Head chamber size.
Turbo Mini Forums

Just want to know what the maximum safe head chamber size is, ill be running around 15psi of boost, but have +20 omega forged flat top pistons. So if i can get away with not having the pistons machined id be much happier! If i do need them dished, is 5/6cc about the limit on these?
Ive worked out with 35cc chambers my compression is still too high, but i don't know the rough limit for the head. Another 5cc would bring me pretty much perfect on CR i think if i can get it.
Thanks in advance.

Edited by Carlzilla on 11th Nov, 2011.

I am sure one of the resident head butchers will advise. But I would say forget 40cc in a head for starters.

Remember CR is not the total story. You also need a good shape in the combustion chamber.

Personally if you want the best performance knock out the flat tops to someone that needs high compression for an NA motor and get the right pistons for the job.

Again I've heard of 30cc, but not much more. I did once see a head with a totally round combustion chamber that I have a feeling was over 30cc. But performance wise it was poor.

I think that you're going to need different pistons.

Fastest 998 mini in the world? 13.05 1/4 mile 106mph

i was going to say, could carl not dish out the pistons?

35cc has been done, but you need to start with a selected casting, and mill the chamber deeper. Far easier to do the pistons.

Well, as i don't have a head yet, and will be buying one to the spec i need i can't see the casting being a problem. I'd rather take the head out to something safe and dish the pistons to something safe and run a few less psi of boost if that's what is needed to be done.
So if we can say 30cc for the head (playing it safe) what do people think for the omega forged? I know med do these in 6cc dish so im presuming that is what is the safe maximum?
And if those are right, could somebody point out a safe boost level?
It'll be intercooled (4x4 cooler) Undercrown oil jetted and oil cooled.

Omegas come in 15cc forged. However they are almost certainly a different forging and so don't think you can machine 15cc into flat top pistons.
I really think you are missing the point. If you think you can just bolt together a motor with what are essentially the wrong pistons, but as long as the CR is right then you will make the power, then you've a lot to learn.

Like others have said really, you need to balance one with the other to get the right package. With my N/A motor I only need .2 cc taking out the chambers to get my CR how I wanted it. I know its completely different with FI but the principles are still the same when choosing the right spec :)

Remember piston dish and head chamber size are not the only factors that make your CR

I would of thought "a safe boost level" would be different from engine to engine, no 2 engines are the same usually, all have different characteristics?

Edited by Pauly on 12th Nov, 2011.

ok to elaberate on my last post now i'm at a PC...

35cc has been done certain castings, but is has to be a certain type (austin america rings a bell) but even then its dificult and not guaranteed, on a std casting from what ive heard about 28's ok 30 can be done and 33 is at the limit. not going to be cheap though,

as Tom says there are differences in the basic piston shapes, some can take machining some cant, a general rule of thumb is you can probably get an extra 5cc on top of what the piston is tops, although saying that Ive taken a 10cc powermax to 17.5cc,

it looks like you might be able to get down to 9:1 just, but you'll need to have a good measure of the crown thickness to see what you can get out of them,

I do realise things like the head gasket, how much the pistons sit down in the block, stroke etc etc.
But like i said, its a rough guide and not the definitive answer, i know it can be done with flat tops as that is what kean was running once upon a time, but i don't see how i can keep the pistons as is for the CR i need.

Carl, ditch the pistons use 10cc cast omegas

and get 26ccs in the head job done!

like tom says those flat top pistons are a diffrent casting

What cr are you actually aiming for? I've read the full post but didn't seem to see that factor

Weren't the flattops that Kean ran quite a way down the bore at TDC though?

Fastest 998 mini in the world? 13.05 1/4 mile 106mph

I'm interested in the flat tops if you decide to sell.

Edited by alaskanow0 on 12th Nov, 2011.

Class C 3rd Place Avon 2011 14.18 @101mph

The Omegas I had were the short height type that ran down the bore as Wil stated above.

The block has to be decked to get the CR sensible but it is doable and does work.

With the range of pistons available now, I'd get some decent dished pistons and do it that way.

Around 8.5:1 or as close as possible i think seems sensible. Looking at Phil's site.
Im not entirely sure of the in's & outs of these pistons to be perfectly honest.
Would BMC 4 ring +30's with a 14cc dish be far more suitable or do people think im better off using a performance piston?

Because id rather build the engine to accept a bit more earlier on, and not have to worry about it, than have to spend another lump sum later on when i eventually get bored of the power (it probably will happen)
Ive been doing a lot of reading, and within reason id rather have a reliable quick engine than an engine that is stupidly quick but doesn't last long. Im trying to think of whatever i can to gain every last bit of power for a little bit of work so that the engine isn't as stressed, as much balanced as possible, cooling i think i have covered, fuelling and ignition i think i have covered, and what instrumentation i need to keep an eye on things. So I'm now at the point where i ultimately need to decide what i need to do with the pistons and head, to be getting the most out of a t3 for the time being until i get used to it, but build with the capability to accept another turbo later on.

I'm a big fan of not doing everything like the norm, for the sake of it, but in this case, you need new pistons. Seriously. Flat tops and 30cc chambers WILL not get you where you want to be.

It's so much easier to get another set of pistons, and then if you want to change anything at a later date, this is all bolt on.

Seriously!

Im not dismissing the fact i need new pistons, I've already decided im going to let them go for what i paid for them.
I would still much rather use the +30 14ccs i have and buy rings for them, Or is there 1275 Pistons you can buy as the block i have is a fresh 1275 liner?

4 ring pistons are not a performance piston.

No BL piston with four rings is suiteable for turbo application. Buy some 10cc cast Omegas and be done with it.
Now lock this thread

You shall be pleased to know the omega pistons are on their way to a new home!