3.9 on LPG - porting, polishing & compression ratio.

[aRRon]

Hi all,
I've just put my stripped down 3.9 (1990 RR EFI 200,000kms, 10 yrs/ 100,000km + on LPG - Impco system) into the machine shop after a blown head gasket, one gouged No.1 bore (bits of head gasket or water blasted bits of spark plug tip) and previously very low manifold vacuum from sticking valves.
Had some discussion with the engine reconditioning guy who has done many Rover V8s. His advice was that the gouged bore will need resleeving because the water ingress there may also have caused cracking in the block behind the liner. More generally the motor will need a rebore, crankshaft reground and new valves. Valve guides are good.

He agreed with my plan to increase the compression ratio to 9.35:1. I also asked about porting and polishing the heads and he advised that this will yield additional torque at low revs - good for initial take-off and 4wdriving but the motor will lack go at high revs and feel breathless. Since my driving is city traffic, towing and occasional sand work I would vote for the port and polish.

I figure on ordering the engine rebuilding kit from Rimmer Bros in the UK which provides all the valve gear, valves, pistons, rings, bearings, seals and gaskets for a great price, and specify the 9.35:1 pistons at +0.20. New standard camshaft and timing chain.

I'm interested in the range of opinions from those who've been down these paths, especially re the port and polish effects and whether I'm on the right track for setting up for gas?

[bee utey]

Don't know about the porting/polishing but here's a tip: want 25% more low down torque? Chuck the Impco mixer, its a strangler. Go for a ring mixer system as they stay fully open at all revs. 9.35 compression is good.

[Tank]

Match porting the manifold and head ports to the gasket will help a little, polishing the combustion chamber will make no difference, don't polish the inlet runners, leave them rough finished. Blending the valve guide bosses and flowing the bowl below the valve heads will also help. But with the standard cam, exhaust manifolds and standard size valves I doubt you would notice any difference, higher compression will do more for horsepower gains than the other items mentioned. I would opt for a decent RV type cam (low to mid Revs torque increase), match ports and flow valve guide bosses and bowl area for a small but noticeable gain. To get the benefits of LPG I would fit a Haltech chip to handle the ignition needs of LPG and petrol. LPG benefits mainly from correct ignition and a higher compression ratio, good luck, Regards Frank.

[aRRon]

Thanks mate, is there a website or brand name for a supplier/s of "ring mixer systems" where I can check them out?

[Blknight.aus]

you can up the compression more than that if you want to stick to premium fuel and primarily LPG.

[bee utey]

Thanks mate, is there a website or brand name for a supplier/s of "ring mixer systems" where I can check them out?

Can't help you with a website, I do custom systems and the pics show parts I will use for a D1. AMR brand mixer M352, Gastec FP50DS processor and Ford oxy sensor, OMVL or MGA/Zavoli converter. The M352 mixer requires slight modification to fit the D1, minor lathe work and filing required. It is 46mm bore compared to typically 38-40mm bore used by other fitters. The processor has dual level mixture control depending on throttle and gives excellent economy/power delivery. It is not legally required for a D1 but works great to extend your range by 10 to 20%.
All this stuff should be available through your local LPG parts channels but PM me if you want any help.

[big guy]

If you stripping down the motor and doing all that work, may I suggest a stroker kit instead with a better cam. Or perhaps a cam for towing so the torque comes in early in rev range, increase compression to 9.35 is great and just get head tidied up. Leave the exhaust as is, will make little impact in my experience.
I did use a Haltech system and gas on a 4.6 that I had and power was almost same on gas/petrol.
Timing was as suggested at 10 BTDC.

The bigger motor will actually be more economical as the 3.9 and the extra power is quite amazing.

There is also a 4.9 or 5.3L stroker kit available it just depends on budget.
You could just get a complete motor from TRS or some mob in UK.

After all that, don't neglect the cooling system though, it will need to be in perfect condition.

I presume you car is a auto as the manual will not handle much of a power increase.

