Outer Limits 4x4 Board

Just need to find out if the combustion chambers differ in volume to each other?

I do know the earlier heads have an extra set of bolt holes, but I don't have a clue if this is the only difference or about the chamber sizes.

I have a 8.17:1, 3.9l engine and will use the heads off that on to my 9.35:1, 4.6 litre engine.

Obviously it is a clever thing to check that I don't end up running an 11:1 compression ratio as it's hard to get alcohol or 110RON avgas everywhere.......

36CC tin
28CC composite (4.6 head) equals tin head minus 50 thou.
I have both 4.6 heads and 3.9 heads and have measured both plus my 3.9 heads after 50 thou shave.
That is really most of the difference between the two castings. The 4.6 head has better exhaust ports (slightly) due to more accurate casting. That would not be hard as the 3.9s are TERRIBLE.
Regards Philip A

SOOO if you use a tin 3.9 head with composite gaskets you will bring the CR down by about 0.6 or to about 8.8 :1 which is perfect for normal ULP.
BUT watch the tappet preload. You will have to mill the rocker posts.

Sorry I read your later post before this one.
Reagrds Philip A

G`day , 8.17:1 is an odd ratio ? 8.13 is the norm though there are a variety of odd ratios .
What are your heads off ?
Are they 10 or 14 bolt ?
What is the engine number prefix and suffix ?
What are the casting numbers on the heads ?

We have a 4.0 in our 85RR using comp gaskets and machined 3.5 heads with a ratio around 10:1 , its factoy stamp is 8.23:1.

It`s up to you but i need answers to all to be of any use .

All the best , peter .

My mistake, it's an 8.13:1. Was going off memory.........


The 4.6 block is 10 bolt.
The engine number for this is:
CR9.35:1
S46D4XXXXA

The heads are off a 3.9 14 bolt engine.
The engine number for this block is:
CR8.13:1
37D0XXXXA

Casting numbers might be a little harder to get at, haven't stripped the old engine yet.

I'd love to be 8.8:1 using composite, rather than steel, gaskets.

So, from what I can gather if I glue the heads on, as is, with a composite gasket I will run around 8.8:1? And I will need to machine the rocker posts down by around 50 thou?

Thanks , the nos tell it`s 3.9 from RR 8.13 ratio and if std heads should
have the larger chambers .

When you get it apart it should either have ERC 0216 or HRC 2210 , main diff is HRC have cap type stem seals ( norm ) as opposed to the flat washer type used on ERC type .

If when you pull it apart you find it has comp gaskets and you know they are factory fitted , some HRC 2210 heads were machined to suit comp gaskets some were 10 and some were 14 bolt but i think this applies more to Disco 3.9 than RR but they should wear a B suffix .( engine no)

If you find it has HRC 2479 heads these are the ones with the smaller chambers .

The 4.0 and 4.6 use the same heads , as in no difference between the 2 .

Each engine is different when it comes to practice , unless a particular engine is measured , it`s only possible to have a general idea .
Depending on the brand of comp gaskets they have different thicknesses and depend on what has been done to the head ( how much machining already) and combustion chambers vary .

It`s possible to work out an approx comp ratio but the figures i have for our 4.0 are no good because a 4.6 and 4.0 can use the same block as in the block is interchangeable but everything else is different .


A bloke on the forum i usually visit has done what your doing , he fitted comp gaskets , machined heads and i did do an very approx ratio , hes was a 4.6 9.35 too .
It was only very approx though because i didn`t have the piston deck height and took the piston dome volume from the web and if any 1 fig is wrong so is the ratio .

This may be sounding a bit more involved than your interested in .

It can be put together without being so involved and will still work ok.

How involved do you want it ?

Then i can suggest and i say suggest what i think .

All the best , peter .

I'll start pulling the 3.9 to bits this weekend, I hope. I'll get back to you then on the castings.

I believe they will be factory tin gaskets and ERC 0216 heads. Will know for sure a bit later.

Thanks a heap for the info!

G`day , made abit of a blue with the HRC 2102 these heads depending if early or later use either flat washer ( early ) type stem seal or cap type ( later )

Had a bit of a look at some figs and it seems to get somewhere around 8.8:1 with comp gaskets it will need combustion chambers around 34CC

I`ll show you the figs and how i got them later ( bit late just now ) , then you can judge for your self if they have merit . ( sort of taking the easy way out )

I read in your other topic that you have spare ERC heads .

If you have a look at them on each corner there is a flat surface opposite the head face . The 2 on the exhaust side are much shorter than the 2 on the inlet side .

I have a few heads here and 1 set measures 0.04MM below what original heads are , as in new un machined heads .
These heads i`ve measured and have various figs for various depths and the CC values .

Do you have some way of giving a fairly accurate measurement of the flat surfaces ( mentioned above ) , preferably the shorter ones on the exhaust side , coarse on the heads you intend to use ?

All the best , peter .