4.2 compression

texpis · Post by **texpis** » Fri Oct 12, 2012 10:07 pm

Hi All I have a 4.2 in bits ready for a rebuild the CR on these I am sure you will know is quite low. What is the best way to increase the CR, I am putting this in a defender that runs mainly on LPG. Will simply using 3.9 pistons in and using later heads do the job?
I know you will ask what Cr I am after but I am not really sure, I believe LPG can run on higher CR but I need it to run on petrol in times when I cant get LPG. So what would you recommend?

Mick

RoverP6B · Post by **RoverP6B** » Sat Oct 13, 2012 4:25 am

Hi Mick,

Is the original CR not stamped into the block adjacent to where the dip stick tube enters? From memory it was around 8.13 : 1.

The pistons determine the CR, if all else remains unchanged. The 4.2 pistons run with an offset gudgeon pin whereas the 3.9 pistons do not.

The LC engines actually deliver their power and torque more quickly at lower revs compared to HC engines, so with an automatic transmission or heavy four wheel drive, this is advantageous. Only at higher revs do the HC engines have an advantage, thus they favour manual transmissions etc.

Ron.

landrovernuts · Post by **landrovernuts** » Sat Oct 13, 2012 6:38 am

I think the original 4.2 compression ratio is 8.95:1 or around that figure. When my 4.2 was originally built by V8D the pistons were left alone other than cutting valve inserts and the block was decked so as to bring the compression up to 9.5:1. As said previously the pistons on a 4.2 run off set gudgeon pins, although you here of people running 3.9 pistons with the crowns machined - not sure how long they last however.

Toby

texpis · Post by **texpis** » Sat Oct 13, 2012 4:17 pm

Hmm thanks for the reply's yes the CR is probably on the side but its all packed away in boxes. I have just bought a 3.5 Land rover 90 withmanual gearbox and i want to upgrade the engine to this 4.2. I was going to leave the SUs because it mainly runs on gas so putting on EFI is probably not worth the bother. Its now running on megajolt mainly for waterproofing. As per the first post I was going to up the CR thinking it was the right thing for performance, it is sounding like I may be better off leaving as is.
What do you recon?

mick

kiwicar · Post by **kiwicar** » Sat Oct 13, 2012 4:49 pm

Hi
with LPG you want as much CR as you can get, aim at 10.5:1 or more.
Best regards
Mike

sidecar · Post by **sidecar** » Sat Oct 13, 2012 4:59 pm

I guess "mainly" LPG means not totally on LPG. Really super unleaded in these old lumps with their old combustion chamber design does not want to be squeezed more than 10:1 Maybe 10.25:1 if you are really careful when setting up the engine.

With regards to measuring the CR I would not trust any markings on the block or any pistons that you fit. The ONLY way to be sure what the CR is is to measure everything with a burette then work it out. With any other way you are taking some risk, this would not matter if the CR was 9:1 but it does matter when you are pushing the CR high.

I guess you could go high on the CR if you use a clever ignition that can retard the ignition when on petrol. Mind you I think LPG needs the ignition retarded anyway?

texpis · Post by **texpis** » Sat Oct 13, 2012 6:26 pm

Soo I do want to raise the cr how do I just dec the block? If so by how much? LPG likes advanced timing I am told I will only use petrol if I do a journey where I cant get LPG which will be very few if any, I go to Drumclog off roading once a year may need petrol there. The 3.5 has headman exhaust manifold I am guessing they will be reusable?

Mick

sidecar · Post by **sidecar** » Sat Oct 13, 2012 10:20 pm

texpis wrote:Soo I do want to raise the cr how do I just dec the block? If so by how much? LPG likes advanced timing I am told I will only use petrol if I do a journey where I cant get LPG which will be very few if any, I go to Drumclog off roading once a year may need petrol there. The 3.5 has headman exhaust manifold I am guessing they will be reusable?

Mick

Decking the block will raise the CR. Personally I would be careful of any figures that anyone offers up as the amount to remove. How can anyone give you a figure over the web? You need to get hold of a burette and measure it all. Also bear in mind that once you start removing metal then other things will start to be altered, namely the lifter pre-load and possibly the fit of the inlet manifold.

texpis · Post by **texpis** » Sun Oct 14, 2012 6:08 pm

Hi Sidecar and thanks for the advice. I would have thought the cc of rover V8 heads would be well documented are they that much different? The use of the burette is for measuring the head capacity. I was thinking that some one could tell me that decking the head by X would reduce the cc of head by Y. I understand and always check the lifter preload the inlet manifold can I not machine them the same as the head or am I in a world of my own

Mick

sidecar · Post by **sidecar** » Sun Oct 14, 2012 7:24 pm

I can help you out with some of the measurements, the heads are not 'full circular chambers' therefore it is not easy to use maths to work out how much the chamber volume will be reduced if you skill X off the heads. Having said that I know that every 8 thou off the heads will reduce the chamber volume by 1cc. I was told this by V8 Developments and I had quite a few heads skimmed which did work out with this formula.

The block is a different kettle of fish, you can use maths to work out whats going on. Basically it is R X R X 3.14 X SKIM.

The bore of the larger engines is 9.4cm so the radius is 4.7cm

Lets say you wanted to skim 20 thou off the block which is 0.5mm the formula would be as follows:-

4.8 x 4.8 x 3.14 x 0.05 which is 3.46 cm3

So skimming 0.5mm (0.05cm) off the block will reduce the volume above the piston by 3.46 cm3

Don't forget the head gasket volume when working things out!

Cobratone · Post by **Cobratone** » Mon Oct 15, 2012 11:45 am

sidecar wrote:I can help you out with some of the measurements, the heads are not 'full circular chambers' therefore it is not easy to use maths to work out how much the chamber volume will be reduced if you skill X off the heads. Having said that I know that every 8 thou off the heads will reduce the chamber volume by 1cc. I was told this by V8 Developments and I had quite a few heads skimmed which did work out with this formula.

The block is a different kettle of fish, you can use maths to work out whats going on. Basically it is R X R X 3.14 X SKIM.

The bore of the larger engines is 9.4cm so the radius is 4.7cm

Lets say you wanted to skim 20 thou off the block which is 0.508mm the formula would be as follows:-

4.7 x 4.7 x 3.14 x 0.0508 which is 3.525 cm3

So skimming 0.5mm (0.05cm) off the block will reduce the volume above the piston by 3.53 cm3

Don't forget the head gasket volume when working things out!

Sorry to correct you Pete, but it might make an important difference.

sidecar · Post by **sidecar** » Mon Oct 15, 2012 3:34 pm

Cobratone wrote:
sidecar wrote:I can help you out with some of the measurements, the heads are not 'full circular chambers' therefore it is not easy to use maths to work out how much the chamber volume will be reduced if you skill X off the heads. Having said that I know that every 8 thou off the heads will reduce the chamber volume by 1cc. I was told this by V8 Developments and I had quite a few heads skimmed which did work out with this formula.

The block is a different kettle of fish, you can use maths to work out whats going on. Basically it is R X R X 3.14 X SKIM.

The bore of the larger engines is 9.4cm so the radius is 4.7cm

Lets say you wanted to skim 20 thou off the block which is 0.508mm the formula would be as follows:-

4.7 x 4.7 x 3.14 x 0.0508 which is 3.525 cm3

So skimming 0.5mm (0.05cm) off the block will reduce the volume above the piston by 3.53 cm3

Don't forget the head gasket volume when working things out!
Sorry to correct you Pete, but it might make an important difference.

Yes Tone, 0.065cc may make all the difference!