V8 engine modifications for LPG

[100I]

I'm just planning ahead here for some more serious mods to suit LPG a little better,,

I will want to up the compression ratio - probably to around 10:1 seems appropriate without being radical as it will still be dual fuel, albeit ULP only which I don't mind. I'll also give it a more appropriate camshaft profile.

A few questions; As the rover V8 is renowned for it's longevity - likely due in part to it's low compression, low stress design - is it up to this CR in the long run or will bottom end durability suffer?
Is it simple enough to just shave the heads to achieve this? Is there enough valve to piston clearance? Especially considering the intention of fitting a high lift cam. Will I have to change pistons to achieve this? For that matter will the OE hold up to it or am I up for forged items?
My engine is only 150k young so I couldn't justify a full house rebuild yet. (SWMBO would also take some serious convincing)
What sort of other consequences such as manifold fitment, lifters etc?

Has anybody done 'the package' to their 3.9?

Yes I know all this info is probably either somewhere on here or out there, and I have searched a bit but it's scattered and I'm as lazy as anyone else

[PLR]

G`day Dan

I thinks it`s probably a B suffix engine but is it A OR B suffix ?

I`ve got a 3.5 with that type ratio was 9.35 std

Also a 4.0 with similar ratio was 8.23 std

Almost have enough bit to do a 3.9 but lacking a block at present

Starting point is which 3.9 A or B suffix

Cheers

[100I]

G`day Dan

I thinks it`s probably a B suffix engine but is it A OR B suffix ?

I`ve got a 3.5 with that type ratio was 9.35 std

Also a 4.0 with similar ratio was 8.23 std

Almost have enough bit to do a 3.9 but lacking a block at present

Starting point is which 3.9 A or B suffix

Cheers

Gday Peter,

It's engine# is 38D38437C and it's currently a low comp.

So with experienec of 2 engines similar, presumeably on gas, did you find it a worthwhile exercise? Is that CR about on the money? Have you cammed them or anything else?
I don't think I could go much more on PULP could I?
Plus I read a post where JC mentioned problems with an engine of his on LPG. He had around 11-odd and found it was too much due to a change of LPG formula to a formula with dramatically reduced octane rating nowadays.

cheers

[PhilipA]

You will have big problems shaving heads to get CR, the ports will not line up with the manifold,and you will have to machine the manifold. Also you will have to machine a rocker pedestals. A lot of maths.
Everything will then be unique to your motor.
By your question you probably should not attempt this.


The best /correct way to get CR in a 3.5 is to fit HC pistons. I think you can readily get 9.75 (TVR)and maybe over 10:1 from UK as the P6B had over 10:1.
It should be all right with over 10:1 as the first engines were this when they had lots of lead.
Regards Philip A

[100I]

Thanks Phil,
yes I was afraid of that, oh well have to do it properly

[walker]

If you could keep us all filled in on how you go about it it would be appreciated. I am building a Rangie ute at the moment and once the body is finished I will start on the motor.
At the moment I have a 4.6 block with 3.5 heads on it. The ute is going to run straight gas and like you I want to up the CR to about 10:1 at least. I was going to jsut do it by getting the heads shaved but it sounds like that is not possible.

[100I]

Adam, I might be keeping an eye out for your engine build instead. With a house build keeping me busy there is an extremely good chance you will beat me to it if I have to go to all the effort of tearing the engine down...

Now I just had a thought, do I have to completely strip it, can it be done in-chassis?
Do the pistons clear the crank?

[rovercare]

Adam, I might be keeping an eye out for your engine build instead. With a house build keeping me busy there is an extremely good chance you will beat me to it if I have to go to all the effort of tearing the engine down...

Now I just had a thought, do I have to completely strip it, can it be done in-chassis?
Do the pistons clear the crank?

Why bother even building a motor if you just going to short cut it all the way

Buy yourself a dial indicator and some good instructional books on blueprinting and engine building and do the job properly

30though of the heads will not lift compression that much, you need to do the math for exact amounts, its a simple formula

[100I]

Such as? It's barely got 150k, mostly lazy highway work. As far as I can tell the engine is in very good shape, the cams aparently show wear at 200plus, what else should I do? Maybe pull the heads off anyway and get them serviced, but aside from perhaps some carboning they should be in good shape too I'd like to hope. Should the crank have any wear at this mileage?

[PhilipA]

By the way Dan, What I said about head shaving applies to an 8.35 or whatever your 97 Disco is, for BIG shaves such as the 100thou that you would need.

There is no problem with say 20-30thou, but that only gives you about .6 or .8 additional compression.
Even then I have a few problems with the gaskets all lining up. ( I have taken 50 thou off tin heads and use composite gaskets)
I have a leak at the rear valley seal, I think because the rubber split, and its a dog of a job for a small oil leak, so I have deferred it.
Regards Philip A