My stg3 heads

Post by **Coops** » Wed Oct 29, 2008 5:34 pm

been sat here thinking, and its just clicked im more down on power and comp ratio than i thought,

the pistons are 9.35:1
the heads have 36ish cc chambers,
and im using comp gaskets

so it must be something like 9.2:1 ??? or lower?
could this assist with only hitting 230bhp on the rollers?

maybe when the heads goto V8D after crimbo to have the valve springs change to doubles i should get them to look into this,
what may need doing to get the ratio back up????
chamber size changed?
faces skimmed?

Post by **katanaman** » Wed Oct 29, 2008 5:56 pm

you will be a lot lower than 9.2 ish T. You ran lowish comps before but you didn't have anything like as big a cam in there. I would reckon you need to be up around 9.75-10 maybe more unless you cant for NOS reasons. No idea about that stuff to be honest. Options, use tin gaskets least you will be at standard comps, skim heads/block depending how high you want to go. If you put deep enough pockets in the pistons you will still be clear after the skim. A lot of people ignore compression ratios saying it wont make that much diff but when it comes to lumpy cams it means loads. Plug you cam specs into dyno 2000 and play with the CR figure and watch what happens

Post by **Coops** » Wed Oct 29, 2008 6:04 pm

i was running 4.6 heads then mate, with comp gaskets so was around the stock 9.35 mark,

i have 5mm cut outs in the crowns if i remember correctly mate.
will speak to Rob friday regarding what to do, as while they have the heads they may as well do what they can to them,
im still not sure about tin gaskets, call me a worry head but hey thats me

so my results are not as bad as first thought then,
wonder what she will do on gas now when rebuilt

HairbearTE · Post by **HairbearTE** » Wed Oct 29, 2008 7:01 pm

The problem is you gotta have some compromise if you're running gas. Your cam probably bleeds off more than the old one did too. I'm sure you could easily run 10.1 or higher on optimax with that cam. You might want to think about just using race gas when you're on the nitrous to allow a higher static cr for everyday use.

JP. · Post by **JP.** » Wed Oct 29, 2008 7:46 pm

Tony you might have something about this info,

Our Merlin heads are 31cc which in combination with the composite head gaskeds yield 9.02:1 cr.

Thats what Real Steel calculated when he advised us which of the Merlin heads to take in combination with our supercharger.

To remember its a 3.5 V8 we have if it matters....

You'll might find some interresting calculators overhere :
http://www.wallaceracing.com/Calculators.htm
Cr calculators can be found under CID - Engine Calculators

It looks like you'll have a nice set of heads that's in the need to be boosted............
Yeah I know its that wallet thing.

JP

Post by **Coops** » Wed Oct 29, 2008 8:10 pm

cheers mate,
yeah its a wallet thing,

not sure what to do,
1- keep them and when i can afford boost the car,
2- get V8 D to machine them, to bring comp ratio up a bit more,
3- sell them as is and buy a set pre machined etc with 28cc chambers,

ian.stewart · Post by **ian.stewart** » Wed Oct 29, 2008 9:09 pm

Static comp ratios mean nothing, as soon as you introduce a valve that is open on the comp stroke or on the firing stroke the static comp goes out the window, I think my static is around 15.1, but the dynamic is around 11.1, as you are well aware it seems to work,

Post by **Coops** » Wed Oct 29, 2008 9:23 pm

yes mate yours does,
but im defo down on power, which needs addressing

monty man · Post by **monty man** » Wed Oct 29, 2008 10:13 pm

i see your issue m8, if you work out how much you want to skim off your heads or block i can give you a price.

Post by **Coops** » Wed Oct 29, 2008 10:17 pm

rather have the heads done as they are 36cc chambers, would like them back to 28cc like the 4.6 heads i had on before these stg3 ones,
can change the comp ratio by using tin gaskets then etc,
how much to come off, aint a clue mate, maybe someone can assist

CastleMGBV8 · Post by **CastleMGBV8** » Wed Oct 29, 2008 11:28 pm

Tony.

Doing a quick calculation I think your compression ratio with the comp gaskets to be approx 8.95/1 not taking into account the effect of the overlap on the cam which could lower it substancially more.

Don't know how you calculate a reduction for the cam effect, is there any known formula.

I would have thought that your spec engine with a C/R in the high nines would have got you near 250BHP

Kevin

Post by **Coops** » Thu Oct 30, 2008 5:16 am

250 was what i was hoping for mate,

sidecar · Post by **sidecar** » Thu Oct 30, 2008 6:29 am

I was told that evey 8 thou off the heads is worth 1cc reduction in the chamber vol. My 36cc heads are now around 25-26cc, the above formula seemed to be quite accurate. (My heads are on a 4.6 with 9.35 pistons).

You'll have to sort out the pre-load but that's do-able by several methods.

Pete

CastleMGBV8 · Post by **CastleMGBV8** » Thu Oct 30, 2008 8:58 am

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.

sidecar · Post by **sidecar** » Thu Oct 30, 2008 9:49 am

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