what cr

[ford100ev8]

I have a rv8 3.9, i am fitting twin turbos, what cr do i need, i wa t bout 450 hp, can i use compresen plates or do i need forged pistons, thanks

[sidecar]

You'll need an awful lot of stuff to get 450 BHP out of a 3.9 if you want it to hold together of any length of time. Forced induction ain't my thing but forged pistons with a low CR and decent con rods would be a start. You will probably need O ringed heads and ARP studs. The cooling system is going to be have to be something special too.

Clutch, gearbox, ignition system, induction system (EFI)....they are all going to have to be top notch too.

[mgbv8]

I'm using cometic mls head gaskets on my current build for the blower.

The comp gaskets dont last long when you start stuffing shed loads into the engine.

Forged pistons will be important as well. And try to keep the CR down a bit.

I'm still running 9:1 cr on my 4.6 and it makes 398hp with about 10psi of boost.

Stock heads and other internals though + a real steel blower cam.

[stevieturbo]

I have a rv8 3.9, i am fitting twin turbos, what cr do i need, i wa t bout 450 hp, can i use compresen plates or do i need forged pistons, thanks

What fuel ? how efficient is your intercooler ? Do you have proper control over fuel and sparks ?

All of these will play a part.

A compression plate would be an absolute last resort and could require longer pushrods.

You can achieve different CR's various ways. Head chamber, gasket, piston dish.
Pistons dont need to be forged, they just need to be suitable.

[ford100ev8]

as it stands i have a standard rv8 3.9 ,i have two td04 turbos, i am making some stainless manifolds to fit these,

[ford100ev8]

What other parts would i need, thanks

[kiwicar]

hi
I would look at a CR of 8.75 to 9:1, avoid decompression plates like the plague, set the squish band (piston to head clearance) to .035" to .040" use the real steel blower cam, forged or at least hyperutetic pistons, use ARP bottom end and head studs, lightly ported heads with bigger exhaust valves, a wasted spark ignition and EFI, a nice big intercooler and probably a light weight flywheel. Chamfer all the oilways especially the ones in the crank.
Best regards
Mike

[stevieturbo]

hi
I would look at a CR of 8.75 to 9:1, avoid decompression plates like the plague, set the squish band (piston to head clearance) to .035" to .040" use the real steel blower cam, forged or at least hyperutetic pistons, use ARP bottom end and head studs, lightly ported heads with bigger exhaust valves, a wasted spark ignition and EFI, a nice big intercooler and probably a light weight flywheel. Chamfer all the oilways especially the ones in the crank.
Best regards
Mike

Budget dependant of course. Most of the above does make sense.

I wouldnt say larger valves are needed though, but depends how much he wants to spend. Again a light port cleanup can do no harm. Cam choice may dictate new or at least shortened valve guides depending how much lift. I would fit a decent set of valve springs though, I think Neal had issues on his with standard springs.
I also would say that a light flywheel offers no real benefit to make it worth having.

I would say that 8.5-9:1 would be a decent enough choice for CR, but you will need a decent intercooler and you will need proper control mover fuel and timing. If you are lacking in either of these areas, a lower CR will make more sense.
Also assuming using super unleaded. If opting for 95, err on the side of caution with a lower CR.