Compression ratio and volumetric efficiency.

[JAlanko]

I will say that ultra high static cr results in theory somewhat smaller cylinders when compared to same engine with heads with bigger combustion chambers. Incoming air-fuel mix have a mass, actually is quite large, but anyway it can fill cylinders with over 100VE% and if that occurs, so we will get a situation that in engine with bigger combustion chambers aka. lower CR will get more af mix inside of it, compared wit same engine with cylinder head with smaller combustion chambers aka higher cr.

And this is static, actually engine is running with dynamic cr and it depends so much at the camshaft that i don't go to it now...

[Allx]

In my RB26, I have no squish at all. They are completely removed and the chambers looks like a HEMI and I think i have around 7.0:1 CR.
With 264 inlet and 272 exhaust cams with 8.5mm lift and a GTX4088 turbo I make over 700 whp at 1.6 bar of boost.

Compared to my friends RB26, with stock head 8.5 CR and 9.5mm cams, mine makes around 100 hp more at the same boost level

[Stefan Ingi Jonsson]

In my RB26, I have no squish at all. They are completely removed and the chambers looks like a HEMI and I think i have around 7.0:1 CR.
With 264 inlet and 272 exhaust cams with 8.5mm lift and a GTX4088 turbo I make over 700 whp at 1.6 bar of boost.

Compared to my friends RB26, with stock head 8.5 CR and 9.5mm cams, mine makes around 100 hp more at the same boost level

Thats very interesting, do you have more specs on the motors and setup?

[Allx]

In my RB26, I have no squish at all. They are completely removed and the chambers looks like a HEMI and I think i have around 7.0:1 CR.
With 264 inlet and 272 exhaust cams with 8.5mm lift and a GTX4088 turbo I make over 700 whp at 1.6 bar of boost.

Compared to my friends RB26, with stock head 8.5 CR and 9.5mm cams, mine makes around 100 hp more at the same boost level

Thats very interesting, do you have more specs on the motors and setup?

Just forged bottom ends and slightly ported heads. Same manifolds and very similar turbos

[JAlanko]

If you raise your CR then your "possible maximum Ve-%" goes down and vice verca, if you lower your CR then you had potential to bigger Ve-%.

It's kind of consensus type of thing that 2-valve per cylinder cyl.heads benefits cr:s to somewhere about 14-14.5 cr, and if you had pentroof and 2 valves per cyl, you can have same amount of advantage with 12.5:1 cr:s as 14.5 in typical wedge or bathtub shaped combustion chamber.

If i had to build ultimate compromise engine, it would be 4-valve per cyl, intakes are about 17-19% bigger in diameter, hox, in diameter.
It has static cr something about 13.0 and much lover dynamic cr.
I prefer quite a square design in engine, but maybe a little undersquare even could work well, like if cylinder diameter was 86mm i prefer then stroke to somewhere like 94-98mm.

With these kind of measures engine must be something between 2.2-2.3liters in displacement.

[Leiffi]

Higher CR = higher VE