you, no more BS from sellers who do not show dynos etc. So on my XJR I could add the jetting that will allow it any amount of HP the car can handle, in my case that is probably around a total of 600+ HP. I am going to start with the 2 x 30=60 hp jets first, then if that all goes well, try the 2 x +50 hp jets and probably leave it at that. No need to overstress anything. You will also NEED to improve your cooling system when you add more powah.

Installation should take only about 4-8 hours whereas swapping engines could take months if it even works...