Head CC/Compression

[Ralphh85]

im trying to work out what compression ratio i should go for and how to work out how many CC my heads are now an what they need to be.

building a 3.9L, probably a crower 50232, TVR ported heads which have standard valves sizes but have wasted stem inlet valves an bullet valve guides. and have mappable Emerald ECU.

more compression will obviously give more power but are there any down sides? worse fuel economy? obviously its limited by the type of fuel which i have not totally decided on but i expect it will be standard unleaded as its in everyday use.

guessing 10.5:1 is in the region of reasonable compression?

does anyone know what the CC of the pistons are and how many the heads need to be? or do i need to measure it all an work it out?


Ralph

[ian.stewart]

bit of a grey area here with the cam overlap playing a major role in deciding the CR, Im running nigh on 14:1 Static CR, but taking into account the bleed off of my silly overlap cam it sems to work out about 11.5;1

Compression calks made easy here
http://www.csgnetwork.com/compcalc.html

[SuperV8]

I think if you have lots of unknowns you should work it out. Quite easy, just need a syringe and some clear perspex.

How would you plan on changing the CR if its not what you want?

Tom.

[sidecar]

I think if you have lots of unknowns you should work it out. Quite easy, just need a syringe and some clear perspex.

How would you plan on changing the CR if its not what you want?

Tom.

I agree, the safest way is to measure the piston at TDC, (use a DTI to get the piston at TDC), measure a chamber then workout the gasket vol then work out the CR.

A handy tip is 8 thou off the heads equals 1cc out of the chambers, this is not the same for a block being decked.

These lot might help you get hold of a burette:-

http://www.lab3.co.uk/contact_us

Pete

[kiwicar]

As a rough guide, an engine with a standard cam will take a 10.5:1 cr (mechanical worked out as if the inlet cam closes fully at BDC)
If you want a guide to what your new cam will take then instead of using the full stroke of the crank to work out the CR from when the inlet valve is effectivly closed at .05" lift. work out the effective CR by using the formula, new stroke = actual stroke X( (1+ cos (angle at which valve has .05 lift))/2) for the original cam, this will give you a number less than 10.5
Now do the same for the new cam with the new and it's new closing point (.05 lift point), it will give you a new CR even lower. You can now work out the chamber volume that will give you a CR that matches what the original cam's CR was when the valve closed.
The formula for the new mechanical CR is
required mechanical CR = 10.5 X original cam effective CR/ new cam effective CR
It is aproximate because you are not taking into account improvments in exhaust pulse tuning or higher volumetric efficiency but it will get you close (say +-0.5to.75 CR)
Best regards
Mike