Thread: V8 engine modifications for LPG

Originally Posted by PLR

G`day Dan

Yep or C suffix , all i was trying to do was work out what you have and yep the engines are for gas .

The heads you have are the smaller combustion chambers and also have the stem seals that fit onto the top of the guides .

The stem guides are longer on these heads ( outside the port ) and the valve/piston clearance is less of a concern than the spring retainer fouling the top of the stem guide

But only when using a high lift cam , the earlier heads , eg ERC 0216 used on early EFI and later carby engines are able to take more lift without modification .

The heads can be machined to raise the ratio , for the inlet manifold fitting , the heads are machined again on the port face .

There`s a formula to work out how much need to come off the port face from the amount taken off the head surface so the manifold will fit as normal .

Because the heads sit lower the rocker pedestals will probably need shims but again a formula to work out how much .

So it can be done and the math is there to get it right .


The best and easiest way to raise compression is with pistons but they need to be got at the right price or not economical . ( i got a high comp 3.9 set for $100 so for me it is )

I could go on for pages but i`ll just put some stuff , lpg/ratio related.

LPG produces more heat than petrol and raising the ratio also produces more heat than std .

EFI need to run within a specific heat range to work properly not as important on carby

In theory the difference 8:1 and 10:1 is little although there is a measureable increase in economy/power till it`s too high .

Below 8:1 the difference is large .

In practice the difference between 8:1 and 10:1 is/could be noticeable . ( about 10/15kph over the top of a local hill )

The ratio has to be physically measured to use the math to get the ratio of any specific engine . ( meaning your 3.9 will be similar to my 3.9 but not the same )

A general idea can be got from factory figures but the physical figures will often not tally with them .

As far as is it worth while i don`t have a comparison because gas and high comp came together with the 3.5 and the 4.0 was done for lpg from new and petrols there but not used much .

I intend to do it to a 3.9 as well if that means anything .

The same heads used on each engine , stainless in/ex valves Kline guides , a little above std lift but little .

I won`t say how much was removed from the heads but the combustion chambers are a bit smaller than the 28cc later heads .

On the 4.0 more metal was removed and it`s been 3 or 4 yrs ( i think ) but did replace the thicker 10 bolt comp gaskets with thin 14 bolt comp gaskets couple of month back and haven`t worked out the ratio but it`s possibly higher than needed .

Don`t know if i`ve actually answered anything or just rambled .

Cheers

Not at all Peter, very informative as usual, and ties in with rovercares advice of blueprinting it rather than just wacking some new slugs in and not neccesarily getting a result.
With a smaller 4cyl engine I've no doubt at 150k it would be tired and want the works, but at the other end of the spectrum I've heard it's not uncommon for large engines in truck or earthmoving equipment to only receive as much as neccesary, say only one piston replaced if damaged. I thought these would fall somewhere in between and the major components like crank & liners etc would be good for well over 300k at least.

Phil, yes mine's not even a high comp to start with so I was afraid it would be a heck of a lot of metal off, with all the knock on effects.

What do you want.To build an engine to get more go or LPG compatible?The standard v8 will run just fine on both so why not save time and a lot of money by just porting the head and a new cam,maybe exhaust and drive it.The v8 is very keen when performance mods are used.I don't think the work and money involved justifies the work you want. Pat

G`day Dan

Just read mine and the hill is a comparison .

The difference was with the 3.5 i fitted Felpro composite gaskets which are around the same thickness as 10 bolt factory composite gaskets 3.9/4.0/4.6 .

By removing the tin gaskets and fitting Felpro the ratio was lowered and the 10/15 kph over the hill was the outcome .

My heads are only any use on my engine by machining them the way they are , they are only any good to raise comp and useless else where .

I only have experence with taking big bits off pre 10 bolt heads and know it can be done .

With later 10 bolt heads ( as yours ) i don`t know what the outcome would be taking big chunks off .

The 10 bolt heads are a different casting as well as method to 14 bolt heads .

In my case for a 3.9 i intend to use 9.35 pistons , 10 bolt heads and the thin 14 bolt comp gaskets .

I haven`t looked too closely yet but it will come out around 10 this way maybe a bit more .

Yep your right , a cared for 3.5 bottom will do many many klms as will a cared for 3.9 .

The 4.0 bottom has yet to really prove its self and being a totally different bottom than either the 3.5 or 3.9 , time will tell .

Cheers

Well, something in between was what I had in mind.
Like I say, I can't justify the works as such.
I don't need a tyre shredding crate motor (I have neither the funds, time, shed space or enthusiasm), but being on LPG by upping the compression I was envisaging a little better efficiency both performance & economy wise. I'm not talking a radical cam job either of course, just what's commonly called a torque cam or an economy cam. Again I'm not an expert but I was of the belief this is just basically higher lift but essentially no change to duration.
If these engines do respond well to porting and a cam, and that is a cheaper & easier way to get the same result, then I'm certainly open to that.

Originally Posted by PLR

G`day Dan

Just read mine and the hill is a comparison .

