Compression ratio.
Turbo Mini Forums

Howdy, im in the process of building up a turbo motor, but when i worked otu the CR usieng a block thats the standard deck height it works out at around 6.68:1 with the pistons iv got! (73.5mm, 16cc dish and only 33.9mm from the centre of the pin to the top of the crown!)

Iv gota bore a fresh block to fit them, so should i get the block decked a few mm? rendering it scrap if i need to move over to normal aftermarket pistons, or do i jsut get new pistons straight off the bat?

What CR should i ideally be aiming for?

Cheers,
Denis.

turbo 16v k-series 11.9@118.9 :)

Denis O'Brien.

Which pistons do you have ? Are they from Japanese engine, 0,5 mm oversize perhaps ? They sure don?t sound like any Mini aftermarket pistons. Normal comp height for Mini pistons is 38 mm.

Jukka

duno, there made by accralite, and they have the pistons on file. asked them for confirmation of the bore they should be put in and running clearances etc.

From what i can gather they were made to order. duno if it was for an individual or company.. or where i could possibly get more, besides asking accralite to knock up another set!

the pin hole is the correct size and the bore is 73.5mm so i can only asume they were origionally destined for the a-series. definately a turbo piston tho, no NA forged piston would ever have a dish that big! its like a soup boul.

Cheers fo rthe input, stil foneing around machine shops to try get a decent quote for the work.. so far im looking at around ?300+vat (includeing offset boreing and decking as its alot of material)

Edited by evolotion on 28th Oct, 2003.

turbo 16v k-series 11.9@118.9 :)

Denis O'Brien.

I am running a turbo engine with 73,5 mm Omegas and currently have CR around 7,7-7,8:1. I ran several years at 10 psi boost but since May when I finally got the Dellorto carb sorted have used 15 psi. No reliability issues.

I think that this is just about the practical limit for 1380 engine, I have thought about re-setting the boost to allow even higher boost, say 20 psi for short sprints (just for curiosity...). However, this will happen after IMM2004. I want to have a car to drive there.

So, if you fit a decent intercooler and limit the boost at about 10 psi, you can leave the CR as high as 8,5:1. To play safe aim between 8-8,5:1. You would need to skim something like 4 mm off the deck. I am not sure how much A+ blocks tolerate skimming before the deck gets too thin and creates head gasket problems. I have used Allegro blocks which are really robust and most likely have more material to accept that much skimming. Anyway, I am not expert in this field (nor anything else ... ) so ask someone who has solid info.

Jukka

Read about your car!.. was the insparation behind my decision to go turbo big-bore.

got a decent quote for the work now :) so hopefully hav the block off for machining very soon.

From my maths 4mm will take me to exactly 8.22:1 which seems very reasonable, for say 10PSI normal running and a little more for short sprints. Wont dare take it very high as i need the car to be reliable.

didnt know about the skimming part. must look into that, worst case il chop a few mm of the block and a little off the head, strike a compromise!

Cheers for the info on the CR though, exactly what i was after!

denis.

turbo 16v k-series 11.9@118.9 :)

Denis O'Brien.