8 replies to this topic

[RonH]

Hi all,
having just 'accidently' won a super modded head on the dreaded ebay, I now find I need advice on compression ratios and how to decrease them!!!
This head has been ported to make my stage 3 door stop look tiny, however it has also been radically skimmed. It doesn't yet have valves or guides so I haven't been able to measure the combustion chamber volume, but the spark plug threads have been cut into by the skim, so possibly 11.5:1 ?

Question: is there a reasonably reliable way of increasing the combustion chamber volume to give a lower ratio that doesn't involve chiseling out more metal. I'm thinking of spacers or similar???
The head is approx 3mm less thick than the door stop (which has crack between valve seats) but as yet I don't know how much of this came off the head surface and how much off the rocker surface.
2nd question: Is there a reliable lead replacement product I could use to compensate for a high comp ration, and to protect the valve seats which are probably not lead free?

Cheers
Ron.

[ace01]

As most people will tell you, measure the combustion space in the head with parafin or similar and then work out the ratio.  To try and guess the amount of dish required in a piston or to reduce the compression ratio in any other way will simply be guesswork otherwise and may well lead to other problems.

[carbon]

Sounds like most of the metal has been skimmed off the lower face of the head. If it is 3mm off the lower face then you're probably looking at about 10cc less than a standard head (if this is a 1275 head), but only way to check this for sure is as Ace has said by measuring.

 

If you are completely rebuilding a motor then you may be able to use pistons with larger dish. Decompression plates are sometimes used, but would likely give rise to other issues.

 

Octane boosters can be useful on occasions, but even they have limits. And if you having to add a lot of octane booster check out how much this is likely to cost before going down this road...

[Cooperman]

You can grind more out of the combustion chambers to reduce the CR so long as it's not been done too much already.

It is VITAL to measure and calculate the CR before building the engine.

A above there are pistons with larger dishes and the Omega ones as used in forced induction engines can have dishes around 4 cc larger than the normal Omega or Hepolite (Nural now?) pistons.

Best thing is to measure everything then post the results on here for more advice.

Castrol Valvemaster Plus is good for lead replacement and octane boosting and my 'S' which has 11.2:1 CR works fine with it, but I use an Aldon custom-curved distributor.

For a road car the highest CR advisable is around 10.7:1.

[ACDodd]

CR's have to be linked with the cam used. Trust me the fuel we have now will not run with a high compression as it used to, even 10 years ago.

 

Cams upto 270 duration 10:1 CR max.

 

Cams upto 260 duration 9.75:1

 

Cams up to 245 duration 9.5:1

 

Cams up to  230 duration 9.25:1

 

AC

[Earwax]

Slight detour, but i am interested in the combustion property of heads, 

 

I think both AC and cooperman make very valid points - with some subtle differences for specific tuning but both look at compression burn property of modern fuel etc. Does anyone know whether squish factor or shaping of ports effects this burn property? Also  The block looks to have a V number- is there significance to this ( i am in Australia and used to 9 F Sa Y etc numbering. good luck with build.

[jaydee]

Its because of the dynamic compression, short valve opening duration and high static CR will create a stupidly high compression, while a long valve opening needs an higher CR to create the optimal compression.

[Vipernoir]

Check the remaining deck height of the head - if it has been skimmed that much, it may be paper thin and prone to problems.

[RonH]

Not sure if anyone is still out there for this question, but...
I now have valve guides, valves and a head space volume of 16.6cc.

My current head (from memory) has 22.1cc headspace and a CR of 9.7. This means that without further modification I wil be running a CR of 11.1.

Anyone with experience, is this feasable with modern fuels, probably with additives???

If not, is there a reasonable way of gaining 5cc of combustion chamber without a total engine rebuild (I was hoping for a straight head swap as I have just re-built the engine and it's not yet fully run in)

Does anyone know where I can find reliable information on where the water chambers are relative to the possible areas of combustion chamber reshaping for a 12G 940 head so I can have a go at increasing the head space?

Many thanks

Ron