The difference was with the 3.5 i fitted Felpro composite gaskets which are around the same thickness as 10 bolt factory composite gaskets 3.9/4.0/4.6 .

By removing the tin gaskets and fitting Felpro the ratio was lowered and the 10/15 kph over the hill was the outcome .

My heads are only any use on my engine by machining them the way they are , they are only any good to raise comp and useless else where .

I only have experence with taking big bits off pre 10 bolt heads and know it can be done .

With later 10 bolt heads ( as yours ) i don`t know what the outcome would be taking big chunks off .

The 10 bolt heads are a different casting as well as method to 14 bolt heads .

In my case for a 3.9 i intend to use 9.35 pistons , 10 bolt heads and the thin 14 bolt comp gaskets .

I haven`t looked too closely yet but it will come out around 10 this way maybe a bit more .

Yep your right , a cared for 3.5 bottom will do many many klms as will a cared for 3.9 .

The 4.0 bottom has yet to really prove its self and being a totally different bottom than either the 3.5 or 3.9 , time will tell .

Cheers

Ok, conversely, can the 3.9 be fitted with tin gaskets for the opposite effect? Perhaps in conjunction with some mild head work etc as above this would probably give that little bit I want. I was all set to give it an exhaust when I bought it as I had driven a few older RRC and one in particular with just a 350 holley and exhaust went a hell of a lot better off the bottom. But then I heard & read that headers do diddly for a 3.9 and I do not want an annoyingly loud tail pipe for minimal gains.

What about computer mods?

Originally Posted by oldzook

Ok, conversely, can the 3.9 be fitted with tin gaskets for the opposite effect? Perhaps in conjunction with some mild head work etc as above this would probably give that little bit I want. I was all set to give it an exhaust when I bought it as I had driven a few older RRC and one in particular with just a 350 holley and exhaust went a hell of a lot better off the bottom. But then I heard & read that headers do diddly for a 3.9 and I do not want an annoyingly loud tail pipe for minimal gains.

Yes the tin gaskets can be used although the 14 bolt comp gaskets wouldn`t be alot thicker .

It had to get a measurement from them as they tend to self-destruct when the head comes off .

Possibly with a tin gasket the torque to yeild head bolts ( as yours ) wouldn`t be required and cheaper ordinary head bolts could be used .

Our 4.0 uses ordinary bolts but part of the reason for the change of head gaskets , mentioned earlier was because i don`t think the ordinary bolts exert as much pressure as the yeild bolts .

Another theory on the yeild bolts is they can be part responsible for liner shift in 3.9/4.0/4.6 because it is put forward that because of the extra pressure they evert .

They can distort the alloy around the liner which can cause the cracks in the block which allows the liner to shift .

For mine this is much more feasable theory than the pourous block theory but i undestand many like the sound of pourous block .

The headers/extractors on ours , made a big difference midrange on the 3.5 replacing std carb single manifolds .

The 4.0 has only ever had them so don`t know .

Cams are a science by themselves , i have limited infomation on them .

I get them when needed from a place that machines them , i`m sure the off the shelf ones work well but cost more than i`m prepared to pay .( last one $180 3or4yrs ago add $100 for a shelfer then )


I`ll stop now before i go on

Cheers

Originally Posted by PLR

G`day Dan

Just read mine and the hill is a comparison .

The difference was with the 3.5 i fitted Felpro composite gaskets which are around the same thickness as 10 bolt factory composite gaskets 3.9/4.0/4.6 .

By removing the tin gaskets and fitting Felpro the ratio was lowered and the 10/15 kph over the hill was the outcome .

My heads are only any use on my engine by machining them the way they are , they are only any good to raise comp and useless else where .

I only have experence with taking big bits off pre 10 bolt heads and know it can be done .

With later 10 bolt heads ( as yours ) i don`t know what the outcome would be taking big chunks off .

The 10 bolt heads are a different casting as well as method to 14 bolt heads .

In my case for a 3.9 i intend to use 9.35 pistons , 10 bolt heads and the thin 14 bolt comp gaskets .

I haven`t looked too closely yet but it will come out around 10 this way maybe a bit more .

Yep your right , a cared for 3.5 bottom will do many many klms as will a cared for 3.9 .

The 4.0 bottom has yet to really prove its self and being a totally different bottom than either the 3.5 or 3.9 , time will tell .

Cheers

PLR, may I suggest that instead of spending big money HC pistons and shaving heads, you have a UNICHIP fitted and have it Dyno tuned to run optimally on ULP and LPG. The UNICHIP will allow the Tuner to adjust fuel and IGNITION Timing to suit both fuels. The big compromise with LPG and Petrol (Dual Fuel Setups) is that timing is usually set for optimum performance on Petrol and performance on LPG falls away because of the difference in IGNITION timing required between the 2 fuels. An LPG system requires MORE initial timing advance and LESS total advance, Petrol is the opposite. When I get my LPG system installed I will be having a UNICHIP fitted, something for you to consider, Regards Frank.