Another "What Compression Ratio" topic
Turbo Mini Forums

Yes, I know, there are several topics discussing the requirements of compression ratio against boost, but, I need something more specific :)

Modern thinking is low boost high CR. But what is considered low boost when there are those running 26psi? and what is a high CR when most run sort of average 8.5:1 CR?

Ok, so what am I asking. I need to make a decision on CR. I have calculated two options for CR, 8.5:1 and 9:1. I intend to run 7psi most of the time with the option of 15psi when I feel the need () Peak power in N/A form is up around 7500rpm, and I will be using the same cams, LT inlet/ RS Exhaust, retarded to around 110degrees. Intercooler is from an audi S3 and will be ducted side mount, no idea on estimated efficiency, but it has to be around 70% I would have thought.



I supose the real question is which one, 8.5:1 or 9:1 at 15psi

I am tempted to run the 9:1.

I would be tempted to go even higher.

You have a lot of things in your favour, so it's different from a carbed 5 port.

Alloy head will provide better cooling and less likley to hold hot spots.

You have an ECU controlling fuel and ignition.

I was going for 10psi at 10:1 on my K100.

It is very dependant on the efficiency and placing of the intercooler though. If you had a 100% efficient intercooler, then you would not need to drop the CR at all.

I reckon you could get away with 9.5:1.

All posts of this nature are made in good faith and infer no liability whatsoever

Edited by Paul S on 4th May, 2010.

Saul Bellow - "A great deal of intelligence can be invested in ignorance when the need for illusion is deep."
Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."

I had an s3 cooler on mine, and it is highly efficient,rarely gone above 35°.
Your choice is:
8.5/1: more power because of correct ignition advance but trade on drivability, lag, of boost response, more animals.
9.2/1:better drivability,less lag, better of boost response, engine more linear but trade on power because ignition a bit retarded.
your choice
I'm with paul, sure I would have tried a bit more c/r on this sort of engine, like 9.5/1, but perhaps 9.2 is finaly a good middle, then water injection can be your light!!!

12:1 and see what it will do

Boom?

If something is worth doing, it's worth doing half of.

boom shake the room

I run a supercharger and I don't care the TB is on the wrong side.
VEMS + 12 PSI + Liquid Intercooler = Small Bore FUN!

tick tick tick boom.

Well yo are you all ready for me yet?

lol is there nothing that doesnt go off at a tangent on TM

Fab, its 9:1, rather than 9.2:1

Nic, I'm trying to get the compression down from 12:1, lol, but i did run 50 shot of nitrous at 12:1 and look where that got me

well my current (5-port carbe'd) build is 9:1 with a proposed boost of 12psi. which isn't far off 15 and you have all the advantages Paul lists above.

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

I am strugling with the same question.
I am trying to get a bit further on my supercharged 1293cc.
I have done some work on a combustion chamber which is now 28,5cc.


I have 21253 pistons with 8,4cc dish.
It will be running an MG metro cam and a small front mounted modified Escort RS intercooler.
Further more I plan on putting injection in it, not made my mind up yet about wet manifold or siamese coded; will make a post under EFI at an other time.
But I will have programmable ignition.

With the above ccs I calculate 8,9:1 CR.
Now I'm not after top HP and plan to run 10-12psi boost, I would like to have an engine that will last longer than a year as I don't have time to rebuild every year (obvious from current mega slow build :$).

Originally I was planning to go for 8,5:1 CR but reading this post I am thinking that might be going to the ultra safe side...

Mind you I would like to run 95 Octane but could use 97 Octane as back-up...
Car is used for weekend driving (if it will ever get finished).

8,5:1 or 8,9:1
Thoughts anyone?

Dazed and Confused....

if your definatly not going more than 12psi go for the 8.9:1

I have run 18psi on 8.5:1

I managed to get my compression down to 6.5:1, it was like driving an 850 on 1 cylinder off of boost, once I managed to get my T34 to boost it went like shit off a shovel lol...

The lowest you really want to go is 8.5:1 anything lower and you start to loose off boost drivability

I plan to run my 21253s with a 28cc head on no more than 10psi with an Rs intercooler as well so you should be fine Yo-Han

Well I was running 8.8:1 and ive now had a skim (only 6thou) lol, changed the gasket to a lower CC, and slapped a bigger turbo on for good measure. Im guessing its around 9:1 now.

Just do it, whats the worst that can happen?:)

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

Matty , It IS being done

I cant help but wank over the new pistons, lol, seems such a shame to put them in an engine they look so fucking good

Wind the boost up and you might see them sooner than expected!

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

Thanks lads!
Will stick to 28cc and see what happens...

Dazed and Confused....

What pistons did you go for in the end Colin?

If something is worth doing, it's worth doing half of.

Spunky modified 18cc Accralites.

They look the bollox straight out of the box, but, they now look fucking awsome!!!!!

Pictures please!

If something is worth doing, it's worth doing half of.

I will when I finaly get home on friday

What size you gone for? +20's?

If something is worth doing, it's worth doing half of.

I need to :(