Outer Limits 4x4 Board

G'day folks,
Just about to start nailing the 4.6 together and wondered if steel gaskets(which I have in my VRS set) is better than the composite gaskets (that were in the engine when I pulled it to bits)?

If steel is better, do you use a sealant like Hylomar when installing?

Appreciate your experiences.

Go with composite, you torque them down and forget. I would have thought that the 4.6 vrs kit would have composite ?

Only thing is OE (Ehrling)composite are 35thou thicker than steel. So you lose quite a bit of CR. You will be down in the 7s, at about 7.8 :1 if I recall . it will run on kero!! . That is if you are using 3.5 or 3.9 tin gasket heads.

You may lose all your lifter preload and have to machine the bottoms of the rocker posts to restore between 20 thou and 40 thou.or they will rattle.

I shaved 50 thou off the 3.9 heads when I fitted composites which gives me about 8.8 :1 which is ideal.

Still I would run composites and shave the heads by either a neat 35thou or more if you like.

By the way 4.6 heads are 50 thou different from 3.9 heads even with the same CR as 4.6 pistons have a different bowl volume than 3.9 pistons to achieve the same compression ratio.
Regards Philip A

By the way 4.6 heads are 50 thou different from 3.9 heads even with the same CR as 4.6 pistons have a different bowl volume than 3.9 pistons to achieve the same compression ratio.

Philip, can you explain this one a bit more for me?
If I understand correctly:
* The 4.6 heads have 50 thou more metal to the rocker shafts than the 3.5/3.9?
* If I shave the 3.9 heads by 35 thou I will end up around 8:2(or so):1?
* If I shave the 3.9 heads by 50 thou I will end up with 8.8(or so):1 with the composite gaskets?
* If I shave the 3.9 heads by 50 thou and use steel gaskets I will be closer to 9.?:1?

Thanks for the help!

Alex

let me see
1 The 4.6 deck is 50thou CLOSER to the rocker pillar. IE it appears that the 4.6 casting is similar to a 3.9 which has been shaved 50thou. Ther may be deck thickness differences but I do not know or think so.
2 If you shave the 3.9 heads by 35 thou and use OE composite gaskets you will end up where you started with tin gaskets 8.13:1
3 Yes if you shave the 3.9 heads by 50thou you will end up with 8.7or 8.8:1
4 I have not measured what happens with this, but I estimate you would end up with about 9.2:1 ( with 8.13:1 pistons). BUT the ports then are getting quite low compared to the inlet manifold and you will definitely need to shim the rocker posts (probably by 50thou)to reduce the lifter preload.

I originally bought 4.6 heads to increase my compression by using tin gaskets in this way but I became nervous of the smaller overlap between the inlet manifold and port at the top of the port. Triumph Rover Spares do this and say there is no problem. At the time there was no info anywhere on whether you could shave heads etc. Peter R and I seem to be the pioneers in this.
Regards Philip A

Reckon I'll need to machine the inlet manifold by removing 50 thou?

You are starting to confuse me.
You stated that you have a 9.35:1 4.6 engine. If you were to shave the heads and use tin gaskets you would end up with something like 10:1 or more.
Why would you want to do this?

If you use 3.9 tin heads with tin gaskets you will end up somewhere around 9:1 as the bowl of a 4.6 piston is relatively bigger compared to the combustion chamber and gasket volume.

If you shave 50thou from a 3.9 head, which I believe you have, you end up with a 28CC combustion chamber, the same as a 4.6 head. So the compression ratio will be 9.35:1 with composite gaskets.

You do not have any real issues with either of these choices. You do NOT have to machine inlet manifolds, but you may have to check preload.

The amount would not be 50thou anyway, as the angle is 90 degrees so it will be only a fraction needed.
I haven't done the maths but I think its 12.5 thou. (90degrees between banks =45degrees from vertical=half the distance, divided by 2) I am probably wrong as geometry was never my strong point. I would have to draft it out and measure it to be sure.

BUT DO NOT DO THIS. You end up with a motor that no one can work out how to work on.
Regards Philip A

We're still talking the same thing.
4.6 , 9.35:1 short engine using 3.9, 8.13:1 tin heads and installing composite gaskets.

Haven't done anything to my 3.9 heads yet, so no machining has been done.

I do have an old set of 3.5 heads that were fitted to the 4.6 when I got it, but I haven't measured them at all and they will not be going back on.

I wasn't planning on using tin gaskets anymore, the last question re the shaving and tin gaskets was for reference.

The main bits I wanted to clarify are:
2 If you shave the 3.9 heads by 35 thou and use OE composite gaskets you will end up where you started with tin gaskets 8.13:1
3 Yes if you shave the 3.9 heads by 50thou you will end up with 8.7or 8.8:1

I like option 3! [/u]

Mate you are confused.
The compression ratio of the engine from new has nothing to do with the heads. All heads are the same for a given model. the difference in compression ratio is in the size of the bowl in the pistons.
My answer to youe questions on Compression ratio assumed you has an 8.13:1 3.9. This was before I read your post about a 4.6.
Now the answeer of what CR you end up with with a 9.35 4.6 block is not straightforward as there is a difference between the piston bowl size for the same compression ratio in an earlier engine.

Your best bet if you want a lesser compression ratio than 9.35:1 is to use your tin gasket heads without shaving them with composite gaskets, BUT you will need to maybe take material from the bottom of the rocker posts to gain the minimum 20 thou Preload. This wil be 8.6 or so .
To answer your question 2 you will end up somewher near 9.35:1 but not exactly because the composite head has a combustion chamber size of 28 cc and by taking off 35thou you end up with 31-32CC. Only by taking off 50thou will you end up with 9.35:1. You will probably have around 9.00:1

3 NO No No . if you shave the 3.9 heads by 50 thou AND use a composite gasket you end up with 9.35:1. ON A 9.35:1 BLOCK.


Look I know its confusing and these are only estiimates because I have not done every permutation that you ask. IIf you are still unclear please say so.
Regards Philip A

Nah, not confusing. I always had the opinion that the comnpression ratio was linked to the actual volume of the cylinder (taking into account dish of piston AND the cylinder head chamber volume. Therefore the shaving of heads, etc, will affect this regardless of what is stamped on the block.

If it relates to piston only then it wouldn't matter a stuff what heads you put on.

So a 4.6 with the heads it was designed to use would be 9.13:1.
Same goes for a stock 3.9, or any other for that matter.
What makes a mess is when you start gluing bits on from different models. If the chamber for the 3.9 head differs in volume by whatever cc it does then this will alter the CR for the engine.

The big but is, if the standard 3.9 8.13:1 heads stuck on a 4.6 9.13:1 engine gives 8.6-8.8:1 CR then I'll be up for that.

Cheers,

Alex