Thanks for commenting I am running the car on lpg thats one reason I'm doing it as you say would have to run on higher octane fuel so should be good will keep people updated when I finish porting the heads and the engine goes back in to the car and also will make a good tow car to

y is it I post what I'm doing to my engine and people say I'm putting 40l parts in a 4.6l
all I'm doing is changing ( THE PISTONS ) not conrods or crankshaft as its a 4.6l engine in the first place
please read my first comment properly and it would make sense. I do no what I'm doing been messing with engines for 30 odd years
sorry if it feels like I'm annoyed but people need to read comments properly first before commenting

Did try to attach pic have now made public

You just have to get the 4.0l piston remove conrod and replace with the 4.6l rods and then off you go with higher compression

(https://drive.google.com/file/d/1nIvcSNCjqCxX4DulFl8VjblJDpgbEemN/view?usp=drivesdk "pic of 4.0l pistons in a 4.6l")The pistons on the 4.6l have a deeper dish than the 4.0l that's the only difference rover did it to keep the compression the same so putting the 4.0l in increase compression the diameter is the same and hight of pistons are to
But I did find that the 4.0l pistons are lighter by 20 grams as I made shor the balance was good over the hole crankshaft

Thanks for the info had to use the 4.6l conrods as people say the 4.0l conrods are longer all I replaced was the piston it just puts the compression up to about 10.5 to 1 instead of 9.35 to 1 so should be good for bottom end tork that's the reason I have done the conversion
I have new head bolts for it and timing chain but will see if the ecu can cope if not will be sent of for a remapping
Thanks for all the information

thanks for letting me join your group
i have a 1999 range rover p38 and I'm in the proses of rebuilding the 4.6 lump that was in it as head gasket went on it so stiped down and was venting exhaust gases into atmosphere as the head was off fined out it has a top hat liner engine. so the big job had started so thought as I'm in there I would swop out the 4.6 pistons for the 4.0 and swap the camshaft for a Kent camshaft H180
in doing the swap with the pistons I find out that the 4.0l pistons are 20grams les than the 4.6l so less mass to move witch should be better for the mass of the bottom end. so I started to clean the ports to give good air flow thru the engine (wished I hadn't started) but work on them so I can feel the benefit later so no pain no gain.
I have spent a lot of time cleaning the engine as it had a fair bit of oil all over it it was looked after very well by the first owner but the next two did it no favours as it must of had head gasket done but must have been done by a monkey as head boults were so tight had to use breaker bar and an extension bar all in all a bar three feet long bar on some boults and some were lose they had managed to brake some of the rockers as not given the right gap settings so its been a bit of a challenge to make it right but so far the bottom end is back together and sorting the heads out