made an arse of my head
Turbo Mini Forums

thort id have a go at shaping my combustion chambers to lower cr, my base car is a spi cooper so has compression ratio of 10.1 with only 6cc dish pistons. i calculated i needed to get the chambers to be 27cc to get comp of 9.4.1.im only wanting to run 7 psi for next year till i do the engine and box done properly.the problem is my heads been skimmed alot more than i thort and i underestermated how must metal needed to come out.this is a pic of it now ,its only at 26cc now and there isnt much left to go at lol. i know its scrap but is there any reason i carnt run this on low boost for next summer with a chamber this shape ?

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There's no reason you can't run that chamber shape, though not necessarily considered ideal, it will work.

Phil.

WWW.TURBO-MINI.COM

yeah that will work, a few good performaers have run similar in the past.

it will make powerr but might not be as good part throttle due to lack of mixing,

looks similar to my astra head ...



working well on boost and off .(using lpg)

Medusa + injection = too much torque for the dyno ..https://youtu.be/qg5o0_tJxYM

But the OEM head will have had ports/chambers designed together etc.

Would have been easier just to get pistons to lower the CR, and IMO better all round

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

thats good to hear,i have a good standard head that i could use but this one has bigger inlets with race type valves and i ported it out to vizards spec last yearso have hours of time in it.

Double check you're CR calculations... The standard pistons used in the SPI (P21263??) that give you 10.1 CR are 8.4CC dish not 6CC.

EDIT: I think I meant P21963 not P21263

Edited by PhilR on 16th Nov, 2014.

the pistons in mine are 21963, iv had a look on minispares and they are the right ones for my car, im sure they are 6cc,il have another look,i hope you are right tho phil

21963 out of my SPI had exactly the same dish and wrist pin height as the common 21253 that I replaced them with. I did a lot of measuring on the 21963 as I was modifying them to see how far I could cut the dish out to.

I was aiming for 26 CC on this (non skimmed) head that I'm doing, and reached it with the shape below. To be honest, I was prepared to start cutting either side of the spark plug (like in Turbo Phil's avatar) or take it as far as you did to get the CR right. There's lots of different ideas about the chamber shapes and I don't fully understand most of it. Needless to say, you need to do what you can to get the CR down. Maybe your's will put the spark plug closer to the centre of the mixture. If the CR is right, just try it

21253

iv just calculated it again with 8.4 cc dish and its 10.2 to one and with 6cc as i thort they were it was 10.9 so looks like your right,im sure i read they were 6cc tho,well it not alot but its better,that will give me 8.9.1

yes all will be well