Please help me decide which pistons to get (and what CR)
Turbo Mini Forums

I need to get my block rebored and trying to decide which pistons to get.
I have been running my current engine at about 11-12psi(no IC) without any problems and thats what I´m aiming to run at after the rebuild to.

I have read all I can find about CR on this forum(and in Vizards book), and have done some calculations:

Std Metro Turbo(just for reference): 21.4(head)+2.8(gasket)+12(piston)+0.78(ring land)+1(deck)= 37.98cc
318.75+37.98/37.98=9.392CR

My current 1275 engine: 25.1+2.8+12+0.78+1= 40.68
318.75+40.68/40.68=8.835CR (I actually thought I was running a lower CR!)

My options:

Mini Spares Mega +0.040 9.38cc piston: 25.1+2.8+9.38+0.78+1=39.06
328+39.06/39.06=9.397CR

Morespeed Mega +0.020 13cc piston: 25.1+2.8+13+0.78+1=42,68
323.5+42.68/42.68=8.579CR

Morespeed Mega +0.040 13cc piston: 25.1+2.8+13+0.78+1=42,68
328+42.68/42.68=8.685

Have I done the calculations right?
What is your opinion on my options?
It looks like the MiniSpares Mega pistons is out of the question (these are the ones I was going to order before I did the calculations!).

Should I go for Morespeed Mega +0.020 13cc piston to get as low CR as possible or should I go for the +0.040 to get some more displacement?

Any help i appreciated!

id say the 'sensible' option would be to go with the +20's, that way you can rebore the block to +40 next time, and have a lower cr for a bit more safety regarding boost.


but tbh its your call...

if it were me (tight arse) id do whatevers cheapest :p

"Turbo's make torque, and torque makes fun"

"did you know you can toast potato waffles?"

Hi,
With an intercooler and mapped ignition system you should be able to run 11psi on the standard 9.4:1 compression.
That would be the absolute maximum. With the compression ratio you've got you should be able to get 15psi intercooled in there.
Road manners suffer with lower compression, but you can ram more boost in there...
Cheers,
Gavin :)

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Edited by turbodave16v on 17th Nov, 2008.

Check your deck height...

From my investigations the +40 and Mega pistons have a lower piston height than standard turbo pistons and therefore have approx 4cc deck volume... Thats how a 9cc piston can give the same CR as a 12cc standard piston (i.e 9.4 in standard form)


Check my post at
http://www.turbominis.co.uk/forums/index.php?p=vt&tid=121851


BTW I hope thats right! I just told Simon at Morspeed to only go 11cc on mine as I am after 9:1 CR (allowing for a small skim)

Edited by Turbo Tel on 10th Jan, 2007.

website:- http://www.terryhunt.co.uk

website:- http://www.terryhunt.co.uk

"I must be missing the point here - but the Pistons that Morspeed machine out to 13cc are the very minispares Mega pistons you say you can't run???"

Because of the high CR.
(If the Morspeed Mega +0.020 13cc pistons are machined MiniSpares Mega pistons, why doesnt MiniSpares have +0.020 pistons? Do Morspeed have a "old" stock of +0.020 pistons?)

I didnt know the difference in deck volume, that why I got so high CR with Minispares Mega pistons.

This should be closer:

Mini Spares Mega +0.040 9.38cc piston: 25.1+2.8+9.38+0.78+4=42.06
328+42.06/42.06=8.798CR

Morspeed Mega +0.040 13cc piston: 25.1+2.8+13+0.78+4=45.68
328+45.68/45.68=8.180

Anybody know the approximate deck volume for Morespeed Mega +0.020 13cc pistons?

Morspeed purchase pistons from minispares in 6cc dish form. ie:
http://www.minispares.com/Product.aspx?pid=35484
http://www.minispares.com/Product.aspx?pid=34806
http://www.minispares.com/Product.aspx?pid=36975


They carefully mount them in a lathe chuck.

They remove material from the dish, until the dish is 13cc in volume.

The piston is now what you have dubbed the 'morspeed mega'.



The minispares +040" 'turbo pistons' are 'new old stock' AE Turbo pistons I believe.


By coincidence / plagiarism / whatever, the minispares mega is a similar design and uses a very similar casting layout as the Factory AE metro turbo piston.

The difference is that the crown is a little lower than the factory turbo piston; the ringsets (same as omegas) are mounted a little higher; and the dish is smaller.



*****TD edit*****
Please see this topic for up-to-date info on the suitability of the minispares 'mega' range of pistons for turbo use:

http://www.turbominis.co.uk/forums/index.php?p=vt&tid=255230

Edited by turbodave16v on 17th Nov, 2008.

website:- http://www.terryhunt.co.uk

Excuse the thread stealing but how much is the discount members get from minispares and can you use it online?

Cheers Matt

I have +20 omega and i went down to 7.8:1 looking to run 16 -18psi and it is too low it is a bit dowy off cam and under 2k or so.

if you only want to run 11psi theni would go 8.5 or so.

Grant

179hp on 12psi 1293cc of pure A series
14.0et @101 mph

I have finally decided to go for Minispares Mega 6cc pistons. And machine them to the desired volume (about 10-11cc).

I have one last question before I order them:

Is it common that a 0.020 rebore isnt enough to take away scratches etc in the cylinders? (I have some scratches and 2 marks(above the top piston ring) from a failed valve but its hard to tell how deep they are)?
Would it be wise to go to 0.040(I would still be able to go to 0.060 in the future?) just to be on the safe side?

/Jonas

jonas, what made you go for the 6cc mega's. im in a similar posistion and looking at the 13cc 21253's from morspeed.
there cheaper (im a tight arse) and already 13cc?

'always keep a spare shot for a stray bird'

Mainly because 13cc is to much for the CR I want.
Also the Megas are considered a bit stronger and not that much more expensive.

so can you use any of the mega pistons from minispares of do they have to be the turbo ones?

'always keep a spare shot for a stray bird'

website:- http://www.terryhunt.co.uk