compression ratio (too low?????)
Turbo Mini Forums

hello peoples, after trying to sell my engine parts my interest in all things turbomini has become thoroughly renewed and with my build restarted i calculated my compression ratio to be 7.24:1 and that is if i flush the pistons to the deck of the block. the sums were as follows; RINGLANDS & GASKET = 5cc PISTON DISH = 18cc HEAD CHAMBER = 29cc I was planning on at least 20 psi boost maybe more later on. have i made a booboo or is what i plan feasible? would i be better of changing my pistons to 10cc omegas? my budget will only allow cast pistons if i change them where what ive got now are forged. i calculate the CR with the 10cc pistons to be 8.37:1 again pistons flushed with the deck. what should i do? any help is appreciated, thank you

Is there much work gone into the head other than increasing chamber size? My first thought would be to put a head with smaller chambers on.

On 12th Nov, 2009 Paul S said:

I think Gary OS has taken over my role as the forum smart arse

yes the head is an MED top spec item offset valves 36/32 ported and it would be cheaper to change the pistons

Make some of your money back by selling your pistons and get another forged set? I assume the engine hasn't run yet

On 12th Nov, 2009 Paul S said:

I think Gary OS has taken over my role as the forum smart arse

Just get the head skimmed a touch to get the CR back up a touch.

You can always added pistons with a bigger dish at a later date to get your CR back down again.

no the engine is yet to be built, i thought a new set of pistons would be easier, i take it from what your saying that 7.24 is too low? thanks gary

i did think of getting the head skimmed but then i thought; would it effect the amount of lift on the valves or is that a silly question? thanks apbellamy

You could also get the block decked, same effect different component

7.24 is awfully low alright. Closer to 8 would be fine. You may have some room to deck the block. It's approx. 0.2mm per cc (as long as my rushed in-head maths is OK) so you would need a big deck height to get the CR near suitable. Skimming doesn't effect lift. If you go mad skimming it can effect rocker geometry but shorter pushrods can be used to compensate

On 12th Nov, 2009 Paul S said:

I think Gary OS has taken over my role as the forum smart arse

7.3:1 is too low tbh, I would change the pistons. If you skim a turbo spec head, you may struggle to sell it on at a later date.

What size bore are you running?

Edited by matty on 6th Aug, 2009.

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1/4mile in 13.2sec @ 111 terminal on 15psi

I would either sell the pistons on to get some with a smaller dish. Or if its been run get a suitable sized turbo and run crazy boost! All depends on what the cars gonna be used for!

Like star mag says ,let people no what you planning on doing with the engine, it may help.

You mention 20+psi ,is that just to get power figures??

Am running 7.5-1 and mines ok, am sure people may tell me different though.

Rick

30+Psi here we come!

As the others have said, it is abit low depending on the intended use.

However if you dont mind lag you could put some rather large boost figures through it and have some fun!

Hello again everyone thanks for all your input, it seems that i have a couple of options to go with and to think about. I was building this engine wiyh the odd 1/4 mile in mind but mainly as a second weekend fun car. i dont mind a bit of lag. the reason for me wanting 20+ psi was just for my love of boost.

sorry matty im running a +40 bore 1312

Are you sure your calcs are right ?

7.24:1 would be ideal.....if you intended to run say 40-50psi lol


But for more mundane levels of boost....aim higher than 8.

9.85 @ 145mph
202mph standing mile
speed didn't kill me, but taxation probably will

What size bore/stroke?

In the 13's at last!.. Just

i used to run 7.9:1 and 20+psi, now i run about 8.6:1 and 15psi, and it drives better and has does better 1/4 mile times

Steve, dont want to jump thread but wot boost could i run with 7.5:1 ??

rick

Im runnning 8.8:1 and have managed 18psi no probs.

Only my map sensor holding me back now.

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1/4mile in 13.2sec @ 111 terminal on 15psi

thanks again everyone, ive decided to go the new piston route, 10cc dishes would give me a cr of over 8;1. what do you all think i should ask for my set of unused accralite 18cc forged pistons? still boxed and wrapped.

Whatever you can get for them. At the end of the day...if they are going to give extremely low compression in most engines.....how easy will they be to sell ?

Although some new pistons can make nice paper weights.

9.85 @ 145mph
202mph standing mile
speed didn't kill me, but taxation probably will

I'll give you a fiver for them.

Old pistons make good paper weights too.

ive got an old set of omega 10cc dishes that ill let you have for a fiver but these forged ones have an 18cc dish which is good for an ashtray aswell as a paper weight

£7?