[Disco_owner]

Hi all,
I've just put my stripped down 3.9 (1990 RR EFI 200,000kms, 10 yrs/ 100,000km + on LPG - Impco system) into the machine shop after a blown head gasket, one gouged No.1 bore (bits of head gasket or water blasted bits of spark plug tip) and previously very low manifold vacuum from sticking valves.
Had some discussion with the engine reconditioning guy who has done many Rover V8s. His advice was that the gouged bore will need resleeving because the water ingress there may also have caused cracking in the block behind the liner. More generally the motor will need a rebore, crankshaft reground and new valves. Valve guides are good.

He agreed with my plan to increase the compression ratio to 9.35:1. I also asked about porting and polishing the heads and he advised that this will yield additional torque at low revs - good for initial take-off and 4wdriving but the motor will lack go at high revs and feel breathless. Since my driving is city traffic, towing and occasional sand work I would vote for the port and polish.

I figure on ordering the engine rebuilding kit from Rimmer Bros in the UK which provides all the valve gear, valves, pistons, rings, bearings, seals and gaskets for a great price, and specify the 9.35:1 pistons at +0.20. New standard camshaft and timing chain.

I'm interested in the range of opinions from those who've been down these paths, especially re the port and polish effects and whether I'm on the right track for setting up for gas?

I have a 3.9L V8 efi with Ported heads , Valve seats machined etc new guides , springs , running on LPG with an IMPCO mixer , my rig seems to have plenty of high revs go . the fuel economy is heaps better with heads ported If I don't push the loud pedal too hard and it has considerably more low end Torque , I took the Advise of the "Gurus" on here ( Namely JC ) and had the heads ported . I've also removed the front cover of the Air Filter and the engine breathes a lot better.

[It'sNotWorthComplaining!]

The 3.9 litre motor no matter how much money you throw at it is still a small V8 lugging around a fair bit of vehicle weight.

[tebone]

Hi all,
I've just put my stripped down 3.9 (1990 RR EFI 200,000kms, 10 yrs/ 100,000km + on LPG - Impco system) into the machine shop after a blown head gasket, one gouged No.1 bore (bits of head gasket or water blasted bits of spark plug tip) and previously very low manifold vacuum from sticking valves.
Had some discussion with the engine reconditioning guy who has done many Rover V8s. His advice was that the gouged bore will need resleeving because the water ingress there may also have caused cracking in the block behind the liner. More generally the motor will need a rebore, crankshaft reground and new valves. Valve guides are good.

He agreed with my plan to increase the compression ratio to 9.35:1. I also asked about porting and polishing the heads and he advised that this will yield additional torque at low revs - good for initial take-off and 4wdriving but the motor will lack go at high revs and feel breathless. Since my driving is city traffic, towing and occasional sand work I would vote for the port and polish.

I figure on ordering the engine rebuilding kit from Rimmer Bros in the UK which provides all the valve gear, valves, pistons, rings, bearings, seals and gaskets for a great price, and specify the 9.35:1 pistons at +0.20. New standard camshaft and timing chain.

I'm interested in the range of opinions from those who've been down these paths, especially re the port and polish effects and whether I'm on the right track for setting up for gas?

Hi,
Just a suggestion you want to run 9.35:1 static compression, I would bump this up to 10.5:1 if you are going to run on lpg. With aluminum heads it absorbs energy so your engine doesn't ping as much as using a cast iron head. The rule to thumb is you can go 1 point higher with aluminum heads ( 10.5) , cast iron head (9.5) as I am lead to believe. I would go bigger valves, new camshaft and a good exhaust. I have researched doing a rover motor up and found it to expensive so I went to a chev stroker instead ( but alot more mucking around ). Once you get the engine going, stick with the gas system you have put a vacuum gauge connected to the inlet manifold and see how much vacuum you have at open wide throttle. Anything over 1.5 - 2.0" vacuum you will know there is a restriction with the gas mixer.