20 replies to this topic

[Dusky]

Hi again!

While I am waitingfor a solution for my camshaft ( someone is intrested in it)

I am wondering what the Ideal dynamic compression would be of a mini 998.

I heard static is ideally 10.1.. But what's the ideal dynamic then? For road use ( don't want to rebuild the engine every year..à

 

 

grtz!

[minimadles]

We are back to cam timing the dynamic cr is measured from when the inlet valve closes and has to be measured once the engine has been dry built and varies from engine to engine but your getting quite technical now aim for 10:1 static and you won't be far away

[ACDodd]

What camshaft and rocker combo are you using? What fuel octane are you intending to use? Typically there are 3 types of compression ratio. Static, effective and dynamic. The compression ratio at valve closure is refered to and effective compression ratio. Dynamic compression ratio is somthing quite different and is when the engine is running. This figure is different for different RPM's.

[ACDodd]

Effective compressions in the order of 8.0 are the order of the day. For Road engines. However this figure is affected by rocker and cam timing. My suggestion is measure both static and effective during the dry build and see what you actually have. From this point sensible suggestions can be made.

[minimadles]

Yes sorry ac is correct got me effectives and dynamics mixed up lol dynamic is effected by lots of issues not just cams and rockers

Edited by minimadles, 06 January 2014 - 03:57 PM.

[Dusky]

What camshaft and rocker combo are you using? What fuel octane are you intending to use? Typically there are 3 types of compression ratio. Static, effective and dynamic. The compression ratio at valve closure is refered to and effective compression ratio. Dynamic compression ratio is somthing quite different and is when the engine is running. This figure is different for different RPM's.

I mixed up effective and dynamic then ^^

Calculated with a calculater that I would have ( with standard rockers), bmc 544 profile cam, 998 engine with 10.1 static compression a effective compression of 7.37....

 

Not that far from 8, but stil... With standard cam it would be 8.77 ..

[ACDodd]

How are you measuring piston position?

AC

[grumpy dad]

Hi

could one of you 

do a list so that when you stick a compression tester in

and it says 130 psi this means a compression of  X

and then some guide

as to you could go X plus a % and that is the best for your motor

[ACDodd]

Erm no, as this depends on too many variables!!! For a good fast road engine and sotck or 1.3 rockers you should be aiming for 180 to 190psi of cranking compression. If you have more than this you will find that with the fuel we have abvailable today the engine will be timing fussy and liable to pinking.

 

AC

[grumpy dad]

Hi 

thanks AC

that now gives people a good bench mark 

[Dusky]

How are you measuring piston position?

AC

Hey!

to be honest.. I just used this calculator : http://www.wallacera.../dynamic-cr.php

grtz

[ACDodd]

Nope you have to measure the actual position at actual valve closure. It is significantly different to using the manufactures cam figures.

Assumption is the biggest killer of engines........

Ac

[Dusky]

Oh, I thought that the calculator used that ( Inlet Valve Closes ABDC )

Hmm, it is (again) more complicated than I thought!

[ACDodd]

Nope this is for figures you put in. You would put in those off of the cam specs. These are not the same as those the engine sees. Those need to be measured.

Ac

Edited by ACDodd, 07 January 2014 - 07:12 PM.

[Dusky]

Learned something again ;)

Thanks Ac, you really are building half of my mini

 

grtz!