My stg3 heads

Post by **Coops** » Thu Oct 30, 2008 3:18 pm

cant believe i was stupid not to take that into account when i bought the heads,
but i was on a deadline and now im paying for it,
i'll speak to rob at V8D on friday like i said and see what they recomend,

cheers for your help so far guys, im paying for my mistake again, the hard way

just a thought,

the 4.6 heads, are they identical to older heads (chamber wise), just the faces machined more to give the 28cc chambers?

sidecar · Post by **sidecar** » Thu Oct 30, 2008 4:38 pm

thev8kid wrote: the 4.6 heads, are they identical to older heads (chamber wise), just the faces machined more to give the 28cc chambers?

I was told that they are the same, the heads were skimmed by Rover to allow them to use the comp gaskets. There maybe some asolutely minor differences but really they are the same.

Pete

kiwicar · Post by **kiwicar** » Thu Oct 30, 2008 5:16 pm

If you skim the heads you will increase width of the squish band a little which could only be a good thing.
Mike

Post by **katanaman** » Fri Oct 31, 2008 10:41 am

The interim 3.9 heads are the same just skimmed more and the outer bolt holes not drilled but the 4.6 heads are a different casting. I would say since your not sure where your going with the engine use tin gaskets with a coat of wellseal. They are absolutely fine when used with ARP studs. You don't do big miles anyway between strip downs so there is next to no chance of them failing. TBH looking through this forum there are far more comp gasket failures than tin anyway. This will save you a few quid until you know if your going forced or not and save you having to buy new heads if you do.

Post by **Coops** » Fri Oct 31, 2008 3:45 pm

hi marki,
would love to go forced mate.
but due to cost its looking mighty doubtful,
will run her on tins next year then to see how she runs.
any certain areas to use the wellseal?
water ways? bore lip?

Post by **katanaman** » Fri Oct 31, 2008 4:58 pm

Na just paint the whole gasket,. clean the original coating off first though. There was another product the guys were on about that was an aluminium type paint. It sounded very good but I couldn't find any. Cant remember the name but have a search for wellseal in the forum and you will find the thread.
If you go machining your heads right now going forced will be less likely because of the cost of new heads I guess. Suppose you could go low comp pistons or one off head gaskets right enough but its still an extra cost.

CastleMGBV8 · Post by **CastleMGBV8** » Sat Nov 01, 2008 12:16 pm

sidecar wrote:
CastleMGBV8 wrote:Pete,

Real Steel quote the volume of a compressed tin head gasket for 94mm bore at 18 thou as being 3.5cc. so with the smaller area of the chamber I would say your advised figure is correct.

To calculate the volume of taking 20 thou off the decks of a 94mm bore engine,

Bore + 9.4 cm divide by 2 = Radius 4.7cm x 4.7cm(squared) x 3.14 (Pi) x .0512cm (20thou) = 3.55cc.

Kevin.

Hi Kevin,

You have to be careful when deciding to skim the block or the heads, skimming say 10 thou off the block will result in a higher CR than skimming 10 thou off the heads. The reason is that the combustion chambers are not round and do not go out to the edge of the bores.

My heads had something like 70 thou off them which is quite a lot but I'm using comp gaskets so that accounts for 20 thou. The CR of my lump is around 10:1.

I know that alot of people try to work out the CR using using the figures quoted for piston dishes etc but really the only safe way to do it is to burette each and every piston at TDC and each and evey combustion chamber. The pistons also need to be at true TDC, not just setup "by eye".

All this takes ages and you end up covered in parafin or whatever you use as the measuring liquid, it takes ages to do and then you have to sit down and calculate all the CR's

You then have to get the manifold to fit and sort out the pushrods (as you well know!)

I had my eddy manifold skimmed but if I was building another motor I get the heads skimmed on the manifold gasket face so that all the machining was "contained" in the heads. (it would have been cheaper to!)

All just my humble....!

Pete

Pete,

Missed your earlier reply as the thread had jumped to page two, I think that was basically what I meant in that the combustion chamber shape is different to the bore the effect of machining the heads by the same amount result in a lesser increase in compression so as usual I was agreeing with you

.

Re; the effect of compression ratio to power production, the standard 3.9 EFI engine with 9.35/1 ratio is rated at I believe 184BHP but I can't recall seeing a rated figure for the low 8.313?/1 engine anyone know this figure, as it may give a more accurate insight into the effect of compression on this specific engine.

Kevin