3.9 Vs 3.5?

Hey guys,

I currently own a 1990 3.5 Disco, it hasnt run for about a year but i had a guy to look at it recently and it starts now, still needs some TLC to get her right but its a work in progress.

But while trawling through forums recently i came across a 3.9 V8 and manual gearbox with less miles than mine and for the money it was going for just grabbed it!

Now im stuck with the problem of deciding what to do!?

I was planning on getting my 3.5 rebuilt and tuned but is it worth just spending the money on giving the 3.9 the same treatment and putting it in instead?

Car is mostly going to be used at weekends for biking trips and a bit of off road.

Any info welcome!

Cheers

Ally

Have you got ecu ect to go with engine?

Sorry, yes it does have an ECU, as far as i am aware its pretty much ready to drop straight in. No transfer box but im pretty sure it would just swap straight in for my 3.5.

it would be fairly easy to fit but 3.9 were prone to cracking block behind liners , check not the reason its available similar symptoms of hgf

jamesmartin said:

it would be fairly easy to fit but 3.9 were prone to cracking block behind liners , check not the reason its available similar symptoms of hgf

Click to expand...

I'll go along with that!! My '93 did it at 87k (interestingly one year & 12k after the fitment of a LPG conversion) & my mate's '91 suffered the same fate, albeit after a longer period of LPG running - coincidence? I've stuck to 3.5's & run them on unleaded ever since, if you want a bit more output there are ways to achieve it without going down the 94mm bore route.

OK cheers for that guys, any one else have anything different to say? I wasnt planning on running it on gas as like i said ill mostly just use it for weekends so i can afford to run it on petrol, i have a quote for a full rebuild of the 3.9 with better parts giving it about 260 bhp but its alot more than i was planning on spending!

my 3.9EFis have always run smoother with more 'umph' than my 3.5EFi ever did. Could be just the examples I have/had, just my personal experience with them. I've only ever run with petrol tho, no LPG

what you have to consider here is the improved breathing of the 3.9 over the 3.5.
the 3.5 is a very strangled engine with regardes to breathing improving the air in to the cylinder will not improve the valve to cylinder shrowding.
now on the 3.9 it is a bigger bore thus you can fit bigger valves to the heads to get more air and fuel in thus making a bigger bang for your bucks!

with my engines they are all tested for porus block if they leak then they are top hat linered and i now treat every engine with a ceramic sealer treatment too so in theory it is 100% good.
you can have mid valves three angle seats, piper cam, fully balanced bottom end making for a very free reving engine

hi Ali how you doing? lol

but for how much ,for most of us a 3.5 hotwire high compression new cam etc would be reliable rugged and enough

Yes a 3.5 would be ok but is never going to develop more than 150-165 bhp in a rebuilt form! So would be no further gain in mpg which is what the engine game is now all about more bhp=mpg once over the 200 bhp on petrol then you see rise in mpg and it will pay for itself at all most the same as a gas conversion

3.5 can be made to make 200 but for the same work you'd be getting 240 out of the 3.9.


rover V8s are prone to damage when you run them on LPG

Hey guys, really good stuff here now, exactly what i was after!

So the general idea is that the 3.9 is just a better engine and by the sounds of things, cheaper to run if you get it running nicely!

Sounds like it would be worth doing the conversion then!

How hard is it to do a full rebuild yourself with little experience? I would quite like to do it as a learning experience so i can trya nd fix them myself in the future?

Ally

POAH said:

3.5 can be made to make 200 but for the same work you'd be getting 240 out of the 3.9.

Click to expand...

I've toyed with this myself as my 3.5efi has only done 46k therefore should be a worthwhile candidate for tuning, however I'd rather settle for 200 from the (generally bulletproof) 3.5 against 240 from a 3.9 & risk liner problems.

if you have one or can get one running smooth, stick with it...3.5 or 3.9 doesn't matter if u got a good one.

Use the 3.9 , for no other reason than it will have a 4 bolt block , try to over rev that one !!!

3.5s sound better i have noticed, dont know why but they do.

is it possible to fit the 3.9 cam and heads to the 3.5 motor?

surely that would make a nice happy medium, not massively powerful but bit more responsive and nicely reliable.

Mr Noisy said:

3.5s sound better i have noticed, dont know why but they do.

is it possible to fit the 3.9 cam and heads to the 3.5 motor?

surely that would make a nice happy medium, not massively powerful but bit more responsive and nicely reliable.

Click to expand...

3.9 cam will give an improvement over the 3.5
the 3.9 heads will fit but as it has bigger valves they will hit the cylinder wall as the bore is smaller on the on the 3.5 not worth the hassel

if you have the 3.9 block tested before using then al should be fine i usualy have a good seasoned 3.9 block instock and can even get the top hat linners fit if you want them installed if it fails after being tested

I suppose LPG is a the biggest liner killer. A 3.9 that's always run petrol would probably be fine I suppose.

Didnt realise the bore/valves were that much bigger on the 3.9

Cheers

won't see any difference between a new 3.5 cam and a new 3.9 cam compared to an old 3.5 cam.

Mr Noisy said:

3.5s sound better i have noticed, dont know why but they do.

is it possible to fit the 3.9 cam and heads to the 3.5 motor?

surely that would make a nice happy medium, not massively powerful but bit more responsive and nicely reliable.

Click to expand...

elsie said:

Use the 3.9 , for no other reason than it will have a 4 bolt block , try to over rev that one !!!

Click to expand...

I don't doubt it, but surely if you want your power at high revs you don't buy a 2 tonne 4x4 in the first place - esp. one with a slush-box