selecting a compression ratio - 10 psi supercharged engine
Turbo Mini Forums

my understanding is that you want the compression ratio as possible without pre detonition. Is there a way of calculating this for a forced induction a series.

i am aiming for 10 psi from my eaton m45, what is the maximum comp ratio i can run without pre detonation????

cheers

lot of factors effect det

realistically on an iron head 'a' series:

cr
timing
charge temp

with more info, others might be able to help you find an answer, but if you consider the stock metro ran 7psi, and alot of guys on here run much more on std cr, (but with ic's and mj).

im not sure how using a sc will affect det, but someone might point you in the right direction

a search will probably reveal a hell of a lot of info

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

"did you know you can toast potato waffles?"

Firstly, no such thing as pre detonation. There is pre-ignition caused by the fuel air mix igniting before the spark. There is also detonation which, put simply, is igntion in the combustion chamber caused by the pressure wave generated by the spark.

However, both have similar causes and effects.

Pre-ignition and detonation are not caused by boost alone but more by a high inlet charge temperature. So you need to take into account the supercharger efficiency and if you have an intercooler.

There is no simple way to calculate the maximum compression ratio. Most people use trial and error and others experiences.

At 10 psi, a wel intercooled turbo engine could run as high as 9.5:1 without problems.

However, with a M45 supercharged engine without an intercooler, I would not start any higher than 8.5:1, simply because of the higher inlet charge temperature.

Just my opinion that may not be correct.....

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 tun 9-10 on a suck through at 8.7:1

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

how does the SC12 compare to the m45 efficiency wise though?

what sort of charge temps do you see?

Surely a modern supercharger like that is machined tight enough to be used blow through allowing intercooling, unless its one of those manifold mounted type?

yes same thoughts as pauls 8.5:1 is a safe margin on 95ron fuel