12g940 again

Post by **turbominor** » Fri Dec 19, 2003 10:55 am

Have fitted a 12g940 unleaded head to turbominor(1098) and all is well, but even after an atack of the dremel the compression is still higher than i want for turbo use.

Both my local garages said use two head gaskets and that will lower the compression. This has been said may time to me....

any thoughts fellow minor owners??

Post by **Kevin** » Fri Dec 19, 2003 1:13 pm

Did they say how much it would be reduced by as I have also heard the same but have not actually met anyone who has done it.

Post by **Peetee** » Fri Dec 19, 2003 1:40 pm

but even after an atack of the dremel

Is that following the guidelines given by Dave Vizard?

Post by **les** » Fri Dec 19, 2003 11:14 pm

You could fit low compression pistons, if you haven't already. maybe you've thought of that though, I don't like the 2 gasket idea.

Post by **ianselva** » Mon Dec 22, 2003 11:33 am

I would not consider two normal gaskets. If you are left with no althernative I would get a new gasket made out of solid copper as thick as two standard ones. I had a manifold gasket made for my Fiat engine that cost £30 and part of that was for the CNC programme to cut it.
I would have thought the best route, if you cant fit low comp pistons, would be to take it to a head specialist who could open out the combustion chamber enough to drop the C/R to whatever you need .

Post by **rayofleamington** » Mon Dec 22, 2003 6:10 pm

If you are considering having pockets in the block (eg to use higher lifitng rockers) then these pockets need to be accounted for when you consider the overall compression ratio.
What worries you about the compression ratio as it is?

Post by **turbominor** » Tue Dec 23, 2003 8:00 am

main worry is the 5psi of boost that i am running i was looking for around 8:1 compression ratio or less

any one know if fitting the 12g940 will greatly increase the compression

Post by **Peetee** » Tue Dec 23, 2003 12:01 pm

any one know if fitting the 12g940 will greatly increase the compression

Try getting hold of someone from a rallycross club. There is a catagory for 1000cc mini's where, I suspect, most would be running 12G940 heads to get the best power within the regs. i realise it's not the same engine but that field would have the biggest choice of experienced engineers etc that may have done similar to the 1098.

Post by **turbominor** » Tue Dec 23, 2003 12:41 pm

Cheers Peetee.

Have already got low compression van pistons, have some metal in the head i could remove, but i dont want to removed too much as the turbo needs lots of metal to dissapate heat

Post by **rayofleamington** » Tue Dec 23, 2003 12:51 pm

the turbo needs lots of metal to dissapate heat

the issue you will get with that is when there are thin parts between the head and water jacket. This will mean some parts of the combustion chamber get cooled more and other parts will be less cooled (extremely hot when you drive hard). I don't know if this is a significant risk on A-series heads, but in theory it will increase the risk of warping and cracking.
12G940's are not too difficult to come by so maybe you can suck it and see.

So is 5psi boost quite a lot?

Post by **Peetee** » Wed Dec 24, 2003 8:58 am

I know there is a heat issue here with the 1275cc heads between cylinders 2 and 3 on the block. IIRC it's something to do with the lack of water cooling in this area. Is this the same for the smaller capacities - where in theory there is even more metal because of the smaller bores?

Post by **les** » Wed Dec 24, 2003 9:28 pm

I'm not too sure about the lack of water cooling, but I do know that the original 1275 A series head, fitted to the cooper 's' had very large inlet valves, the heads of which almost touched the exhausts! Cracking was common between the valve seats. When the 12g940 head was introduced it had smaller inlet valves and the cracking was virtually solved.
Have you considered using a 12g295 its got a pretty good gas flow is designed for small bore engines and if fitted to a 1098 usually needs skimming, so if you do not skim, it could give you the compression ratio you need. The 12g940 was based on this head.

Post by **turbominor** » Fri Dec 26, 2003 5:12 pm

I was looking for a 12g295 but could not find a decent one, eps converted to unleaded.

So a 12g940 from a metro for £20 (50k miles on clock) came my way, have filled the front water way and drilled a new one to match 1300 gasket.

5psi is not much, but enough to cause detonation if compression is too high.